FXUS62 KCHS 182340 AFDCHS
Area Forecast Discussion National Weather Service Charleston SC 640 PM EST Thu Dec 18 2025
.SYNOPSIS...
A coastal trough will continue to move inland today before a cold front arrives Friday morning. High pressure will then prevail Saturday into Sunday. Another cold front will then move across the area Monday, with high pressure then prevailing into the middle of next week.
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.NEAR TERM /THROUGH FRIDAY/...
As widespread rainfall slides north of the region this evening, conditions across coastal GA/SC will remain warm and humid. As a cold front approaches from the west, surface winds will shift from the SW, becoming generally parallel with the coast. The time window from 7PM-4AM may see conditions suitable for sea fog along the coast. By late tonight, the front will arrive and winds should strengthen and become gusty, dissipating any fog. We will monitor trends this evening and overnight for a potential Dense Fog Advisory.
This evening through tonight: The aforementioned southern stream shortwave will continue to phase and absorb into the more significant trough extending across the Great Lakes and toward the southern Appalachians. Larger scale forcing with the trough and jet aloft with coincide with precipitable water values surging into the 1.4-1.6" range during the first part of the evening. At the surface, the main cold front will still be displaced to the west across MS/AL in the early part of the evening. This combination of increasing moisture and deep layer forcing is expected to allow for the greatest coverage of precipitation in the time period generally covering 6pm- midnight. There remains the potential for seeing some embedded thunderstorms as well. Model soundings show a relatively shallow layer of weak instability (CAPE 100-300 J/kg), though not necessarily surface-based. Also of note is the strengthening low-level wind field with 40-45 knots at 850 mb and a nicely veering profile yielding 0-1 and 0-3 km SRH as high as 250-300. Current thinking is that the lack of surface-based instability will prevent any real severe weather threat.
After midnight, the bulk of the rainfall will shift off to the northeast and we will be left with just isolated to scattered showers ahead of the approaching front. Additional rainfall amounts for the evening and overnight are forecast to be mostly in the 0.25-0.50" range with a lower end potential for some areas to see up to 0.75". This could bring the max storm total rainfall for the event up to as much as an inch, primarily where developing convection tracks and produces some locally higher rainfall rates. There could be a final line of showers immediately along the front as it moves through around or just after sunrise Friday, but this should be dissipating and not amount to much. Temperatures will hover in the low 60s for most of the night with the min not occurring until the front arrives around or just after sunrise. Inland areas will likely still cool into the low to mid 50s, while the coast likely doesn't drop out of the upper 50s.
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.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/...
Friday: A surface cold front will be exiting the region to the east with winds turning from the northwest. In the mid-levels of the atmosphere, a potent mid-level trough axis will be directly overhead Friday morning with a trailing shortwave advecting northeast. Much drier air is forecast to move in behind the cold front (with PWATs falling to less than 0.2"), but given the shortwave some of the precip could fall just behind the cold front or anna frontal. By the afternoon though, all precipitation will have exited the region. The LLJ on the backside of the cold front is forecast to be ~40 kt, however the mean PBL winds are near 25 kt. Current temperatures on Lake Moultrie are in the upper 40s which will further weaken lapse rates/ mixing efficiency. As such, will hold off on the Lake Wind during the day Friday. Expect high temperatures in the mid to upper 60s.
Saturday: The mid-level trough axis will exit the region to the east with strong DAVA setting up overhead. The means surface high pressure will center just east of the Appalachians with winds quickly decoupling Saturday morning. Expect low temperatures near freezing inland (Saturday morning) and near the lower 40s at the coast. During the afternoon, surface high pressure will quickly slide east off of the Mid-Atlantic coast with a quasi coastal trough/ sea breeze pushing inland. The main change will be the rising dewpoints and increasing relative humidity values. No precipitation is forecast. Expect high temperatures in the lower 60s.
Sunday: Another potent shortwave will head east just south of the James Bay. Parent surface high pressure will be located across the Midwestern United States and quickly pull east Sunday morning. Locally, Sunday looks like another day with above normal temperatures with highs in the mid to upper 60s (maybe 70s across interior Georgia). Late Sunday afternoon into evening, surface high pressure that was over the Midwestern United States will start to crossover the Appalachian Mountains. As this occurs, cold dense air will start to ooze southwest with winds turning from the northeast. The back door cold front looks to cross the region very early Monday morning. No precipitation is expected.
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.LONG TERM /MONDAY THROUGH THURSDAY/...
Monday: Surface high pressure over the Mid-Atlantic will filter colder air into the region Monday with high temperatures in the mid 50s or below normal for this time of year. Another round of freezing temperatures are possible (~50% chance) Tuesday morning. It should be noted that ensemble guidance is actually in fairly good agreement for the the forecast with the IQR only being 6 degrees on high temperatures for Monday (or day 5)!
Tuesday: High pressure quickly disintegrates with temperatures warming into the mid to upper 60s. Model spread is the largest here as the exact timing of when the surface high loses cohesion is unknown and will impact the resultant high temperatures.
Wednesday (and into Christmas): Extremely good agreement with global ensembles advertising above normal temperatures with lows in the mid to upper 40s and highs near 70 degrees. No precipitation is expected.
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.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
0Z TAFs: As widespread rainfall slides north of the terminals this evening, conditions across the region will remain warm and humid. As a cold front approaches from the west, surface winds will shift from the SW, becoming generally parallel with the coast. The time window from 7PM-4AM may see conditions suitable for sea fog and stratus along the coast. By late tonight, the front will arrive and winds should strengthen and become gusty, dissipating any fog. The TAFs will feature rounds of IFR ceilings and at least MVFR vis for fog through late tonight, highlighted with TEMPOs and FM. In addition, passing llvl jet may result in low level wind shear this evening, ending once gusty conditions develop across the terminals after midnight. A cold front is timed to sweep across the terminals around daybreak, shifting winds from the west and ending restrictive ceilings. Clouds are forecast to push offshore by mid-day, with gusty west winds remaining through Friday afternoon.
Extended Aviation Outlook:
Friday afternoon and Saturday: VFR. Winds will be gusty at times Friday behind a cold front. Surface high pressure will be in control for the start of Saturday with winds out of the northwest. High pressure will then quickly move offshore with winds at the terminals turning from the southeast by Saturday afternoon. Early Sunday morning winds will then turn back from the west as a cold front approaches.
Sunday: West winds with a cold front approaching from the northwest. VFR. Winds veering from the northeast as a cold front approaches from NC.
Monday: MVFR/VFR. Aviation users should be prepared for some MVFR conditions behind a cold front that is forecast to cross the terminals Monday morning. Any MVFR cigs will then slowly rise to VFR Monday afternoon.
Tuesday: VFR with surface high pressure located over NC/ SC/ GA.
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.MARINE...
Today: Winds will gradually start to increase through the afternoon while turning more south-southeasterly ahead of an approaching front. Speeds should mostly top out around 15 knots through sunset, highest in the outer waters where warmer SST's reside. Seas will steadily rise, becoming 3-5 feet and perhaps up to 6 feet in the outer GA waters by the late afternoon. As such, a Small Craft Advisory (SCA) goes into effect for the outer waters.
This evening and overnight: As a cold front approaches from the west and low-level wind fields strengthen, conditions will steadily deteriorate across the local waters. Winds will turn more southerly and then southwesterly with sustained speeds up to around 20 knots and gusts as high as 25-30 knots. Seas will increase accordingly, building as high as 6-8 feet in the Charleston County waters and the outer GA waters. SCA's will be needed for all waters including Charleston Harbor.
Marine fog: As southerly flow and showers spread across the coastal waters this evening and into the overnight, there continues to be a low end chance for at least some patchy fog. As of now it looks like low-level wind fields will be too strong, but it is possible there could be some development before winds increase. We are not explicitly advertising fog in the forecast right now, but it certainly will need to be watched.
Friday: A cold front will have crossed the marine zones Friday with winds gusting 25 to 30 kt. Even over Charleston Harbor some gusts 25 kt or slightly higher are possible. The one exception to this are the nearshore Georgia waters. Shelf water temperatures are in the mid 50s which should limit mixing. Given that, only a 25% - 40% chance exists for winds to reach Small Craft Advisory criteria for the nearshore Georgia waters. Seas up to 6 ft are likely though and therefore Small Craft Advisories are in effect for all waters. Winds and seas will fall below Small Craft Advisory criteria across all waters by Friday evening.
Saturday and Sunday: Seas 2 to 4 ft with winds gusting up to 15 kt at times. No advisories or warnings are expected.
Monday: A cold front will cross the waters in the early morning hours from the northeast with winds gusting 25 to 30 kt at times. Small Craft Advisories will likely be needed for all zones (the one exception being the harbor).
Tuesday: Northeast winds turning from the southeast around 10 kt with seas 4 to 7 ft relaxing to 3 to 6 ft.
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.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None. SC...None. MARINE...Small Craft Advisory from midnight tonight to 10 AM EST Friday
for AMZ330. Small Craft Advisory until 7 PM EST Friday for AMZ350. Small Craft Advisory from 10 PM this evening to 1 PM EST Friday for AMZ352. Small Craft Advisory from midnight tonight to 1 PM EST Friday for AMZ354. Small Craft Advisory until 7 PM EST Friday for AMZ374.
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NEAR TERM... SHORT TERM...Haines LONG TERM...Haines AVIATION...NED MARINE...BSH/Haines