January 23, 2003 Winter Storm

Event Summary
National Weather Service, Raleigh NC

January 23, 2003 Winter Storm

Event Overview - Snow began falling in the mountains of North Carolina during the evening on Wednesday 22 January 2003. The snow spread eastward before ending in the northeast coastal areas along the Outer Banks on Thursday afternoon 23 January. Atypically for winter weather events in North Carolina, all 100 counties in North Carolina received measurable snowfall. Snow accumulations across the state showed considerable variability, ranging form less than an inch to over a foot of snow. The principal intent for this case review is to provide explanations accounting for the distribution of snowfall amounts.

Synoptic Overview - The event’s large scale synoptic scale pattern is not an unfamiliar one for the Carolinas. The combination of strong dynamics in a northwest flow aloft tracking toward a favorable low level baroclinic zone provides a favorable setting for coastal cyclogenesis. A conceptual model indicating the distribution of precipitation amounts in North Carolina generally associated with this pattern shows that the area with the greatest degree of precipitation amount uncertainty lies between precipitation total maximums expected in the mountains near the track of the upper level dynamics and in eastern North Carolina associated with track and intensity of the surface coastal low. This is demonstrated in the December 2-3, 2000 winter weather event.

Snow Accumulation Map - Snow totals across North Carolina ranged from less than an inch to more than a foot. The larger areas of snow accumulation maximums for this event can be seen in both western and eastern North Carolina in a manner generally consistent for the northwest flow dynamics-coastal cyclogenesis conceptual model.

Visible Satellite Imagery from 1622Z Friday January 24, 2003 - High resolution satellite imagery reflect the patterns shown on the snowfall map. One notable possible discrepancy not resolved by the reported snowfall totals appears to be the lesser amounts seen in the imagery on the eastern slopes of the Blue Ridge Mountains. There, down slope effects from the strong northwest flow may have accounted for apparent lesser amounts seen in the imagery.

Visible Satellite Imagery from 1622Z Friday January 24, 2003 - Click to enlarge

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Mountain Barrier -

Snowfall totals in the foothills and far western Piedmont of North Carolina and South Carolina (not shown) were more than expected. These amounts up to a foot of snow were associated with a mesoscale low pressure system that developed there well inland of the principal surface low off the Carolina coasts. It is thought that this low was a result of the direct and indirect effects of topography. The highest mountains in the Appalachians are in North Carolina. These mountains are necessary features for the production of cold-air damming in the Carolinas. They also play an important role in the pattern of cyclogenesis. Under conditions of a deep and strong northwest flow, the mountains often produce a number of effects including:

development of a lee surface trough,
an area of surface wind convergence on the southeast side of the mountain barrier
slowing the progression of arctic air into the foothills and the far western Piedmont of the Carolinas.

North Carolina Topographical Map - Click to enlarge

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Surface Analysis from 12Z Wednesday, January 22 2003.

Notice in the above surface analysis that the inverted lee trough over North Carolina extends well south of the Appalachian Mountains into central Georgia and connects to the surface low in southern Georgia. This portion of the inverted trough was likely caused by the mountains producing a barrier effect between the two surface high pressure centers, one over the Delmarva area and the stronger principle high over the Dakotas. Note the cold dry air to the west of the mountains (temperatures in the teens and dewpoints in the single digits in eastern Kentucky). This cold dry air mass was dammed up against the western slopes of the Appalachians, and the only escape was around the southern end of the mountains over Georgia. Meanwhile the weaker surface high in the Delmarva area provided an east-northeasterly flow over the Carolinas. This created an area of surface wind convergence on the southeast side of the mountain barrier over Georgia and western South Carolina. With a dry arctic air mass from the west converging with slightly warmer and moister air mass from the east, a weak baroclinic zone was generated from northern Georgia to the NC Foothills. The effects of this can be seen in the next figure (surface analysis at 06Z Jan 23), as the dewpoint gradient (dashed green lines) has tightened over northern Georgia and a moisture ridge axis extends north over the NC Foothills. The mountain barrier effect also helped to slow the progression of dry arctic air into the Foothills and the far western Piedmont of the Carolinas. This preserved the low level moisture ridge that was in place to the east of the mountains.

Surface Analysis from 06Z Thursday, January 23 2003.

Surface Analysis from 06Z Thursday January 23, 2003.

Development of the Lee Trough and Foothills Mesolow -

A lee trough can be generated when the air in a northwest flow descends the leeward slope of the mountains. The air column expands vertically and contracts horizontally, gaining cyclonic vorticity. This in turn is reflected at the surface by a low pressure trough. In this case, a lee trough was located over the NC Foothills long before the precipitation event began and is evident in the below surface analyses at 00Z 22 January. This lee trough would become a focusing mechanism for cyclogenesis once the upper level dynamics moved closer to the area.

Upper Level Dynamics and Mesolow Development- The Foothills Mesolow developed in an area located in the region of maximum cyclonic vorticity advection ahead of a shortwave trough rounding the base of the closed 500 mb low (500 mb winds, heights and vorticity (colored) shown at 1200 UTC 23 Jan 2003). The area of strongest cyclonic vorticity advection is over the NC Foothills.

12Z 2003/01/23 500 mb Heights and Geostrophic Absolute Vorticity - Eta Initialized Data, - Click to enlarge

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Upper Level Dynamics and Mesolow Development – The Foothills mesolow also developed in an area favorable for enhanced lift, coincident with the left exit region of the 300 mb polar jet (300 mb winds, heights, and isotachs (colored) shown at 1200 UTC 23 Jan 2003).

12Z 2003/01/23 300 mb Heights, Isotachs, and Winds (kts) - Eta Initialized Data, - Click to enlarge

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Surface Analysis from 12Z Wednesday, January 22 2003.

Surface Analysis from 06Z Thursday, January 23 2003.

Pattern of Surface Cyclogenesis – With a surface lee trough and a weak baroclinic zone in place in the Foothills, the linkage between these surface features and the upper level forcing led to the development of a small surface mesolow. This closed low strengthened as the upper level trough approached and cyclonic vorticity advection increased. At the same time, the polar jet at 300 mb was diving southeast, with the left exit region of the jet located over southwest NC. In this quadrant of the jet, the ageostrophic flow is divergent, resulting in ascending air, and enhanced cyclogenesis. With the jet’s left exit region and the area of maximum cyclonic vorticity advection moving over the surface lee trough and weak baroclinic zone that was already in place, the mesolow formed in the NC Foothills and upstate SC. With the mountains acting as a barrier to preserve the moisture ridge east of the mountains, adequate moisture was available in the area of cyclogenesis, which enhanced the snowfall in this area. Later in the event, the focus for cyclogenesis shifted to the Carolina coastal waters once the upper level dynamics linked with the favorable baroclinic zone found there in the presence of an old synoptic frontal boundary and the enhanced baroclinic effects provided by the warm Gulf Stream.

Java Loop of Surface Analysis from 00Z Wednesday January 22 through 18Z Thursday January 23, 2003.

Effect of mesolow development on snowfall pattern – The time of greatest deepening of the surface mesolow was between 0400 UTC and 0900 UTC. Wind profiler data at Charlotte (CLT), located about 60 miles east of the mesolow, is shown below from 0030 UTC 23 Jan 2003 to 1200 UTC 23 Jan 2003. Note that as the surface mesolow was initiated, the profiler winds below 850 mb can be seen to back in response to cyclogenesis. Precipitation also begin to increase in the area around 0600 UTC. Enhanced low level convergence and an upslope wind component resulting from the mesolow assisted in maximizing the snowfall amounts in the area. The area of maximum snowfall also coincided with the low level moisture ridge that had been in place.

12Z 2003/01/23 VWP Data from Charlotte, NC - Click to enlarge

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Mesoscale snow bands over the Northern Piedmont -

The snow accumulation map shows a narrow west-to-east band of 3 to 5 inch snowfall amounts across the Northern Piedmont of North Carolina. An analysis of radar imagery from 1300 UTC to 1700 UTC shows that some north-to-south oriented mesoscale snow bands moved across this area.

Animation of KRAX radar data showing north-to-south oriented mesoscale snow bands

Band formation occurs in areas of low static stability. In this case, there was low level instability as seen in the 06Z and 12Z GSO soundings. The soundings show that lapse rates below 850 mb are nearly dry adiabatic. Thus the low stability was probably the main contributor to the formation of snow bands.

KGSO RAOB Plot (Skew-T Diagram) from 06Z Thursday January 23, 2003.

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

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KGSO RAOB Plot (Skew-T Diagram) from 12Z Thursday January 23, 2003.

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So why did the bands form only in the Northern Piedmont? Looking at the radar loop, some of the bands appear to form downstream of lakes near the Virginia border. Two bands formed over Person County, where Hyco Lake and Mayo Reservoir are located. A larger band formed over Granville County and western Vance County, downstream of Kerr Lake. Much like the lake-effect snow bands that form in the Greta Lakes region, the warmer waters of these lakes may have created localized areas of lower stability, which led to the formation of mesoscale snow bands.

Visible Satellite Imagery from 1622Z Friday January 24, 2003 -

(Click the image to enlarge.)

12Z 2003/01/23 850 mb Heights and Geostrophic Absolute Vorticity - Eta Initialized Data

12Z 2003/01/23 850 mb Heights and Geostrophic Absolute Vorticity - Eta Initialized Data, - Click to enlarge

(Click the image to enlarge.)

This event produced unusually low temperatures at 850mb – between -12 and -15 degrees C over central NC. Cold advection at this level was also very strong, contributing to the low level instability which helped form the lake-effect snow bands. Because of the unusually cold air, snowfall ratios were unusually high, around 15:1. Normally, rain to snow ratios in central NC are 10:1.

Radar Imagery -

KRAX Base Reflectivity Imagery from 1131Z on Thursday January 23, 2003 - Click to enlarge

Click the image to enlarge.

Overview of the storm - animation of imagery from every hour near the top of the hour

Java Loop of KRAX Base Reflectivity Imagery from 0200Z Thursday January 23 through 2003Z Thursday January 23, 2003.

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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Meteograms -

Surface Meteogram for Raleigh (KRDU)

KRDU Meteogram.

Surface Meteogram for Greensboro (KGSO)

KGSO Meteogram.

Final Thoughts -

A westerly flow over the mountains before the event produced a lee trough in the Foothills of NC, which provided a zone of low level convergence and would become the focusing mechanism for cyclogenesis.

The effect of the mountain barrier was to keep the dry arctic high from intruding east of the mountains, and the weak surface high over Delmarva provided an easterly component. This set up a zone of baroclinicity over western SC and the NC Foothills.

As a strong vorticity maximum and an upper level jet approached this area, cyclogenesis occurred in this area, and a closed low formed. The result of this closed low forming was an increased upslope wind component, which produced enhanced snowfall amounts in the NC Foothills.

As the upper level dynamics shifted east, cyclogenesis occurred in a baroclinic zone offshore, leading to heavy snowfall along the coast.

An enhanced area of snowfall over the northern Piedmont of NC may be explained by strong low level instability created by cold air advection over area lakes.

Case study team -
Jonathan Blaes
Phil Badgett
Joel Cline
Kermit Keeter
Douglas Schneider

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

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