textproduct: Charleston

WHAT HAS CHANGED

The aviation section has been updated for the 18z TAF issuance. Cold temperatures for Wednesday morning continue to be addressed in key message 1. The potential for a winter weather event remains addressed in key message 2. Added key message 3 to address the cold weather early next week.

KEY MESSAGES

- 1) Cold temperatures in the mid to upper 20s across inland areas Wednesday morning. - 2) Widespread precipitation is expected to occur this weekend, bringing increasing potential for a multi-day winter weather event across portions of the region.

- 3) Very cold temperatures will occur early next week.

DISCUSSION

KEY MESSAGE 1: Cold temperatures in the mid to upper 20s across inland areas Wednesday morning.

Dry high pressure will move from the central Appalachians to the Mid Atlantic region overnight, then move to the northeast over the Atlantic on Wednesday. Expect very light northeast flow through the overnight, with cirrus passing aloft. This will yield low temperatures in the mid to upper 20s inland, with mid to upper 30s along the immediate coastline. Given that winds will be light to calm, wind chill values will remain close to the air temperature. Minimum wind chill values are expected to be in the mid to upper 20s away from the coast, so a Cold Weather Advisory will not be needed.

KEY MESSAGE 2: Widespread precipitation is expected to occur this weekend, bringing increasing potential for a multi-day winter weather event across portions of the region.

Guidance continues to show a mult-day winter weather event developing this weekend and potentially lingering into early next week. The period of winter weather begins late Saturday as the center of a Arctic air mass reaches New England. This dry and cold air mass should ridge southwest across the Southeast U.S late Saturday, with cold air damming strengthening through the night. Given the approach of a broad H5 trough and upper divergence associated with a 180 kt jet over the Mid-Atlantic, a low is forecast to develop off the GA/SC coast. As the low develops, deep isentropic lift will develop over the wedged high pressure, producing widespread precipitation. Gusty northeast winds and wet-bulb effects should result in temperatures to cool into the 30s across inland SC Lowcountry into SE GA. By late Saturday night, sfc temperatures are forecast to drop into the upper 20s from Berkeley County west to Jenkins County. Precipitation may transition from a cold rain, to a mix of rain and sleet, then as freezing rain. Accumulations are possible, especially on elevated surfaces across northern Dorchester and Berkeley counties by Sunday morning.

On Sunday, the coastal low should continue to track northeast off the Carolina coast. Sfc winds may turn from the north, strengthening the cold air damming across the region. High temperatures on Sunday were cooled significantly, limiting values to the 30s across the inland tier of counties, elsewhere temperatures in the 40s are possible. These temperatures may need to be cooled further with future updates. Temperatures Sunday night should cool sharply after sunset, with sub-freezing temperatures pushing to the coast across the Lowcountry and potential to the I-16 corridor of SE GA. Forecast soundings within the cold air Sunday night features a warm nose center at H85 with cold BL. Freezing rain is expected across the SC Lowcountry, with a mixture of rain/Freezing rain south to I-16. The forecast become more uncertain by Sunday night and beyond. GFS indicates a much slower scenario with a closed H5 low, opening to a wave over the Southern Plains, potentially keep precip over the forecast area into early next week. Fortunately, GFS and ECMWF AI models indicate a faster scenario, opening the wave as far west as Baja, resulting in dry air to each the forecast area by Monday. At this time, precip in the forecast will end during the daylight hours on Monday.

The Probabilistic Winter Storm Severity Index (WSSI-P) shows chances of minor winter storm impacts across the entire area, with even a 20-40% chance of moderate impacts especially north of I-16. Ice accumulation appears to be the primary threat. We will also need to be on the lookout for temps/wind chills approaching Cold Weather Advisory criteria Sunday night and beyond.

KEY MESSAGE 3: Very cold temperatures will occur early next week.

Arctic air is forecast to spread across the region Monday night into Tuesday morning. Using a blend of guidance, min temperatures are forecast to range in the teens west of I-95 with low to mid 20s along the coast. North to northwest winds may remain across the region, possibly around 10 mph. The combination of cold temperatures and wind may result in widespread wind chill values in the teens.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

VFR conditions will prevail at KCHS, KJZI, and KSAV through 18z Wednesday.

Extended Aviation Forecast: VFR through Friday. Periods of MVFR and/or IFR conditions possible with a rain and/or mix of frozen precip over the weekend.

MARINE

Tonight: With high pressure to the north and northeast, we should see winds turn more northeasterly overnight with speeds increasing into the 10-15 knot range. Seas should average 2-3 feet through the night, with up to 4 feet in the outer GA waters.

Wednesday through Friday: Overall, quiet conditions expected to prevail across the local waters. Wind speeds should remain no higher than 10 knots during this period with seas mostly in the 2-4 ft range.

Friday night through the weekend: The weekend continues to be the period of greatest concern across the local waters. A strong cold front is expected to pass through from the north Friday night as high pressure builds inland. Then Saturday night into Sunday, an area of low pressure is expected to develop offshore and track away to the northeast. Behind the front Friday night and through the development of the offshore low Saturday night, the coastal waters will likely become situated within an area of enhanced gradient and strengthening northeast flow. The potential for frequent gusts into the 25-30 knot range can be expected across all waters, with seas quickly ramping up into the 6-10 ft range (highest from around 10 nm and out). Small Craft Advisories will almost certainly be needed for all waters beginning Friday night and continuing through the end of the weekend. There continues to be a low probability for gale force gusts in the Charleston County waters.

EQUIPMENT

The Beaufort/Green Pond NOAA Weather Radio transmitter (WXJ-23) is operating at low power with diminished service. Repairs are scheduled to be made Wednesday or Thursday to restore full service.

CHS WATCHES/WARNINGS/ADVISORIES

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