Area Forecast Discussion
Issued by NWS Greer, SC
Issued by NWS Greer, SC
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726 FXUS62 KGSP 041724 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 124 PM EDT Thu Jul 4 2024 .SYNOPSIS... Hottest day of the week expected on Friday. A cold front moves in from the north on Friday night, increasing shower and thunderstorm chances into the weekend. A typical seasonal pattern of afternoon showers and storms set in starting Monday. && .NEAR TERM /THROUGH TONIGHT/... As of 1130 AM EDT Thursday: Dewpoints are starting to mix out a tad more than aniticpated. Adjusted temps and heat index forecast to account for the change. Still expecting hot heat index values east of the mountains, but well below any criteria for a heat advisory. Slight adjustments to PoPs as confidence is still low for showers and thunderstorms this afternoon. Over the next 24 hours, upper ridging over the Carolinas and Mid-Atlantic will be gradually flattened by nrn stream short waves tracking from the upper OH Valley to the nrn Mid-Atlantic coast. As the center of the old upper anticyclone gets suppressed farther south, the flow will come around to more southwesterly across the region today. This should allow for temps to climb up another category higher compared to yesterday even as sfc high pressure relinquishes control. The higher temps and slight increase in humidity will push some apparent temps into the lower 100s, still enough below Heat Advisory criteria that one was not issued, but this will have to be monitored. The modification of the air mass through increased low level RH should also provide for more buoyancy this afternoon, and while fcst soundings are not very impressive, they are at least uncapped. Differential heating over the mtns should be able to trigger a few showers and storms this afternoon and the CAMs generally move whatever develops eastward across the foothills and Piedmont, favoring NC and the nrn Upstate and remaining more dry across the Lakelands and lower Piedmont. There looks like a hint of some organization along a low level boundary dropping southeastward over the western Piedmont late this afternoon, so that might be interesting for awhile. Think the risk of severe storms is low. The convection should wane in the middle part of the evening, so most of the region should be without precip shortly after sunset. Temps will be warm overnight tonight. A small precip chance will be ramped up on the TN border toward daybreak Friday in the event that some convective remnant reaches the mtns. && .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/... As of 224 AM EDT Thursday: Subtropical ridging will get shunted off to the east of the forecast area on Friday as a deep trough churning over the Upper Midwest works its way into the upper Ohio Valley and Ontario. Deep layer SW flow will allow the atmospheric column to saturate as Atlantic moisture streams into the area. Sfc instability on the order of 1500-2000 J/kg will develop Friday afternoon based on the new 00z HREF cycle, so any parcels which manage to get a "push" should have no trouble developing into showers and thunderstorms during the afternoon and evening hours. Sans much shear or any dry air aloft, the severe risk looks minimal...but with ensemble PW estimates ranging from 1.8-2.3" for much of the forecast area, some brief heavy downpours will certainly be in the works. Right now, it still looks like the best opportunity for triggering will be over the mountains, so that`s where we expect the bulk of coverage. Temperatures on Friday will also be of some interest - given highs are expected to climb into the upper 90s for most of the non-mountain zones, with soggy dewpoints to boot, we can expect heat index values to surge to 105 degrees for many locations...and a Heat Advisory may eventually be necessary. A lobe of vort energy will rotate across the Ozarks Friday evening, entering the Tennessee Valley by early Saturday and driving a vigorous surface into eastern Tennessee before daybreak. In the resultant prefrontal trough, instability values will surge to 2000-2500 J/kg, more than enough for numerous showers and thunderstorms to develop. A decent amount of activity should escape the mountains as the front crosses the region. In fact, there`s a group of ensemble members, composed mainly of GEFS and ENS members and representing about 1/3 of the 12z LREF cycle, which depict the front arriving so quickly that the westernmost zones of the CWA may actually see a FROPA before afternoon instability can get going. Regardless, those zones which do see afternoon destabilization - likely a majority of the forecast - can expect numerous showers and thunderstorms, with severe risk once again low-end, but localized heavy rain a bit of a concern. Meanwhile, the faster propagation of the cold front continues to have ramifications for the afternoon heat...with highs as much as a category cooler than Friday`s for the western 2/3 of the forecast area. The eastern zones, especially those south and east of I-85 where the front will almost certaintly not have penetrated, can still expect balmy heat index values above 100 on Saturday. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... As of 243 AM EDT Thursday: As its upper support lifts north over the axis of the stagnant ridge over the Atlantic coast, a surface cold front will stall somewhere over the forecast area Sunday, where it`ll gradually weaken to the point of being unresolvable by the start of the new workweek. At that point, we`ll fall into a typical July pattern, with S/SE flow driven by the Bermuda high and ample moisture being pumped into the region off the Atlantic. Seasonal highs in the lower 90s can be expected each afternoon, with diurnally-driven showers and thunderstorms each afternoon. && .AVIATION /18Z THURSDAY THROUGH TUESDAY/... At KCLT and elsewhere: VFR conditions are expected to prevail across all terminals during the TAF period. SCT cirrus and stratocu this afternoon with a few showers or thunderstorms possible at KCLT and KAVL. Confidence is low for severe storms and exactly where these storms could develop. Will keep a TEMPO at KAVL and KCLT this afternoon for TSRA. Winds will remain SW and light this evening and at some sites, become calm overnight. There could be some VSBY issues with developing stratus in the mountain terminals if rain falls this afternoon, though confidence is low. Friday, expect similar conditions with more widespread thunderstorm activity ramping up during the afternoon time frame once again. Winds should remain S/SW through the end of the TAF period. Outlook: Convection will continue into early next week, leading to scattered TSRA and possible restrictions each afternoon/early evening. Fog and/or low stratus may develop each morning, especially for areas that received rain the previous day. && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...CP NEAR TERM...CP/PM SHORT TERM...MPR LONG TERM...MPR AVIATION...CP