National Weather Service Raleigh, North Carolina
Training Information


February 22, 2001 Winter Weather Event
  • Description - A fast moving weather system brought winter-type precipitation to a large portion of North Carolina and Virginia on Thursday, February 22, 2001. In central North Carolina, the precipitation type ranged from predominately a snow/sleet mixture in the far northern Piedmont, to a few hours of freezing rain over sections of the coastal plain and Sandhills. Amounts varied from one to three inches of snow and sleet over the far northern Piedmont, to a thin glaze on exposed objects in the northeast Piedmont and the northern Sandhills. The majority of the precipitation fell in a narrow window of three to five hours Thursday morning.

    Measurable Frozen (Snow/Sleet) totals on 02/22/01





    Liquid Equivalent Precipitation Totals (melted snow/sleet and freezing rain) on 02/22/01
    Amounts are in inches.

    For the Triad Area (Guilford-Forsyth counties), compare the liquid equivalents to the accumulated frozen amounts. Despite daytime highs into the 60s on the day before, there was little melting of the frozen as it fell in the Triad area. The drop in the 1000/850 thickness was 53 meters in 12 hours. The overnight surge of low-level cold air southward was accompanied by gusty surface winds and was very effective in cooling surfaces.




    Maximum Daytime Temperatures on February 22, 2001
    Temperatures are in Farenheight

    The maximum daytime high temperatures were at or below 30 degrees F for those areas where an inch or more of frozen precipitation accumulated.






  • Evolution of the Event- The leading edge of the colder, drier air was marked by a cold front that moved south across central North Carolina on Wednesday afternoon. Surface dewpoints fell from the mid and upper 40s into the 20s in a matter of a few hours. The parent high quickly translated eastward Wednesday afternoon evening, moving from the Upper Mississippi river valley to southern New England in a span of fifteen hours. The OOZ Thursday upper air analysis suggested that the rapid eastward movement may slow as the 500mb flow was slightly confluent along the eastern seaboard. In addition, a dual jet structure was noted aloft over the eastern United States (primary jet stretching from Pennsylvania to Long Island, with a secondary jet over the southeast U.S.). While the 850mb flow was not indicative of a damming scenario (mainly northwest versus a favored southerly flow), the 06Z Greensboro sounding depicted that the 850mb flow had become southerly.

    The 06Z surface analysis shows a distinct dry air ridge across eastern Virginia and the eastern Carolinas. This feature is known to be a sigani for the potential for cold air damming. The precipitation overspread the western Piedmont between 09Z-12Z. By 15z, the wedge signature was evident on the analysis over the -typical damming region as the precipitation falling through the dry sub-cloud layer had increased the hydrostatic pressures, causing the dry air ridge to migrate into the western Piedmont.

    The 12Z upper air analysis depicted a scenario reminiscent of previous cold air damming episodes. The aforementioned 850mb southerly flow had become established across the Mid Atlantic coast states. The best 500mb confluence was positioned offshore, suggesting that the parent high would continue to migrate east. The 300mb jet was exiting off the northern Mid Atlantic coast states, placing central North Carolina in the favorable right entrance region. The divergence aloft coupled with the strong low level convergence supplied by the low level jet crossing the Deep South likely enhanced the precipitation rates across central North Carolina Thursday morning.

    The 18Z surface analysis suggest that the evolution into a residual cool pool was beginning. While the surface flow was still northeast in the damming region, the flow was no longer feeding colder drier air into the Piedmont. This became more evident by the OOZ analysis. By 06Z the surface flow across the favored damming region had become west-southwest, and skies have cleared. Thus, it appears that the actual damming occurred for three to six hours with a residual cool pool lasting six to twelve hours. Since the wedge signature did not develop until after precipitation had been falling for a few hours, this is probably a hybrid damming event of short duration.





  • Upper Air Data-



    KGSO Raob from 00Z 02/22/2001





    KGSO Raob from 06Z 02/22/2001





    KGSO Raob from 12Z 02/22/2001





  • Surface Analyses -
    18Z 02/12/01 Surface Analysis





    06Z 02/22/01 Surface Analysis





    12Z 02/22/01 Surface Analysis




  • Model Performance -





    Observed heights and thickness values at KGSO compared to forecast data




  • Model Trends (dProg/dt) -

  • Evaluating Precipitation Types -



















  • Case study team -
    Phil Badgett
    Jonathan Blaes
    Gail Hartfield
    Kermit Keeter
    Scott Sharp

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


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