Event Summary
     National Weather Service, Raleigh NC


February 1, 2007 Winter Storm
Updated 2007/02/11






Event Headlines

...An average of one half to one inch of snow accumulated across most of central North Carolina with only trace amounts in the far Northwest Piedmont. There were localized heavier amounts of up to 2 inches reported across portions of the Southern Piedmont and Sandhills...
...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...
...Much lower than expected precipitation amounts were realized on the northwestern edge of the precipitation shield which was a significant factor in less than expected snow and freezing accumulations...


Event Overview

Central North Carolina experienced its second winter weather event in as many weeks on Thursday, February 1, 2007. A cold front moved through North Carolina during the evening hours of January 30. Behind the front, a bitterly cold and very dry high pressure system built over the region on January 31st. Dewpoints under the surface high were in the single digits during the day and into the evening hours across the Piedmont and Foothills regions. Temperatures only fell a few degrees during the evening as the "blanket" effect of the thickening cloud cover kept temperatures from falling more rapidly. In fact, temperatures and dewpoints rose overnight with temperatures ranging in the upper 20s to around 30 degrees at around daybreak with dewpoints in the teens. Even though the surface temperatures were moderating, the airmass over the Piedmont of North Carolina was still very cold as shown in the 00Z, February 1, 2007 RAOB from Greensboro, NC.

Meanwhile, a strong upper level jet at 300 MB extended across the Southern Plains and Southeast. A jet max of over 140 knots was observed at 300 MB at 12Z on February 1 with an area of upper level divergence over the western Carolinas and southern Appalachians. At the lower levels of the atmosphere, a general southwesterly flow at 850 MB developed which resulted in considerable warm air advection. The southwesterly flow at low and mid levels also resulted in increasing amounts of low and mid level moisture. The broad lift produced by the upper level jet energy combined with the lift associated with the transport of warm, moist air at lower levels resulted in a relatively large area of precipitation across the Southeast. With a cold and dry airmass in place over central North Carolina early on the morning of February 1, the precipitation primarily started as snow across much of central North Carolina.

The progressive surface high pressure system responsible for the cold air was centered off the Virginia coast, in a location typical of an "In-Situ" Cold Air Damming (CAD) event. The "In-Situ" type CAD is characterized by a 1) a high pressure system that is located in an unfavorable location to support the CAD, 2) an event where diabatic processes are essential, and 3) where there is little or no cold air advection. A southwesterly wind flow continued at low levels through the morning and this gradually warmed up the lower levels of the atmosphere over central North Carolina during the morning and early afternoon hours. The lack of a cold and dry airmass to resupply the fledgling CAD over the Carolinas and the warming just above the surface resulted in a rather quick transition from mostly snow to mostly rain, with a rather brief period of mixed wintry precipitation during the transition. Surface winds across the Piedmont during the early morning and late morning were from the south and southwest, which assisted in the moderation of the airmass at the surface and the transition from mostly snow to rain.

The 24 hour liquid equivalent precipitation amounts with this storm varied considerably across central North Carolina. Liquid precipitation amounts over an inch were observed across the southern tier of central North Carolina in Richmond, Scotland, and Wayne Counties with radar estimates providing similar amounts. The precipitation amounts averaged around a half inch across the Raleigh-Durham area with considerably less precipitation across the Triad and Virginia border counties. Snowfall amounts (see map below) averaged from one half to one inch across most of central North Carolina with localized amounts up to 2 inches reported across the Southern Piedmont. Freezing rain amounts across central North Carolina were generally light with most locations reporting no more than a light glaze.


Snow Accumulation Map

Snow accumulation map



Surface Analysis

The surface analysis from 09Z (400 AM) Thursday, February 1, 2007 shown below depicts the weather pattern as the event unfolds across central North Carolina. The weak area of high pressure (1022 MB) is transitory and is centered just off the Virginia coast. A developing coastal front is located off the Carolina coast with a weak surface wave developing near the Mississippi River delta.

A Java Loop of surface analysis imagery from 00Z Wednesday January 31 (700 PM Tuesday) through 21Z Thursday (400 PM Thursday) shows the evolution of event.

Surface analysis from 09Z Thursday, February 1, 2007




Satellite Imagery

Satellite imagery was used to monitor the track and intensity of the mid and upper level wave moving out of the Mississippi Valley toward the Carolinas. The water vapor satellite image below is from 1415Z (915 AM EST) on Thursday, February 1, 2007. The dark area, stretching from north to south across Tennessee into Alabama is the back edge of the wave that was enhancing the precipitation across the Carolinas.

A Java Loop infrared satellite imagery from 0615Z (115 AM EST) on Wednesday, January 31, through 2315Z (615 PM EST) Thursday, February 1, 2007 is available.

A Java Loop water vapor satellite imagery from 0615Z (115 AM EST) on Wednesday, January 31, through 2315Z (615 PM EST) Thursday, February 1, 2007 is available.

Water vapor satellite imagery from 1415Z on 
Thursday, February 1, 2007




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.

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 14Z on Thursday, February 1, 2007.

A Java Loop of Partial Thickness, Surface Wet bulb and Weather Imagery from 00Z through 21Z on Thursday, February 1, 2007 is available.

Partial Thickness, Surface Wet bulb and Weather Imagery from 14Z on Thursday, February 1, 2007




Precipitation Type Forecasts and TRENDs

Predominate Precipitation Type Nomogram - click to enlarge The TRENDs technique, in which the thermal structure of the atmosphere can be used to indicate the most likely predominate precipitation type, clearly showed a transition from snow to rain during this event. Model forecasts of partial thickness values a before the event proved reasonably accurate. This technique indicated that the predominant precipitation type would initially be snow with a brief change over to a sleet/snow/freezing rain mix, then to mostly freezing rain for several hours over much of the Piedmont, before the warming aloft reached down to the ground to push surface temperatures above the freezing mark. However, the duration of the period of a wintry mixture ended up being very short, typically an hour or less over much of the area. Why was this?

First, the effects of the warming aloft may have been underestimated. An 18Z (100 PM EST) sounding from Morehead City, NC, showed this significant warming aloft, with temperatures over 6°C less than 2000 feet aloft. While the Greensboro, NC, sounding at 18Z (100 PM EST) was notably colder aloft, much of the eastern and southern Piedmont and Coastal Plain may not have had enough of a cold surface-based layer to overcome the warm rain falling from above.

A second, and likely more significant factor, was the lack of a continuous feed of low level dry air into the area, which may have severely limited the potential for ice accrual. To get freezing rain, temperatures must be at or below freezing in a thin layer near the surface, with a significant layer of above-freezing air aloft. Once a coating of ice, or glaze, forms on surfaces near the ground, the phase change from liquid water to ice (as the rain freezes on contact) releases a small amount of heat (called latent heat). The release of this heat is often just enough to push surface temperatures that are initially just below or near freezing up to the freezing mark. Keep in mind that the freezing temperature is also the melting temperature and that freezing rain without the resupply of cold, dry air will often result in a balance where there is some freezing of rain and some melting as well as the temperature hovers near 32 degrees.

Historically, to get a significant coating of ice from freezing rain in central North Carolina, such as the December 2002 ice storm, there must be a steady feed of cold, dry low level air into the region. Typically this feed of cold, dry air originates from with a surface high centered to our north. As precipitation falls into it, evaporative cooling will offset the release of latent heat and allow the ice to build up. In this event from February 1, 2007, the high pressure center that had deposited the cold and dry air had shifted well offshore and the surface wind flow became southerly. Once the warming aloft allowed the snow to transition to sleet and freezing rain, the release of latent heat almost immediately caused a changeover to mostly rain at the surface.



Reduced Precipitation Amounts in the Northwest Piedmont and its Impact

One of the most significant factors that resulted in the lack of winter storm conditions across the northwest Piedmont and Triad area was the lack of appreciable precipitation. The majority of forecast models used by forecasters at the NWS over forecasted the amount of precipitation on the northwest side of the precipitation shield. Liquid equivalent precipitation forecasts from the operational numerical weather prediction models ranged from 0.25 to nearly 0.75 inches across the northwest Piedmont including the Winston-Salem, Greensboro, and High Point (images of the GFS liquid equivalent precipitation forecasts for the 12 hour period ending at 00Z Friday, February 2, 2007 issued 00Z on 1/31 |  issued 12Z on 1/31 |  issued 00Z on 2/1 |  issued 12Z on 2/1). The liquid equivalent precipitation forecasts from the Hydrometeorological Prediction Center (HPC) were better than the suite of numerical weather prediction models and the HPC forecasts were correctly trending toward less precipitation with each issuance (HPC liquid equivalent precipitation forecasts for the 6 hour period ending at 18Z on Thursday, February 1, 2007 issued 18Z on 1/31 |  issued 00Z on 2/1 |  issued 06Z on 2/1 |  issued 12Z on 2/1 ). The last HPC forecasts still predicted between 0.25 and 0.10 inches of liquid equivalent precipitation during the 6 hour period ending at 18Z (100 PM) on Thursday.

24 hour precipitation estimate - click to enlarge The storm total liquid equivalent totals reached only a few hundredths of an inch across much of the northwest and northern Piedmont of North Carolina. This was far less what the models had forecast and not nearly enough to create winter storm conditions. There are likely several reasons for the limited observed precipitation with the greatest inhibitor likely being the very dry air mass in place. This dry airmass limited the spread of precipitation from eastern Tennessee and northern Georgia across the NC mountains and into the Foothills and Northwest Piedmont. Other factors limiting precipitation across the Northwest Piedmont include the elongated precipitation shield and the sharp precipitation gradient both of which were influenced by the strong upper level jet at 300 MB extended across the Southern Plains and Southeast.

Meteogram from Greensboro (KGSO) - click to enlarge Forecasters were confident that the atmosphere would warm at mid levels during the event. This meant that the precipitation would initially fall as snow for a few hours and then transition to sleet and then freezing rain by mid afternoon. The lack of precipitation (much less than 0.25 of an inch) obviously reduced the amount of snow that would fall initially. In addition, the reduced precipitation amounts did not allow diabatic cooling processes to have a significant impact. These processes include the cooling from evaporation of precipitation and from the melting of snow. Therefore, with only very light precipitation during the morning hours (less than 0.05 of an inch at KGSO and 0.01 at KINT), the surface temperatures warmed steadily throughout the morning, reaching 32 degrees by the early afternoon hours (meteogram from Greensboro (KGSO), meteogram from Winston-Salem (KINT), and meteogram from Raleigh-Durham (KRDU).

The surface wet bulb temperatures across the Northwest Piedmont were in the mid 20s as the light precipitation began at around 12Z (700 AM). Model guidance from the NAM and GFS did not bring the wet bulb temperatures to 32 degrees until the mid to late afternoon hours. Had there been more precipitation during the morning and early afternoon hours, the affects of diabatic cooling would have been much greater in the Northwest Piedmont. This would have allowed surface temperatures to fall more significantly after the onset of precipitation. The initial period of snow still would have eventually changed to liquid with the strong warming aloft. However, surface temperatures would likely have remained below 32 degrees for a much longer duration allowing a significant period of freezing rain to occur. Although the amount of time that snow and sleet would have been the predominate precipitation type would likely not have changed much, the amount of time that freezing rain would have occurred would have been much longer.


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 31 were only in the mid 30s to near 40 while the low temperatures on Wednesday night and Thursday morning were in the 20s. Ground temperatures during the few days preceding the storm ranged in the upper 30s during the afternoon and in the mid 30s during the overnight hours. 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 Lake Wheeler Road Field Lab in Raleigh, NC from 00Z on January 29, 2007 (700 PM EST January 28) through about 07Z on February 3, 2007 (200 AM EST February 3, 2007).

The precipitation began at the Lake Wheeler Road Field Lab just before 12Z on Thursday, February 1, 2007. Note that the ground temperature at the 4 inch depth level was just a few degrees above freezing in the hours just preceding and at the onset of precipitation. The snow began to accumulate fairly quickly on exposed earth during the event. 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 average hourly soil temperature at a depth of about 4 inches at at the Lake Wheeler Road Field Lab in Raleigh, NC



Raleigh - KRAX Radar Imagery and the Bright Band

Raleigh, KRAX WSR-88D base reflectivity imagery from 1358Z (858 AM EST) Thursday, February 1, 2007 is shown below. Note the area of enhanced reflectivities in the 35 dBZ - 45 dBZ range (shown in yellow and orange) stretching from near Wadesboro east to just south of Fayetteville to Clinton and Kenansville. This area of enhanced reflectivities was primarily produced by the "bright band."

radar bright band diagram - click to enlarge 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.

At time of the reflectivity image below, snow was reported across most of central NC, including Fayetteville and Kinston which were located just north of the "bright band." A little over an hour later, the bright band had moved north and the precipitation had changed to freezing rain and rain at Fayetteville and Kinston.

A Java Loop of KRAX Radar imagery from 0904Z (404 AM EST) Thursday, February 1, 2007 through 2100Z (400 PM EST) Thursday, February 1, 2007 is available.

Raleigh, KRAX Radar imagery from 
1358Z (858 AM EST) Thursday, February 1, 2007



Southeast Regional Radar Reflectivity

A Java Loop of Southeast Regional Radar Imagery from 0108Z (808 PM EST Wednesday) through 2158Z (558 PM EST) Thursday, February 1, 2007 is available.

Southeast Regional Radar 
Reflectivity from 1458Z (958 AM EST) on Thursday, February 1, 2007



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 Brandon Dunstan and Jonathan Blaes.
(Click the image to enlarge.)


Capability Drive and the parking lot for Research III on NC State’s Centennial Campus - Click to enlarge           Bridge from Research III to the Textiles building on NC State’s Centennial Campus - Click to enlarge           Snow and red berries on NC State’s Centennial Campus - Click to enlarge

A creek and wooded area on NC State’s Centennial Campus - Click to enlarge           A lake on NC State’s Centennial Campus - Click to enlarge           Snow and red berries on NC State’s Centennial Campus - Click to enlarge

Snow and holly bushes on NC State’s Centennial Campus - Click to enlarge           A creek and wooded area on NC State’s Centennial Campus - Click to enlarge                 Research III sign on NC State’s Centennial Campus - Click to enlarge


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.

Forecasters were able to use the Area Forecast Discussion (AFD) to share their analysis, expectations, and confidence in the forecast effectively. The content of AFD's leading up to this event were very good.

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.



Case Study Team

Gail Hartfield
Phillip Badgett
Michael Strickler
Brandon Dunstan
Jonathan Blaes


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


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