AFD_CHS

FXUS62 KCHS 162242 AFDCHS

Area Forecast Discussion National Weather Service Charleston SC 542 PM EST Tue Dec 16 2025

.SYNOPSIS...

High pressure will prevail through Thursday before a cold front pushes offshore Friday morning.

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...

For tonight, the mid-level ridge axis slides across the region, with flow aloft becoming southwesterly. Skies will initially be clear and with weak winds at the surface, radiational cooling will bring temperatures back down into the lower 30s inland to mid 40s along the coast. With dewpoints rising up into the mid 30s to lower 40s along the coast, some patchy fog can't be ruled out. The HREF and NBM both show low probabilities (<15%) for visibilities below 1 mile, though the REFS is more convinced with a narrow corridor for probabilities near 50% for 1 mile visibility up along the southeast Georgia coast and up to Beaufort, SC. Did adjust the forecast to be closer to what the REFS is showing given the low dewpoint depression and light winds, so be on the lookout for patchy fog Wednesday morning along the southeast Georgia coast and up towards Beaufort, SC. However, mid-upper level cloud coverage does increase overnight, so it's possible that may act against fog formation.

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH FRIDAY/...

Wednesday: Weak ridging aloft will begin to break down as the mid- level ridge axis propagates to the east and well offshore as a broad, southern stream shortwave ejects east across the Southern Plains and Northern Mexico. A fast, southwest flow will generally prevail aloft with ongoing warm air advection helping to keep temperatures moderating. High pressure over the Atlantic will gradually push east with what appears to be a weak coastal trough developing just offshore. A few showers could develop over the Atlantic in the vicinity of the trough Wednesday night, primarily near the west wall of the Gulf Stream, but dry conditions should generally prevail both Wednesday and Wednesday night as cloud steadily thicken. Highs will warm into the lower-mid 60s (a bit cooler at the beaches) with overnight lows ranging from the lower- mid 40s inland to the mid 50s at the beaches.

Thursday: Rain chances will quickly increase Thursday as the storm stream shortwave propagates across the region and PWATs surge to around 1.50". Models cross sections suggest forcing for ascent/UVVs ahead of the shortwave will be modest owing to increased DPVA and ongoing warm air advection/isentropic ascent within a column of deep moisture with some secondary contributions from a passing subtropical jet streak. Rain chances look the greatest from late morning into the afternoon hours as the coastal trough offshore works inland and gradually dissipates with the increasing southerly flow ahead of an approaching cold front. Pops were increased into the likely to categorical range with the greatest coverage occurring along the lower South Carolina coast. Highs will warm into the upper 60s/lower 70s. Similar to yesterday, a rumble or two of thunder can not be completely ruled out anywhere given the degree of forcing aloft, but the risk looks highest along parts of the lower South Carolina coast where some very marginal instability could work inland from off the Gulf Stream. A slight chance for tstms was introduced along the lower South Carolina coast to address this potential. Probabilities still look too low to justify a mention of tstms elsewhere at this time.

Thursday Night and Friday: The initial surge of shower activity could tapper off during the evening hours Thursday, only to increase again overnight as the cold front to the west begins to move into the local area. There remains some uncertainty on how quickly the front clears the coast late Thursday night with some guidance members still keeping some degree of measurable rainfall risk until mid-late morning Friday with a slightly delayed FROPA. The drier 16/13z NBM solution which keeps conditions rain-free during the day Friday was still used as the basis for the forecast, but trends are being watched closely. Breezy conditions could develop across the coast Thursday night into Friday morning with a 45-50 kt pre-frontal 925 hPa low-level jet passing through. Winds on Lake Moultrie could also surge a bit, but poor mixing profiles over the cold lake waters should keep winds just below Lake Wind Advisory criteria. Lows Thursday night will range from the upper 40s/lower 50s well inland to the upper 50s at the beaches. Friday will feature clearing conditions with highs peaking in the upper 50s/mid 60s.

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...

High pressure will prevail for the weekend with signals that another stronger area of high pressure could wedge into the region early next week as a coastal trough/coastal front develops offshore. Temperatures will be near normal with a brief period of above normal temperatures possible Sunday.

.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...

00Z TAFs: Dewpoints have begun to recover across the terminals this evening. High resolution guidance indicates that dewpoints may narrow towards dawn to yield a period or two of fog, primarily near KCHS and KJZI. However, mid and high clouds will stream across the region late tonight, possibly pushing over KCHS and KJZI prior to sunrise. The TAFs at KCHS and KJZI will limit fog mention to ground fog during a TEMPO from 8-12Z. Otherwise, conditions will remain VFR with light winds.

Extended Aviation Outlook: There is an increased risk for MVFR or lower ceilings and visibilities in showers Thursday into Thursday night. A risk for low-level wind shear could develop Thursday night as a strong, pre-frontal low-level jet (45-50 kt) pushes east through the area.

.MARINE...

Through tonight: No marine concerns as surface high pressure continues across the region.

Wednesday through Sunday: The main forecast concern still centers on the Thursday night to Friday morning time period where both winds and seas could push Small Craft Advisory thresholds ahead of an approaching cold front. Winds do tend to struggle a bit in warm air advection regimes with cold sea surface temperatures helping to temper mixing a bit, but LREF probabilities for winds >25 kt continue to increase with each cycle and are still highest over the South Santee-Edisto Beach nearshore and Georgia offshore leg. Probabilities range from 40-80% in these areas, highest in the eastern portions of the Georgia offshore waters closer to the west wall of the Gulf Stream. Seas could get as high as 4-7 ft Friday morning, again, highest in the areas noted above.

.CHS WATCHES/WARNINGS/ADVISORIES...

GA...None. SC...None. MARINE...None.

NEAR TERM... SHORT TERM... LONG TERM... AVIATION...NED MARINE...