Event Summary
     National Weather Service, Raleigh NC


January 17, 2008 Winter Storm
Updated 2008/03/08






Event Headlines

...Model guidance was remarkably consistent for several days preceding the event...
...The synoptic pattern leading up to and including the event was consistent with patterns that typically do not produce major winter storms in central North Carolina...
...The parent high pressure system was progressive and was located well offshore in a location that results in "In-Situ" cold air damming...
...AMDAR aircraft soundings were once again a great resource for forecasters to better understand the changing thermal profile...
...The Renaissance Computing Institute (RENCI) deployed a mobile Micro Rain Radar (MRR) to Winston-Salem, during this event. The (MRR) provides a vertical view of reflectivity and Doppler velocity which can be used to better understand and anticipate precipitation type changes at the surface...


Event Overview

A 1036 mb high pressure system dropped south from Canada into the Northern Plains at 00Z on Monday, January 14, 2008. The high pressure system moved south and east behind a low pressure system that was moving across the Great Lakes. On Monday morning, a significant trough in the southern stream was moving across the Southwestern U.S.. The cold front associated with the Great Lakes low pressure system approached the Appalachians on Monday afternoon before weakening on Monday night and Tuesday morning. By Tuesday afternoon, January 15, the high pressure system had moved to the lower Mississippi Valley and weakened to 1025 mb.

By Tuesday evening, the upper trough axis associated with the Great Lakes storm system was moving off the New England coast. The trough over the Southwestern U.S. was moving across northern Mexico and western Texas. At 300 mb, a 150 kt jet was located over the Carolinas in a westerly flow driven by the phasing of the northern and southern jet streams. The surface high pressure system was now located over the Ohio River Valley with a weak surface low developing over the far western Gulf of Mexico.

Forecast guidance was relatively consistent in depicting a weak surface low moving from the Gulf of Mexico northeastward along the Carolina coast. Although the pattern was not a classic setup for a significant winter storm, forecasters believed that the air mass would initially be cold enough to support a brief period of wintry precipitation before a strong southerly flow in the 850 to 700 mb layer would change the precipitation over to freezing rain and eventually rain. Forecasters on Wednesday morning, January 16, issued a Winter Storm Warning for the Northwest Piedmont and Triad area and a Winter Weather Advisory across the Northern Piedmont, western Sandhills, and Southern Piedmont.

On Wednesday morning, a large area of divergence aloft can be seen at 300 mb across the Southeast while the trough at 500 mb across Texas had weakened. A water vapor satellite image at 12Z on Wednesday morning clearly shows the trough over southeast Texas along with a developing area of convection across the Gulf of Mexico. Light rain was falling along much of the northern Gulf of Mexico at 12z. The 12z surface analysis shows the surface high was now centered over the Appalachians with a low pressure system and warm front in the Gulf of Mexico.

Models on Wednesday afternoon continued to be relatively consistent in showing a strong low level flow ahead of a weak low pressure system that would move up the Carolina coast on Thursday. At this time, the NAM showed a thermal pattern that was slightly colder and with a more robust QPF over central North Carolina than the GFS. But both models suggested a brief period of snow was possible after midnight with the precipitation changing to freezing rain and rain a little after daybreak. The surface temperature pattern suggested that freezing rain would persist in the Northwest Piedmont well into the morning.

By Wednesday evening the parent surface high (1028 mb) extended from Pennsylvania southward into the Carolinas. The high pressure system had deposited a cold dry air mass over the area. The trough at 500 mb had sheared out and weakened taking much of its energy away from central North Carolina. A developing area of isentropic lift and warm advection was generating a rather large area of precipitation across South Carolina and Georgia. Precipitation rapidly moved northeast across South Carolina during the evening hours. The precipitation seemed to have a hard time saturating the dry air mass over North Carolina. By the time precipitation spread into central North Carolina, the high pressure system was moving offshore.

The precipitation moved into western and far southern North Carolina just after midnight with most of the precipitation falling as light snow. A wave in the upper atmosphere along with low level isentropic lift provided the forcing that generated much of the precipitation across North Carolina. The snow steadily advanced northward spreading into the NWS Raleigh forecast area by 300 am. By 500 am, the snow had spread into much of the NWS Raleigh forecast area with the precipitation mixing with or changing to rain near the South Carolina border. As warm air surged northward in the 850 mb to 700 mb layer600 am NC Weather Roundup shows that the precipitation had changed over to rain or freezing rain across much of central North Carolina. During the next few hours the precipitation became lighter and more intermittent and the surface temperatures rose to near or just above freezing.

The accumulating snow was over by around daybreak with snow accumulations ranging around an inch to an inch and half across the western Piedmont where the snow started the earliest. Further east, the atmosphere was a little milder and the precipitation was lighter which limited the amount of snow accumulation. There were a few reports of sleet but it was generally brief. The precipitation rate was a big factor in determining the precipitation type. Light precipitation resulted in light rain, while the cooling associated with the melting of heavier precipitation aloft resulted in a changeover to sleet and snow. An hour or so of freezing rain was observed across the southern and eastern Piedmont before temperatures moderated above freezing. Across the Northwest Piedmont, temperatures were still around 32 degrees through the late morning hours which resulted in a longer period of light freezing rain. The strong southerly flow continued to warm the atmosphere above 950 mb and the release of latent heat in the freezing process limited the ice accretion. The freezing rain ended in the Triad during the early afternoon hours with all of the precipitation over by the late afternoon.



Snow Accumulation Map

Snow accumulation map



Surface Analysis

The surface analysis from 12Z Thursday, January 17, 2008 shown in the image below depicts the weather pattern when wintry precipitation was falling across central North Carolina. Two weak low pressure centers were located along a frontal system that stretched from just off the Southeast coast into the Gulf of Mexico. A moderately strong high pressure system (1031 mb) was centered over New England. The area of high pressure was progressive and was moving offshore.

A Java Loop of surface analysis imagery from 12Z Tuesday, January 15, 2008 through 06Z Friday, January 18, 2008 shows the evolution of event. Note the area of high pressure was centered over the Carolinas and Virginia on Wednesday when high temperatures were only in the lower to mid 40s. By the time precipitation spread into central North Carolina, the high pressure system was moving offshore and was not in a position to supply additional cold air into the region. The surface gradient was easterly which allowed temperatures near the surface to moderate during Thursday morning.

Surface analysis from 12Z Thursday, January 17, 2008




Satellite Imagery

Water vapor satellite imagery was used to monitor the track and intensity of the short wave that moved across northeastern Mexico and southern Texas early on Wednesday, January 16. This shortwave gradually shears out along the northern periphery of the ridge located across Florida and the Bahamas. Infrared satellite imagery was used to monitor the intensity of precipitation across the Gulf Coast and the drying of the upper atmosphere.

A Java Loop water vapor satellite imagery from 0015Z on Wednesday, January 16, 2008 through 2215Z Thursday, January 17, 2008 is available.

A Java Loop infrared satellite imagery from 0015Z on Wednesday, January 16, 2008 through 2215Z Thursday, January 17, 2008 is available.

Water vapor satellite imagery from 1515Z on Wednesday, January 16, 2008




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. The Java loop shows the changes in the thickness levels and their associated wintry precipitation types.

Precipitation arrived in northern South Carolina and western North Carolina by 03Z on Thursday, January 17th. The precipitation in these areas initially fell as snow with thickness values generally less than 1300 meters in the 1000 - 850 mb layer and between 1540 and 1550 meters in the 850 - 700 mb layer. The precipitation arrived in the Northern Piedmont including the Triad and Triangle areas between 07Z and 10Z. At that time, the partial thickness values for the lower and mid levels were generally parallel with a noticeable ridge of cooler low level thickness contours ridging southward across the Piedmont.

The transition from frozen precipitation to freezing and then liquid precipitation occurred rather quickly between 10Z and 14Z across much of the Piedmont region. During this period from 10Z to 14Z both the low level and mid level thickness values warm significantly. The freezing precipitation was essentially over by 12 pm or 17Z on Thursday as thickness values continued to rapidly warm into the 1305 / 1565 meter range.

The image below depicts the partial thickness values for 1000 - 850 MB and 850 - 700 MB along with surface wet bulb temperatures and the observed weather at 10Z Thursday, January 17, 2008.

A Java Loop of Partial thickness, surface wet bulb temperatures and Weather from 21Z on Wednesday, January 16, 2008 through 21Z Thursday, January 17, 2008 is available.

Partial thickness, surface wet bulb temperatures and Weather from 10Z Thursday, January 17, 2008



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 data 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. During the 27 hour period from 21Z on 01/16 through 00z on 01/18 there were a total of 23 AMDAR soundings available at RDU and GSO.

The image below contains a loop of AMDAR soundings at GSO during the event. There are 6 soundings in the loop that runs from 2311Z on 01/16 through 1446Z on 01/17. A Java Loop of AMDAR soundings from 0158Z through 2325Z on Saturday January 19 that can be stopped, controlled and zoomed is also available.

An atmosphere that has a temperature profile that is near or just below freezing except for the lowest 3,000 feet or so is shown in the AMDAR sounding from GSO at 2311Z on 1/16. There are two AMDAR soundings from around 0330Z at GSO (0316Z and 0341Z) and they show an atmosphere that has cooled slightly. Note the surface temperature had cooled to around 3 degrees C and that a light northeasterly flow has developed at the surface. The precipitation has already been falling in the Greensboro area for around an hour or two by the time the 1137Z GSO AMDAR sounding is observed. A near freezing isothermal layer between 875 mb and 810 mb can be seen on the 1137Z sounding with a surface temperature that had fallen below freezing. Greensboro reported a couple hours of snow before the precipitation changed to freezing rain at 11Z or 600 am (hourly weather reports across NC on January 17). By 1231Z an above freezing layer can be seen on the AMDAR sounding between 920 and 820 mb with the near surface layer just slightly below freezing. By 1446Z the warm layer has expanded to cover a greater depth and it has warmed slightly with the air near the surface moderating to around freezing.

The utility of the AMDAR was noted in the forecast discussion on January 17 from RAH at 1502Z or 1002 am





The Micro Rain Radar

The Renaissance Computing Institute (RENCI) deployed a mobile Micro Rain Radar (MRR) to Winston-Salem, North Carolina during this event. The (MRR) is a vertically pointing Ku-band radar that is commercially available from METEK Inc. The output from the MRR can be processed to provide the user with a vertical view of reflectivity and Doppler velocity. The high temporal and vertical spatial resolution of the MRR along with its unique data makes it an invaluable tool to view precipitation type changes during winter storms. In addition to the data from the MRR, RENCI deployed a handful of other meteorological instruments including a disdrometer, temperature, and wind sensors during this event.

There are two primary facets of MRR output, radar reflectivity in dBZ and the Doppler velocity. These datasets are displayed with a vertical axis depicting height in thousands of feet and a horizontal axis depicting time.

MRR reflectivity imagery - click to enlarge The reflectivity data can be used to determine precipitation intensity, when precipitation will reach the surface, and potential bright banding. In the MRR reflectivity imagery to the right, black shading represents no reflectivity, blue/purple colors depict light reflectivity returns or light precipitation, with red/orange colors representing greater reflectivity returns and typically heavier precipitation or bright banding.

Examining the reflectivity imagery to the right more closely, a very brief period of precipitation reaches the ground at around 245 am local time. A longer and more sustained period of precipitation reaches the ground between around 400 am and 530 am. Data from a disdrometer confirms that precipitation particles were reaching the ground during this period. Additional periods of precipitation, often very short in duration, continued for the next several hours through 1000 am.

The fall velocity data can be used to determine precipitation type based on the fall velocity of the hydrometeor. Snowflakes have a much slower fall velocity than sleet or rain (including freezing rain). The location where a slower fall speed changes into a faster one can be inferred as the melting layer.

MRR fall velocity imagery - click to enlarge The fall velocity imagery from the MRR paints a vivid picture of the precipitation type changes over Winston-Salem during the morning hours of January 14. Based on the reflectivity imagery shown above and the disdrometer data, there was a short period of precipitation at 245 am and a longer, more sustained period of precipitation between 400 am and 530 am. The fall velocity during this period is consistent and rather slow, generally less than 8 mph, indicating the precipitation was likely falling as snow.

The fall velocities are noticeably higher just before and around 530 am and then again at around 600 am. The faster fall velocities are indicative of a precipitation type other than snow. The sudden increase in fall velocities begins just above 2,500 feet and this is indicative of the melting layer. When snow flakes encounter temperatures above freezing, they do not melt immediately, it typically takes several hundred feet for the snow flakes to melt. Approximately 15 miles to the east, an AMDAR aircraft sounding is recorded at GSO at 1137Z or 637 am and the sounding shows a small warm layer centered at 850 mb with temperatures just above freezing.

Later that morning, the MRR fall velocity data easily shows the melting layer increasing with height. At 730 am local time the melting layer can be estimated at around 4,000 feet. An hour later, the melting layer had increased a couple thousand feet with the melting layer estimated to be over 6,000 feet. Note the significant warming indicated by the AMDAR soundings in the 850 mb to 900 mb layer at GSO between 1137Z or 637 am and 1231Z or 731 am. The mid levels continue to warm markedly during the late morning and midday hours with the MRR fall velocity data continuing to indicate a rising melting layer. This is largely confirmed with the 1446Z or 946 am AMDAR aircraft sounding at GSO.


Analysis of MRR reflectivity and velocity imagery - click to enlarge


Snow Accumulation and Ground Temperatures

In addition to the forecaster determining the overall meteorological pattern, the predominant precipitation type, the spatial distribution of precipitation types, the mechanisms for precipitation, the amount of precipitation, and the snow to liquid equivalent ratio; other factors still play an important role in the amount of snow accumulation. These factors included the rate of snowfall as well as the temperature and moisture characteristics of the ground and road surfaces.

High temperatures on Wednesday, January 16 were in the mid 40s, ranging from around 42 to 47 degrees. This is about 3 to 5 degrees below normal. High temperatures on Tuesday, January 15 were in the lower to mid 40s. Low temperatures on Tuesday night/Wednesday morning were in the upper teens to lower 20s.

Ground temperatures on the Monday afternoon preceding the storm were in the lower 40s. Cold temperatures on Monday night and Tuesday allowed soil temperatures to fall into lower 30s on Tuesday morning and in the lower 40s on Tuesday afternoon. The image below, provided by the State Climate Office of North Carolina, is a plot of the average hourly soil temperature at a depth of about 4 inches at the NC A&T SU Research Farm Field Lab in Greensboro, NC from 00Z on January 15, 2007 through about 00Z on January 22, 2008.

Note the very limited diurnal rebound on January 16. This was likely as result of the extensive cloud cover, precipitation, and near steady temperatures in the lower to mid 30s.

The snow began to accumulate fairly quickly on exposed and elevated surfaces. The snow was a bit slower to accumulate on roadways (especially asphalt) as the preceding day was sunny and the blacktop likely absorbed heat before the storm.

Plot of the soil temperature at a depth of about 4 inches at at the NC A&T SU Research Farm Field Lab in Greensboro, NC



Southeast Regional Radar Reflectivity

The regional radar image below shows the area of precipitation across the Southeast at 0658Z (158 AM EST) on Thursday, January 17, 2008. The precipitation area is characterized by an initial surge of precipitation and then periods of generally light precipitation. A Java Loop of Southeast regional radar imagery from 1158Z on Wednesday, January 16, 2008 through 1458Z Thursday, January 17, 2008 is available.

Southeast Regional Radar 
Reflectivity from 0658Z (158 AM EST) on Thursday, January 17, 2008



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


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Final Thoughts

The synoptic pattern leading up to and including the event was consistent with patterns that do not produce major winter storms in central North Carolina. The parent high pressure system was progressive and was located well offshore in a location that can result in "In-Situ" cold air damming. Since the cold air support from the high pressure system was lacking, other factors for generating cold air become critical for supporting frozen precipitation in central North Carolina. These factors include dynamic cooling, melting and cooling from evaporation. The dynamic cooling and the cooling from evaporation was quickly overcome by the warm air advection. The reliance on these factors typically results in a less confident forecast that frozen precipitation will occur.

Special tools were utilized by forecasters to improve short term forecasting during the event including the Micro Rain Radar (MRR), AMDAR (MDCRS/ACARS) aircraft sounding data, DOT web cameras, and a situational awareness display web page and screen.

AMDAR aircraft soundings were once again a great resource for forecasters to better understand the changing thermal profile. NOAA/NWS forecasters can access this data at http://rucsoundings.noaa.gov/

The Renaissance Computing Institute (RENCI) deployed a mobile Micro Rain Radar (MRR) to Winston-Salem, during this event. The (MRR) provides a vertical view of reflectivity and Doppler velocity. Numerous details of the vertical thermal profile and can be inferred from the MRR data. This allows forecasters to better understand and anticipate precipitation type changes at the surface.



Acknowledgements

Many of the images and graphics used in this review were provided by parties outside of WFO RAH. The Renaissance Computing Institute (RENCI) deployed a mobile Micro Rain Radar (MRR) during this event and they provided the MRR base graphics as well. The surface analysis graphic was obtained from the Hydrometeorological Prediction Center. Upper air analyses were obtained from the Storm Prediction Center and the National Center for Environmental Prediction. Satellite data was obtained from National Environmental Satellite, Data, and Information Service. AMDAR aircraft sounding data was obtained from the Earth System Research Laboratory - Global Systems Division. Some radar imagery was obtained from the National Weather Service web site. The skew-T imagery was obtained from the University of Wyoming.


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Jonathan Blaes


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