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Area Forecast Discussion
Issued by NWS Greer, SC
Issued by NWS Greer, SC
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267 FXUS62 KGSP 200721 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 321 AM EDT Sat Jul 20 2024 .SYNOPSIS... A nearly stationary front will meander across our region through next week providing a focus for above-normal chances of mainly afternoon and evening showers and thunderstorms. Temperatures will be near normal or just a bit cooler than normal. && .NEAR TERM /THROUGH TONIGHT/... As of 315 AM: Low clouds and patchy fog, along with scattered showers will continue this morning. The forecast area remains between a trough over the central CONUS and the Atlantic sub-tropical ridge. This keeps southwesterly flow aloft over the area through the period. A weak short wave moves through the flow and across the area today. The right entrance region of the upper jet is over the area as well. This will keep weak synoptic forcing in place through the day. This weakens tonight as the short wave moves out of the are and the jet weakens. At the surface, a stationary boundary remain in place near or over the area but the low pressure center along the front moves to our east this morning. Copious deep moisture also remains in place over the area. Instability is weak, but shear does increase, especially across the Piedmont, as the short wave moves through. With these features, expect scattered to numerous showers across the area through the day, with scattered thunderstorms, especially this afternoon and evening. Despite the weak instability, the increased shear could lead to a few severe storms, especially across the Piedmont, with damaging winds the main threat. The isolated heavy rainfall and localized flooding threat also continues. Highs will again be around 5 degrees below normal. Convection tapers off this evening as the forcing diminishes, but isolated showers will linger across the mountains overnight. Low clouds and patchy fog will again develop overnight. Lows will be near to slightly above normal. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... As of 221 AM Saturday: The synoptic pattern will remain relatively unchanged Sunday into Monday with a tall upper ridge extending from the Great Basin through the Rockies and into Alberta/British Columbia. A large subtropical ridge is also progged to remain centered over the Atlantic. Between these two ridges, broad upper troughing will extend from the Great Plains to the Ohio Valley with convergent southwest flow entrenched across the southeast states. A plume of deep moisture with above average PWATs reaching from the Gulf of Mexico across the Appalachians will help to foster numerous to widespread showers and thunderstorms each afternoon and evening along with below average temperatures. Forecast soundings depict very moist profiles with high freezing levels and near moist adiabatic lapse rates. Tall skinny CAPE profiles and only modest instability will likely preclude a threat for severe thunderstorms, although locally gusty winds cannot be ruled out with any water loaded downdrafts. The primary focus, however, will be for locally heavy rainfall. Any slow moving/training storms or clusters focused along cold pools could result in a quick couple inches of rain that could result in minor flooding of low lying/poor drainage areas. && .LONG TERM /TUESDAY THROUGH FRIDAY/... As of 236 AM Saturday: The synoptic pattern is forecast to remain stagnant through much of next week as upper ridging meanders over the Great Basin and the Atlantic ridge remains locked in place. This will continue to keep troughing centered just to our west with periodic shortwaves diving across the Great Plains and Midwest. Deep- layer southwest flow will thus persist through much of the period with above normal chances for showers and thunderstorms persisting each afternoon and evening. Temperatures will also remain below average with the majority of the area likely to stay below the 90 degree mark, which would be an impressive feat for late July. As with any summertime convection, a few stronger storms cannot be ruled out with wet microbursts. However, poor thermodynamic profiles within a very moist airmass will generally preclude a greater severe threat. Locally heavy rainfall may eventually become an issue, especially after several days of convection. The main period of note that would be worth keeping an eye on would be mid to late week. The 00z suite of global model guidance is in fairly good agreement on a sharp shortwave trough swinging across the Great Lakes on Wednesday and across the Ohio Valley Thursday into Friday. This will in turn strengthen the height gradient between the trough and upper ridge over the Atlantic and thus increase flow through the column. IVT values ramp up in turn with a plume of deep moisture advecting across the area with PWATs surging to 1.75-2.25" and dewpoints in the low to mid 70s. While the best synoptic forcing will remain displaced north of the area, any mesoscale features that could focus convection could be sufficient to realize a greater threat for flash flooding. A pattern change may finally be on the horizon by next weekend as a potent trough moving into the Pacific Northwest helps to suppress and breakdown the Great Basin ridge. Troughing over the Ohio Valley also lifts northeast and allows subtropical ridging to gradually build back in across the southeast states. If realized, this would allow for a return to more typical summer conditions with isolated to scattered pulse convection and warmer temperatures more common to late July. && .AVIATION /08Z SATURDAY THROUGH WEDNESDAY/... At KCLT and elsewhere: Conditions vary from VFR to LIFR across the area early this morning along with scattered showers. Expect LIFR cigs to continue where they area and IFR to LIFR cigs to develop before daybreak where they currently aren`t. Vsby will be highly variable with generally VFR to MVFR, but IFR or lower is possible, especially where showers move through. Expect VFR vsby to develop after daybreak, but a much slower improvement in cigs with MVFR by late morning then low VFR by early afternoon. Although convective coverage expected to be in the scattered range this afternoon, chance is high enough for PROB30s. Convection should taper off during the evening with only a small chance of lingering SHRA overnight. Expect cigs to drop back to MVFR then IFR overnight, with vsby restrictions possible. Light wind early this morning becomes S to SW through the morning into the afternoon. Light wind overnight. Outlook: Expect scattered to numerous showers and thunderstorms each afternoon/evening through the middle of next week. Morning fog/low stratus will be possible each day, especially in the mountain valleys and across areas that received heavy rainfall the day prior. && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...TW NEAR TERM...RWH SHORT TERM...TW LONG TERM...TW AVIATION...RWH