Event Summary
     National Weather Service, Raleigh NC


January 19, 2008 Winter Storm
Updated 2008/11/14






Event Headlines

...The synoptic pattern leading up to the event trended toward a pattern that is not typically associated with major winter storms in central North Carolina...
...Light to sometimes moderate snow fell across portions of central North Carolina for several hours during the storm. Despite the multiple hours of falling snow, the snow did not accumulate very efficiently. Several factors are believed to be responsible for the less then expected accumulation of snow including: generally light and inconsistent precipitation rates, surface temperatures at or just above freezing, and rather mild soil temperatures...
...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...
...In events where the surface low is weak, cold air is lacking, and there is little cold advection, secondary factors such as diabatic processes become much more important making it difficult to received Winter Storm Warning criteria accumulations...
...This case provides an example where the dProg/dt analysis approach with QPF and partial thicknesses could have led to an improved forecast...


Event Overview

This storm followed a minor winter storm that impacted portions of central and western North Carolina on Thursday, January 17, 2008. The first winter storm was moving away from the Mid Atlantic during the evening hours of January 17. At the same time a cold front was moving across the Mississippi Valley with an associated 850 mb trough. Temperatures at 850 mb of -10 deg C or below were as far south as southern Missouri. At the same time, an Arctic cold front and Alberta Clipper system were located over the far Northern Plains.

By the daybreak hours of Friday, January 18, the cold front had reached the western Appalachians and colder 850 mb temperatures were moving into the Ohio Valley. An impressive trough was located at 500 mb over the south-central U.S. with a closed low over northern Mexico in the southern stream. A broad southwesterly flow was present across the southeastern U.S. A strong upper level jet at 250 mb stretched from Texas across the Ohio Valley and into the Northeast with winds up to 150 knots.

During the late afternoon hours on Friday, the cold front crossing the Appalachians was dissipating and the Arctic cold front was entering the upper Mississippi Valley. A weak surface low was developing across the western Gulf of Mexico. The 850 mb temperatures across central NC had fallen a few degrees in 12 hours to around 3 deg C with a west-southwesterly flow and some weak cold advection. The 500 mb closed low in the southern stream had opened up as the trough approached the Gulf of Mexico.

Precipitation generated in the strong southwesterly flow aloft rapidly advanced northeast during the overnight and early morning hours on Saturday, January 19. The KGSO RAOB from just after midnight at 06Z showed that the lower levels of the atmosphere were still dry and above freezing. The bulk of this precipitation remained south and east of KGSO in a slightly milder air mass.

The first round of precipitation moved into central North Carolina during the predawn hours on Saturday. This precipitation was largely driven by divergence aloft and mid level frontogenesis. At the same time, the trough in the southern stream over the Gulf of Mexico was shearing out as a larger northern stream trough approached the lower Mississippi Valley. Temperatures at 850 mb across central North Carolina had cooled slightly to around 1 deg C with a light westerly flow still producing some weak cold advection from the Tennessee Valley where 850 mb temperatures where in the -4 to -8 deg C range. The KGSO RAOB from around daybreak at 12Z showed that the low level flow had become northerly and the profile had cooled. The first round of precipitation largely remained east of Interstate 85 and the precipitation fell as rain with surface temperatures in the upper 30s to lower 40s. The precipitation dissipated by around midday and the break in precipitation allowed temperatures to warm into the lower to mid 40s at many locations in central North Carolina.

A second round of precipitation developed across Georgia and South Carolina and rapidly spread northeast into the Piedmont of North Carolina between 100 and 400 PM. During the late morning and early afternoon hours, a cold and dry air mass was observed moving south from western Virginia into the Northwest Piedmont of North Carolina. Dew points in this air mass dropped into the teens to lower 20s. The KGSO RAOB from just after noon at 18Z showed that the lower levels of the atmosphere had cooled and the profile supported snow with a possible near freezing layer near the surface. Further east at KRDU, an AMDAR aircraft sounding from just after noon at 1724Z showed a significant above freezing layer in the lowest 3,000 feet or so of the atmosphere. Precipitation in the Triangle area started as rain at around 200 PM and then changed over to wet snow between 200 and 400 PM as the melting snow aloft cooled the above freezing layer near the ground. Eventually the melting snow aloft cooled the lower level profile to near freezing isothermal which allowed wet snow to fall over all of the RAH CWA by the early evening hours. Despite many locations receiving several hours of snow, the snow had a difficult time accumulating as temperatures remained just above freezing in the 33 to 34 degrees F range. A significant gradient in precipitation amounts also resulted in diminished snow accumulations as the least amount of precipitation fell where the air was coldest and most supportive of snow. All of the precipitation tapered off from southwest to northeast during the evening hours. The Arctic cold front moved across the Appalachians during the evening hours and began crossing central North Carolina after midnight.

After the event, snow accumulations across the CWA were around a half inch with two areas of more significant accumulation. The larger and more impressive area stretched from the Northern Piedmont southwest into the northern portions of the Southern Piedmont snow accumulations in this area ranged from 1 to 1.5 inches with localized amounts of 2 to nearly 2.5 inches in northern Durham, Orange, southern Alamance and northern Chatham counties. Further south across the southern Sandhills and Southwestern Coastal Plain, a localized area of snow accumulations of1 to 2 inches was located across Scotland, Robeson, and southern Cumberland counties. The more significant snow accumulations in this area likely resulted from heavier and more persistent precipitation rates. The snow accumulation was visible the following morning via the MODIS visible satellite imagery.

After the event, snow accumulations across the CWA were around a half inch with two areas of more significant accumulation. The larger and more impressive area stretched from the Northern Piedmont southwest into the northern portions of the Southern Piedmont snow accumulations in this area ranged from 1 to 1.5 inches with localized amounts of 2 to nearly 2.5 inches in northern Durham, Orange, southern Alamance and northern Chatham counties. Further south across the southern Sandhills and Southwestern Coastal Plain, a localized area of snow accumulations of1 to 2 inches was located across Scotland, Robeson, and southern Cumberland counties. The more significant snow accumulations in this area likely resulted from heavier and more persistent precipitation rates. The snow accumulation was visible the following morning via the MODIS visible satellite imagery. This was a very difficult storm to forecast with inconsistent model guidance, a sharp gradient in the amount of precipitation, a complicated timing scenario between the precipitation and the arrival of colder air, and the expected impact of diabatic processes such as cooling from evaporation and melting snow.


MODIS Visible Satellite Image Showing Snow Cover

MODIS visible satellite image showing snow cover - click to enlarge


Snow Accumulation Map

Snow accumulation map


Liquid Equivalent Precipitation Map

Liquid equivalent precipitation map



Surface Analysis

The surface analysis from 15Z Saturday, January 19, 2008 shown in the image below depicts the weather pattern a few hours before a second round of precipitation that would eventually change to snow, spreads across the central North Carolina. An arctic cold front is spreading east across the Ohio and Tennessee Valleys at this time. An area of high pressure across southern Virginia is associated with a cold and dry air mass that was moving south across Virginia into the Northwestern Piedmont of North Carolina. Finally, two weak areas of low pressure are noted along the baroclinic zone that stretches from the Gulf of Mexico to off the Southeast coast.

A Java Loop of surface analysis imagery from 00Z Friday, January 18, 2008 through 12Z Sunday, January 20, 2008 shows the evolution of event.

Surface analysis from 15Z Saturday, January 19, 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 Friday, January 18. This shortwave gradually shears out along the northern periphery of the ridge located across Florida and the Bahamas. Another short wave embedded in the northern jet stream can be seen moving through the upper Midwest and Ohio Valley, just out of phase with the southern stream.

A Java Loop water vapor satellite imagery from 0015Z (715 PM EST Thu) on Friday, January 18, 2008 through 1215Z (715 AM EST) Sunday, January 20, 2008 is available.

Water vapor satellite imagery from 1515Z on Saturday, January 19, 2008



Southeast Regional Radar Imagery

Regional radar imagery shows the evolution of the event across the Southeast. Precipitation initially spread into central North Carolina at around 1000Z and then persisted for a few hours before tapering off from west to east. A lull in the precipitation moved across central North Carolina at around midday before the second surge of precipitation spread into the state. The significant precipitation began to come to an end toward sunset.

A
Java Loop of Southeast Regional Radar Imagery from 1158Z (658 AM EST) on Friday, January 18, 2008 through 0858Z (358 AM EST) on Sunday, January 20, 2008 is available.

Southeast Regional Radar 
Reflectivity from 1801Z (101 PM EST) on Saturday, January 19, 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 loop depicts the partial thickness values for 1000 - 850 MB and 850 - 700 MB along with surface wet bulb temperatures and the observed weather from 18Z Friday, January 18, 2008 through 12Z Sunday, January 20, 2008. Low level thickness values (shown in blue) were falling in the Piedmont region as snow aloft initially evaporated and then melted as it approached the surface. This process resulted in a rapid cooling of the lowest few thousand feet of the atmosphere.

A Java Loop of partial thickness, surface wet bulb temperatures and Weather from 15Z Saturday, January 19, 2008 through 06Z Sunday, January 20, 2008 is available.

Partial thickness, surface wet bulb temperatures and Weather loop from 15Z Saturday, January 19, 2008 through 06Z Sunday, January 20, 2008




TREND’s Predominant P-type Nomogram

The nomogram below shows the distribution of p-type trends 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 trend 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 from 00Z January 19 through 00z January 20 at KGSO during the event. The second image shows the observations from KGSO from 1454Z through 2354Z on January 19. When the precipitation started just after 18Z it fell as snow at KGSO and continued for a little over 4 hours.







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. During the 25 hour period from 23Z on 01/18 through 00z on 01/20 there were a total of 14 AMDAR soundings available at RDU and GSO.

The image below contains a loop of AMDAR soundings at RDU during the event. There are 8 soundings in the loop that run from 0158Z on 01/19 through 2325Z on 01/19. A Java Loop of AMDAR soundings from 0158Z through 2325Z on Saturday January 19 that can be stopped, controlled and zoomed is also available.

The cooling from melting snow aloft is first obvious on the 1219Z sounding plot where the temperature profile is near freezing isothermal from about 810 mb to around 885 mb. After a brief lull between 1500Z and 1700Z, the precipitation resumed and the 1724Z sounding data showed that the melting level had dropped slightly to around 920 mb with a near freezing isothermal layer above it. During the next two hours light rain developed at RDU and quickly mixed with snow before changing to a very wet snow by around 20Z. The next aircraft sounding did not arrive until 2216Z at which time the sounding was subfreezing down to just a few feet above the surface. On and off periods of wet snow fell during the next hour before the entire column cooled before freezing as a result of some weak cold advection. By 2325Z, the entire profile was below freezing the precipitation.






Model Trends (dProg/dt)

A dProg/dt analysis can be used to assess the consistency of the models, comparing the solution for a given time from the latest model with several previous runs valid for the same time, for a number of fields. With each new forecast, forecasters may notice that the specific event is changing character in a systematic way. Trends in model solutions may show a more accurate solution than a given specific model run. Sometimes there may not be a reliable trend with each successive model run. Each new model run could reverse trend or revert back to an earlier solution, but this typically does not occur since newer guidance in general tends to be more accurate.

This case provides an example where the dProg/dt analysis approach could have led to an improved forecast. While dProg/dt has limitations, using this approach (especially with the conventional 00Z and 12Z runs) with low level thickness values and QPF implied a less significant event.

In the chart below, note the change in expected precipitation totals from the NAM40 at RDU and GSO as the event gets closer. At GSO, the trend, especially with the conventional 00Z and 12Z runs, is for less and less precipitation. Note that it is not until the 00Z run on 01/19 (Friday night) that a significant decrease in QPF is noted at RDU. This is well after the Winter Storm Watch and Winter Storm Warnings were issued. Over a half inch of QPF was expected at RDU and over an inch at FAY until the 00Z 01/19 NAM run.



The partial thickness values (1000-850 mb and 850-700 mb) verifying at 18Z on Saturday 01/19 followed a similar trend as the QPF as shown in the table below. Note the NAM had some significant trouble with the partial thickness values at GSO as they increased a moderate amount as the event gets closer. This is indicative of a NWP solution that is showing a noteworthy warming trend in the lower portions of the atmosphere. The partial thickness values at RNK and MHX are much more consistent.




A more dramatic look at the gradual warming of the lower levels with each successive run of the NAM40 can be seen in the images below. The first image is a comparison of the BUFR forecast soundings during 5 consecutive NAM runs at RDU valid at 18Z with an 18Z AMDAR aircraft sounding at RDU serving as the observed. Note that the AMDAR sounding did not provide a dew point trace and it should be noted that the lower levels were not saturated at 18Z. A user could assume that if saturated, the 18Z AMDAR sounding (shown in green) would look very similar to the 6 hour forecast from the 01/19 12Z NAM run (shown in orange). Note the significant differences between the 18Z AMDAR sounding and the 01/19 12Z run with the model runs from the night and day before on the 18th. The 18Z time period is important at RDU since it was the time in which the changeover from rain to snow was expected to occur and it turned out to be the time which a second area of precipitation was spreading across the Triangle area.




The image shown below is a comparison of the BUFR forecast soundings during 5 consecutive NAM runs at GSO valid at 18Z with an 18Z special RAOB release at GSO. The 18Z GSO RAOB was remarkably dry, and some cooling should be expected when the profile becomes saturated but the clustering of forecast surface temps near or below freezing during previous runs was significantly colder then the observed surface temperature of around 40 degrees.






The Role of Melting

In events such as this one where the surface low is weak, horizontal thermal advection is weak, secondary factors such as diabatic processes, ground temperature, precipitation rates, and topography become much more important. Melting is a process that can cool the temperature to freezing. Relative to evaporation, melting is a less effective means for cooling. Cooling from melting is typically an order of magnitude smaller than cooling from evaporation.

The effectiveness of melting to lower a temperature to freezing is dependent upon the precipitation rate as well as the depth of the melting layer. As the precipitation rates increases (decreases), melting increases (decreases) and the snow level is lowered (raised). This explains why there can be relatively frequent changes between rain and snow as the precipitation rate changes. The combination of weak highs and lows at the surface, weak horizontal thermal advection and an active jet aloft is another scenario where increased snow rates can erode a marginal melting layer. This situation results in the infrequent pattern of snow islands embedded in a cold rain.

In the long term, precipitation rates are hard to anticipate. In the short term, you can use the WSR 88D to monitor precipitation rates and look for evidence of the melting “bright band”.

In the base reflectivity image below, from the krax WSR-88D at 2139Z, note the area of enhanced reflectivities in the 25 dBZ - 40 dBZ range (shown in the pink and purple colors) stretching from south of Fayetteville to east of Rocky Mount. This area of enhanced reflectivities was primarily produced by the "bright band." Keep in mind that snow flakes are typically larger than rain drops but they reflect a lesser amount of radar energy. So despite their larger size, the reflectivities associated with snow flakes compared to rain drops at similar intensities is typically equal to or less than rain drops. However, when snow flakes initially or partially melt, they keep their larger diameter while possessing a coating of water. Weather radars see this and display an area of higher reflectivities which are often oriented along the rain/snow line in a band called the bright band.




The reflectivity cross section shown below is from the krax WSR-88D at 2013Z on Saturday, January 19 along with the LAPS temperature analysis from 20Z. The vertical axis is in thousands of feet up to around 15,000 feet. The cross section extends from Greensboro, NC on the left to Jacksonville, NC on the right (you are looking northeast across central NC).

Note the correlation between the reflectivity intensity and the LAPS analysis temperature. While the LAPS analysis is only hourly and the impact of microscale processes such as melting may be difficult to analyze, it should server as a reasonable approximation of the thermal profile. The enhanced reflectivities near and below the 0 deg C isotherm show the melting layer where snow flakes have begun to melt. Keep in mind that snow flakes are typically larger than rain drops but they reflect a lesser amount of radar energy. So despite their larger size, the reflectivities associated with snow flakes compared to rain drops at similar intensities is typically equal to or less than rain drops. However, when snow flakes initially or partially melt, they keep their larger diameter while possessing a coating of water. Weather radars see this and display an area of higher reflectivities which are often oriented along the rain/snow line in a band called the bright band.






Reduced Accumulations and Soil Temperatures

Light to sometimes moderate snow fell across portions of central North Carolina for several hours during the storm. Despite the multiple hours of falling snow, the snow did not accumulate very efficiently. Several factors are believed to be responsible for the less then expected accumulation of snow including:
  • generally light and inconsistent precipitation rates
  • surface temperatures at or just above freezing
  • rather mild soil temperatures

    The less than expected amount of precipitation reduced the potential for snow accumulations by lessening the amount of snow available to accumulate. The generally light and unsteady precipitation rates prevented the total potential cooling from melting snow from being realized. This was one contributing factor that kept the surface temperatures from falling to or below freezing. As noted previously, the advancement of colder air from the west failed to develop because of the weak horizontal cold advection and the disruption of the cold air by the higher terrain of the Appalachians (surface map of wind, pressure, and theta-e at 12Z on Saturday | U.S. topographic map.)

    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 Friday, January 18 through noon on Sunday, January 20. Note that for most locations the soil temperatures were as low as the lower 40s or colder on the day before precipitation began. On Saturday morning, 4 inch soil temperatures were in the mid 40s across much of central N.C. although soil temperatures were notably cooler, around 40 degrees, in the Triad and far Northern Piedmont regions (NCAT and OXFO). Around the midday hours, the soil temperatures rose a few degrees as a lull in the precipitation moved across the area and the cloud cover thinned. The temperature rise was more significant across the Triad where there was limited if any precipitation earlier in the day resulting in a dry ground. Another area of precipitation spread across the region during the mid to late afternoon hours. The precipitation started as rain across much of central N.C. before changing over to snow. As the rain fell and melting snow cooled the air temperatures near the surface, the soil temperatures dropped notably (around 2 to 3 degrees F) during the late afternoon and early evening.

    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
    SILR - Siler City Airport, Siler City, NC
    CLIN - Horticultural Crops Research Station, Clinton, NC


    Hourly 4 inch (0.1m) Soil Temperatures

    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 is 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.

    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. North Carolina joined the CoCoRaHS network in 2007. The snow and liquid equivalent precipitation 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.

    Snow Accumulation Totals

    click to enlarge



    Liquid Equivalent Precipitation Totals

    click to enlarge



    Warning Decision Process

    WFO RAH winter weather philosophy includes a targeted lead time for Winter Storm Watches of 24 hours and for Winter Storm Warnings of 12 to 18 hours. These lead times can be increased if confidence is unusually high, if sociological factors require it (such as a holiday weekend or first storm of the season) and if collaboration among adjacent offices necessitates it.

    The Watch/Warning/Advisory decision making during this event was very difficult. Generally, forecasters knew that the arctic cold air arriving from the west with the northern stream trough would be chasing the moisture associated with the southern stream upper trough and distant surface low. This was a classic case of relatively limited precipitation in the cold air over the western Piedmont, with a lack of cold air (at least initially) where moisture was more plentiful over the Coastal Plain. Model thicknesses and forecast soundings indicated a transition from rain to snow from northwest to southeast during the day Saturday, as cooling in the lowest 5 thousand feet pushed the thermal profile to slightly sub-freezing isothermal.

    One weak winter storm was already threatening central North Carolina on Tuesday January 15 when forecasters recognized the upcoming weather pattern would potentially lead to southern stream short waves that could produce cyclogenesis across the Gulf of Mexico and East Coast during the weekend and early the next week. On Wednesday, forecasters monitored the system and it appeared that the track of the surface low and moisture would remain largely southeast of the area.

    Forecasters on Thursday recognized that this storm would fit the “Miller A” pattern of limited freezing precipitation (freezing rain and sleet) and a narrow transition zone. Forecasters accepted a GFS/ECMWF solution that brought system closer to the coast but kept the center offshore. Model guidance was inconsistent and portrayed a variety of solutions. Details regarding the potential phasing of the southern stream trough with the northern stream were a significant unknown and forecasters kept the forecast conservative. Snow was first introduced to the weekend forecast for the RAH CWA in the 400 PM ZFP issuance on Thursday with a rain/snow mix in the north an northwest portion of the CWA in the morning and then with the rain mixing with snow eastward during the day Saturday. The 18Z HPC snow accumulation guidance included a potential for 4 to 6 inches of snow accumulation across parts of the RAH CWA.

    During the midnight shift on Friday morning, the event was close enough to warrant the issuance of a Winter Storm Watch for the northern Piedmont and Coastal Plain region for the potential of 2 to 4 inches of snow. Snow accumulation guidance from HPC issued at around 07Z agreed with the potential for up to 4 inches of snow from the NC mountains to the Piedmont largely because of concern over an intense 150 to 170 knot jet at 250 mb. The Heavy Snow Discussion from HPC issued at 451 AM on Friday morning noted the continued inconsistency and the southeastward trend in the NWP guidance. In addition, the HPC Model Diagnostic Discussion issued Friday morning noted the importance of the potential phasing of the northern and southern jets.

    Forecasters during the day shift on Friday were impressed with the intense dynamics observed with the system (note the impressive upper trough south of Texas in the water vapor imagery at 1815Z.) This short wave trough over the Mexico was still digging south during the midday hours and convection was ongoing across the Gulf of Mexico. This in part led forecasters to believe that the developing storm system had the potential to intensify more than the NWP guidance was suggesting; resulting in stronger cyclogenesis and greater amounts of precipitation (closer to the NAM solution.) Colder air was expected to be edging into the area during the day from the northwest and the low levels were expected to be cooled by diabatic processes (evaporation and melting.)  As noted by HPC in the 412 PM Heavy Snow Discussion, the 12Z Friday model guidance was coming into better agreement (the NAM was still an outlier) with a slower and more amplified solution. Interestingly, the HPC Heavy Snow Discussion mentioned that snow accumulations would be limited because of above freezing surface temperatures. After collaborating with neighboring offices, WFO RAH issued a Winter Storm Warning for portions of the northern Piedmont and northern Coastal Plain with a Winter Weather Advisory for the remainder of the RAH CWA. Snow accumulations were expected to be greatest in the Interstate 95 corridor when amounts were forecasted to range between 3 and 5 inches.

    During the evening shift on Friday, forecasters noted some potential points where the predicted accumulations could be overdone. Snow to liquid equivalent ratios were expected to be 8:1 or even 6:1 because of the relatively warm atmosphere. In addition, the above freezing temperatures at and just above the surface were expected to initially melt the snow and a portion of the falling snow would go toward cool the near ground temperature and the ground itself. During the midnight shift on Saturday morning, forecaster confidence at RAH was low and decreasing despite the fact that the event was about to begin. The diminished confidence stemmed from several factors, including an evolving depiction of QPF values across central NC with each model run and the fact that the 00Z models were having great difficulty in forecasting the observed weather just 6 hours after model initialization time.  The 00Z Saturday GFS/NAM were too slow and too far south with the precipitation across the Carolinas (note the difference between the 3 hour NAM20 QPF ending at 12Z and the 1 hour RTMA observed precipitation analysis over SC and NC which already showed up to 0.10 inches in 1 hour.) Forecasters realized that the primary lifting mechanisms will be divergence aloft from a strong upper level jet and mid level frontogenesis, but the various ways these fields were progged by the models was disconcerting. In addition, observed temperatures and dew points were still in the upper 30s to lower 40s during the predawn hours.

    With the poor model performance in handling the short term and uncertainty in the QPF, forecasters decided to leave the Warnings and Advisories as they were. The midnight shift did note that this would be a borderline Warning event and that there was potential for Warning criteria snow in the southern Coastal Plain.  The ZFP issued at 407 AM on Saturday morning generally included 1-2 inches in the advisory area with 2-3 inches in the Warning area with potentially higher amounts.

    Forecasters on Saturday morning were troubled with mixed signals when analyzing the observed and guidance data. The 12Z proximity soundings came in on average 3 to 6 m higher with 1000-850 and 850-700 mb thicknesses than progged by the models. In addition, the NAM and GFS BUFR soundings were too cool in the lowest 5,000 feet relative to the observed 06Z and 12Z KGSO soundings as well as too cool over RDU relative to AMDAR aircraft soundings between 11 and 13z. Colder and drier air was believed to be on its way with the observed soundings displaying an increasingly backing wind profile in the lower layers. Dew points over southwestern VA and northwestern NC were dropping into the teens which meant the potential for further near surface cooling via evaporation.

    The subsidence on the back side of the short wave that produced the initial round of precipitation across central NC was noted during the late morning hours in the drying shown on satellite imagery and a large lull in the precipitation across the western Carolinas. This break in the precipitation allowed surface temperatures to climb into the lower 40s before the onset of the second round of precipitation.

    The Advisory was allowed to stand across the Triad because of the models poor performance with precipitation amounts upstream, indications that the cold air is showing signs of infiltrating the Triad and snow ratios will accordingly increase closer to 9:1 or 10:1, and forcing for ascent was expected to continue into the late afternoon hours.

    Across the Warning and the remainder of the Advisory areas, the forecast rationale and resultant snowfall amounts were generally unchanged from the previous package. It was noted that the accumulations would likely be on the lower end of the Advisory/Warning range. Uncertainty in the location of an expected sweet spot where the arrival of the colder air will co-exist with the higher QPF made it difficult for forecasters to trim back the Warning. In fact, the locations in which the forecasters contemplated downgrading the Warning (the northwestern tier of counties – Person, Orange, Alamance, and Chatham) were the locations that received the most snow. Finally, the forecasters realized that most of the accumulation will occur on grassy surfaces, cars, and rooftops given the above freezing near surface temperatures and already wet ground.

    Forecasters during the afternoon Saturday were relieved to see the second area of precipitation develop and spread across central NC. The precipitation had already developed or changed to snow in the Triad area around 200 PM and the changeover was moving into the western portions of the Triangle area by 300 PM. Snow accumulations on the order of a coating to a third of an inch were already reported in the Triad area by 300 PM. With conditions rapidly changing over to snow, forecasters were still hesitant on pulling the warning despite the fact they realized it may be a long shot. While it was not clear at the time the warning decision was made (1-3 PM), the fact that the surface temperature was failing to fall below 33 degrees in the RAH CWA during the afternoon and early evening hours was a chief reason Warning criteria snow accumulations were never realized. The Winter Weather Advisory was dropped across the Triad area at 752 PM and the remainder of the Advisories and Warnings were updated. All of the Advisories and Warnings were cancelled at 1158 PM.



    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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    Selected Photographs of the Winter Weather Event

    Photos courtesy of Helen Needham (NOAA'S Southeast Regional Climate Center), Sean McManus and Jonathan Blaes.
    (Click the image to enlarge)


    Snow covered field and foreast adjacent to a pond near Graham - photo courtesy of Helen Needham - Click to enlarge           Snow falling near Graham - photo courtesy of Helen Needham - Click to enlarge           Snow falling near Graham - photo courtesy of Helen Needham - Click to enlarge

    Snow falling near Graham - photo courtesy of Helen Needham - Click to enlarge           Snow falling near Graham - photo courtesy of Helen Needham - Click to enlarge           Cardinal feeding with the snow falling near Graham - photo courtesy of Helen Needham - Click to enlarge

    Snow started to accumulate on the roads near Graham - photo courtesy of Helen Needham - Click to enlarge                 Snowy scene near Graham - photo courtesy of Helen Needham - Click to enlarge                 Snow falling near Graham - photo courtesy of Helen Needham - Click to enlarge

    Roadway with a light coating of snow near Mebane - photo courtesy of Sean McManus - Click to enlarge           Field and forested area during the snowfall near Mebane - photo courtesy of Sean McManus - Click to enlarge           Measurement of around 1.5 inches on Saturday evening near Mebane - photo courtesy of Sean McManus - Click to enlarge

    Small pasture with livestock in the snow near Mebane - photo courtesy of Sean McManus - Click to enlarge           Trees covered with snow near Mebane - photo courtesy of Sean McManus - Click to enlarge           Field in north Raleigh with a light snow cover the following morning - photo courtesy of Jonathan Blaes - Click to enlarge


    Final Thoughts

  • The lack of cold and dry, low level air during the event was critical. NWP guidance was too cold and too aggressive in bringing in the cold air into the area and the overall pattern suggested the cold air would be hard to acquire.
    • The amount of cold air being transported into central NC was not expected to be great, but some minor transport of colder and drier air from the west was expected. Initial analysis suggests that the higher terrain of the Appalachian Mountains across NC, VA, and WV may have inhibited the transport of this cold, drier air. Reliance upon cold air crossing the Appalachians to catch up to moisture to produce frozen precipitation is risky.
    • The presence of a sufficiently strong cold air high to the north and/or the ability of the disturbance to tap into cold air via horizontal cold air advection are key ingredients to winter storms in central NC.
    • With the NWP guidance generally trending toward a more sheared upper level system and a weaker surface system, the amount of low level lift was diminishing with each run. Dynamic cooling of the atmosphere from upward vertical motion was reduced with each model run and the lower levels of the atmosphere were warmer.
  • In events where the surface low is weak, precipitation amounts are expected to be moderate, cold air is lacking, and there is little cold advection, secondary factors such as diabatic processes become much more important. Reliance solely upon these factors to produce warning criteria snow accumulation should be avoided.

  • This case provides an example where the dProg/dt analysis approach could have led to an improved forecast. While dProg/dt has limitations, using this approach (especially with the 00Z and 12Z runs) with low level temperatures and QPF implied a less significant event.

  • When there is some concern that Winter Storm Warning conditions may not occur after the Warning is issued, the recommended practice at RAH is to not be over-reactive and drop the warning prematurely. During past winter seasons the benefit of holding onto the Warnings a little longer has proven itself, even when there are significant doubts about the event before it occurs. The prudent course of action is to wait until the system begins affecting the area and then thoroughly evaluate.

    Obviously there are situations in which dropping the warning is necessary. The forecaster needs to weigh the risk vs. reward of dropping the warning when there is still a chance that it will need to be issued.

    A reasonable criterion to use in determine the need to hold onto the Warning can be evaluated when writing the text of the Winter Storm Warning. If it becomes difficult to include the Warning amounts in the body of the text, even with some caveats, then the Warning can be dropped.

  • Our Winter Storm Warning criteria is 3 " in 12 hours or 4 " in 24 hours. For those cases when forecast snow amounts may approach 3", but forecast confidence in that amount of snow occurring over a large area within the county is less than 80% it is advised to issue a Winter Weather Advisory. The Advisory can mention snowfall amounts up to 3 inches along with the need for future warnings should forecast amounts increase or confidence in the snow event become more certain.

  • 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 to garner access to all of these soundings when they did not appear in AWIPS.


  • Acknowledgements

    Many of the images and graphics used in this review were provided by parties outside of WFO RAH. The surface analysis graphic was obtained from the Hydrometeorological Prediction Center. The upper air analysis images were obtained from the University of Wyoming. Satellite data was obtained from National Environmental Satellite, Data, and Information Service. 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. Photos are courtesy of Helen Needham (NOAA'S Southeast Regional Climate Center), Sean McManus and Jonathan Blaes.


    Case Study Team

    Phillip Badgett
    Jason Beaman
    Rod Gonski
    Ron Humble
    Michael Moneypenny
    Jeff Orrock
    Barrett Smith
    Michael Strickler
    Brandon Vincent
    Jonathan Blaes


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


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