Area Forecast Discussion
Issued by NWS Greer, SC

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000 FXUS62 KGSP 040008 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 708 PM EST Sun Mar 3 2024 .SYNOPSIS... Above normal temperatures and an active weather pattern will persist through the week. A frontal system will bring heavy rain and possibly a few thunderstorms to the area on Wednesday, while another strong system impacts the area later in the week. && .NEAR TERM /THROUGH MONDAY/...
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As of 700 PM EST Sunday: No major changes were needed as the forecast remain on track. Otherwise, low clouds are creeping northward into extreme southern tier of the forecast area this evening, while cirrus tracking east across the western half of the forecast area. Cloud cover should gradually increase again through the overnight hours, becoming broken to overcast. Otherwise, another tricky forecast is on tap tonight regarding the return of low stratus and fog. Weak shortwave trough centered over SE GA will swing northeast off the Carolina coast overnight. The coastal feature will back low- level flow which suggests upslope lift will become mainly a factor in western NC. With skies starting off mostly clear and permitting radiational cooling, and given the abundant moisture in the PBL, another night with widespread fog and stratus is expected particularly in our North Carolina Piedmont zones and the mountain valleys. Areas of dense fog appear possible again, and another Dense Fog Advisory could be needed for some of those areas later tonight. If the stratus/stratocu do persist long enough they could congeal and inhibit cooling, keeping fog much more localized. Depth of saturation on prog profiles late tonight is not as high as it was this morning, so insolation and mixing should be more effective at scouring out the low clouds for tomorrow. However, we will retain a bit of onshore flow, particularly as the influence of the low weakens as it moves up the East Coast. Some cloud cover is expected to linger much of the day. Temps should trend warmer, but given expectation of some cloud cover, have leaned toward cooler end of guidance. That still suggest highs in the upper 60s to around 70 for the Piedmont and mtn valleys. A few spotty showers could result with subsidence inversion weaker and slightly better (but still shallow) SBCAPE.
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&& .SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY/...
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As of 230 PM Sunday: The aforementioned coastal low will be moving towards Delmarva late Monday night into Tuesday morning as a shortwave trough develops over the Lower MS River Valley. Low-level flow is expected to gradually veer from easterly to southeasterly during the day on Tuesday as a weak surface low develops over the Deep South in response to the amplifying shortwave trough. PoPs will gradually increase during this time frame from west to east during the day on Tuesday, but will only peak at chance during the daylight hours and total QPF for areas that do receive showers will be less than a tenth of an inch. Therefore, Tuesday will be a transition day marked primarily by increasing cloud cover and continued warm temperatures, with highs about 8-10 degrees above normal. PoPs become likely to categorical overnight Tuesday night as the surface low pressure deepens modestly over GA. There remains disagreement between the operational runs regarding the exact low track as the attendant cold front approaches the Carolinas early Wednesday morning. However, a review of all of the individual members of the GEFS and EPS, suggests that the low will likely track somewhere over the northern half of SC before noon on Wednesday as a strong, negatively tilted shortwave trough axis pivots over the southern Appalachians, possibly closing off briefly as it does so. The track is important because there will likely be "triple-point" cold front/low/warm front forcing as the system propagates north and east. The models are in general agreement that bulk-shear Wednesday morning will briefly peak to an impressive 50-60 kts as the triple-point moves along or south of the I-85 corridor. The challenge, as is often the case, is that instability is going to be hard to come by with model soundings suggesting poor to possibly positive lapse rates with an inversion aloft. Nevertheless, given the forecast shear, it will take very little CAPE to create an environment favorable for a few strong to severe thunderstorms, especially across our southern zones. Instability looks to increase later Wednesday morning into the early afternoon hours as the southeastern half of the area is briefly in the warm sector of the system before the cold front moves through later Wednesday. However, this is after the best forcing and shear moves off to the northeast so impacts are less likely. Additionally, favorable upper-level diffluence, low-level convergence/forcing, and PWs at or above the 90th percentile Wednesday morning will likely support heavy precip rates, especially from the southern NC mountains to the south and east across northeast GA and Upstate SC, where widespread 6-hr accumulations of 1-1.5" are possible. However, thanks in part to the speed of the system, EPS, SREF, and GEFS ensembles all suggest very low to near zero probabilities of total QPF exceeding 2 inches during this event. Therefore, given the forcing and rates, a threat of isolated nuisance flooding cannot be ruled out, especially in urbanized areas, but the likelihood of a more notable hydrologic response is very low at this time. The cloud cover and rainfall will suppress the diurnal range on Wednesday, with morning lows starting off in the mid- to upper 50s outside of the mountains, or about 15 degrees above normal and afternoon highs reaching the mid- to upper 60s east of the mountains or about 6-8 degrees above normal.
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&& .LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/... As of 230 PM Sunday: The aforementioned low-pressure system will be rapidly moving north and east into VA and the Mid-Atlantic overnight Wednesday into Thursday morning as the cold front slides east of the area, resulting in weak northwest flow. This may allow a few rain showers to develop along the NC/TN ridgetops overnight into Thursday morning, so a slight chance PoP was maintained along the state line. Weak CAA will support low temperatures in the upper 40s to near 50 in the valleys and east of the mountains, though this is still 8-10 degrees above normal. Otherwise, Thursday looks to be a brief respite in this active pattern as shortwave ridging propagates overhead while the next weather maker ejects out of the southern Rockies into the southern Plains. Partly to mostly sunny skies should develop across the area during the daytime hours, and highs should rebound back to near 70 east of the mountains. Cloud cover and PoPs once again increase Thursday night into Friday as numerous shortwaves amplify the longwave trough over the Plains and the Deep South is underneath broadly diffluent southwesterly flow. The 12Z operational guidance is slower with the QPF onset than the NationalBlend which is not unusual in the extended. Therefore, appreciable rainfall is not expected at this point until later Friday into Friday evening, as a cold front and, possibly, a weak secondary surface low track across the Tennessee River Valley and the Deep South. The GFS and the Canadian operational runs are still very aggressive with an axis of several inches of rainfall progged somewhere between the I-20 and I-85 corridors in AL and GA overnight Friday, with heavy rainfall moving into SC early Saturday morning. Moisture transport and PWs will be more robust with this system, exceeding the 90th and 95th percentiles, respectively, especially across our southern zones, while right-entrance region jet dynamics and strong low-level convergence support higher rainfall rates. Area soils will be primed given the previous system and the wet winter overall so unfortunately a flood risk is likely if the current 2-3.5" storm-total forecast verifies. Current ensemble probabilities of greater than 2 inches of rain are above 50 percent for the southeastern half of the area. Furthermore, bulk shear values may reach 60-70 kts as the system moves across the forecast area on Saturday, but like with the previous system, instability is currently very hard to come by. While the parent low will be well to our north and west over the Ohio Valley, model development of a secondary surface low over the Deep South is occurring. Its resultant track may result in much of the best convection and rainfall rates staying to our south, intercepting moisture flux into our region, while a track over the area could increase impacts. Obviously, there is plenty of time for the forecast to evolve, but the system will bear watching. Conditions will quickly improve Saturday night into Sunday as flow aloft becomes zonal. Light northwest flow snow showers cannot be ruled out along the TN border and adjacent ridgetops overnight Saturday into Sunday morning. The airmass behind the system will be relatively cooler so expect high temperatures to be slightly above normal Saturday and near to slightly below normal on Sunday, when high temperatures will drop into the 40s and lower 50s in the mountains and lower 60s east despite mostly sunny skies. && .AVIATION /00Z TONIGHT THROUGH FRIDAY/...
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At KCLT and elsewhere: Starting out VFR as of 00Z this evening, although another tricky fog and low stratus forecast is on tap again tonight into early Monday morning. The latest guidance has come in less pessimistic for some terminals but more pessimistic for others. Right now, it looks like the locations along I-77 in the North Carolina Piedmont will have the best potential to see fog and low stratus again overnight into early Monday morning. However, guidance is now depicting higher chances of low stratus and patchy to dense fog development at KAND, KGSP, and KGMU, while depicting lesser chances at KHKY. All this to say, confidence remains high that patchy to dense fog and low stratus will develop tonight, but the exact locations remains the main question at this time. Went with IFR vsbys around daybreak everywhere with the exception of KCLT, as this terminal has the best chance to see dense fog develop. Thus, went with LIFR vsbys at KCLT. Went with IFR cigs at KAVL, KCLT, and KAND overnight into early Monday morning as these areas have the best potential to see low stratus development. Went with MVFR cigs elsewhere during this timeframe. Any fog and low stratus that forms should gradually lift between 14Z and 16Z on Monday, with VFR cigs returning by the early afternoon hours. Cloud cover will gradually increase becoming BKN to OVC later this evening through Monday morning before gradually decreasing in coverage and becoming SCT to BKN by Monday afternoon. Wind direction will gradually turn N/NE this evening into tonight east of the mountains. Winds will remain SE at KAVL through this evening before becoming light and VRB overnight. Winds will remain NE east of the mountains on Monday, with KAVL`s winds picking back up out of the SE after daybreak. Dry conditions should prevail across the terminals through the 00Z TAF forecast period. Outlook: Low confidence in flight category Monday night, but as coastal low moves toward Chesapeake Bay and its influence diminishes in our area, near-sfc flow would appear to become southerly once again and could permit restrictions to develop by daybreak Tuesday. Clouds and precipitation will expand over the area Tuesday in advance of the next low pressure system which will pass early Wednesday, with restrictions associated with stronger onshore flow and heavy rain, perhaps along with TSRA.
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&& .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...JMP NEAR TERM...AR/Wimberley SHORT TERM...JMP LONG TERM...JMP AVIATION...AR

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