textproduct: Charleston

WHAT HAS CHANGED

24/23z temperatures were running a bit cooler than expected as the CAD wedge becomes more firmly established and diabatic processes initiate with the onset of isentropically induced showers. Hourly temperatures were nudged down a bit based on the current trends and match pretty close with a blend of the NAM12, H3R and NBM25. This brings freezing temperatures in a bit quicker by a few hours across parts of northern/eastern Berkeley County with freezing temperatures expanding west into far interior areas as daybreak approaches. The initial batch of isentropic showers across coastal Georgia will move north into the Lowcountry over the next few hours with additional activity developing over the Atlantic and streaming into the Charleston Tri-County overnight. The main precip area over the Deep South will likely only brush far interior areas prior to daybreak. Given the above trends, the risk minor impacts from freezing rain or freezing drizzle still look centered in the current Winter Weather Advisory area, but ice accums were nudged up slightly in northern and eastern parts of Berkeley County where the longest juxtaposition of precip with freezing temps will occur through daybreak.

KEY MESSAGES

- 1) Widespread precipitation is expected tonight through Sunday night, with freezing rain possible tonight into Sunday morning across inland portions of the SC Lowcountry and SE GA.

- 2) Arctic air will yield very cold temperatures Monday night into next weekend, with Cold Weather Advisories possible.

DISCUSSION

KEY MESSAGE 1: Widespread precipitation is expected tonight through Sunday night, with freezing rain possible tonight into Sunday morning across inland portions of the SC Lowcountry and SE GA.

Infrared satellite today is very interesting, showing the Arctic surging southward from central Canada over the U.S., spanning the area from the Great Plains to New England. By this afternoon, the center of the 1040 mb high will slide over New England, ridging south along the east facing slopes of the Appalachians. Cold Air Damming (CAD) will continue to build through tonight, rapidly cooling temperatures and deepening the CAD across the forecast area this evening. Near-term guidance indicates that a sfc low will organize and develop over the Gulf Stream this evening as H5 longwave trough approaches from the west. Once the low develops, it is expected to track northeast off the SC coast tonight.

As the low passes off the coast, isentropic lift will increase across the forecast area, especially along and west of the I-95 corridor. NAM12 shows isentropic lift increasing between 285-295K this afternoon, generally remaining into the daylight hours Sunday morning. Forecast soundings indicated saturated conditions below H7, especially below a strong warm nose centered at H9. The combination of the PW in excess of an inch with rounds of isentropic lift should yield rounds of drizzle or light precip within the wedged air mass. The falling precip should lead to some degree of diabatic cooling as temperature approach the wet bulb. In addition, steady NNE winds will advect cooler temperatures across the SC Lowcountry and SE GA tonight. High resolution guidance indicates that the freezing line will push over the interior counties, Jenkins County north to Berkeley County, by daybreak Sunday. This corridor of counties is expected to see an overlap of light precip with sub-freezing temperatures, resulting in rounds of freezing rain or freezing drizzle. Guidance as trended a bit drier, but at least a light glaze to a few hundredths of an inch of ice is possible by early Sunday morning. A Winter Weather Advisory will remain for a tier of counties from Jenkins County east to Berkeley County from 7 PM this evening until 1 PM Sunday.

Guidance continues to indicate a strong cold front approaching the CWA from the west on Sunday, timed to reach the inland counties during the early evening and the coast by midnight. High resolution guidance shows a solid band of showers with embedded thunderstorms remain along and ahead of the cold front. Forecast soundings indicate that weak instability may develop ahead of the line, resulting in 100-200 J/kg of CAPE across portions of SE GA. However, wind shear remains very strong, featuring 0-1 km SRH in excess 250 m2/s2 and bulk shear around 50 kts. 12Z HREF indicates STP values approaching one unit across extreme SE GA. It is possible that a severe thunderstorm or two could reach SE GA Sunday evening, some possibly feature QLCS structure with rotating updrafts. Elsewhere, strong thunderstorms remain possible, with wind gusts as the primary threat. Cold air will follow the cold front, however all moisture is expected to be offshore by the time the cold air arrives.

KEY MESSAGE 2: Arctic air will yield very cold temperatures Monday night into next weekend, with Cold Weather Advisories possible.

As a strong arctic high pressure ushers cold, dry air into the region behind a cold front on Monday night/Tuesday morning, temps. are forecast to drop into the teens and low 20s west of I-95 and across much of the Francis Marion Forecast. It'll be a tad warmer near the coastline with temps. ranging in the low to mid 20s. Mixed the NAM12/NBM10 into the overnight temperatures as the NBM appeared to be struggling to realize the presence of the cold-air damming (CAD) across the region. These conditions combined with light west- northwesterly winds could result in widespread wind chill values in the mid to upper teens from interior counties all the way down to the coastline. Some guidance has been indicating that minimum wind chill values could drop range around 10 degrees across portions of inland SE GA. A Cold Weather Advisory should eventually be needed for the entire area across southeast South Carolina and southeast Georgia. Forecast remains a bit warmer on Tuesday night/Wednesday morning as temps. range in the low to mid 20s across the interior counties with low 30s along the coastline. Thus, confidence with needing a Cold Weather Advisory seem less likely for this time period with probabilities for temps. less than 20 degrees range from 20-40%. It's important to note that model confidence continues to be fairly high as the IQR shows a 1 to 2 degree spread for lows Tuesday and Wednesday morning.

This cold airmass will likely stick around through late this week as a reinforcing cold front moves through on Thursday and pushes another shot of cold air into the region from the northwest. It's reasonable to say that we could have a few hours of wind chill values below 20 every morning into the weekend and additional Cold Weather Advisories might be needed.

AVIATION /01Z SUNDAY THROUGH THURSDAY/

25/00z TAF Discussion: KCHS/KJZI/KSAV: MVFR cigs will give way to IFR cigs overnight as over-rrunning atop the wedge intensifies. Pockets of light rain will likely meander around the terminals through the night, but no meaningful impacts are expected. VCSH was highlighted to account for this. Any risk for FZRA/FZDZ will remain well to the north and west of the terminals. Gusty winds will linger through early morning, then begin to diminish a bit as the coastal front offshore draws closer to the coast. Cigs may lift to MVFR at KSAV by early/mid-afternoon Sunday, but confidence on timing is poor given models are notoriously fast in eroding cold air damming.

Extended Aviation Forecast: Expect a cold front to move across the region Sunday night and yield VFR conditions through early next week.

MARINE

Through tonight: As is typically the case in strong CAD events, the models have underplayed the strength of the pinched gradient along the coast. Winds are solidly near gales with gusts approaching storm force at times in the eastern portions of the South Santee-Edisto Beach nearshore leg. Winds should peak over the next few hours, only to slowly weaken overnight as the coastal front offshore tries to meander closer to the coast. However, models are likely bringing winds down too quickly given their bias to eroding the CAD too quickly. A Gale Warning continues for all waters except the Charleston Harbor where a Small Craft Advisory continues.

Sunday through Wednesday: As a strong cold front approaches the region from a low pressure system moving across the Southeast, hazardous marine conditions continue to persist through early next week. Northeasterly flow will veer out of the south Sunday afternoon with wind speeds of 20 to 25 kt with gusts up to 30 kt possible. Seas remain a bit chaotic with wave heights 6+ ft on Sunday and Monday, and then expect the swell to retreat on Monday night and seas to slowly diminish. Small Craft Advisories remain in effect until Tuesday morning given the elevated winds and seas.

EQUIPMENT

The KCLX radar will remain out of service. Critical parts are on order and should arrive on Monday. Users should use adjacent WSR-88D sites, including KCAE, KLTX, KJAX, KVAX and KJGX.

CHS WATCHES/WARNINGS/ADVISORIES

GA...Winter Weather Advisory until 1 PM EST Sunday for GAZ087-088. SC...Winter Weather Advisory until 1 PM EST Sunday for SCZ040- 042>045. MARINE...Small Craft Advisory until 1 AM EST Sunday for AMZ330. Gale Warning until 1 AM EST Sunday for AMZ350-352-354-374.