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An example of a Northwest Flow Warm Anomaly and Low Level Wind Maximum Affecting the Lower French Broad River Valley and South Carolina Upstate

Bryan P. McAvoy
NOAA/National Weather Service
Greer, SC

Author's Note: The following report has not been subjected to the scientific peer review process.

1.  Introduction
Frequently, when a significant trough is exiting the region to the east, 
a temperature and wind maximum will develop over the lower French Broad 
River Valley of North Carolina and parts of Upstate South Carolina.  To 
a lesser extent this also happens over parts of the North Carolina 
Foothills.  This results from channeling of winds through the French 
Broad Valley, as well as through other, smaller valleys throughout the 
southern Appalachians.  These events make for rather difficult forecast 
decisions, especially considering that two of the larger population 
centers in the county warning and forecast area of the National Weather 
Service office at Greenville-Spartanburg are directly in the path of this 
low level area of mixing.  For example, during the 17 October event the 
low level flow relaxed enough to allow many areas to decouple.  In the 
mixing maximum, the winds briefly calmed, allowing the temperature to fall 
to around 40 oF, but then increased again, raising the temperature into the 
middle 40s with winds gusting in excess of 20 knots.  Just a few miles 
miles away the temperature fell into the mid 30s with frost.
A loop of IR satellite imagery and surface observations shows the warm 
pocket developing and expanding across the Greenville and Spartanburg metro 
areas on the morning of 17 October.  As we have very limited data on these 
events, it is difficult to draw many conclusions from one event.  However, 
it is interesting to notice the Eta model did pick up on the increase in 
low level winds and subsidence in the lee a little before 1200 UTC.  Looking 
at a plan view of 925 mb subsidence, the values at 0900 UTC were rather weak 
compared to those at 1200 UTC, which extended further east and were a few 
microbars stronger.  The Eta also showed an increase in the lower tropospheric 
flow at this time, as can be seen by time sections take from Greenville, 
Anderson and Asheville.
The 850 mb Eta plot from 0900 UTC and 1200 UTC did not show quite the change 
that the 925 mb surface did.  This implies that the 925 mb surface may be a 
good forecast tool for diagnosing a low level mixed jet in favored areas of 
the North Carolina Mountains and the South Carolina Upstate.  More surface 
data from this event needs to be retrieved and plotted through time.  While 
there are many cooperative observer reports in the Upstate, their static 
nature makes plotting the changes in time from 0900 UTC to 1100 UTC impossible.
Something else interesting to note in the IR loop is that the winds picked 
up at the Asheville Regional Airport about two hours before they did at the
Greenville-Spartanburg International Airport.  Considering that the two 
airports are separated by about 50 miles, it is obvious that this area 
of mixing was the result of a weak density current (basically a cold front) 
which was difficult to see in height or pressure fields.  While this is an 
interesting feature, in the stronger mixing events, the wind tends to stay 
up all night, resulting in even larger temperature spreads than were seen
in this event.

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Page last modified: August 26, 2011

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