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.
(Click the image to enlarge.)
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).
(Click the image to enlarge.)
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.
(Click the image to enlarge.)
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