textproduct: Charleston

WHAT HAS CHANGED

The first key message was updated to mention only this afternoon for low relative humidity values. For the second key message, wind gusts should remain within Small Craft Advisory levels Thursday into Thursday evening. The risk for Gale force wind gusts has diminished.

KEY MESSAGES

1) Dry high pressure will remain across southeast Georgia and South Carolina today, yielding low relative humidity values this afternoon.

2) Dangerous fire weather and marine conditions possible as gusty winds and low humidity values arrive behind a mostly dry cold front Thursday into Friday.

3) Very cold temperatures will occur Friday morning, with lows in the upper teens to lower 20s away from the beaches.

DISCUSSION

KEY MESSAGE 1: Dry high pressure will remain across southeast Georgia and South Carolina today, yielding low relative humidity values this afternoon.

Dry sfc high pressure will linger across the Southeast U.S. today. Forecast soundings indicate that mixing will range between 2.5 to 3 kft this afternoon. As mixing begins, sfc dewpoints should remain in the 20s through mid-day. However, weak southwest winds may result in a slow increase in dewpoints from south and near the coast, increasing into the 30s. Minimum RH values should occur during the early afternoon hours, before the dewpoint recovery. Min RH values are forecast to range from the mid 20s inland to the mid 30s along the coast. High temperatures are forecast to range around 10 degrees warmer than values reached on Monday, generally in the low 60s.

KEY MESSAGE 2: Dangerous fire weather and marine conditions possible as gusty winds and low humidity values arrive behind a mostly dry cold front Thursday into Friday.

On Thursday a rex block will be established over the Western United States allowing for a long wave trough axis to amplify across the Eastern United States. In the mid-levels of the atmosphere, a closed mid-level low will be moving east across the Great Lakes with a strong push of lower heights approaching the area from the west (120 m height falls in 12 hrs). This magnitude of DCVA/ height falls is rather impressive, but precipitation chances are limited (<15%) with the surface cold front Thursday morning/ afternoon. There are several reasons for this. The main reason being the Wednesday wave. This wave will be moving away early Thursday with the LFQ of the subtropical jet streak moving off of the coast. During this type of setup, most of the precipitation gets going off of the South Carolina/ North Carolina coast.

Behind the surface cold front, strong low level CAA will take hold with 850 mb temperatures dropping from 0 degrees C to -8 degrees C. This, in combination with peak mixing and downslope flow in the low and mid levels, will likely result in wind gusts Thursday afternoon on the higher end of the guidance envelope, with 25 to 30 mph gusts likely, with some gusts to 35 mph possible over land. A Lake Wind Advisory will likely be necessary with probabilities of reaching 25 kt around 55%.

Additionally, dewpoints will rapidly fall behind the cold front allowing relative humidity values to approach 25%. It should be noted that the Wambaw and Glissons Pond RAWS sites have both had fuel moisture values in the single digits Sunday and Monday afternoons.

Surface high pressure will settle overhead Friday, with dewpoints likely mixing out to near the single digits in the afternoon. The latest LREF probabilities show a 50 - 70% chance of RH values less than 25% away from the immediate coast. Winds will be much weaker Friday though given the proximity of the surface high pressure to coastal South Carolina and Georgia.

KEY MESSAGE 3: Very cold temperatures will occur Friday morning, with lows in the upper teens to lower 20s away from the beaches.

Surface high pressure now looks to move east faster, or allow winds to decouple quicker Thursday night/ Friday morning. As such, low temperatures will likely be in the upper teens for areas of rural Georgia away from the coast (and closer to the surface high pressure), and low 20s for the rest of coastal South Carolina. Right at the beaches (including the Charleston Peninsula), expect lows in the upper 20s. Wind Chill to temperature spread will likely be greatest Thursday evening, with the differential becoming less and less as winds slacken by Friday morning. Still though, enough wind is forecast that a Cold Weather Advisory might be required for some of the area Friday morning. Rural Georgia has ~ a 60- 80% chance of needing a Cold Weather Advisory, while coastal South Carolina and Georgia has ~ a 20-50% chance of needing a Cold Weather Advisory.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

VFR conditions will prevail at CHS/JZI/SAV terminals through 12Z Wednesday.

Extended Aviation Outlook: VFR conditions will continue for KCHS, KJZI and KSAV through late week. NW winds could gust to around 20 kts in the wake of cold front.

MARINE

Broad high pressure will remain across the marine zones today and tonight. The pattern should yield around 5 kts of wind today, gradually increasing to 5 to 10 kts tonight. Seas are forecast to remain between 2 to 3 ft.

Wednesday, the marine zones will remain between a developing low pressure system over the Gulf Stream and a cold front over the western Carolinas. A weak gradient on the west side of the low keeps benign marine conditions in place despite increased shower chances.

A strong cold front will cross the area Wednesday night. Northwest winds should develop in the wake of the front, with gusts around 25 kts across the Atlantic waters of GA and SC on Thursday into Thursday evening. As a result, wave heights will build during the daylight hours on Thursday, peaking between 3-5 ft across the nearshore with 6ft beyond 30 NM Thursday evening. Small Craft Advisories will likely be needed for all marine zones outside the CHS Harbor Thursday and Thursday evening. The center of a large area of high pressure will arrive on Friday, supporting winds around 10 kts with 1-2 ft seas. Winds should gradually strengthen as a dry cold front sweeps across the region on Saturday, with gusty winds and building seas, possibly reaching Small Craft Conditions across a portion of the waters.

CHS WATCHES/WARNINGS/ADVISORIES

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