AFD_CHS

FXUS62 KCHS 170629 AFDCHS

Area Forecast Discussion National Weather Service Charleston SC 129 AM EST Wed Dec 17 2025

.SYNOPSIS...

High pressure will prevail through Thursday before a cold front pushes offshore Friday morning. High pressure returns during the weekend, followed by another cold front arriving early next week.

.NEAR TERM /THROUGH TONIGHT/...

Prior to Daybreak: Aloft, a weak/broad ridge remains directly over the Southeast while sfc high pressure centered across the western Atlantic extends across the Southeast. Latest sfc obs indicate a light/calm wind across most areas, which is likely to persist through daybreak. Strong radiational cooling during the first half of the night has helped temps dip into the low-mid 30s inland to upper 30s/around 40 near the coast. Temps could lower another 1-2 degrees during the next few hours, but further cooling will likely be limited due to an increase in high clouds arriving from the west late night. High clouds will also play a limiting factor in fog potential within a few hours prior to daybreak, especially across inland areas where temps are coldest. However, sfc dewpt temps increasing into the upper 30s across southern areas, a light/calm wind, along with favorable condensation pressure deficits depicted on some model guidance, suggests patchy fog could develop late across Southeast Georgia and persist for a few hours prior to high clouds thickening across the local area approaching daybreak.

Today: Aloft, the axis of a weak/broad ridge over the Southeast will shift offshore early, favoring a southwest flow across the region ahead of h5 vort energy traversing the Deep South. At the sfc, high pressure extending across the area from the Atlantic will remain the dominant weather feature, resulting in dry/warmer conditions with a light southwesterly wind for the day. Although high clouds are expected to spread across the region and thicken, temps should still reach the mid 60s for most areas, and potentially the upper 60s south of I-16 across Southeast Georgia.

Tonight: Aloft, little change in the overall pattern occurs, but the first signs of h5 vort energy should enter the region after midnight. The most notable change occurs at the sfc, with guidance supporting the development of a coastal trough along the southwestern edge of the Atlantic high offshore. Increasing low clouds will likely accompany this feature, spreading across Southeast Georgia, then Southeast South Carolina starting during the evening. By midnight, sufficient low-lvl moisture and forcing supports few to scattered showers developing across nearby coastal waters and shifting onshore, impacting most beaches south of the Charleston vicinity across southern Southeast South Carolina and Southeast Georgia. Activity should eventually make a push inland, resulting in rainfall amounts around 1/10 of an inch in spots prior to sunrise. Overnight lows will be considerably warmer than the previous night, generally ranging in the mid-upper 40s inland to low- mid 50s near the coast.

.SHORT TERM /THURSDAY THROUGH SATURDAY/...

Thursday: Precipitation chances will gradually increase as a powerful mid-lvl shortwave and associated cold front propagates towards the Southeast. Ahead of this feature, low-lvl south- southwesterly flow will advect warm and humid air into the region, yielding PWAT values ~1.5 inches by the afternoon. Model cross sections suggest forcing ahead of the shortwave will be modest as DPVA increases + the passing of the subtropical jet streak aloft. It's important to note that SBCAPE values remain quite low (~200-300 J/kg), but this might not matter with enough lift along the cold front. This should at least result in some low-topped convection right along the boundary. Expect precipitation to develop in the early morning hours, and then slowly expand across the region throughout the daytime. Highest chance of hearing a few rumbles of thunder will be in the afternoon into the late evening as the frontal zone interacts with the warm, moist pre-frontal airmass. A slight chance of thunderstorms has been introduced across the region to address this potential. Rainfall amounts remain light with ~0.5 inches expected in the span of 24 hrs. Overnight temperatures will remain mild due to the overcast skies with lows ranging from low to mid 50s.

Friday: Broad mid-lvl troughing will extend into from the Bluegrass region into the Deep South as the associated cold front progresses across the region. It's possible to a continuation of some light showers in the morning before the cold front passes through. There has been a bit of uncertainty in the timing of the front amongst guidance, however the early morning hours (before 10AM) look the most reasonable. Thereafter, high pressure is expected to build into the region from the west and skies should gradually clear throughout the day. This will yield temperatures in the low to mid 60s across the Lowcountry. Overnight lows will be a bit cooler than the previous couple nights with temps. dipping into the low 30s across the interior counties and upper 30s to low 40s closer to the coastline.

Saturday: Expect a return of quasi-zonal flow aloft as the aforementioned shortwave continues to progress offshore. At the surface, high pressure will situate itself across the region and yield cool and dry conditions. Expect sunny skies + light winds as temperatures moderate back to near normal. It looks like another cold night is in-store with lows dropping into the upper 30s to low 40s.

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

Expect high pressure to prevail for the weekend with signals that another cold front to approach the region late on Sunday, however confidence remains low with the timing and impacts of this at the moment. Temperatures will moderate back to above normal on Sunday into early next week.

.AVIATION /06Z WEDNESDAY THROUGH SUNDAY/...

VFR conditions will prevail at CHS/JZI/SAV terminals through 06Z Thursday. However, shallow ground fog could develop at the terminals between 09Z-12Z this morning, primarily at CHS/JZI where high clouds are latest to arrive.

Extended Aviation Outlook: Periods of MVFR possible on Thursday and Thursday night as increasing cloud cover and showers might result in lower ceilings and vsbys. A risk for low-lvl wind shear could develop Thursday night as a strong, pre-frontal low-level jet (45-50 kt) pushes east through the region. Conditions should improve on Friday behind the passage of the cold front.

.MARINE...

Prior to Daybreak: High pressure will remain in control for the night with no marine concerns expected across local waters.

Today and Tonight: High pressure will prevail across the western Atlantic, leading to quiet marine conditions through the day. In general, north winds will turn more easterly this afternoon, remaining around 10 kt or less. Seas will range between 1-3 ft. Overnight, a coastal trough is anticipated to develop along the southwestern edge of the Atlantic high, which should lead to scattered showers and possibly a few thunderstorms across local waters during the second half of the night. Additionally, a slight uptick in easterly winds and seas are possible, with wind gusts in the 10-15 kt range and seas between 2-4 ft (highest across outer Georgia waters).

Thursday through Sunday: Situated between a weakening high offshore and an approaching shortwave from the west, expect the pressure gradient to gradually tighten across the local waters on Thursday. The period of largest concern remains to be on Thursday night into Friday ahead of the approaching cold front. Southwesterly winds will become elevated with speeds of 15 to 20 kts with gusts of 25 kts possible. Also, east-southeasterly swell will mix into the waters with seas ranging from 3 to 5 ft with some 6 footers reaching into nearshore South Carolina and outer Georgia waters. Small Craft Advisories will likely be needed on Thursday evening into Friday.

.CHS WATCHES/WARNINGS/ADVISORIES...

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

NEAR TERM...DPB SHORT TERM...Dennis LONG TERM...Dennis AVIATION...Dennis/DPB MARINE...Dennis/DPB