Event Summary
     National Weather Service, Raleigh NC


March 2, 2009 Winter Storm
Updated 2009/04/21






Event Headlines

...Model guidance was generally good and forecasters took advantage of the good forecast data and their meteorological reasoning to issue Winter Storm Watches which verified with an average lead time of nearly 40 hours. Winter Storm Warning lead times verified around 18 hours with no missed events...
...Isolated bands of snow that moved across the Northern Piedmont of North Carolina during the morning of March 2, 2009 appeared to be a result of thermal instability in the boundary layer that produced horizontal convective rolls...
...Forecasters used NCDOT and other highway traffic cameras to supplement the limited traditional observations during the storm. Collecting information on snow accumulation and snow impact during the overnight hours is often very difficult...
...Forecasters utilized AMDAR aircraft soundings to provide thermal profiles at GSO and RDU in between RAOB times. This data set is an important resource during critical winter weather forecasts.
...Several CoCoRaHS intense snow reports were received by the NWS Raleigh. These reports are especially important since it is very difficult to get accurate snow accumulation reports late at night...
...Warm weather preceeding the evwnt along with warm ground temperatures resulted in some melting of the snow as it accumulated across the Eastern Piedmont and Sandhills...
...The CIPS Winter Storm Analog Guidance data was archived during the event and a brief summary of the methodolgy and the data for this event are available.
...A notable mountain shadow effect can be seen in the snow accumulation analysis and in the high resolution MODIS visible satellite picture...


Event Overview

The winter storm that produced significant rain and snow across central North Carolina during the end of February and early March was a result of three features that moved across the Carolinas. The first feature was a surface low pressure system centered over the Ohio River Valley at 00 UTC on February 27th, and its associated cold front. The second feature was a high pressure system moving across southern Canada that led to the development of a cold air damming event over North Carolina. Finally, the third feature was a 500 mb low located over the northern Rockies on February 27th that would eventually be the main driving force for the snowfall seen across the Carolinas.

On February 28th at 00 UTC, an area of low pressure moved from the Ohio Valley northeast into southern Quebec. The associated cold front moved across the Appalachians and into the western Carolinas. The 00 UTC 02/28 Greensboro RAOB showed a saturated layer up through 700 mb with above freezing temperatures from 750 mb down to the surface. Meanwhile, the 500 mb low in the northern Rockies had moved east into the northern Plains. While the upper level low was most apparent aloft, it was visible at 700 mb but rather disorganized in the lower levels. By 12 UTC on the 28th, the cold front had advanced across western and central North Carolina before becoming nearly stationary in the Coastal Plain. By 18 UTC, a high pressure system moving eastward across the northern Great Lakes extended into Virginia and North Carolina in a position favorable for classical cold air damming. Along the front, a well-defined surface low emerged over central Alabama and an 850 mb low located in central Tennessee with snow falling in Memphis and portions of the Deep South. The first period of precipitation across central North Carolina during this event was produced by isentropic lift or warm advection over the southeastward advancing cold air mass during the early morning through mid afternoon hours on Saturday, February 28.

By 00 UTC on March 1st, an upper level trough formed in the jet stream with an upper level low now visible at 300 mb over northern Arkansas. The upper low was generally vertical down through 700 mb with the 850 mb and surface low still ahead of the system over western South Carolina. With a surface high now positioned over Quebec, low level northeasterly winds had increased across the Foothills and Piedmont of North Carolina, an indication of cold air damming. The 00 UTC 03/01 Greensboro RAOB confirmed this with 20-30 knots of northeasterly flow near the surface. In addition, the low level temperatures dropped considerably from the previous RAOB and a cold nose began to form near 900 mb.

The cold air flowing into central North Carolina pushed the cold front off the coast by 12 UTC. The upper level trough began to dig further south into the Gulf of Mexico with the upper level low over Alabama beginning to catch up to the surface low. Winter precipitation over northern Georgia began to slowly move northeast as the parent high pressure system that initiated the cold air damming lifted off to the northeast. The 12 UTC 03/01 Greensboro RAOB showed additional cooling in the low levels as surface temperatures dropped closer to zero. This The second period of precipitation arrived around daybreak and was driven by significant upward motion ahead of the approaching upper level low including low level warm advection, and impressive upper level divergence.

By 18 UTC, snow was falling upstream in a deformation zone across Georgia, portions of Alabama, and western South Carolina. In addition, a layer of elevated instability was producing thundersnow and snow fall rates of 1 to 2 inches per hour in the area.

The main surface low had moved offshore and began to deepen as a surface trough extended back into central North Carolina by 00 UTC on March 2nd. The upper level low was now centered over western South Carolina. The precipitation across the eastern Carolinas diminished as a pronounced dry slot (water vapor imagery from 2115 UTC on March 1  | 700 mb analysis from 21 UTC on March 1) moved across the Carolinas between 18 UTC on March 1 and 03 UTC on March 2. Upstream across northwestern South Carolina, the precipitation was becoming more convective with a cold pocket of temperatures at 500 mb associated with the upper low. A moist adiabatic absolutely unstable layer or MAUL can be seen in the 00 UTC 03/02 KGSO RAOB in the 750 to 650 mb layer, indicative of the increasing instability at mid levels.

The
temperature at 850 mb had cooled dramatically across central North Carolina by 06 UTC. At 0600 UTC the main surface low continued to move northeast as snow began to overspread much of central North Carolina from the west. This third area of precipitation was associated with a TROWAL feature or deformation zone located to the north and northwest of the 850 mb and 700 mb low track that produced a relatively persistent band of light to moderate snow. A few hours later, the 12 UTC Greensboro RAOB showed freezing temperatures throughout the profile.

The storm system moved offshore and tracked up off the eastern seaboard by 12 UTC. Most snowfall associated with the system ended by 1800 UTC on March 2nd and the Greensboro RAOB at 00 UTC on March 3rd indicated that dry air had moved into the area between the 850 and 500 mb.

Snow accumulations were generally stratified from northwest to southeast primarily because of the temperature distribution in the lower portions of the atmosphere and the placement of the greatest forcing for ascent. Outside of the mountains where 12 inches or more of snow was reported, the heaviest snowfall amounts of 6-10 inches fell in a discontinuous band running from west of Charlotte to west of Greensboro. An average of 2 to 4 inches of snow fell across the eastern portions of the Piedmont and an inch or less fell in the Coastal Plain.

Across central North Carolina, the greatest snowfall reports were of 7 inches in Person County. Snow accumulations generally decreased from west to east with 4-6 inches of snow in the Triad, 2-4 inches along the U.S. Highway 1 corridor and a Trace–2 inches falling along the I-95 corridor. Officially, the RDU airport measured 3.2 inches of snow and Greensboro had 5.7 inches.

A significant minimum in snow accumulation amounts was noted just east of the mountains where a considerable down slope flow and the associated subsidence resulted in reduced snow accumulations.


Snow Accumulation Map

Snow accumulation map


MODIS Visible Satellite Image Showing Snow Cover from 2009/03/03

MODIS visible satellite image showing snow cover - click to enlarge




Surface Analysis

A strong, cold area of high pressure located over the Northern Plains separated into two high pressure centers early on February 28, with one area of high pressure moving into southeastern Ontario. The high pressure system was of sufficient strength (>1035 mb) and located in a location supportive of the "Classical" cold air damming sub-type. By 12 UTC on March 1, the damming pattern was evident on the surface analysis and a local analysis of low level partial thicknesses. By 00 UTC on March 2, the surface low was redeveloping near Cape Hatteras. The 850 mb low and trough axis and the 700 mb low and trough axis were moving east into the Piedmont of the Carolinas allowing the tight thermal gradient to shift east resulting in a rapid cooling of the lower portions of the atmosphere. By 12 UTC on March 2, the surface low was well off the DELMARVA coast with a strong northerly flow and cold advection across central and eastern North Carolina.

The surface analysis from 03 UTC on Monday, March 2, 2009 shown in the image below, depicts the surface weather pattern as a large area of snow moved eastward across the western Carolinas. During the next few hours, the snow would shift eastward producing fairly significant snow accumulations across much of the Northern Piedmont and Coastal Plain.

A Java Loop of surface analysis imagery from 00 UTC Saturday, February 28 through 00 UTC on Tuesday, March 3, 2009 shows the evolution of event.



Surface analysis from 03 UTC on Monday, March 2, 2009 - click to load a loop of Surface analysis




Satellite Imagery

Water vapor imagery was used to monitor the track and intensity of the upper low as it moved across the mid Mississippi Valley into the Deep South and then across the Carolinas. The Water vapor imagery was also used to monitor the development and progression of a significant dry slot that moved across the Carolinas from around 18 UTC on March 1 through around 03 UTC on March 2. The dry slot produced a significant break in the precipitation across the region on Sunday evening.

A Java Loop water vapor imagery from 1215 UTC on Saturday, February 28 through 1815 UTC on Monday, March 2, 2009 is available.

Water Vapor satellite imagery from 1215 UTC on Saturday, February 28 through 1815 UTC on Monday, March 02, 2009 - click to control



Southeast Regional Radar Imagery

The regional radar imagery shows the multiple rounds of precipitation that moved across the region during the event. Each area of precipitation was generated by different forcing mechanisms. For example, the first area of precipitation was produced by isentropic lift or warm advection over the southeastward advancing cold air mass that arrived across North Carolina during the early morning hours on Saturday, February 28 and persisted into the mid afternoon hours. Another area of rain advanced into central North Carolina just before daybreak on Sunday, March 1. This area of precipitation was driven by significant upward motion ahead of the approaching upper level low including low level warm advection, and impressive upper level divergence. This area of precipitation diminished as a pronounced dry slot (water vapor imagery from 2115 UTC on March 1  | 700 mb analysis from 21 UTC on March 1) moved across the Carolinas between 18 UTC on March 1 and 03 UTC on March 2. Finally, a third area of precipitation moved across North Carolina during the evening hours on March 1 and overnight into the morning hours of March 2. This area of precipitation was associated with the deformation zone located to the north and northwest of the 850 mb and 700 mb low track.

A Java Loop of Southeast regional radar imagery from 2358 UTC on Friday, February 27 through 1458 UTC on Monday, March 2, 2009 is available.

Southeast regional radar reflectivity from 0158 UTC on Monday, March 2, 2009 - click to control




Partial Thickness Values

The thickness of a layer of air is proportional to the layer's mean temperature. The warmer the layer, the greater it's thickness. The layer of air bounded by a pressure of 1000 - 850 MB is used by forecasters to monitor the average temperature in the lower level of the atmosphere (roughly from the surface to 5,000 ft). The 850 - 700 MB layer is closely followed by forecasters to monitor the average temperature in the layer of air (roughly from 5,000 to 10,000 ft) where elevated above freezing temperatures may exist. NWS forecasters at Raleigh have for decades now, correlated the thickness of these layers as observed in RAOB's to the observed wintry precipitation types. The technique has proven to be helpful in anticipating the frequent precipitation type changes often associated with winter storm events in central North Carolina.

A loop of partial thickness values and weather depicts the evolution of the thermal pattern in the lower levels of the atmosphere during the event. A cold air damming signature is visible in the 1000-850 mb thickness pattern by 00 UTC on March 1 with the signature becoming even more prominent by around 12 UTC on March 1. The transition from rain to snow across central North Carolina can be seen in the analysis from 00 UTC on March 2 to the analysis from 04 UTC on March 2. Note the rapid cooling of the 850-700 mb layer which essentially eliminated the warm nose on the 00 UTC RUC sounding from KRDU. Note the dramatic cooling of the 1000-850 mb layer between midnight (05 UTC) and daybreak (12 UTC). Surface temperatures during this period fell well into the 20s by 700 AM EST across the northern and especially Northwestern Piedmont.

A Java Loop of partial thickness and weather from 12 UTC February 28, 2009 through 12 UTC March 2, 2009 is available.


Partial thickness, surface wet bulb temperatures and Weather loop from 00Z March 2, 2009




TREND’s Predominant P-type Nomogram

The nomogram below shows the distribution of precipitation (p-type) TREND's as a function of partial thickness values. Close examination of precipitation events over the past 30 years accounts for the boundaries on the nomogram separating the various p-type areas. Mid level thickness values increase from left to right along the x axis. Low level thickness values increase from bottom to top along the y-axis.

The first image below displays the observed thickness values from the 6 hourly RAOB's at KGSO from 01/00 UTC through 02/00 UTC on March 2nd. Initially the atmosphere cooled in the lower levels as shown in the nearly vertical drop in the TREND's line produced by a significant drop in the 1000-850 mb thickness between 12 UTC on 2/28 and 00 UTC on 3/1. The lower levels of the atmosphere do not cool much between 00 UTC and 12 UTC on 3/1 with some cooling in the mid levels indicated by a drop in 850-700 mb thicknesses. More significant cooling occurs between 12 UTC and 18 UTC with the air mass changing little between 18 UTC and 00 UTC on 3/2. A significant drop in the 1000-850 mb and 850-700 mb thicknesses occurs after 00 UTC on Tuesday as the upper low approaches. The second image contains a list of observations from KGSO from 12 UTC on 2/28 to 12 UTC on 3/2. Note the changeover from rain to snow that occurs around 00 UTC to 01 UTC on 3/2.







Shallow Convective Snow Bands


TRDU base reflectivity loop Just before daybreak on March 2, 2009 as a large area of precipitation was moving away from central North Carolina, forecasters began to notice bands of snow developing and extending southward across central North Carolina (loop of KRAX WSR-88D reflectivity imagery and loop of TRDU TDWR reflectivity imagery). The bands were also visible across southern Virginia and northern North Carolina on the visible satellite imagery available just after daybreak.

The snow was expected to be on the wane during the early morning hours as a low pressure system developed off the Mid Atlantic coast and a 1041 mb high pressure system moved into the western Great Lakes and extended into the Mississippi Valley. These systems combined to produce a moderate pressure gradient that resulted in steady northerly winds across central North Carolina. The northerly flow combined with a cold air mass over the Great Lakes and Ohio Valley produced cold advection across the Carolinas and Virginia both at 850 mb and at the surface. At the same time, air temperatures in the mid to upper 20s were moving across 4 inch soil temperatures in the 39 to 41 degree range in the Triangle area (ground temperature map |  soil temperature text product).

The base reflectivity radar loop from the krax WSR-88D shown to the right shows the appearance of the bands after a larger area of snow moves northeast away from central North Carolina. In the wake of the larger precipitation area, numerous bands of light precipitation appear. During the loop which runs from 1004 UTC through 1431 UTC on March 2, 2009, the narrow bands consistently extend from north to south as the number and character of the bands themselves evolve. The krax base reflectivity image from 1044 UTC shows numerous bands extending southward from near the Virginia border to just south of the Triangle area. By 1239 UTC, the band structure became fairly regular with north south bands separated by approximately 5 km (3 miles). The narrow bands became more difficult to find as a slightly larger area of precipitation associated with an approaching short wave moved southeast across the region and changed the narrow bands into larger and broader areas of precipitation.

Visible satellite imagery from 1245 UTC showing the across southern Virginia and northern North Carolina - click to enlarge A previous study, Snowbands during the Cold-Air Outbreak of 23 January 2003 (Schultz, D.M., D.S. Arndt, D.J. Stensrud, and J.W. Hanna, 2004) investigated similar events and hypothesized that the bands were produced through two processes: 1) thermal instability in the planetary boundary layer that produced horizontal convective rolls (HCR's) over widespread areas, and 2) lake-effect processes downstream of small lakes which produced localized bands. The horizontal convective rolls are a strong candidate for the production of the bands since the bands were associated with a strong cold-air outbreak, at times they were regularly spaced over a large area, and occurred within the planetary boundary layer. The study noted that the updrafts associated with HCR's in the planetary boundary layer can saturate and produce parallel cloud bands or cloud streets if the updrafts are sufficiently deep and moist. HCR development can be result of either thermal or dynamic instabilities. The March 2, 2009 event was associated with cold advection over a relatively warm land surface suggesting that the thermal instability mechanism likely produced the HCR. The same study noted that for this mechanism, the buoyancy provides the energy for the circulations, and the vertical wind shear organizes the circulations into bands provided the instability does not become too great. Finally, HCR's due to the thermal instability mechanism are frequently observed in slightly unstable environments with some sensible heat flux from the surface.

In the study by Schultz, Arndt, Stensrud, and Hanna, they state that HCR's in the planetary boundary layer form when the environmental lapse rate is near neutral or slightly unstable, the mean wind speed in the roll layer exceeds some relatively small value (2–5 m/s), the vertical wind shear is nonzero, and a modest value of surface sensible heat flux exists (Atkinson and Zhang, 1996).

Four panel RUC analysis soundings for RDU - click to enlarge The four panel image of RUC analysis soundings shown to the right (click on the image to enlarge) for KRDU on March 2 at 09 UTC, 12 UTC, 15 UTC, and 18 UTC shows that the boundary layer became slightly unstable. In fact, the RUC soundings at 12 UTC and at 15 UTC indicates that a layer at or just above the surface which was around 4,000 to feet deep had become unstable with the top of the boundary layer at around 850 mb or 5,000 feet.

The winds in this layer ranged up to around 30 knots and they were from the north-northwest with very little directional shear. Wind speeds were light at the surface and then increased in speed up to around 3,000 feet. While there was very little backing of the winds observed in the lower portion of the RUC analysis soundings, backing was noted above 850 mb in the frontal inversion. Cold advection was noted in the SPC 850 mb temperature advection analysis and in the SPC 3 hour surface temperature change analysis.

Some sensible heat flux should be expected with surface air temperatures ranging between 26 and 30 degrees at 12 UTC and soil temperatures ranging between 39 and 41 degrees at 12 UTC. Following a similar methodology used in the study by Schultz, Arndt, Stensrud, and Hanna, the difference between the ground temperature at 4 cm below the native vegetation (TS04) and the air temperature (TAIR) was calculated. Across the Northern Piedmont where the snow bands occurred, the ground temperatures at 12 UTC ranged between 39 and 41 degrees while the air temperatures at 12 UTC ranged between 26 to 30 degrees. For example, at Oxford in Granville County, the ground temperatures at 12 UTC was 39 degrees while the air temperatures at 12 UTC was 26 degrees, a difference of 13 degrees. Similar differences between the ground and air temperatures were observed in the Northern Piedmont with differences of 13 degrees at Reedy Creek in western Wake County and 13 degrees at Clayton in northwestern Johnston County. Goldsboro reported a difference at 12 UTC of 10 degrees.

TRDU Base reflectivity from 1259 UTC on March 2 - click to load a loop The bands of light snow during the morning and into the midday hours on Monday generally produced only light snow accumulations with most locations in the Northern Piedmont reporting an additional light coating of snow on Monday morning. The snow was rather persistent through much of the morning across portions of the area (observations from Raleigh-Durham and Henderson) with light snow falling for several hours in North Raleigh with only a light dusting of new snow observed. This period of snow was generally unanticipated and was likely a surprise to many residents in the area.

Another interesting aspect of the snow bands is that some of the bands appear to have at least a limited connection with the relatively large lakes that straddle the North Carolina - Virginia border including Kerr Lake and Lake Gaston. The reflectivity loops from the KRAX WSR-88D radar located 10 miles southeast of Raleigh and the TRDU Terminal Doppler Weather Radar (TDWR) located just north of the Raleigh-Durham International Airport can be stopped and advanced to allow improving viewing of the phenomenon. While many of the bands do no appear to originate or even be located downwind of the lakes, there are a couple of bands that may be enhanced by the additional flux of moisture heat from the relatively mild lake surface.

Oftentimes the precipitation on the back edge of a departing winter cyclone ch0nages from light snow to a light drizzle or freezing drizzle as the deep moisture exits and the only lingering moisture is shallow and below the favored growth zone for dendritic ice crystals between -12 ° C and -18 ° C. The RUC analysis soundings from March 2 at KRDU indicate that for much of the morning after daybreak the low level moisture was generally confined below (warmer) the -10 ° C to -12 ° C layer (RUC soundings for KRDU on March 2 at 09 UTC, 12 UTC, 15 UTC, and 18 UTC. The moisture appeared to be sufficient at 09 UTC with considerable drying noted at 12 UTC. The low level moisture extended up to -14 ° C at 15 UTC with only some slight drying in the sub -10 ° C layer at 18 UTC.

The bands of snow that moved across the Northern Piedmont of North Carolina during the morning of March 2, 2009 were an unusual event. The bands may be a result of thermal instability in the boundary layer that produced horizontal convective rolls (HCR's). The instability is largely the result of large scale cold advection over a relatively warm ground that produces a modest sensible heat flux. The HCR's may have been enhanced by additional fluxes of sensible heat and moisture from the relatively large lakes near the North Carolina - Virginia border including Kerr Lake and Lake Gaston. It is somewhat uncommon in winter across central North Carolina to get the conditions necessary for snow producing HCR's including a boundary layer that is very moist, increasingly unstable, and sub freezing.


Using Analogs as Medium Range Guidance for Organized Snow Events



Note - much of the text below was provided by the CIPS Winter Storm Analog Guidance page.

Researchers at the Cooperative Institute for Precipitation Systems (CIPS) along with the NWS St. Louis, MO and the NWS Springfield, MO are engaged in a project to show how the knowledge of past events that exhibit similar characteristics to the current forecast can assist forecasters with a range of potential scales and intensities of the upcoming event. The project is entitled "Using analogs as medium range guidance for organized snow events."

The goal of the analog forecast approach is not to make a forecast; but to provide medium-range guidance for events by using a historical dataset. In addition, a forecaster can quickly gain historical experience and become familiar with the meteorological patterns associated with certain events. The analog forecast approach can be applied to any meteorological event as long as a control run can be created.


Methodology

The methodology below is used by the CIPS Winter Storm Analog Guidance Project which currently uses 36, 48, 60, and 72 hour 40 km GFS forecasts.

  • The basis for analog guidance is to search a climatological dataset for maps that resemble the current forecast, and then assume that the atmosphere will evolve similarly to the historical analogs.

  • Search the 28 year North American Regional Reanalysis (NARR) dataset against the model forecasts provided by the 40km GFS (GFS212) for potential analogs based on the constraints below:
      6 months over the winter season (October - March)
      6 hour temporal resolution
      Potential database of 20,384 analogs (28 winters, 6 months, 4 per day)
  • Remove “duplicate” times for the same analog event by choosing the best analog over a 24-h period.

  • Refine and rank the resulting analogs to create products that are useful for medium range guidance.

  • Determining what constitutes an analog is done statistically using the following techniques:
      Pattern Correlation
      Mean Absolute Error
      Root-Mean-Square Error
      Anomalies
  • During the first pass through the NARR dataset, the statistics are computed on a large domain (REGN) and then on a smaller domain (MESO). If certain thresholds are not exceeded, the date/time is not considered a potential analog.

  • To reduce the approximately 20,000 potential analogs, threshold values were determined based on the control run for the following fields:
      300 HGHT COR 0.85 REGN
      500 HGHT COR 0.83 REGN
      850 TMPC MCOR 0.88 MESO
      850 TMPC MMAE 3.8 MESO
      PMSL MCOR 0.83 MESO
      850 HGHT MCOR 0.70 MESO
  • Once the approximately 20,000 potential analogs are reduced, “duplicate” times are removed. “Duplicate” times occur due to the variability in system speed (e.g., a slow historical system may exhibit similar patterns to the forecast over a longer period of time).

  • The best analog is found over a 24-h period by using the following formula:
      SUM(COR) - SUM(MAE/3)
  • After the potential analogs are reduced, the program is rerun to find statistics on the following variables:
      300 HGHT COR REGN
      500 HGHT COR REGN
      700 FRNT COR MESO
      850 HGHT COR MESO
      850 TMPC COR MESO
      850 TMPC MAE MESO
      850 FRNT COR MESO
      850 THTEADV COR MESO
      2m TMPC COR MESO
      2m TMPC MAE MESO
      PMSL COR MESO
      PWTR COR MESO
  • After new statistics are determined, a results score is computed using the following formula:
      850HGHTCOR*3 + PMSLCOR*2 + SUM(COR) - SUM(MAE/3)
  • Finally, in order to catch possible system propagation, statistics are computed for ± 12 h from the time of the best analog using the matching forecast (the 48 hour forecast is used in the example below):

    (-12 h Analog: GFS 036h FCST, Analog: GFS 048h FCST, +12 h Analog: GFS 60h FCST). Using statistics from the ± 12 h times, results scores are also computed.

  • The final analog rank is determined by using the results scores in the following formula:
      AVERAGE(m12ANALOG, ANALOG, p12ANALOG)

    Example Analog for this Event

    The CIPS Heavy Snow Analog Guidance for 00 UTC on 3/2 across the East Coast 2 domain was examined for this event and a brief review of the process and data is shown below. The analog was based off the 48 hour forecast from the 40km GFS (212 grid) valid at 00 UTC on 3/2. The four panel GFS 48 forecast indicated an impressive 500 mb cutoff low centered just west of Charleston, SC (lower left panel) and a modest 1004 mb surface low off the North Carolina coast with a band of snow falling across the western Carolinas and Virginia (upper left panel).

    The CIPS Heavy Snow Analog Guidance Table for the East Coast 2 domain from the GFS 40km 00 UTC 20090228 48 hour forecast is shown below (click to enlarge). Note the statistics for GFS212 20090228/0000F048 forecast:

    Total number of potential analogs: 20044
    Number of potential analogs that did not exceed set thresholds: 19954
    Number of potential analogs that exceeded set thresholds: 90
    Number of distinct analog events: 42

    Percentage of potential analogs that did not exceed the set threshold at each step of the analog process:
    300-mb HGHT correlation: 80.58 %
    500-mb HGHT correlation: 1.72 %
    850-mb TMPC correlation: 5.32 %
    850-mb TMPC mean-absolute-error: 5.94 %
    PMSL correlation: 4.48 %
    850-mb HGHT correlation: 1.95 %


    CIPS Heavy Snow Analog Guidance Table for the East Coast 2 domain from the GFS 40km 00 UTC 20090228 48 hour forecast - click to enlarge


    The 19890223/1800 analog had the greatest final analog rank based on the averaging of that forecast along with the statistics from 12 hours prior to and after the 0223/18 forecast. The CIPS Heavy Snow Analog page provides analysis maps of the COOP snowfall from the various analogs along with some probabilities based on the analogs.

    The COOP snowfall analysis maps from the CIPS Heavy Snow Analog page for 02/23/1989, 02/24/1989, and 02/25/1989 are shown below. In addition, the mean and median of the top 15 analogs are also provided on the CIPS Heavy Snow Analog page.

    Analysis map of the COOP snowfall from 02/23/1989 - click to enlarge Analysis map of the COOP snowfall from 02/24/1989 - click to enlarge Analysis map of the COOP snowfall from 02/25/1989 - click to enlarge

    The CIPS Heavy Snow Analog page also provides images displaying measures of center and spread guidance for various fields including mean sea level pressure, 850 mb height, 500 mb height, 300 mb height, 850 mb temperature, and 2 meter temperature.

    500 mb height mean based on the top 15 analogs - click to enlarge  5460 meter spaghetti based on the top 15 analog - click to enlarge

    850 mb height mean based on the top 15 analogs - click to enlarge 1410 meter spaghetti based on the top 15 analog - click to enlarge

    Mean sea level pressure mean based on the top 15 analogs  - click to enlarge 1008 mb spaghetti based on the top 15 analogs - click to enlarge

    Some of the more insightful products are the probabilistic forecasts of COOP snow greater then various thresholds. The probabilistic forecast of COOP snow greater then 2 or 4 inches based on the top 15 analogs is shown below. Additional probabilities for COOP snow amounts greater then 6 or 8 inches are also available along with probabilities of snow to liquid ratios from COOP observations greater then 10:1, 12:1, 14:1, and 16:1.

    Probabilistic forecasts of COOP snow greater then 2 inches based on the top 15 analogs - click to enlarge Probabilistic forecasts of COOP snow greater then 4 inches based on the top 15 analogs - click to enlarge


  • AMDAR Aircraft Soundings

    AMDAR is an acronym for Aircraft Meteorological DAat and Reporting (AMDAR) which is an international effort within the World Meteorological Organization to coordinate the collection of environmental observations from commercial aircraft. In the United States, we often refer to the Meteorological Data Collection and Reporting System (MDCRS) which is a private/public partnership facilitating the collection of atmospheric measurements from commercial aircraft to improve aviation safety.

    AMDAR is very useful for short term forecasting situations where conditions are changing rapidly and in particular for aviation forecasting. Regarding winter weather events, AMDAR data can provide forecasters with the height of the freezing level, the presence of elevated warm layers, indications of thermal advection and dry layers. All of these are necessary for accurate precipitation type forecasts. The availability of this upper air data at times and locations where RAOB data may be lacking is invaluable.

    The image below contains a loop of AMDAR soundings at RDU during the event from 18 UTC on 03/01 through 15 UTC on 03/02. During the 21 hour period from 18 UTC on 03/01 through 15 UTC on 03/02 there were a total of 14 AMDAR soundings available at KRDU. A Java Loop of AMDAR soundings from 1819 UTC on Sunday, March 1 through 1406 UTC on Monday, March 2, 2009 that can be stopped, controlled and zoomed is available.

    Forecasters used AMDAR data to supplement other observational data to monitor the erosion of the mid level warm nose and to determine the depth of the surface based shallow layer of cold air. Use of AMDAR data was noted in the 1145 AM AFD on March 1 and the 345 PM AFD on March 1.






    Snow Accumulation and Soil Temperatures

    Light to moderate snow fell across much of central North Carolina for several hours during the storm with a varied snow accumulation pattern. A subjective review of the event, notes that the snow accumulation was most efficient in locations with a cooler soil temperature along with slightly cooler but subfreezing surface temperatures and greater precipitation rates. The snow accumulated most readily in the Triad area where accumulations ranged between 4 and 7 inches with 4 cm soil temperatures in the upper 30s and surface temperatures falling to around 30 degrees during the time in which snow was falling from the late evening of 3/1 through the early morning hours of 3/2. Further east, across the Triangle area, the snow did accumulate although there was considerable melting at the base of the snow accumulation near the ground. Soil temperatures in the Triangle area ranged between 39 and 42 degrees during the time in which snow was falling from the late evening of 3/1 through the early morning hours of 3/2.

    Snow accumulated on most area roadways during the event with the degree of accumulation varying based on the air temperature, ground temperature, precipitation amount, precipitation rate, and the degree of direct sunshine available to the roadway during days preceding the event (is the road in the shade for example). One of the biggest limiting factors for snow accumulation was the warm ground temperatures resulting from the warm and somewhat sunny days preceding the event (highs at Raleigh-Durham were 65, 72, and 58 on February 26th through 28th). The snow that accumulated on the roads in the Triangle area was very wet and melted from below during the morning as shown in these photos from around 900 AM in North Raleigh (example 1 | example 2 | example 3). Most Triangle area roadways were just wet by Monday afternoon with little or no slush or snow.


    Hourly 4 inch (0.1m) Soil Temperatures

    The image below (click on it to enlarge) shows the hourly 4 inch soil temperatures at 6 locations across central North Carolina from midnight on Saturday, February 28 through midnight on Wednesday, March 4. Note that for most locations the soil temperatures were in the upper 40s to lower 50s just 36 hours prior to the snow starting.

    CLAY - Central Crops Research Station, Clayton, NC
    CLA2 - DAQ Clayton Profiler, Clayton, NC
    REED - Reedy Creek Field Laboratory, Raleigh, NC
    NCAT - NC A&T SU Research Farm, Greensboro NC
    OXFO - Oxford Tobacco Research Station, Oxford, NC
    GOLD - Cherry Research Station, Goldsboro, NC

    click to enlarge




    CoCoRaHS Observer Network

    CoCoRaHS is a grassroots volunteer network of weather observers of all ages and backgrounds working together to measure and map precipitation (rain, hail and snow) in their local communities. By using low-cost measurement tools, stressing training and education, and utilizing an interactive web-site, CoCoRaHS aims to provide the highest quality data for natural resource, education and research applications. The only requirements to join are an enthusiasm for watching and reporting weather conditions and a desire to learn more about how weather can effect and impact our lives. North Carolina joined the CoCoRaHS network in 2007.

    The CoCoRaHS Web page provides the ability for CoCoRaHS observers to see their observations mapped out in "real time", as well as providing a wealth of information for our data users. The snow accumulation maps from the CoCoRaHS web site (shown below) were a great resource for WFO RAH.

    For more information, visit the CoCoRaHS web site at www.cocorahs.org.


    CoCoRaHS Intense Snow Report

    The CoCoRaHS intense snow report below was received by the NWS Raleigh just after 100 AM EST. The snow report from Forsyth County indicated that heavy snow had fallen in that area with 3 inches of snowfall in the past 2 hours bringing the storm total to near 4 inches. The report also provided some information on the temperature and the local accumulation trends. The intense snow report is especially important since it is very difficult to get accurate snow accumulation reports late at night. The value of this kind of report cannot be overstated.
    NZUS45 KBOU 020617
    CCRAHS
    
    Intense snow report from CoCoRAHS spotter:
    
    03/02/2009 01:00 AM local time
    County: Forsyth NC
    Lewisville 4.2 N (number NC-FR-4)
    Latitude: 36.154362
    Longitude: -80.405506
    3.00 inches of snowfall in the past 2 hrs
    4.00 inches of snow on the ground
    Comments: Snow was heavy most of the time from about 01Z through about
    05Z but since then has steadily dropped off and is now very
    light.  Accumulation depends heavily on how exposed the
    location is - 4" on fully exposed lawn away from trees and
    the house, 3" on wooden deck near the house.  Temperature has
    just dropped below freezing and the wind has risen again
    (from NNW) after being nearly calm most of the evening so
    occasionally there's a little snow blowing and drifting off
    the roof.  Most of the trees are bent over and looking very
    stressed, and there are quite a few small branches down
    (about an inch in diameter).
    
    Received NWS Boulder Sun Mar  1 23:17:51 2009 MST
    Sent to WFOs: RAH,GSP,RNK
    
    All of today's CoCoRAHS observations are in WRKCCR (Boulder and CRH only)
    Or at http://www.cocorahs.org (click on reports)
    


    Central North Carolina Snow Accumulation Totals

    CoCoRAHS central North Carolina snow accumulation map - click to enlarge



    Triangle Area Snow Accumulation Totals

    CoCoRAHS Triangle area snow accumulation map - click to enlarge



    Archived Text Data from the Winter Storm

    Select the desired product along with the date and click "Get Archive Data."
    Date and time should be selected based on issuance time in GMT (Greenwich Mean Time which equals EST time + 5 hours).


    Product ID information for the most frequently used products...

    RDUAFDRAH - Area Forecast Discussion
    RDUAFMRAH - Area Forecast Matrices
    RDUHWORAH - Hazardous Weather Outlook
    RDUNOWRAH - Short Term Forecast
    RDUPFMRAH - Point Forecast Matrices
    RDUPNSRAH - Public Information Statements (snow/ice reports among other items.)
    RDUWRKDRT - Soil Temperature Data from the NC State Climate Office
    RDUWSWRAH - Winter Storm Watch/Warning/Advisory
    RDUZFPRAH - Zone Forecast Products


     from 



    Selected Photographs of the Winter Weather Event

    Photos courtesy of Jonathan Blaes.
    (Click the image to enlarge)


    Snow falling near the NWS office before daybreak - photo courtesy of Jonathan Blaes - Click to enlarge           Snow falling near the NWS office before daybreak - photo courtesy of Jonathan Blaes - Click to enlarge           Snow falling near the NWS office before daybreak - photo courtesy of Jonathan Blaes - Click to enlarge

    Snow covered landscape near the NWS office just after daybreak - photo courtesy of Jonathan Blaes - Click to enlarge           Snow covered landscape near the NWS office just after daybreak - photo courtesy of Jonathan Blaes - Click to enlarge           Snow covered landscape near the NWS office just after daybreak - photo courtesy of Jonathan Blaes - Click to enlarge

    Snow covered landscape near the NWS office just after daybreak - photo courtesy of Jonathan Blaes - Click to enlarge           The snow melted quickly on major roads like Six Forks Road in North Raleigh shown here - photo courtesy of Jonathan Blaes - Click to enlarge           The snow melted fairly quickly on many roads like Lead Mine Road in North Raleigh shown here - photo courtesy of Jonathan Blaes - Click to enlarge

    Wet snow and a steady north wind produced this wintry scene in North Raleigh - photo courtesy of Jonathan Blaes - Click to enlarge           Wet snow produced this wintry scene in North Raleigh - photo courtesy of Jonathan Blaes - Click to enlarge           Wintry scene in North Raleigh - photo courtesy of Jonathan Blaes - Click to enlarge

    Snow covered camellia plant - photo courtesy of Jonathan Blaes - Click to enlarge           Dafoldils looking cold - photo courtesy of Jonathan Blaes - Click to enlarge           Wet snow produced good conditions for making snow men - photo courtesy of Jonathan Blaes - Click to enlarge


    Final Thoughts

    Winter Storm Watches verified with an average lead time of nearly 40 hours and Winter Storm Warning lead times were around 18 hours. There were no missed events and the Winter Weather Advisory area was well defined and verified very well.

    Forecasters used NCDOT and other highway traffic cameras to supplement the limited traditional observations during the storm. Collecting information on snow accumulation and snow impact during the overnight hours is often very difficult. Traffic camera pictures (example 1 | example 2) were very helpful in gauging the degree of now accumulation and its potential impact.

    This event was another case in which forecasters utilized AMDAR aircraft soundings to provide thermal profiles at GSO and RDU in between RAOB times. This data set is an important resource during critical winter weather forecasts. Forecasters during the event utilized the http://rucsoundings.noaa.gov/ website and AWIPS to view these soundings during the event.

    Briefings were provided to local emergency managers and decision makers beginning on Friday, February 27, via the new NWS Raleigh Briefing Web Page and via other online conferencing software. Since this was an event that occurred during and just after the weekend, the ability to share information with users who may be at home or away from the office was invaluable.

    Several CoCoRaHS intense snow reports were received by the NWS Raleigh. These reports are especially important since it is very difficult to get accurate snow accumulation reports late at night. The value of these reports cannot be overstated.

    The National Operational Hydrologic Remote Sensing Center provides comprehensive snow observations, analyses, data sets and map products. These products can be used in the analysis of snow fall, snow cover, snow water equivalent and in the anticipation of snow melt.

    Warm weather preceeding the event along with warm ground temperatures resulted in some melting of the snow as it accumulated across the Eastern Piedmont and Sandhills.

    Isolated bands of snow that moved across the Northern Piedmont of North Carolina during the morning of March 2, 2009 appeared to be a result of thermal instability in the boundary layer that produced horizontal convective rolls.



    References

    Atkinson, B. W., and J. W. Zhang, 1996: Mesoscale shallow convection in the atmosphere. Rev. Geophys., 34, 403–431

    Schultz, D.M., D.S. Arndt, D.J. Stensrud, and J.W. Hanna, 2004: Snowbands during the Cold-Air Outbreak of 23 January 2003. Mon. Wea. Rev., 132, 827–842



    Acknowledgements

    Many of the images and graphics used in this review were provided by parties outside of WFO RAH. The upper air analysis images and Skew-T diagrams were obtained from the University of Wyoming. Satellite data was obtained from National Environmental Satellite, Data, and Information Service. Surface observations provided by the University of Wyoming. The surface analysis graphic was obtained from the Hydrometeorological Prediction Center. Partial thickness analysis charts are courtesy of Dr. Michael Brennan and the N.C. State Meteorological Analysis and Prediction Laboratory. AMDAR aircraft sounding data was obtained from the Earth System Research Laboratory - Global Systems Division. Radar imagery was obtained from the National Weather Service web site. CoCoRaHS maps were provided by the CoCoRaHS organization. Analog medium range guidance and background provided by the CIPS Winter Storm Analog Guidance page. Ground temperatures and adjacent air temperature data provided by CRONOS from the N.C. State Climate Office. Photos are courtesy of Jonathan Blaes.


    Case Study Team

    Phillip Badgett
    Jason Beaman
    Ryan Ellis
    Jeff Orrock
    Barrett Smith
    Jonathan Blaes


    For questions regarding the web site, please contact Jonathan Blaes.


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