textproduct: Charleston

WHAT HAS CHANGED

The Aviation section was updated for the 17/12z TAFs.

KEY MESSAGES

- 1) Below normal temperatures and gusty winds expected this weekend as a powerful nor'easter develops off the North Carolina Outer Banks and high pressure prevails inland. - 2) High surf and dangerous rip currents possible into early next week.

DISCUSSION

KEY MESSAGE 1: Below normal temperatures and gusty winds expected this weekend as a powerful nor'easter develops off the North Carolina Outer Banks and high pressure prevails inland.

Through Tonight: As daybreak approaches, the cold front is now offshore. Model output and early morning surface data show the front's thermal and moisture profiles are consistent with a boundary that has a had a strong interaction with Mount Mitchell and the highest terrain of the Southern Appalachians, but these pressure and low-level moisture kinks will continue to smooth out as the front moves farther offshore and strong post frontal cold air advection (CAA) initiates. A few showers look to occur along behind the front and ahead of the approaching upper trough owing to modest quasi-geostrophic forcing with the best chances for a few hundredths of an inch occurring across far inland areas adjacent to the CSRA/Southern Midlands and over the Charleston Tri- County Area. The risk for showers should end by mid- morning, possibly lingering to about noon over upper portions of the Charleston County coast. The day's high temperatures have likely already occurred with temperatures poised to fall into the 60s later this morning with the onset of CAA. In fact, thermal falls may be somewhat accelerated over the Charleston Tri-County where diabatic processes will be maximized and low- level cloud cover will linger the longest. Would not be surprised to see a few spots north of I-26 drop into the upper 50s later this afternoon.

It will become breezy by daybreak with winds 15-25 mph with gusts 30-35 mph common. Winds may be a bit higher over elevated brides and overpasses, especially across the Charleston and Savannah Metro Areas. Wind gusts could approach 40 mph across coastal portions of Liberty and McIntosh Counties late this afternoon into tonight as the base of the upper trough swings through and directions become a bit more onshore relative to the shape of the Georgia coast. The situation looks marginal for a Wind Advisory, so the need for an advisory will be reevaluated later today once shorter term trends can be established. Winds may also gust 35-40 mph at Tybee Island late this afternoon into tonight, given that island's exposure in north/north-northeast flow regimes.

A pretty chilly night is ahead as high pressure builds in and the nor'easter off the North Carolina Outer Banks deepens. Lows look to bottom out into the mid 40s inland with lower 50s at the beaches. Winds will remain elevated through the night with a tight gradient between the inland high and the nor'easter forecast to persist, especially at the coast. Wind chills are expected to drop into the upper 30s/lower 40s away from the beaches--the coldest in quite some time.

Sunday: Low pressure will be forming well off the coast of the Outer Banks of NC in the morning. Meanwhile, strong High pressure will be located over the Mid-Atlantic States, with it's periphery stretching down into the Southeast U.S. The interaction between these two synoptic features will continue to bring our area breezy NE winds (highest along the coast). Additionally, they will usher cold air advection throughout the region. Despite abundant sunshine, high temperatures will struggle to reach the mid to upper 60s. Low temperatures will range from the 40s inland, to the lower/mid 50s along the immediate coast.

Monday through Wednesday: High pressure should persist well to our north on Monday, then gradually release its hold across our region through the middle of next week. This will cause a gradual rise in temperatures. Highs should be in the upper 60s to lower 70s on Monday, then the mid to maybe upper 70s by Wednesday, which is within a few degrees of normal for this time of year.

Lake Winds: Winds on Lake Moultrie look reach 20-25 kt with gusts to 30 kt later today into tonight with waves reaching 2-3 ft with locally higher sets. Some minor lakeshore erosion and overwash induced by significant wave action could along occur along the south end of the lake, including Pinopolis, Lions Beach and Short Stay. Winds will diminish at for Sunday, but will remain elevated with gusts to 25 kt. A Lake Wind Advisory remains in effect through early Monday morning.

KEY MESSAGE 2: High surf and dangerous rip currents possible into early next week.

High Surf: Prolonged gales and a continuous north-northeast fetch will generate large breaking waves along our beaches Sunday into Monday. The highest breakers should be along our Georgia beaches. High Surf Advisories are most likely for this area, but can't be ruled our for our SC beaches as well.

Rip Currents: Increasing winds and building waves will support a moderate risk for rip currents at area beaches today. The combination of strong/persistent gales and elevated seas will generate a high risk of rip currents at all beaches on Sunday, with it potentially extending well into next week.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

17/12z TAFs: KCHS/KJZI: MVFR cigs will spread into both terminals soon as a cold front pushes farther offshore. A few showers will occur along and behind the front, but intensities should be very light. No meaningful impacts are expected. Risk for showers is highest 13-1630z. Widespread MVFR cigs will linger well into the afternoon, improving to VFR by sunset with clearing occurring quickly thereafter. Gusty north winds will prevail behind the front with gusts 22-27 kt likely, diminishing a bit by late afternoon/early evening.

KSAV: MVFR cigs will spread into the terminal prior to daybreak as a cold front pushes through. A few showers may occur along and behind the front, but the best chances look to remain north of the terminal. VCSH was highlighted 13-15z. MVFR cigs should improve to VFR by early afternoon as clouds behind the scour out. Gusty north winds will occur with gusts 22-27 kt, diminishing a bit by late afternoon/early evening.

Extended Aviation Outlook: Expect gusty winds Sunday into Monday.

MARINE

Through Tonight: Dangerous marine conditions will develop across the waters later this morning as a cold front pushes offshore. The risk for gales will increase quickly through the day with the onset of strong post frontal cold air advection. North/northeast gales of 30-35 kt with gusts to 40-45 kt will persist across the waters through tonight and Gale Warnings remain in force for all nearshore and offshore waters. A few storm-force gusts could occur beyond 40 NM closer to the west wall of the Gulf Stream late this afternoon into this evening as the base of the upper trough swings through. The frequency of such gusts look to remain pretty infrequent with the 17/01z NBM probability for 48 kt gusts holding into the 10-30% range, highest over far outer portions (40-60 NM) of the Georgia and far southern South Carolina offshore waters. There is not enough evidence to support an upgrade to a Storm Warning at this time. Seas will build to 4-6 ft nearshore waters and 6-8 ft offshore waters by this afternoon, increasing to 6-9 ft nearshore waters and 9-12 ft offshore waters overnight. For Charleston Harbor, conditions look to remain below gale-force with winds 20-25 kt with gusts to 30 kt and waves 2-4 ft. A Small Craft Advisory remains in effect through tonight.

Sunday through Wednesday: Low pressure will be forming well off the coast of the Outer Banks of NC Sunday morning. Meanwhile, strong high pressure will be located over the Mid-Atlantic States, with it's periphery stretching down into the Southeast U.S. The interaction between these two features will continue to generate strong cold air advection and pressure differences across our coastal waters, generating gale to strong gale conditions. We have Gale Warnings in effect for our Atlantic waters on Sunday, and a Small Craft Advisory for the Charleston Harbor. Expect steep, wind-driven waves close to the coast. Though, the highest winds and steepest/largest seas will be further offshore. Winds should start to ease late Sunday night, gradually easing further Monday and Monday night. The Gales should end for our waters within 20 nm Monday morning, but persist for the 20-60 nm waters, possibly into Monday evening. Afterwards, there may be several evening/nightly surges of winds through mid to late next week. Additionally, wave heights will stay elevated during this time frame. The end result could be Small Craft Advisories for portions of our waters for a significant portion of next week.

CHS WATCHES/WARNINGS/ADVISORIES

GA...None. SC...Lake Wind Advisory until 2 AM EDT Monday for SCZ045. MARINE...Small Craft Advisory until 2 PM EDT Monday for AMZ340. Gale Warning until 2 PM EDT Monday for AMZ360-362-364. Gale Warning until 2 AM EDT Tuesday for AMZ380-382-384.