Area Forecast Discussion
Issued by NWS Charleston, SC
Issued by NWS Charleston, SC
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284 FXUS62 KCHS 070802 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 402 AM EDT Sun Jul 7 2024 .SYNOPSIS... High pressure will remain over the western Atlantic while a weak surface trough exists just inland of the area through the upcoming week. && .NEAR TERM /THROUGH TONIGHT/... Morning: Latest 500 mb hand analysis from 07.00z revealed a broad trough axis centered across the central United States with a strong mid-level ridge located over California. Compared to 24 hrs earlier, the mid-level ridge over California is more amplified with mild height rises observed across the west coast of the United States. As such, the mid-level trough is also slightly more amplified and nudged east towards South Carolina and Georgia. This is reflected in the 500 mb wind field with winds at Atlanta now being out of the southwest. Weak height falls were also analyzed at Atlanta and Charleston. The other item of note was the sharp moisture gradient across the region. The 07.00z KCHS sounding recorded a PWAT of 1.71" with the Atlanta sounding recording a PWAT of 2.16". This moisture/ PWAT gradient is further supported by the CIRA ALPW product which shows a pronounced 700 - 500 mb dry slot in the shape of an inverted V just off of the coast of Charleston. This is due to the eastward moving trough in the central United States and the retrograding mid-level low near Bermuda. The ALPW product shows the dry slot finally being shunted south this morning and global guidance hints this trend in the observational data to persist. Simultaneously, a remnant MCV from the convection on Saturday is currently over Orangeburg county (seen on the KCLX radar) progressing east. The general thought for today is that this MCV will continue to move northeast and approach the North Carolina/ South Carolina state line by late this morning. Some convective development along and south of the MCV is possible, which would be mainly across the Tri- County of South Carolina. Afternoon: Showers and thunderstorms will form along an inland moving sea breeze with slight convective suppression occurring on the immediate backside of the exiting MCV. More of the region will likely see precipitation than did Saturday with PWATs being 2.00" + across all zones. However, the best coverage again looks to be near the Savannah River and southwest where the best instability is forecast (ML CAPE values ~2000 J/kg) and maximum distance from the exiting MCV. Temperatures will also be a degree or two lower today compared to Saturday with 1000/ 850 mb thicknesses falling to 1420/ 1425 m along with plentiful clouds. Dewpoints have also fallen a degree or two across the board and this is forecast to continue. The data above all points to high temperatures in the low to mid 90s with heat index values in the 105 to 108 degree range this afternoon (or just shy of Heat Advisory criteria). Mean cloud bearing winds are also forecast to be around 10 kt this afternoon which should keep storms moving slightly more than Saturday. As such, HREF exceedance probabilities are much less today compared to Saturday. Only a few locations in Georgia are showing probabilities of greater than 50% for 3" of rainfall. As is the case most times in the summer though, a few isolated totals greater than 3" are possible. Tonight: Showers and thunderstorms will persist into the early evening hours thanks to the remaining islands of instability and finally come to an end around midnight. Expect low temperatures in the mid 70s (near 80 degrees along the beaches). && .SHORT TERM /MONDAY THROUGH WEDNESDAY/... A weak mid-level ridge will remain over northern Florida Monday into Tuesday, then weaken in response to the approaching upper low that has been gradually moving through the western Atlantic. Additional upper ridging will occur on Wednesday from the western Atlantic. Despite the continued influence of the ridge, guidance continues to indicate a few weak shortwaves will move through the local area during this period. At the surface, the main sources of convergence will be a persistent lee trough across the SC/GA Piedmont and a robust daily sea breeze that pushes well inland during the afternoons. An incredibly moist airmass will set up over the area early in the week, with surface dewpoints in the upper 70s to around 80 and PWATs ranging from 2.3 to 2.5". Scattered to numerous mainly diurnal showers and thunderstorms are expected each day. The greatest coverage will likely be farther inland, closer to the lee trough. Convection could continue into the overnight hours, though it should generally trend downward with the loss of daytime heating. The main concern from the thunderstorms will be torrential rainfall and localized flooding. High temps will mainly be in the lower 90s. Typically these temperatures would not produce excessive heat concerns, but with forecast dewpoints approaching 80 degrees, we could be dealing with pockets of 108+ heat indices, particularly Tuesday and Wednesday. Heat Advisories may be needed. && .LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/... Shortwave ridging is expected on Thursday with a slightly drier airmass. Less coverage of afternoon convection is expected, though there should still be enough moisture and forcing for scattered showers/tstms. A wetter pattern is possible Friday into Saturday due to upper disturbances moving in from the Atlantic, deeper moisture spreading in, and a weak surface trough over the area. && .AVIATION /08Z SUNDAY THROUGH THURSDAY/... This morning: Showers and thunderstorms from Saturday evening have continued to rapidly decay as they approach KSAV this morning. This is thanks to stubborn mid-level dry air over KSAV to KCHS/ KJZI (observed on the ALPW product). Expect winds to be mostly light and out of the south this morning with VFR cloud cover. Today: The mid-level dry slot that was noted above will begin to be pushed south which should allow for better coverage of thunderstorms at the TAF sites this afternoon. Highest confidence of observing a thunderstorm will be at KSAV which will firmly be in the moist/ high PWAT air (2.4"). Confidence is lower at KCHS and KJZI. For now, have introduced thunder at KSAV and left the mention out at the Charleston terminals. This might have to be included in future TAF packages. Tonight: Showers and thunderstorms will persist into the evening hours once again and slowly come to an end by early Monday morning. Mostly VFR conditions are forecast. Extended Aviation Outlook: Brief flight restrictions possible in showers and thunderstorms, particularly during the afternoon and early evening. && .MARINE... Today and Tonight: Southerly flow across the waters will begin to back from the southeast this afternoon with an uptick in speed near the land thanks to an inland moving sea breeze. Expect gusts in the various harbors and inlets 15 - 20 kt this afternoon. Seas 2 to 3 ft. Winds will then slowly decrease this evening as the sea breeze circulation relaxes. Monday through Friday: Typical summertime conditions expected with southerly flow at or below 15 kt and seas mainly 2-3 ft. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM...Haines SHORT TERM...JRL LONG TERM...JRL AVIATION...Haines/JRL MARINE...Haines/JRL