textproduct: Charleston

WHAT HAS CHANGED

The aviation section has been updated for the 18Z TAF issuance. Key Message 2 has been introduced to address cold weather concerns next week. Key Message 1 was updated.

KEY MESSAGES

- 1) Unsettled and warm conditions will linger into the weekend bringing above normal warmth and a risk for several potentially impactful hazards.

- 2) Cold weather will return Sunday night into Tuesday night.

DISCUSSION

KEY MESSAGE 1: Unsettled and warm conditions will linger into the weekend bringing above normal warmth and a risk for several potentially impactful hazards.

A fairly active period is ahead. A cold front will approach the Southeast U.S. Friday as occluded low pressure weakens over the Great Lakes region and secondary cyclogenesis occurs off the Mid- Atlantic coast. The front will gradually slow and stall across the region Saturday as it becomes increasingly parallel with the mean flow aloft. Low pressure is forecast to develop along the north- central Gulf coast Saturday afternoon, which will quickly translate across central Georgia/the South Carolina Midlands Saturday evening before exiting off the North Carolina Crystal Coast early Sunday morning. As the low approaches Saturday evening, the stationary front will push north bringing much of the Lowcountry and Southeast Georgia back into the warm sector before a cold front finally sweeps offshore Saturday night. Cold high pressure will build in behind the front then prevail through the middle of next week. This storm system will being much much needed rainfall to the area along with a risk for several impacts/hazards.

Rain Chances/QPF: Confidence is high that some much needed rainfall will fall across the South Carolina Lowcountry and Southeast Georgia Friday into early Sunday morning as a storm system impacts the region. A region of scattered to numerous showers, mostly induced by isentropic ascent/warm air advection 30-40 kt low-level jet, will develop Friday brining a risk for mostly light rain amounts. Heavier, potentially more widespread rains are likely Saturday into Saturday night as low pressure approaches and the aforementioned stationary front over the Lowcountry begins to meander north. Vertical cross sections show modest deep-layered UVVs induced by DPVA ahead of a shortwave that is forecast to dig across the Tennessee Valley into the southern Appalachians. This coupled PWATs climbing above 1.50" and the presence of some surface-based instability should easily support some spots receiving over an 0.50" over a 6-hour period. The storm total QPF from Friday through Sunday morning is 0.40-1.15" with the highest totals occurring across far interior Southeast Georgia into areas adjacent to the Southern Midlands and the Charleston Tri-County. While not excessive, these amounts will certainly help with the severe to extreme drought conditions that are in place.

Severe Weather: The risk for severe weather Saturday into Saturday night is conditional on how much instability can develop in the warm sector. Low-level wind fields look strong and sufficiently sheared with 0-6km bulk shear forecast to increase to 50-70 kt by late Saturday afternoon. These values would certainly be supportive of a damaging wind/tornado threat with any organized convective clusters, but the main uncertainty, as is typical this time of year, is instability. The 19/13z NBM shows the risk for SBCAPE >500 J/kg by late Saturday afternoon is greatest over interior Southeast Georgia in the far southern parts of the Lowcountry with probabilities running 70-95%. Probabilities are quite a bit lower (25-50%) farther to the north across the interior parts of the Lowcountry and the Charleston Tri-County where there is much more uncertainty on how far north the warm front will progress. At this time, confidence in at least isolated severe thunderstorms is greatest across interior Southeast Georgia, roughly in the area south of I-16 and west of I- 95 with confidence dropping off quite a bit across the Lowcountry due to uncertainties in warm front positioning and where more southerly trajectories from off the colder Atlantic waters could have a more stabilizing influence. Day 3 WFO guidance from the SPC currently outlines the entire forecast area in a marginal risk (level 1 of 5) for severe tstms which looks appropriate for the current set up and level of uncertainty. Should the risk for higher instability be realized, then the risk level may be increased in future forecasts.

Above Normal/Near Record Warmth: Well above normal temperatures are likely to persist through Saturday. High temperatures Friday could get close to record territory at KCHS and KSAV, but current indications are they will fall just short. Warm overnight lows tonight and again Friday night could challenge the record high minimums, again most likely at KCHS and KSAV. See the record section below for the specific records.

KEY MESSAGE 2: Cold weather will return Sunday night into Tuesday night.

Cold high pressure will build into the region Sunday night and linger through the middle of next week. This will usher in a return of more winter-like conditions with sub-freezing temperatures likely both Monday and Tuesday mornings. Lingering cold air advection and enhanced pressure gradient between the high building in a departing low pressure off the Mid-Atlantic coast could drive wind chills down into the lower-mid 20s. There is about a 20-35% chance for wind chills to drop below the Cold Weather Advisory criteria of 20 degrees, so this is something that will continued to be watched closely. Highs will only recover to the upper 40s/lower 50s Monday with lows Tuesday morning dropping into the mid-upper 20s away from the beaches with wind chills in the mid 20s just about everywhere. Temperatures will begin to moderate Tuesday into Wednesday.

AVIATION /19Z THURSDAY THROUGH TUESDAY/

Gusty southwest winds will continue this afternoon before subsiding to 10 kt around sunset. A light isolated shower could pass near the terminals this evening into tonight, especially at KSAV, supporting no more than VCSH. Guidance trends toward lowering cigs Friday morning due to marine stratus pushing over KSAV and KJZI, bringing MVFR/IFR conditions or perhaps LIFR. Visibilities should remain 5SM or greater, being that we do not anticipate dense fog over the terminals. Rainfall chances increase Friday early afternoon with scattered showers, as well as a return of gusty southwest winds. Refinements on timing of rainfall Friday will be needed in future TAFs.

Extended Aviation Outlook: Flight restrictions are likely at times through the weekend due to potential advection of marine stratus inland as well as scattered showers/thunderstorms Friday and Saturday.

MARINE

Sea fog has encompassed areas within 10 NM of the coast based on webcams and Pilot Boat reports. A Marine Dense Fog Advisory has been issued until 7 PM.

The marine zones will remain between high pressure centered over the western Atlantic and low pressure over the MidWest. This pattern will support southwest winds generally around 15-20 kt with 3 to 5 ft seas through tonight, with some 6 ft seas moving into the Charleston County nearshore waters. The pressure gradient pinches on Friday, and wind gusts across the coastal waters will range 20-25 kt. Small Craft Advisories have been issued for the Charleston County nearshore waters, including the Harbor, and the offshore Georgia waters from 20-60 nm for gusty winds through Friday evening.

A frontal wave is forecast to develop over the I-20 corridor of the Deep South Friday night, tracking east on Saturday. The associated cold front is timed to push over the marine zones Saturday evening, with CAA and strengthening winds Saturday night. On Sunday, Small Craft Advisory conditions should spread across the nearshore SC/GA waters with gales possible across the outer GA waters. Gusty winds and elevated seas could linger into Monday.

Sea Fog: The surface pressure pattern will support southwest winds, generally parallel to the coast through the rest of the week. Southwest winds will continue to usher in moist air with dewpoints in the upper 50s to low 60s through Saturday. Buoy observations indicate that the nearshore water temperatures generally range in the low 50s. Given parallel winds and dewpoints nearly 10 degrees warmer than the water temperatures, at least patchy sea fog is expected to develop. Probabilistic guidance shows likely values of visibility less than 0.5 miles at times Friday and Saturday. On the other hand, the sea fog potential will be limited by gusty conditions and rounds of showers which could mix out any fog for periods of time. Marine Dense Fog Advisories could be needed.

Additionally, sea fog could advect onshore and impact the beaches and nearby coastal locations, with visibilities dropping to 1 mile or less at times. We will monitor the need for Dense Fog Advisories over land.

CLIMATE

Record High Temperatures:

February 20: KCHS: 82/2018 KSAV: 84/1991

February 21: KSAV: 84/2018

Record High Minimum Temperatures:

February 20: KSAV: 64/1939

February 21: KCHS: 62/2023 KSAV: 63/2023

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...None. MARINE...Small Craft Advisory from 6 AM to 7 PM EST Friday for AMZ330. Dense Fog Advisory until 7 PM EST this evening for AMZ350-352- 354. Small Craft Advisory from 1 AM to 7 PM EST Friday for AMZ350. Small Craft Advisory from 9 PM this evening to 3 PM EST Friday for AMZ374.