Forecast Discussion
Summary
SYNOPSIS
... A cold front will bring showers and thunderstorms to the area through this morning. Very windy conditions will develop behind the front late this morning and remain in place through early Sunday morning. An unsettled pattern returns for next week as multiple days of showers and thunderstorms will be possible.
NEAR TERM /THROUGH TONIGHT/... As of 325 AM Saturday: Quiet for the time being, with wind gradually increasing, but the situation upstream still has our attention with a few severe thunderstorms tracking eastward across middle TN as of 07Z. The plan remains for the storms to outrun the weakening instability and diminish steadily as they move east across the Cumberland Plateau and Great Valley of TN over the next few hours. That plan might already be working as the number of severe warnings has dropped over the past hour or so. Nevertheless, the organization and low level forcing might already be sufficient to sustain some of the stronger storms far enough east to reach western NC in the pre-dawn hours, with mainly a wind damage threat. In the mean time, scattered showers and thunderstorms have developed in the Great Valley and they should brush across the TN/NC border region ahead of the main band through 10Z or so. We will continue to monitor.
The CAMs are in good agreement with a loosely organized band of showers and thunderstorms pushing steadily east into the mtns before daybreak. The air mass ahead of the band will have poor to non-existent sfc-based CAPE unless something strange and dramatic happens in the next hour or two. There should be just enough elevated CAPE to keep the storms going, though, such that the first tier of NC counties on the TN border might end up in a SVR Warning as the line moves in, but beyond that all bets are off. The band of convection should make steady eastward progress across the rest of the fcst area through the late morning, and should exit the fcst area by around noon. Will keep a mention of thunder everywhere as the band moves through. Expect some gusty winds ahead and along this band. In the wake of the storms, clearing should be rapid, so we should have no problem warming up in the afternoon. Guidance indicates a bit of a lull in the winds aloft in the early/mid afternoon, but then another westerly low level jet spreads across in the late afternoon. So, similar to last time, we might not be especially windy well into the afternoon and it might look like the Wind Advisory/Warning is a bust, but keep in mind the best potential is centered on 21Z. The wind potential will start to fall off in the mid/late evening. The rest of the fcst will be dominated by sfc high pressure building in from the west tonight, with a cooler and drier air mass.
SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... As of 200 AM EDT Saturday: Winds should begin to subside after daybreak Sunday as surface high sets up shop across the CFWA, with low-level ridging developing over the Eastern Seaboard. Expect a nice day with a very dry vertical profile, supporting some fire weather concerns with low RH values (20-30%), but winds will be light during peak heating. A Fire Danger Statement may be needed for portions of the area as a result, especially over northeast Georgia. Otherwise, surface high will gradually shift offshore as the low-level ridge axis follows pursuit and a return flow commences over the area by Sunday night. With quasi-zonal flow aloft settling over the region, height falls won't be significant enough for a true dropoff in temperatures, with values forecasted to be near-normal for Sunday and Sunday night. An embedded shortwave will travel within the quasi-zonal flow aloft and should allow for enough upper forcing and isentropic lift to force PoPs into the forecast early Monday morning in the western zones. Model guidance are showing better precip coverage across the CFWA and thus, an uptick in PoPs from the previous forecast will be in store for the daytime period Monday. Temperatures will remain near-normal on Monday as a result due to slightly higher PoPs and extensive cloud cover. The shortwave will slip offshore by Monday night and allow for PoPs to decrease, but low-level moisture will continue to filter in with south-southwesterly flow. Temperatures will be 5-10 degrees above-normal Monday night as low clouds remain in place despite drier conditions.
LONG TERM /TUESDAY THROUGH FRIDAY/... As of 315 AM EDT Saturday: Upper ridging noses in across the Southeast Tuesday into Wednesday as the CFWA becomes sandwiched between the ridge as it pulls slightly offshore and an upper low digs into the north-central High Plains. Deep layer south-southwesterly flow will continue to transport moisture into the area through the middle part of the week. With ripples of shortwaves riding within this regime, expect chance PoPs to be in the forecast during this timeframe. Model guidance depict some form of instability in this pattern, so thunderstorms can't be ruled out, but shear parameters are not impressive. Thus, the severe threat will remain low at this time due to low confidence in the overall threat this far out.
Global models continue to send a trailing cold front into the area late Wednesday into Thursday as the upper low lifts into the Northern Great Plains. The struggle continues regarding the forward progression of the front as it enters the CFWA by Thursday as models continue to stall the boundary either in the CFWA or just to our south. This will bring an unsettled pattern to the area through much of the medium range, but timing everything out is virtually impossible at this point. However, there has been run-to-run consensus on the possibility of CAD developing into the area as a stout surface high sets up shop over the northeastern CONUS by the end of the forecast period. Temperatures will be well above-normal during the first half of the extended, but with better rain chances and extensive cloud cover associated with the stalled boundary, expect these values to dip to near-normal values by the latter half of the period. If CAD can become established at some point late next week, expect these values to dip below-normal by D6/D7.