textproduct: Charleston

WHAT HAS CHANGED

The aviation section has been updated for the 06z TAF issuance. There are now three key messages, removing the previous first key message for Tuesday low relative humidity values. Small Craft Advisories have been issued for the nearshore SC/GA and outer GA waters from this evening until Thursday night.

KEY MESSAGES

- 1) Dangerous fire weather and marine conditions possible Thursday into Friday as gusty post-frontal winds combine with low humidity.

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

- 3) Below normal temperatures for the second half of the weekend into early next week.

DISCUSSION

KEY MESSAGE 1: Dangerous fire weather and marine conditions possible Thursday into Friday as gusty post-frontal winds combine with low humidity.

Today, a band of isolated to scattered light rain showers may develop ahead of a passing H5 vort max and jet divergence. High resolution guidance indicates that the showers may develop by mid- morning, forming a thin broken line of showers this afternoon. Using a blend of guidance, some swaths of 0.10" of rainfall is possible, especially across the inland counties. Any rainfall will be beneficial ahead of the very dry conditions expected on Thursday and Friday.

A strong cold front is timed to sweep across the Coastal Plain of GA and SC this evening. In the wake of the front, winds should veer from the northwest and develop gusts between 20-25 mph. Winds will be a bit stronger over the Atlantic waters, see the marine section below for details on winds and wave heights. On Thursday, guidance indicates active CAA through the day, with H85 temperatures reaching -5 to -10C by Thursday afternoon. The air mass will be further dried by downslope flow in the lee of the southern Appalachians and deep mixing to 4.5 to 5.5 kft. As a result, sfc dewpoints will fall through the day, pushing well into the teens by Thursday afternoon. RH values between 25 to 30 percent should be common. The combination of gusty northwest winds, low RH, and mostly sunny conditions should rapidly dry fine fuels from any rainfall today. Depending on fuel moisture levels, a Fire Danger Statement could be needed on Thursday.

The center of the dry air mass will build across the Southeast U.S. on Friday. Fortunately, the weak pressure gradient should only support winds between 5 to 10 mph. Dewpoints could be as low as the single digits at a few locations mid-day Friday, most areas between 10 to 15 degrees. As temperatures reach around 50 degrees Friday afternoon, RH values may range from the upper teens west of I-95, to the 20s to the east.

KEY MESSAGE 2: Very cold temperatures will occur Friday morning, with lows in the upper teens to lower 20s away from the beaches.

Ensemble guidance continues to be in rather impressive agreement for Friday morning with some of the coldest temperatures of the season forecast. The 500 mb wave will be quickly exiting the region Thursday evening with surface high pressure approaching from the west. As this occurs, the pressure gradient will drastically weaken with winds going near calm as the PBL decouples. PWATs are also forecast to be only around 0.10" (or near daily minimum values) which will further enhance radiational cooling.

Expect low temperatures in the upper teens west of Interstate 95 in Georgia and inland rural areas of South Carolina (such as the Francis Marion National Forest). Along and east of the Interstate 95 corridor to about US Highway 17, expect low temperatures in the lower to mid 20s. Along area beaches, expect low temperatures in the upper 20s (including the Charleston Peninsula). Temperatures will be below freezing for 8 to 12 hours.

The other main concern, is the potential need for a Cold Weather Advisory. Winds will quickly slacken Thursday night/ Friday morning with the wind chill to temperature differential decreasing. Still though, Cold Weather Advisory criteria is the temperature or wind chill expected to fall between 20 and 11 degrees F. There is no minimum wind speed requirement. This means even though winds will be mostly calm, we will be close to reaching criteria. Along and west of a Interstate 95 to US Highway 17 line has a 50 to 75% chance of reaching Cold Weather Advisory criteria. To the east of this line, has a 20 to 50% chance of reaching Cold Weather Advisory criteria.

KEY MESSAGE 3: Below normal temperatures for the second half of the weekend into early next week.

The rex block will start to loose cohesion as a wave over the eastern Pacific approaches from the west. The downstream effects of this are another potent shortwave is forecast to dive southeast out of Manitoba and approach the region Sunday morning. This will drive another cold front through South Carolina and Georgia Sunday. Model confidence on this overall pattern remains very low at this point though and the main reason appears to be the shortwave coming out of Manitoba. In particular, how does the rex block evolve and change the placement of this wave (supported by the WPC Ensemble Sensitivity Analysis-- ESA-- webpage). There is also a small chance (~20%) that as the shortwave approaches, a coastal low will form with 500/ 700 mb frontogenesis commencing out in front. Temperatures will be borderline for potentially supporting snow or a rain/snow mix. The probabilistic Winter Storm Severity Index (WSSI-P) does show a 5-10% chance for a trace of snowfall from the colder solutions. The latest run of the GEFS does show more members supporting the wetter solution, but caution should be exercised here as a lot can change with the evolution of the rex block and the downstream shortwave. Any small change could move the precipitation off of the coast and keep most of the area dry. As observed from the ESA webpage, model solutions will likely continue to vary until the rex block and shortwave interaction can be better resolved, which wont likely be until Friday at earliest.

Behind the cold front, temperatures will again be seasonably cold Monday with low temperatures in the mid 20s. However, there is a much larger spread in the interquartile range (IQR) range (around 8 to 10 degrees) compared to Friday morning of this week and that makes sense. Ahead of the approaching high pressure, winds may remain just strong enough (near 5 knots) where wind chills may dip into the 15 to 20 degrees range, with highest probabilities again for the furthest inland areas right at 50%, dropping to 25% along the coast. A Cold Weather Advisory may be needed. Highs on Monday remain below normal, in the lower to upper 50s.

AVIATION /06Z WEDNESDAY THROUGH SUNDAY/

VFR conditions will prevail at CHS/JZI/SAV terminals through 06Z Thursday. Bands of light showers are forecast to track across the terminals this afternoon ahead of upper air disturbance. These showers have been highlighted with a PROB30 in each terminal, generally between 19-22Z. Winds should favor a west-southwest direction around 10 kts.

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 on Thursday.

MARINE

Today, 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. Shower coverage may gradually increase across the Atlantic waters this morning, with a band of light showers expected to sweep across the nearshore waters late this afternoon.

A strong cold front will cross the area tonight. West-northwest winds should develop in the wake of the front, with gusts around 25 kts across the Atlantic waters of GA and SC tonight until Thursday night. As a result, wave heights will build tonight through the daylight hours on Thursday, peaking between 3-5 ft across the nearshore with 6 ft beyond 30 NM Thursday evening. Small Craft Advisories have been issued for all marine zones outside the CHS Harbor from this evening through Thursday night. The center of a large area of high pressure will arrive on Friday and may linger into Saturday, supporting winds around 10 kts with 1-2 ft seas. Winds should gradually strengthen as a dry cold front sweeps across the region on Sunday, 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...Small Craft Advisory from 7 PM this evening to midnight EST Thursday night for AMZ350. Small Craft Advisory from midnight tonight to midnight EST Thursday night for AMZ352-354. Small Craft Advisory from midnight tonight to 4 AM EST Friday for AMZ374.