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