Area Forecast Discussion
Issued by NWS Charleston, SC
Issued by NWS Charleston, SC
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444 FXUS62 KCHS 291926 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 326 PM EDT Sat Jun 29 2024 .SYNOPSIS... Atlantic high pressure will prevail this weekend, while a trough of low pressure resides inland. A cold front will impact the area Monday Monday into Tuesday before dissipating offshore. High pressure will return for the latter half of next week. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... Rest of this afternoon and into the evening: Radar continues to show a gradual increase in convection within the seabreeze, and then well inland from convection approaching from the west. Coverage still seems lower than CAMs originally predicted. Kept generally chance PoPs most areas along/west of I-95 through early evening. Still expect convection to die off after sunset as heating decreases. Temperatures have generally reached their max values in the lower to mid 90s. Low level south-southeast winds of 5 to 10 mph will persist into the evening. By tonight, convection should be mostly gone, but skies will remain partly cloudy with winds becoming light and variable. Another very warm and muggy night in store, with lows in the lower to mid 70s most areas, and upper 70s to around 80 closer to the coast. Some patchy late night fog possible within areas that experienced rainfall, but confidence and coverage not expected to be high enough to put in forecast. Note:the minimum temperatures over the past several nights have flirted with record high minimum levels. The only reason the records have not been tied/broken is mostly due to convection/rainfall helping to keep them just below records. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH TUESDAY/... Sunday: The area will be positioned along the far eastern periphery of an expansive upper-level anticyclone centered over the Central Plains. A pronounced shortwave is progged to cross eastern North Carolina late in the afternoon with its tail end crossing eastern South Carolina during the evening/overnight hours. A cold front will meander to the southeast during the afternoon hours, but looks to hold well to the northwest. The airmass south of the front will remain very moist (dewpts >73 F) and moderately unstable within a ribbon of PWATs >2 inches. Weak shear (0-6km bulk shear averaging <10 kt) within a region of little forcing aloft suggest convection will be mostly airmass driven, only concentrating along mesoscale boundaries and the inland moving sea breeze. Forcing aloft will begin to increase a bit by late afternoon and especially during the evening and overnight hours as the tail end of the aforementioned shortwave pushes through and the low-level flow becomes somewhat confluent ahead of a weak frontal wave that develops over the CSRA into central Georgia. This synoptic setup could delay the greatest coverage of showers/tstms until the evening hours and before the onset of slow nocturnal cooling. The temporal distribution of highest pop were skewed in this direction, averaging 30-50% during the afternoon (highest inland) with 50-60% during the evening hours. Convection will slowly wane overnight, but some scattered coverage could linger well into the early morning hours Monday as the front approaches and continues to interact with the warm and very moist environment. Highs look to reach into the mid-upper 90s away from the coast before convective initiation occurs. Dewpoints in the mid-upper 70s could support heat indices of 108-112 with the highest values occurring along/east of the I-95 corridor. A Heat Advisory has been issued for areas roughly along/east of an Allendale-Springfield- Richmond Hill line, including both the Savannah and Charleston Metro Areas as this areas where confidence in reaching heat indices of 108 are the highest. Far interior Southeast Georgia and locations across the far south near the Altamaha River looks a bit too marginal for hitting 108 for a 2nd period advisory. Adjustments to the advisory area may be needed in later forecast cycles. Lows Sunday night will range from the mid 70s well inland to the lower 80s at the beaches. Monday: The approaching cold front will slow down over Southeast South Carolina and Southeast Georgia Monday as the eastern extension of the upper high over the Plains erodes and broad troughing occurs along much of the U.S. East Coast. The setup looks favorable for the development of numerous to widespread showers/tstms with the front interacting with a very moist and moderately unstable atmosphere. PWATs look to surge well into the 2" range within a region of very weak shear. This will support a somewhat elevated risk for locally heavy rainfall and localized flooding with even a few instances of flash flooding being possible. This should help the current D0 and D1 drought conditions that are in place. Pops were increased to 70- 80% with highs ranging from the upper 80s across Southeast South Carolina with lower 90s over Southeast Georgia. Overnight lows will range from the upper 60s/lower 70s far inland to the mid-upper 70s at the beaches. Tuesday: The front will meander offshore and hug the coast as it sags into northern Florida. Drier air will attempt to advect in from the north, but a good portion of the area looks to hold PWATs >1.5 inches. The best chances for showers/tstms look to occur across the Southeast Georgia where the deepest moisture and highest instability will be found. Pops were nudged to 30-70%, highest south of the I-16 corridor, including the Savannah Metro Area. Shear looks to remain fairly week with slow mean storm motions, so a risk for locally heavy rainfall and flooding will continues, especially over Southeast Georgia into far southern South Carolina. It will be considerably cooler with highs only reaching the mid-upper 80s. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... The front will gradually dissipate offshore by mid-week with high pressure nosing in from the northeast through much of the period. Expect a typical diurnal convective pattern to prevail with shower/tstms mainly concentrating along/ahead of the sea breeze each day, possibly enhanced by mesoscale boundary interactions. After a cool start Wednesday, temperatures will return to above normal levels by Thursday and persist into Saturday. && .AVIATION /20Z SATURDAY THROUGH THURSDAY/... KCHS/KJZI/KSAV: Have not made any significant changes to the previous forecast for the remainder of this afternoon and evening. In general, VFR conditions still expected to prevail through much of valid 18Z TAF cycle. However, tempo MVFR cigs/vsbys are possible at CHS/SAV terminals for the next several hours due to the vicinity of the seabreeze. Think chances for convection for KJZI are now very low given seabreeze is just inland of that site now. Any convection is expected to dissipate after sunset with a return to VFR conditions. Extended Aviation Outlook: Widespread shower/tstm impacts are possible Monday afternoon/evening at all terminals. Otherwise, Brief flight restrictions are possible in showers and thunderstorms, mainly in the afternoon and evenings through the period. && .MARINE... Tonight: No highlights are expected. A ridge of high pressure at the surface will remain in place to the east and southeast of the waters. This will keep south-southeast winds of 5 to 10 knots and seas 2 to 3 feet or less. This time of year it is not uncommon for isolated showers/thunderstorms to redevelop late at night. Sunday through Thursday: Southerly winds will persist into Monday. A front will sink south into the waters Monday night into Tuesday resulting in a shift to the east to northeast with a slight uptick in wind speeds. Winds will return more southerly Wednesday into Thursday as the front dissipates and high pressure noses in from the northeast. Winds look to hold below 15 kt through the period. Seas will average 2-4 ft, except they could build as high as 5 ft Tuesday behind the front. Widespread showers/tstms will impact the waters Monday, possibly reducing vsbys to 1 NM or less at times. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...Heat Advisory from noon to 7 PM EDT Sunday for GAZ101-116>119. SC...Heat Advisory from noon to 7 PM EDT Sunday for SCZ040-042>045- 047>052. MARINE...None. && $$ NEAR TERM...RFM SHORT TERM... LONG TERM... AVIATION...RFM MARINE...RFM