Area Forecast Discussion
Issued by NWS Greer, SC
Issued by NWS Greer, SC
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686 FXUS62 KGSP 301424 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 1024 AM EDT Sun Jun 30 2024 .SYNOPSIS... Hot and humid conditions will arrive today, as well as showers and thunderstorms, ahead of a robust cold front. Drier and cooler conditions will set in behind the front Monday and Tuesday, before the summer heat returns Wednesday and beyond. && .NEAR TERM /THROUGH TONIGHT/... As of 1000 AM Sunday...The biggest changes needed for this update were to the sky and PoPs to line up with latest satellite and radar trends. Skies have really cleared out this morning, and combined with a light downslope NWLY flow are allowing temps to warm a little faster than expected. Also, convection is not firing across the mountains quite yet, so have delayed the ramp up of PoPs with latest hourly NBM grids. So I have bumped up highs a deg or two to reflect the better heating. As for dewpts, the downslope and more intense heating should allow for some mixing. In these situations, blending in the 10th percentile NBM dewpts usually verifies well. And that did lower dewpts some. So with all that being said, the higher temps and lower dewpts net about the same heat indices, and the Heat Advisory still looks good. No major changes in thinking as regards the severe weather threat. Still expecting a few severe storms with damaging winds being the main threat. Otherwise...The next 24 hours should be a little more active across the region as the nearly zonal flow we`ve been under for the past several days finally buckles as a short wave digs down over the Appalachians today and helps to amplify a broad mid-level trof along the East Coast. The upper wave will help to drive a cold front from the OH Valley southeastward toward the region today. Before it arrives, we have the matter of the Heat Advisory to contend with. And then there is the possibility of better convective coverage this afternoon, as forcing should be sufficient to bring the best chances that we`ve had in the past few weeks for some needed rainfall across the western Carolinas. The CAMs are in general agreement that a lee trof will be the focus of storm development east of the mtns early this afternoon. This could also be aided by an outflow boundary from the ongoing convection to the WNW. The extra cloud cover and storm coverage could limit the high temps and keep us from reaching Heat Advisory criteria in some places. The trend in the guidance was slightly down on the Heat Index. Suffice to say, no indication the Advisory needs to be expanded. However, we won`t shrink it either at this point, even though confidence in reaching criteria is trending downward. The front should bring another round of showers and storms into the mtns this evening, and then east of the mtns overnight. Weak shear could help to organize this activity. As to the severity, the lapse rates remain relatively weak and we lack mid-level dry air. A Marginal Risk on the Day 1 from SPC looks okay just on general principles with a boundary moving across the area, but it might be a stretch to get more than a few strong storms. For tonight, the front will move to the east after midnight and help to keep a chance over the eastern zones through daybreak Monday. Low temps will continue to run above normal. && .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/... As of 215 AM EDT Sunday: The short term is shaping up to be a dry few days as ill-defined surface high pressure migrates out of the Great Lakes and across the Upper Mid-Atlantic. The resulting subsidence aloft will hold any prospects of convection at bay, while ushering in cooler temperatures and drier dewpoints after the oppressive weekend heat. A CAD-like regime will set up on Tuesday, further cooling and drying the forecast area. On Monday, afternoon temps will only reach the mid- to upper-80s with RHs in the upper 40s; on Tuesday, they`ll stay in the lower 80s with RHs in the lower 40s. It`s worth noting that forecast soundings from both the GFS and the NAM depict a steep dropoff in dewpoints around 750mb each afternoon...which hints at the possibility of a dewpoint crash. For now, accounted for this by giving added weight to the raw model output for the dewpoint forecast...as this tends to perform better in these scenarios. An afternoon breeze is likely on Monday as what remains of constrained postfrontal flow aloft mixes to the surface; by Tuesday even fleeting wind gusts will be difficult to come by. By all accounts, both days should be beautiful! && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... As of 224 AM EDT Sunday: An increasingly summer-like pattern will set in for the medium range. Deep upper ridging will set up over the southern Great Plains and into the Southeast, while Atlantic high pressure allows a southerly moisture flux to develop with a weak Gulf fetch. In the latest round of operational guidance, the more potent moisture transport is depicted remaining displaced to our west. So forecast profiles remain basically dry through Wednesday, and only begin to exhibit modest moistening on Thursday. With minimal synoptic forcing and a questionable amount of available moisture, it therefore seems like any diurnally forced showers and thunder will be focused over the mountains, and limited in coverage, through Thursday afternoon. By Friday, better moisture should be working into the forecast area...with even the driest members of the latest LREF cycle at least as moist as the consensus on Thursday in the 850-700mb layer. So, somewhat better coverage of mountain showers and thunderstorms is expected, and convection is more likely to break containment and start developing over the Piedmont and Upstate by the end of the week. At this point, temperatures will also be on the rebound beneath the ever-intensifying heat dome. On Friday and Saturday, temperatures may surge into the upper 90s, paired with downright soupy dewpoints. So, heat index may once again become an issue toward the end of the 7-day period. && .AVIATION /15Z SUNDAY THROUGH THURSDAY/... At KCLT and elsewhere: The bulk of the day will be VFR with a light S to SW wind, altho ceiling/vis could be lower and wind variable near any thunderstorms. The CAMs remain in decent agreement with the timing for the best chances of storms, and that is generally mid-afternoon through mid-evening. This will be handled with TEMPO to hold the restrictions. Thereafter, will employ a PROB30 to cover the lingering convective precip. The cold front is expected to push southeast late this evening or early Monday. There is some indication that a restriction might form around daybreak Monday. Outlook: The cold front will push thru and stall just south of the area, resulting in quieter weather for Monday thru mid-week. && .GSP WATCHES/WARNINGS/ADVISORIES... GA...Heat Advisory until 8 PM EDT this evening for GAZ018-026-028- 029. NC...Heat Advisory until 8 PM EDT this evening for NCZ070>072-082. SC...Heat Advisory until 8 PM EDT this evening for SCZ008>014-019- 104>109. && $$ SYNOPSIS...MPR NEAR TERM...ARK/PM SHORT TERM...MPR LONG TERM...MPR AVIATION...ARK/PM