textproduct: Charleston

WHAT HAS CHANGED

The Aviation section was updated for the 20/12z TAFs.

KEY MESSAGES

- 1) Below normal temperatures this week as high pressure prevails and a powerful nor'easter meanders off the Mid- Atlantic coast.

- 2) High risk for rip currents along the Georgia beaches through this evening.

DISCUSSION

KEY MESSAGE 1: Below normal temperatures this week as high pressure prevails and a powerful nor'easter meanders off the Mid-Atlantic coast.

Through Daybreak: Early morning surface observations show the boundary layer as mostly decoupled inland with calm winds being reported. Strong radiational cooling will continue through daybreak with calm winds, a dry airmass and clear skies all in place. Temperatures are expected to bottom out in the lower-mid 40s inland with mid 50s at the beaches. A few upper 30s will possible in some of the normally colder spots such as a Francis Marion National Forest.

Today and Tonight: Sprawling high pressure (1035 hPa) centered over southern Ontario will maintain its influence across the Southeast U.S. and Deep South through tonight as the nor'easter well offshore of the Outer Banks meanders farther to the north. The airmass will remain very dry with PWATs holding <0.35" with zero rain chances. Full insolation low-level thickness schemes support highs in the 74-77 degree range this afternoon away from the beaches which is consistent the forecast soundings and all of the 20/00z statistical guidance. 20/01z NBM point interquartile ranges (IQR) are also quite small, only averaging about 1-2 degrees across the quartiles with no outliers. This yields a high confidence forecast for the afternoon period.

Quiet conditions will persist into tonight. The boundary layer should decouple inland again inland after sunset resulting in fairly steady temperatures falls with light/calm wind, clear skies the very dry airmass remaining in place. The strong radiative process away from the coast may be interrupted somewhat by early Wednesday morning with models showing a slight tightening of the pressure gradient between the high and the deepening nor'easter well east of the Virginia Tidewater. This may allow the boundary layer to recouple to some degree with a few knots of wind possible several hours prior to sunrise. 20/01z NBM lows Wednesday morning look too warm away from the beaches (a well documented bias in radiational cooling dominated regimes) and manual downward adjustments were made consistent with output from the both the H3R and RAP hourly temperature progs in the 09-12z time frame. These lows are about a degree or two below the NBM's lower quartile in the IQR (25% percentile).

Wednesday through Thursday: A nor'easter will be located well off the coast of the Mid-Atlantic States Wednesday morning. Meanwhile, strong High pressure will be located north of New England, with it's periphery stretching well down into the Southeast U.S. The nor'easter will very slowly shift away from our region through Thursday, then accelerate away Thursday night. Though, the periphery of the distant High will persist across the Southeast U.S. This will bring our area dry conditions with sunny skies. High temperatures are forecasted to be in the middle 70s Wednesday, and the middle to maybe upper 70s on Thursday. However, the dry conditions, clear skies, and light to calm winds at night will cause decent radiational cooling. This will lead to low temperatures that are several degrees below Wednesday and Thursday nights.

Friday through Monday: High pressure will strengthen over the Mid- Atlantic States while a coastal trough develops along the Southeast U.S. coast. This will cause NE winds to increase and become gusty, especially closer to the coast and over the weekend. While it should remain dry, temperatures may be several degrees below normal.

KEY MESSAGE 2: High risk for rip currents along the Georgia beaches through this evening.

Today: Powerful, long period back-swell (periods 12-13 sec) being generated by persistent northeast winds and the nor'easter offshore of the North Carolina Outer Banks will keep a high risk for rip currents in place along the Georgia beaches with a moderate risk for the South Carolina beaches through this evening.

Wednesday: Our internal rip current calculator along with RCMOS indicates a borderline Low/Moderate risk for our SC beaches, and a borderline Moderate/High risk for our GA beaches. We anticipate waves to be subsiding, so we maintained the Low and Moderate Risks respectively for each state.

Thursday through Sunday: An elevated risk of rip currents should persist for our GA beaches Thursday, followed by an increasing risk for all of our beaches Friday into the weekend.

AVIATION /10Z TUESDAY THROUGH SATURDAY/

20/12z TAFs: KCHS/KJZI/KSAV: VFR through 21/12z.

Extended Aviation Outlook: Gusty winds are expected on Saturday.

MARINE

Through Tonight: Elevated seas and slightly enhanced north to northeast winds will persist across the local marine area through tonight. Winds will generally average 15-20 kt with gusts to 25 kt with seas 3-5 ft nearshore waters and 6-8 ft offshore waters. Small Craft Advisories remain in effect for all Atlantic waters, not including the Charleston Harbor where winds will remain below advisory thresholds.

Wednesday and Thursday: A nor'easter will be located well off the coast of the Mid-Atlantic States Wednesday morning. Meanwhile, strong High pressure will be located north of New England, with it's periphery stretching well down into the Southeast U.S. The nor'easter will very slowly shift away from our region through Thursday, then accelerate away Thursday night. The interaction between these two synoptic features will be generating strong and gusty NNE winds Wednesday morning. Though, winds are expected to ease into Wednesday afternoon, then maintain intensity Wednesday evening into Thursday morning, followed by easing again Thursday afternoon. The Small Craft Advisories are in effect for the waters within 20 nm through Wednesday evening. Though, they may end sooner if the sea state is lower than expected. But 6+ ft seas will prevail in the GA 20-60 nm waters through Thursday.

Friday and Saturday: High pressure will strengthen over the Mid-Atlantic States while a coastal trough develops along the Southeast U.S. coast. The interaction between these two features will cause marine conditions to rapidly deteriorate. Winds and seas will increase, with Small Craft Advisories likely, and Gale Watches/Warnings possible for all of our coastal waters.

CHS WATCHES/WARNINGS/ADVISORIES

GA...High Risk for Rip Currents through this evening for GAZ217-219-239- 241. SC...None. MARINE...Small Craft Advisory until 9 PM EDT Wednesday for AMZ360-362- 364. Small Craft Advisory until 8 AM EDT Sunday for AMZ384.