Area Forecast Discussion
Issued by NWS Greer, SC

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000 FXUS62 KGSP 171011 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 611 AM EDT Wed Apr 17 2024 .SYNOPSIS... Very warm and relatively humid weather will continue through Friday. Scattered showers and thunderstorms are possible this afternoon and tonight ahead of a weak cold front, but Thursday looks dry. More widespread showers and thunderstorms are expected with another front reaching the area Friday. Much cooler weather returns over the weekend behind that front, with a chance of rain returning by Sunday. && .NEAR TERM /THROUGH TONIGHT/...
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As of 606 AM Wednesday: A band of sprinkles and occasional light rain continues to slide east across the Piedmont this morning, but most of these radar returns are not reaching the ground owing to a dry sub-cloud layer that was well sampled on KCLT ACARS soundings. The ongoing forecast remains on track and was blended in with current observations. Otherwise, an occluded and vertically stacked mid-latitude cyclone spinning over the Midwest early this morning will become absorbed into a large northern stream longwave trough dropping across the Canadian Prairies by tomorrow morning. An associated trough axis currently extends down the Mississippi Valley with a tall shortwave ridge centered over the Appalachians. With time, upper ridging will shift east through the day with weak height falls overspreading the Southern Appalachians. The cyclone is expected to undergo marked weakening with only nebulous forcing for ascent. The 00z suite of CAM guidance is in relatively good agreement that a band of showers and a few embedded thunderstorms will push across Tennessee and make a run at the area this afternoon ahead of a surface cold front. The overall lack of forcing and limited instability, however, will greatly limit coverage and intensity. The greatest rain chances will be across the mountains and Upstate where scattered showers and an isolated thunderstorm are most likely. An isolated strong storm or two cannot be ruled out across southwest portions of the area with locally gusty winds, but no severe weather threat is anticipated owing to poor thermodynamic profiles in near-storm forecast soundings. Any lingering showers should clear the area during the evening hours with dry conditions tonight. Otherwise, cloud cover will limit heating this afternoon with highs a couple degrees cooler compared to the last few days.
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&& .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
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As of 415 AM Wed: Thursday still looks like another exceptionally warm day for the area, with mid to upper level ridging developing overhead. Weak cold front will have passed and led to low-level drying, while midlevels warm via subsidence under the ridge and/or downslope flow aloft. Diurnal convection looks unlikely in the Piedmont due to resultant capping, although models generate some SBCAPE in the afternoon. The NAM and NAMNest show the cap mixing out on prog soundings from KAVL, but given the other factors and CAPE still being limited to a few hundred joules, this does not appear worthy of a mentionable PoP. Still anticipate near-record highs in the Piedmont. The ridge will be transient, already shifting east of the CWA Thursday evening as a developing cold front and associated frontal wave advance out of the western Ozarks and across the lower OH and TN valleys. North of the Ohio River, the front will orient meridionally, with frontal wave being bolstered by the trough over the northern Plains. The portion of the front that approaches the southern Appalachians Thursday night will have less initial dynamic support, but height falls may occur upstream and result in a weak shortwave reaching the area with the front. Associated convection likely will wane nocturnally before it reaches the CWA Friday morning, and the fropa itself is not all that dramatic; in fact the true surface front probably will not pass the mountains until Saturday. Nevertheless PoPs do ramp up overnight and to categorical near the TN border Friday morning with activity along the front aloft. Weak low-level capping is likely to persist and make thunderstorms unlikely, but if an updraft does manage to go up, storms could pose a risk of strong to severe microbursts or marginally severe hail. Not sure this conditional probability warrants the SPC Marginal Risk. Temps generally will drop back a few degrees owing to cloud cover; the mountains will drop a bit more with their higher elevation and exposure to the first push of colder air. Models are a little more supportive of QPF in the Piedmont Friday, but the subsident air still will have to be overcome in that area. With WAA continuing into the SC Midlands and central NC and some runs even suggesting a weak frontal wave or MCV will persist, we will advertise at least a chance PoP for the whole CWA, lasting into evening for our lower Piedmont. Upslope PoPs linger behind the front along the TN border into Friday night also. Mins will trend cooler in the mountains as noted above, but still well above normal across the board.
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&& .LONG TERM /SATURDAY THROUGH TUESDAY/... As of 230 AM Wed: A weakly forced quasi-zonal flow regime is expected for the Southeast this weekend. Low-level convergence will persist across the Piedmont Saturday near the elevated cold front. Airmass change still looks unlikely to occur for our area until late in the day or Saturday night, with most models showing the surface front still just pushing across the Appalachians at 00z Sun. The model spread which had once limited confidence in Sunday`s highs now is greater on Saturday, depending on the advance of the front. It does look likely temps will fall back to more seasonably warm weather, with highs only 3-6 degrees above normal. Drier air is likely to have arrived aloft, although the low levels may still be moist. Midlevel subsidence is expected to limit convection although some prog soundings still show small diurnal CAPE. Thus we will advertise a mostly dry forecast Saturday. As the front pushes further through the area Saturday night, a weak shortwave may develop near the western Gulf Coast, turning the zonal flow slightly cyclonic across the Southeast. The wave will stall and/or reactivate the front to our south, possibly Saturday night but more likely Sunday into Monday. The exact timing remains uncertain, as well as the position of the incipient frontal wave. The consensus solution puts most of our area on the cool side of the front under dry northerly flow, which implies low-end QPF. Thunderstorms now look unlikely with instability more likely shunted south by the front. Sunday and Monday do look likely to be cool days with below-normal temps. Within the cyclonic flow, a compact shortwave may move out of the mid-MS Valley, possibly enhancing lift and QPF for Sunday night or Monday. Weak high pressure may give us a break Tuesday along with milder/sunnier weather, but another shortwave may approach by Tuesday night. && .AVIATION /10Z WEDNESDAY THROUGH SUNDAY/...
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At KCLT and elsewhere: VFR conditions will continue through the morning as high clouds stream across the region. A band of showers is still on tap to approach the area from the west during the afternoon to early evening hours. A brief period of light rain will be possible at any given terminal as these showers move through. Confidence is highest at KAVL/KGSP/KGMU/KAND where brief MVFR visibilities will be possible with any moderate rainfall rates. Elsewhere, coverage and anticipated lighter precipitation rates should allow for VFR visibility. Light winds overnight will become gusty out of the southwest this afternoon before becoming light again tonight. Outlook: VFR generally will persist through midweek. An unsettled pattern with precip chances and periodic restrictions may develop from Thu night into the early part of the weekend, but confidence remains low.
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&& .CLIMATE... RECORDS FOR 04-18 MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 90 1896 44 1921 60 1927 28 1905 1891 KCLT 93 1896 45 1983 66 1896 32 2001 KGSP 89 2002 51 1983 66 1927 28 1905 1967 && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...Wimberley NEAR TERM...TW SHORT TERM...Wimberley LONG TERM...Wimberley AVIATION...TW CLIMATE...

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