AFD_CHS

FXUS62 KCHS 202341 AFDCHS

Area Forecast Discussion National Weather Service Charleston SC 641 PM EST Sat Dec 20 2025

.SYNOPSIS...

High pressure will prevail over the weekend, with another cold front forecast to impact the area Monday.

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

Early this evening: Aloft, nearly zonal flow will prevail with a broadly cyclonic flow regime. At the surface, high pressure will prevail with a very weak gradient allowing for full decoupling and calm winds by the early morning hours. The only forecast challenge for the overnight is the fog and low stratus potential model guidance seems quite bullish about along the southeast GA coast, potentially extending up into the lower southeast SC coast as well. Virtually every source of hi-res model guidance including probabilistic guidance from the NBM and HREF suggests that fog will begin developing in the early morning hours across McIntosh County and will then steadily expand up the coast and across southeast GA through sunrise. Furthermore, much of the guidance actually gives relatively high chances (30-50%) of dense fog across an area that includes Darien, Hinesville, Savannah, Ridgeland, and potentially even up to Beaufort. The fog potential seems reasonable given the calm winds, clear skies, and some moisture pooling in this area where dewpoints are in the mid to upper 40s this evening, suggesting we will fall below crossover temperatures with lows. As such, we have trended the forecast to include patchy fog along the entire southeast GA coast up through coastal Colleton County. Also, we have added a tier of areas of fog along the GA coast up through western Beaufort County. We could eventually need to include some dense fog, but will wait until we can see fog development begin and make sure it is tracking with model guidance. Lows shouldn't be quite as chilly as last night, but still should see widespread upper 30s inland ranging to the low 40s along the coast.

.SHORT TERM /6 AM SUNDAY MORNING THROUGH TUESDAY/...

Quasi-zonal upper level flow and a weak surface high pressure will continue the quiet conditions, with afternoon highs in the mid 60s to lower 70s across the area. A weak cold front pushes through during the afternoon hours, though the colder air lags behind overnight into Monday. Overnight lows in the upper 30s to mid 40s expected.

Upper level ridging pushes a surface high pressure onto the east coast north of the area, increasing the surface pressure gradient. This will result in breezy conditions building overnight into Monday, with gusts into the teens to lower 20s throughout the day. With the fresh influx of cooler air, afternoon temperatures will be below normal in the upper 50s to lower 60s.

Winds decrease overnight into Tuesday as the surface high pressure weakens and moves into the Atlantic. As they swing around to become out of the south-southwest, warmer temperatures will be advected back across the region with afternoon highs in the upper 60s to mid 70s expected.

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...

The aforementioned upper level ridge continues into the extended, with another surface high pressure expected to move down into the region end of this holiday week. This will result in above normal temperatures and dry conditions. Afternoon highs look to be in the upper 60s to mid 70s range.

.AVIATION /00Z SUNDAY THROUGH THURSDAY/...

The 00z TAF period begins with VFR conditions at KCHS, KJZI, and KSAV. The main forecast challenge is the potential for fog and low stratus primarily at KSAV. Model guidance continues to trend toward a more foggy solution and we have also introduced it into the KSAV TAF. We have included a TEMPO group from 10-14z for 3SM fog and an IFR ceiling. There certainly is the potential for the fog to become dense and this will need to be a consideration for the 06z TAF's. If the fog development is really aggressive it could expand up to KJZI, but for now we have just included shallow ground fog there. Fog and low stratus at KSAV should mix out by mid morning and VFR conditions will then prevail at all 3 sites through the rest of the period.

Extended Aviation Outlook: VFR expected to prevail through Sunday, with a period of MVFR cigs possible Monday morning in the wake of a frontal passage. Northeasterly winds are expected to be breezy on Monday, with gusts in the 14-18 kt range.

.MARINE...

Tonight: Prevailing high pressure will yield northeast winds 5 to 10 kt with transient gusts up to 15 kt across all waters. Seas 2 to 4 ft. Also of note, model guidance is suggesting that fog and very low ceilings could develop along the southeast GA coast late tonight and potentially spread out into the coastal waters. Fog could impact the waters, primarily the nearshore GA waters and the lower SC waters. As such, we have added patchy fog to the forecast. It is possible that fog could become dense, but confidence is low right now.

Sunday - Monday: Tranquil conditions continue into Sunday. Overnight into Monday, north-northeasterly winds strengthen with wave heights similarly building. Confidence for wind gusts greater than 25 knots is high for all near and offshore waters, with both the HREF and REFS showing 90%+ probabilities for wind gusts greater than 25 knots, though both ensembles show very low probabilities (<10%) for Gale force winds. For the Charleston Harbor, probabilities for these winds is in the 60-80% range, highest near the entrance and decreasing into the harbor. Wave heights of 6-8 feet are also expected to build into the region overnight into Monday. Thus, a Small Craft Advisory will be needed for the early overnight hours into Monday, though unsure if the Charleston Habor will need to be included, with advisory level winds and seas tapering off overnight into Tuesday. Some indications that the Savannah to Altamaha Sound water from 20-60nm out may hang onto the SCA conditions until late Tuesday afternoon or evening.

Tuesday - Wednesday: No high confidence marine concerns at this time.

.CHS WATCHES/WARNINGS/ADVISORIES...

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

NEAR TERM...BSH SHORT TERM...APT LONG TERM...APT AVIATION...BSH/APT MARINE...CPM/BSH/APT