Event Summary
     National Weather Service, Raleigh NC


February 27-28, 2005 Winter Storm
Updated 2005/03/29


Event Headlines

...The precipitation type distribution was characterized by a narrow transition zone in association with a Miller Type “A” pattern of cyclogenesis...
...The surface high located to the north provided limited low level cold air...
...Heavy snow fell just northwest of the Triad...


Event Summary Objectives

This case event summary has two primary objectives. First, we will share the reasoning that went into the forecast process. Secondly, we will use this case to provide additional details concerning the physical processes and associated meteorological patterns that generate cold air.



Event Overview

An upper level jet generated an intense surface low over the central Gulf of Mexico early on February 27th. A High pressure system centered across the Northeast provided a limited supply of cold, low level air. The upper level 500 mb trough lagged behind the 700 mb trough and the close circulation at 850 mb. The surface low tracked northeast across northern Florida and then along the southeast U.S. coast.


Snow Accumulation Map


Snow and Freezing Rain Accumulation Map



Freezing Rain/Glaze Accumulation Map


Snow and Freezing Rain Accumulation Map



Objective 1: Sharing the Forecast Process

The Forecast

On Sunday 27 February 2005, a “Heavy Snow Warning” was posted by the National Weather Service Forecast Office in Raleigh (NWS RAH) for six counties in the northwest Piedmont of North Carolina, including the Greensboro, Winston-Salem, High Point Triad area.



Verification

None of the six counties, including the Triad area received heavy snow. Late Sunday on 27 February into Monday morning on 28 February, a mixture of snow, sleet, and freezing rain fell in the warning area. Most locations received little or no frozen precipitation. The more notable accumulations of frozen precipitation in the warning area included one half inch of sleet at the Greensboro Airport, and 1.5 inches of snow and sleet at the Winston Salem Airport.



Location of Heavy Snow

Though there was no heavy snow in the warning area, heavy snow did fall in nearby locations. Four inches of snow accumulated about 20 miles north and west of the Winston Salem Airport in nearby central Stokes and Yadkin counties. The snow amounts increased through 6-8 inches in the northern foothills. Snowfall amounts then increased to a foot of snow in Boone (northern mountains) which is located about 80 miles west of the Winston-Salem Airport.



The Forecast Process

Area Forecast Discussions (AFD) issued on 27 February 2005 by NWS RAH revealed that the key components of the station’s winter storm forecast process had been well evaluated. For example, forecasters correctly anticipated that there would be a narrow zone of mixed precipitation oriented from southwest to northeast in association with a Miller Type “A” pattern of east coast cyclogenesis (see footnote below). The forecasters were also correct in recognizing that heavy snow would occur just north and west of the snow/rain line. The forecast discussions cited concern about a suitable cold air source, noting that the colder and drier air in northern Virginia may not be available to the warning area; however it was noted that the NWP models were indicating a mid level closed circulation, implying strong dynamic cooling and a high potential for banded precipitation. It was then correctly reasoned that the combination of dynamic cooling and cooling from melting could well compensate for the lack of significant cold air support from the surface high to the north.



NWP Model Errors

Using the Weather Event Simulator (WES ) to compare archived NWP data from the 12Z 2/27/05 run to the real time data valid at 12Z 2/28/05, some key model errors were found. Rather than a closed circulation at 700 mb, as forecast by the GFS and NAM, there was an open and unimpressive short wave. Hence the models had overstated the potential for banded precipitation and the associated cooling from melting that would have occurred. The 500 mb short wave trough lagged well west of the open 700 mb short wave and the closed circulation at 850 mb. The model of choice for this event, the GFS, over forecast the 500 mb height field by ~ 50 M, and hence overestimated the contribution from dynamic cooling. Though these model errors were not especially large, the errors were enough to limit dynamic cooling and cooling from melting that might have compensated for the limited cold air support from the surface high. Hence the significant snowfall developed and remained just north and west of the Winston-Salem, Greensboro, and High Point Triad area.

Relative to the forecast soundings and partial thickness values, the GFS was too cold in its 24 hour forecast of 850-700 mb thickness while its forecast soundings had portrayed a near isothermal near freezing layer. The NAM’s higher 850-700 mb thickness values and its forecast of a small melting layer aloft was more accurate. While the NAM's thickness forecast had been correct, it was for the wrong reason since it had incorrectly called for the surface low to track inland. In time the NAM's surface low track came more in line with the GFS which correctly called for the surface low to move along the Carolina coast line.

Finally the impressive 992 mb surface low, located along the southeastern coast of North Carolina at 12Z on 28 Feb 2005, appears to be largely driven by the kinematics associated with the left forward quadrant of a 300 mb jet.



Forecast Process Summary

Given the NWP model guidance and the pattern of cyclogenesis, a forecast calling for 2-4 inches of wet snow in the climatologically favored areas of central North Carolina was reasonable. The limited cold air support from the surface high had been well noted in the Area Forecast Discussion. NWS RAH forecasters showed an extensive understanding of winter storm physical processes and their associated meteorological patterns to foresee the potential for significant snowfall in the northwest portion of Central North Carolina. For several model runs preceding the event, the NWP guidance consistently pointed to more dynamic cooling and the potential for additional cooling via melting from moderate to heavy banded precipitation.

Pinpointing the location of a snow/rain line to within ten's of miles requires both an extensive understanding of winter weather science and precise guidance from the NWP models. The area forecast discussions indicated the forecasters level of understanding was substantial, while the NWP guidance, though reasonable, was less than precise.



Product Head Lines

NWS’s policy regarding the use of product headlines has changed. The “Heavy Snow Warning” has again returned as an option. Given the variability of winter weather, the frequent occurrence of mixed and changeable precipitation in central North Carolina, and the users’ tendency to have their own definition for heavy snow, the “Heavy Snow Warning” banner should be seldom used. High confidence in both p-type and snowfall total forecasts, extensive collaborative agreement with other nearby NWS offices, and a prediction of a snowfall total that well exceeds the minimum warning criteria are factors that should be met before using the “Heavy Snow Warning” banner.



Footnote on the Pattern of Cyclogenesis & the Distribution of Precipitation Types

Empirical studies at NWS RAH have shown that a narrow transition zone of mixed precipitation is often associated with a so-called Miller Type “A” pattern of east coast cyclogenesis. In North Carolina, this pattern is characterized by a well organized single surface low developing along a cold front and tracking northeastward while surface high pressure is located north of the low. Differential (i.e. cold and dry vs. warm and moist) horizontal thermal advection between the cold air surface high and the developing surface low plays a large role in establishing the location of the snow/rain line. With this pattern of cyclogenesis, the transition from rain to snow occurs over relatively short distances with the transition zone of mixed precipitation confined to a narrow corridor, typically ranging from 10 to 70 miles wide.



Objective 2: Generating Cold Air

Principal Cold Air Source

Polar and Arctic air masses associated with surface high pressure systems represent the principal cold air source for supporting frozen and freezing precipitation types in North Carolina. A high pressure system configuration characteristics including strength (central pressure), intensity (pressure gradient), location of pressure center, and orientation of the ridge axis are all qualitatively indicative of the degree of cold air support it provides. Indeed, the mean sea level pressure analysis is a surrogate for inferring low level thermal structure and thermal advection. Of particular importance is the orientation of the surface high’s ridge axis. The ridge axis indicates the primary direction of the greatest cold air advection. It also represent the deepest and coldest air emulating from the high’s center, becoming more shallow and less cold as it slopes southward from the center and as the distance increases away from the ridge axis.



2004-2005 Winter Season - Limited Cold Air Support

Frozen precipitation was considerably less than normal this winter season (2004-2005) across central North Carolina. Seasonal snowfall totals were 1.7 inches at Greensboro and 0.9 inches at Raleigh. The normal snowfall totals for Greensboro and Raleigh are 8.9 and 7.3 inches respectively. While there are likely several factors that account for the well below normal snowfall totals, limited low level cold air advection played a prominent role. Particularly important was the orientation of the ridge axis extending from the parent highs located to the north. Moreover, a northwest to southeast ridge axis was a persistent feature this winter season, driving the deepest and coldest air into the Mid Atlantic region. The northwest to southeast surface high ridge axis pattern limited snowfall in central North Carolina while supporting several significant snowfall events in locations just north and west of the Greensboro and Raleigh areas.

With respect to limited low cold air support from an unfavorably oriented surface ridge of high pressure, the 28 February 2005 minor snowfall event in the northwest portion of central North Carolina is a case that typified much of this winter season. As previously noted, the snow/rain line barely extended into the Winston-Salem-Greensboro area, while significant and heavy snow fell in locations just to the north and west. Note that at 09Z on 27 February 2005, a northwest to southeast ridge axis extended from a 1030 mb parent high over the Great Lakes region southeastward into the DELMARVA area. Clearly, the surge of coldest air and lowest dew points was occurring through the Mid Atlantic region. By 15Z as an area of low pressure approached northern Florida, a secondary axis of high pressure had developed from central Virginia southwestward through northern Georgia into northeastern Alabama. This secondary axis was in response evaporative cooling and the developing inverted surface coastal trough. Meanwhile the principal ridge axis indicative of the best cold air advection support was still oriented from northwest to southeast through the Mid Atlantic region.

The 28 February 2005 mean sea level pressure pattern is in contrast to the evolution of classical cold air damming (CAD) for North Carolina where the dry air ridge (i.e. principal surface high ridge axis) first extends well into eastern North Carolina before sliding westward to present a classical CAD wedge signature in the western Carolinas. By 03Z 28 February, the mean sea level pattern shows a rather typical appearing CAD wedge just east of the Blue Ridge Mountains as a well developed low pressure system tracks along the coast of Georgia. While a narrow transition zone from rain to snow has developed as expected with this Miller Type “A” pattern of cyclogenesis, there is limited low level cold air support provided from the cold air high located over the Northeastern U.S. If the snow/rain line is to moved farther southeastward into central North Carolina, there must additional sources for generating cold air.



Additional Sources for Generating Cold Air

When cold air advection from a surface high to the north is limited, other factors for generating cold air become critical for supporting frozen precipitation in central North Carolina. These factors include dynamic cooling, melting, cooling from ageostrophic circulation associated with the boundary between snow and rain, and cooling from evaporation. Obviously, when the low level cold air support is limited from the surface high to the north, the more frozen precipitation becomes dependent upon these other factors.



Raleigh - KRAX Radar Imagery

Raleigh, KRAX WSR-88D base reflectivity imagery from 0558Z (1258 AM EST) Monday, February 28, 2005 is shown below. At this time, rain was reported across most of central NC with some sleet in the Triad and snow across the Mountains and Foothills.

A Java Loop of KRAX Radar imagery from 0016Z (716 PM EST Sunday) Monday February 28, through 15034Z (1034 AM EST) Monday February 28, 2005.

Raleigh, KRAX Radar imagery from 0558Z (1258 AM EST) Monday, February 28, 2005



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


1. The 2/28/05 minor snow event in the Triad area with heavy snow located just to the north and west typified much of the 2004-2005 winter season.

2. While several factors accounted for the well below snowfall totals in Central North Carolina this past winter, a lack of low level cold air support from high pressure located to the north was prominent.

3. While high pressure was favorably located to the north for supporting snow in Central North Carolina on 3/28/05, the principal surge of cold air was into the Mid Atlantic region in association with a northwest to southeast oriented ridge axis. This ridge axis pattern typified much of the 2004-2005 winter weather season.

4. When cold air support from high pressure located to the north is limited, other factors for generating cold air become critical for supporting frozen precipitation in central North Carolina. These factors include dynamic cooling, melting, cooling from ageostrophic circulation associated with the boundary between snow and rain, and cooling from evaporation.

5. Greater height falls are associated with greater dynamic cooling. In a qualitative sense, forecasters can look for negatively tilted or cutoff mid to upper level systems and as well as stronger and more intense systems in general as indicators of significant dynamic cooling.

6. Cooling from melting becomes more substantial as the precipitation rates increase as would be the case with banded precipitation. Banded precipitation is associated with a number of meteorological features including mid level circulations, dual jet structure, and rapidly deepening surface lows.

7. Cooling from melting also becomes more substantial as horizontal thermal advection weakens. This explains in large part why the northwest quadrant of a mature surface low is a favorable location for supporting deep near freezing isothermal layers. Mature means that the surface low has progressed through the baroclinic phase of cyclogenesis toward a barotropic and vertically stacked system.

8. Pinpointing snow/rain lines and the location of nearby heavy snow areas within tens of miles requires both a substantial knowledge of winter weather science and precise guidance from the NWP models.

9. The lack of cold air support from high pressure to the north in general results in less confidence for the occurrence of frozen precipitation in Central North Carolina since other factors must then generate the needed cold air.

10. NWP models in general have better precision with larger scale systems as opposed to the short wave system which occurred with the 3/28/05 winter weather event. In general forecast confidence for generating enough cold air for snow from dynamic cooling increases as the scale of the weather system increases.



Case Study Team

Kermit Keeter
Phillip Badgett
Jonathan Blaes


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


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