417 FXUS62 KCHS 171807 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 207 PM EDT Wed Mar 17 2021 .SYNOPSIS... A stalled front will remain over Southeast Georgia today, before retreating north as a warm front tonight. A strong cold front will then push offshore Thursday evening, followed by a reinforcing cold front late Friday. High pressure will then build in from the north and prevail through early next week. && .NEAR TERM /THROUGH TONIGHT/... This Afternoon: Aloft, the area will remain along the northwest edge of a ridge well in advance of a strong trough advancing across the Central United States and into the Deep South. At the sfc, a slow moving cold front will advance as far south as the Altamaha River with cooler high pressure remaining across northern and inland areas through much of the afternoon. Weak isentropic lift across the front should support a few showers across parts of southeast Georgia, followed by a slight uptick in coverage well inland heading into evening hours as weak mid-lvl energy rounds the ridge aloft. A few thunderstorms are also possible, mainly along and south of I-16 where modest instability develops as sfc temps approach the mid-upper 70s, before activity then transitions well inland as mid-lvl energy increases from the west late. Further north, sfc temps will be cooler, but recent breaks in cloud cover will result in afternoon highs approaching the mid 60s. Tonight: The stationary front will move north as a warm front overnight as the powerful storm system to the west draws closer. A few showers could develop along/north of the warm front, but activity should remain fairly limited and mainly driven by weak isentropic assent/warm air advection. Temperatures are expected to steady out or even rise after midnight as the region becomes fully embedded with the warm sector and a strengthening low-level jet begins to increase nocturnal mixing. Lows will generally bottom out in the lower 60s then rise mid 60s by daybreak Thursday. An approaching squall line should remain west of the forecast area through the night. && .SHORT TERM /THURSDAY THROUGH SATURDAY/... The focus of the short term period will be the potential for severe weather on Thursday. A closed mid level low and associated surface reflection will shift into the mid-Atlantic, eventually sending a cold front through the local area. A healthy low level jet will help bring PWats to around 1.5 inches and with ample large scale forcing, a line of showers and thunderstorms are expected along and ahead of the front. The line should cross the area in the afternoon and early evening. Perusing severe parameters...wind fields are certainly impressive with 0-6 km bulk shear increasing to 50+ knots in the afternoon, and models are indicating a moderately unstable airmass (generally 750-1250 J/kg on average). 0-3 km storm relative helicity is forecast to be upwards of 250 m2/s2 with Significant Tornado Parameter (STP) progs of 2-3 units. All in all, the environment is supportive of severe thunderstorms. The primary hazards are damaging wind gusts and tornadoes, a few potentially strong. Hail is possible as well but appears to be the lesser threat at this time. The new SPC Day 2 Outlook highlights the vast majority of the area within a Moderate Risk. Otherwise, expect breezy conditions even outside of convection with gusts around 30 mph. It will get unseasonably warm despite a fair amount of cloud cover. Highs will peak in the upper 70s to low 80s. The main line of convection and axis of deepest moisture will push offshore in the evening. Rain chance will diminish quickly thereafter. Lows Thursday night are forecast to mainly be in the low 50s. An amplified mid level trough will persist over the eastern CONUS on Friday. A secondary, reinforcing cold front will approach from the north, then drop through the area Friday afternoon or evening. There is a bit of timing discrepancy of how fast this occurs which will have implications on the temperature forecast. Current forecast features highs largely in the mid to upper 60s. Couldn't rule out a couple light showers with the front, but the chance for measurable rainfall will remain limited. Lows Friday night will drop to the upper 30s/low 40s as cold advection ensues. High pressure will build in from the north Friday night and will prevail through Saturday. Enhanced low level convergence within a developing coastal trough will bring a risk for showers, mainly along the coast and offshore. It will remain gusty through the day, especially at the beaches, with a tight pressure gradient in place. Temperatures will be much cooler, near 15 degrees below seasonal norms. Highs will top out in the mid 50s. Lake Winds: Winds will increase on Thursday ahead of a cold front. Gusts to 25 kt will be possible, and a Lake Wind Advisory could be needed. Friday night into Saturday, gusty northeast winds are expected to develop behind a secondary cold front. Another advisory could be needed. && .LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... High pressure will continue to extend south across the region through early next week. Models have come into a bit better agreement showing an area of low pressure developing well off the Southeast coast. This could bring a risk of showers, mainly to coastal areas, late in the weekend. Forecast is dry thereafter. Temperatures will be below to near normal. && .AVIATION /18Z WEDNESDAY THROUGH MONDAY/... CHS: VFR conditions will prevail at the CHS terminal for the next few hours, but MVFR, then IFR cigs should spread across the area late afternoon into the early overnight period as clouds return in advance of a front beginning to lift north overnight. Once the front passes overnight, conditions should improve to MVFR late tonight, then VFR during late morning into early afternoon, but southwest winds will become quite gusty, upwards to 20-25 kt by noon. SAV: Clouds associated with a front will maintain MVFR cigs for a few hours early this afternoon, but should improve to VFR around 21Z and persist into early evening. IFR cigs are then expected to return tonight until the front lifts north of the terminal. Once that occurs, conditions should improve to MVFR late tonight, followed by VFR late morning. Southwest winds are expected to become gusty late morning and heading into early afternoon, upwards to 20-25 kt by noon. Extended Aviation Outlook: Flight restrictions are possible in showers and thunderstorms Thursday afternoon and evening, then again in low clouds Friday night into the weekend. Gusty winds also expected Thursday through Sunday. && .MARINE... This Afternoon: Light east-northeast winds are expected as high pressure remains to the north and a low pressure system advances across the Deep South. Some localized sea breeze enhancements are possible in the Charleston Harbor and along the land/sea interface early to mid afternoon, but winds will generally remain at or below 10 kt. Seas will average 2-3 ft nearshore waters with 3 ft over the Georgia offshore waters. Tonight: East winds will become southerly overnight a warm front lifts north. Speeds will increase to 10-15 kt, except 15 kt over the Georgia offshore waters as the marine area becomes fully embedded within the warm sector. There could be a better chance for some patchy sea fog late tonight/early Thursday morning as dewpoints begins to rise, but again, the Td-SST spread is not overly high. Seas will average 2-3 ft nearshore waters and 3-4 ft Georgia offshore waters. Thursday through Monday: Elevated marine conditions are expected through much of the period. Winds and seas will increase ahead of and behind a passing cold front Thursday into Thursday night. Small Craft Advisories have been hoisted for the Charleston county and outer Georgia waters, although this will likely need to be expanded to other waters, including the Charleston Harbor. Conditions will briefly improve on Friday, before deteriorating again Friday night behind a secondary cold front. Another round of advisories are expected for all marine zones. Confidence is also increasing in the potential for gale force wind gusts, particularly Friday night into Saturday. Gale Watches or Warnings could eventually be needed. Conditions will remain supportive of advisories into early next week. Rip Currents: Strong onshore winds will result in a moderate risk along south coastal South Carolina and possibly even along north coastal Georgia Thursday. An enhanced risk for rip currents will likely persist into early next week. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...Small Craft Advisory from 11 AM Thursday to 11 AM EDT Friday for AMZ350-374. && $$ NEAR TERM...DPB SHORT TERM... LONG TERM... AVIATION...DPB MARINE...DPB