December 4-5, 2002 Winter Storm

Event Summary
National Weather Service, Raleigh NC

December 4-5, 2002 Winter Storm

Introduction -

A cold front moved across North Carolina on Monday night (12/2) and Tuesday (12/3). A very cold arctic airmass moved into the state behind the front, as a high pressure system moved into the Great Lakes region (see map). Low temperatures on Wednesday morning (12/4) fell into the teens across the mountains, the 20s across much of the Piedmont and 30s across the Coastal Plain and Coastal Region. Indicative of an arctic airmass, dew points across central North Carolina ranged from -2 to 5 degrees F.

Meanwhile a storm system slowly developed across the lower plains states Tuesday and Tuesday night (12/3), (see map). By Wednesday morning (12/4), a “wedge” of high pressure extended southward through the Carolinas (see map). The “wedge” (i.e., axis of high pressure) is indicative of cold air damming (CAD), depositing and locking cold dry air in place over the Carolinas.

Moisture streamed into North Carolina Wednesday (12/4) lifting over the surface based cold dry air. As frozen and freezing precipitation was falling across central North Carolina, the Wednesday evening surface map revealed a so-called Miller type “B” pattern of cyclogenesis. This pattern is characterized by a CAD wedge separating dual lows i.e, a well inland and a developing secondary coastal low along a commonly shared frontal boundary.

A developing Miller “B” cyclogenesis pattern is often associated with mixed wintry precipitation where corridors of predominant precipitation types are distributed across North Carolina. For the Dec 4-5 winter storm, the total freezing rain accumulation map shows the maximum amounts of freezing rain extended from southwest to northeast across central North Carolina. Lesser amounts of freezing rain fell to the northwest where measurable amounts of snow and sleet transition to mostly snow and sleet to then nearly all snow (see total snow/sleet accumulation map). Lesser amounts of freezing rain occurred southeast of the maximum freezing rain corridor where liquid rain predominated the event.

By midday Thursday (12/5) (see map), most of the precipitation had ceased as the storm system moved off coastal Virginia. By this time, the dual low configuration had evolved into a single low which served to erode the CAD wedge.

Various images of the Raleigh-Durham area after the winter storm (Click the image to enlarge.)

Slide show of images of the Raleigh-Durham area after the winter storm

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.

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Surface Data - Surface Analysis and Meteograms

NCEP Surface Analysis at the height of the storm across central North Carolina. Image is from 06Z Thursday December 5, 2002.

NCEP Surface Analysis from 06Z Thursday December 5, 2002.

Java Loop of Surface Analysis from 12Z Monday December 2 through 00Z Friday December 6 2002.

Surface Meteogram for Raleigh (KRDU)

KRDU Meteogram.

Surface Meteogram for Greensboro (KGSO)

KGSO Meteogram.

RAOB Data - Selected KGSO RAOB Plots (Skew-T Diagrams)

KGSO RAOB Plot (Skew-T Diagram) from 12Z Wednesday December 4, 2002.

KGSO RAOB Plots (Skew-T Diagram) - Click to enlarge

Click the image to enlarge.

KGSO RAOB Plot (Skew-T Diagram) from 00Z Thursday December 5, 2002.

Click the image to enlarge.

KGSO RAOB Plot (Skew-T Diagram) from 12Z Thursday December 5, 2002.

Click the image to enlarge.

Nomogram -

Leading up to this event, the WFO RAH forecasters used model thicknesses with the TREND technique’s universal precipitation type nomogram to forecast the predominant p-types and changeovers across the CWA. The models’ thicknesses proved to be quite accurate and exhibited good run-to-run consistency, which raised the forecasters’ levels of confidence in the p-type forecasts.

Below is an animation showing the actual thickness values from the GSO soundings taken at 18Z on December 4, 2002, and 00Z, 06Z, and 12Z on December 5, 2002, plotted on the TREND technique’s universal p-type nomogram. As the nomogram predicted, precipitation at GSO began as measurable snow and sleet, followed by a changeover to measurable sleet with freezing rain, and finally to an all-freezing rain event.

RAH P-type Nomogram from 00Z Thursday December 5, 2002.

Click the image to enlarge.

Radar Imagery - Selected KRAX Base Reflectivity Imagery

KRAX Base Reflectivity Image from 2022Z Wednesday December 4, 2002.
Note the band of heavier precipitation (yellow and orange) between Raleigh and Greensboro. This is an area of brief heavy snow and sleet. Burlington (KBUY) and Chapel Hill (IGX) are reporting moderate to heavy snow at this time (see METAR's below).

SPECI KBUY 042020Z AUTO 10003KT 1/4SM +SN FZFG BKN005 OVC023 M04/M06 A3045 RMK AO2 P0000 SPUS70 KBUY 042030
SPECI KIGX 042029Z AUTO 05004KT 1/2SM SN FZFG VV004 M04/M04 A3046 RMK AO2 P0000 SAUS70 KIGX 042100

KRAX Base Reflectivity Imagery - Click to enlarge

Click the image to enlarge.

KRAX Base Reflectivity Image from 0159Z Thursday December 5, 2002.
Note the areas of heavier precipitation (yellow) near the Virginia/North Carolina border. This is an area of light to moderate sleet.

Click the image to enlarge.

Java Loop of KRAX Base Reflectivity Imagery from Approximately 1830Z Wednesday December 4 through 14Z Thursday December 5 2002.

Final Thoughts -

The NWS forecast office in Raleigh issued winter storm warnings for 22 counties in central North Carolina. Advanced lead time for the warnings ranged from 13 to 28 hours. All 22 warnings verified with total ice accumulations from freezing rain exceeding the warning threshold of 1/4 inch. Here are some of the points that NWS forecasters emphasized to issue the accurate warnings:

Beginning days before the storm reached North Carolina, NWS Raleigh forecasters consistently verified the numerical weather prediction models forecasts of vertical thermal structure and quantitative precipitation forecasts. This practice allowed forecasters to determine a measure of confidence in the model solutions. The relatively high degree of confidence was then used to issue warnings with advance lead time well beyond the targeted goal of 12 hours.
Plunging surface dew points to around zero degrees F ensured there would be plentiful low level cold dry air in place when the gulf moisture arrived, adding confidence to the forecasts.
NWS Raleigh forecasters skillfully used the station’s locally developed precipitation type forecast scheme (TRENDs) to diagnosis the atmosphere’s projected vertical thermal structure and to geographically place and time the phase changes in precipitation types.
Forecasters identified a “seeder - feeder” signature in the forecast soundings for GSO lending additional confidence to the expectation that measurable snow/sleet will accumulate before changing to freezing rain.
Given the Miller type “B” pattern of cyclogenesis with an accompanying strong “wedge” from cold air damming, forecasters expected a precipitation type distribution characterized by corridors of predominant precipitation types.
The forecaster's good understanding of the physical processes controlling precipitation types generated additional confidence in the forecasts as the event was unfolding and ensured that forecast updates remained on track.
Understanding the problems associated with the numerical models representation of freezing prevented forecasters from extending the significant accumulations of freezing rain too far eastward and from over-forecasting the maximum ice accumulation.

Case study team -
Jonathan Blaes
Phillip Badgett
Gail Hartfield
Kermit Keeter

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

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