AFD_CHS

FXUS62 KCHS 021127 AFDCHS

Area Forecast Discussion National Weather Service Charleston SC 627 AM EST Tue Dec 2 2025

.SYNOPSIS...

A storm system will pass through the region late today bringing much needed rainfall to coastal South Carolina and Georgia. High pressure then returns for the middle of this week before the next frontal system impacts the area Friday into the weekend.

.NEAR TERM /THROUGH TONIGHT/...

Early this morning, satellite water vapor showed a strong shortwave advancing over the Mississippi River Valley, with a deep plume of moisture across the Southeast U.S. Regional radar composite indicated widespread rain extending from the coast of GA/SC west to the AL/MS line. The widespread rain will slide northeast today, supported by the shortwave, strong jet divergence, and deep isentropic lift. In addition, PW values across the forecast area will increase through early this morning, peaking during the mid to late morning hours around 1.8 inches. Coverage of light rain will increase across the CWA early this morning, temperatures should remain generally steady in the low to mid 50s. Rainfall rates are forecast to peak during the mid to late morning, yielding at least moderate rainfall. NBM indicates isolated thunderstorms along and east of the I-95 corridor this morning. However, given the coverage of the rainfall, cold air damming should reach peak strength this morning, limiting deep instability to less than 200 J/kg.

This afternoon, near term guidance times the sfc low to reach eastern NC, sweeping the cold front across the CWA. Showers should shift east with the front, allowing the forecast area to dry from west to east by late this afternoon. Rainfall totals from today are forecast to range from 1-1.75 across the far inland counties of GA and SC to around .75 east of I-95. After a morning of steady temperatures in the low to mid 50s, temperatures should peak in the 60s early this afternoon, then cooling through the rest of the afternoon.

Tonight, a dry and cool air mass will spread across the forecast area in the wake of the cold front. Linger low level moisture will likely support some degree of stratocu across the region through late tonight. Using a blend of guidance, low temperatures are forecast to range from the mid to upper 30s inland to the low 40s along the coast.

.SHORT TERM /WEDNESDAY THROUGH FRIDAY/...

Wednesday: Surface high pressure will be centered over the Ohio River Valley Wednesday, with cold dense air slowly making its way over the Appalachian Mountains. As the dense air begins to spill east, a reinforcing cold front will surge south and then southwest down the east side of the Appalachians. The front looks to cross the area Wednesday morning before washing out. No precipitation is expected through the period as dry air remains entrenched with temperatures below normal for this time of year.

Thursday: The high pressure from Wednesday will quickly collapse as the ageostrophic convergence transitions neutral to divergence as a potent mid-level wave moves east over the Hudson Bay. Low level winds turn from the southwest with overrunning commencing. Looking at the 300 or 295 K surface though shows a relatively dry air mass remaining in place with condensation pressure deficits large. Even where saturation is implied, moisture profiles below the 300 K surface are extremely dry. As such, most of the area will likely (>80%) remain dry Thursday.

Friday: Model consensus really starts to diverge Friday, with local forecast spread widening rapidly on Saturday. Taking a look at the upper level pattern, a long wave trough axis will center over the central United States with wave breaking potentially occurring over the Baja of California. The latest run of the WPC Ensemble Sensitivity Analysis webpage, reveals that this is indeed what causes the forecast spread. Some guidance has the wave completely breaking off from the long wave trough, while other guidance has the wave being more progressive. Either way for Friday, precipitation will spread over the region as a coastal low forms in the northern Gulf and heads northeast. Precipitation looks likely by Friday afternoon (>55%). Given the widespread precipitation and cloud cover, high temperatures will mostly be in the mid 50s, or well below normal for this time of year.

.LONG TERM /FRIDAY NIGHT THROUGH MONDAY/...

Saturday: Forecast confidence on when the precipitation comes to an end is relatively low for Saturday. As mentioned above, this is likely predicated on the evolution of the coastal low near the Baja of California. As of current thinking, it appears that precipitation will slowly come to an end Saturday evening. As mentioned above, confidence is low though. About 55% of the guidance has most of Saturday dry, while 25% of the guidance has precipitation coming to an end by Saturday evening, with the final 20% of the guidance shows precipitation continuing through Saturday.

Sunday and Monday: A cold front will then cross the area late Saturday or early Sunday, with dry weather forecast. Expect temperatures below normal.

.AVIATION /12Z TUESDAY THROUGH SATURDAY/...

Prior to the 12Z TAFs, KCLX detected widespread rain advancing north across the terminals, accompanied by IFR ceilings. Cold air damming will be reinforced by the widespread rain fall this morning, supporting north-northeast winds and IFR conditions through mid-day. In addition, strengthening llvl jet and directional shear should yield LLWS at each terminal this morning. A period of heavier rainfall rates should pass over the terminals this morning after sunrise, reducing visibility down to 2SM at times, highlighted with TEMPOs from 12-17Z. Rainfall is timed to shift east of the terminals during the mid to late afternoon hours as the front sweeps east. In the wake of the front, winds will shift from the northwest and cloud bases will gradually lift, improving to MVFR by this evening.

Extended Aviation Outlook: Wednesday and Thursday: VFR.

Friday and Saturday: Cloud bases will lower on Friday and reach MVFR by the late morning with eventual IFR conditions forecast. Widespread precipitation forecast. At this time, no thunder is forecast.

.MARINE...

South winds should continue to strengthen through this morning, with gusts reaching 25 kts or more across the outer GA and Charleston County nearshore waters. Wave heights will increase today, with 6 ft or greater seas developing with the SC nearshore and outer GA waters today. The wind and sea conditions will be highlighted with Small Craft Advisories today into this evening. A cold front should sweep across the marine zones late this afternoon, resulting in winds veering from the northwest from 10 to 15 kts, decreasing to 10 kts during the pre-dawn hours Wednesday morning. Seas should gradually subside during the night, ranging between 3-4ft after midnight.

Wednesday and Thursday: Northwest winds veering from the north Wednesday morning 10 to 15 kt. Seas 2 to 4 ft. No precipitation is expected.

Friday and Saturday: Winds will veer from the northeast 10 to 15 kt. Widespread showers and possibly a thunderstorm expected Friday and Saturday morning. Winds will then turn from the northwest behind a cold front. Expect seas 2 to 4 ft.

.TIDES/COASTAL FLOODING...

Upcoming 5 AM high tide cycle: Current tidal trends at the Charleston Harbor tide gage indicate that the tide departure has steadily increased since the afternoon high tide and has remained around 1 ft following the ~10:30 pm low tide. This morning's high tide is at ~5 am and the astronomical tide is 6.23 ft MLLW, meaning we would need to see at least a 0.77 ft departure to reach the minor coastal flooding threshold of 7 ft MLLW. Over the next 2-3 hours, northeast flow along the coast will continue but is expected to weaken slightly as an elongated area of low pressure stretches along the coast and the pressure gradient relaxes. Though wind speeds will come down, we should see the ambient pressure decrease by 7-9 mb which should counter the loss of wind force. All this to say, we think a departure of a little less than 1 ft is likely around the time of high tide and the forecast is for a peak tide value of 7.0-7.2 ft MLLW. A Coastal Flood Advisory has been issued for Charleston and coastal Colleton counties.

Astronomical tides will then continue to rise with the morning high tide cycles through Friday and remain elevated through the weekend. Coastal Flood Advisories will likely be needed for morning high tide cycles, primarily at Charleston Harbor (Charleston and Coastal Colleton counties) through late week.

.CHS WATCHES/WARNINGS/ADVISORIES...

GA...None. SC...None. MARINE...Small Craft Advisory until 1 AM EST Wednesday for AMZ350-374.

Small Craft Advisory from 10 AM this morning to 7 PM EST this evening for AMZ352.

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