Area Forecast Discussion
Issued by NWS Charleston, SC

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000 FXUS62 KCHS 251601 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 1101 AM EST Sat Feb 25 2017 .SYNOPSIS... A cold front will move through the area late today, with colder high pressure to build over the southeast later tonight into Sunday. The high will shift offshore early next week as a warm and moist air mass builds from the Atlantic and Gulf of Mexico. Another cold front will move through Wednesday night into Thursday, followed by high pressure for the end of next week. && .NEAR TERM /THROUGH TONIGHT/... Today: The weak ridge hugging the SE coast is beginning to move offshore in advance of a cold front currently crossing the Appalachians. The front should reach the westernmost portion of our area around 18Z, before it passes offshore around 21z or shortly thereafter. There remains a minimal risk of showers with the front due to shallow moisture, a strong inversion between 800 and 700 mb, and little to no upper dynamics. Radar trends to our west show the showers dissipating as they are moving eastward. However, there are subtle mid level height falls and low level lapse rates are good by February standards. We`re maintaining nothing more than isolated light showers across our interior areas from noon to 3 pm. Compressional heating in advance of the cold front and a westerly flow through about the first 2 miles or so of the troposphere, plus 925 mb temps near the maximum for this time of year, will allow for temps to climb to the upper 70s and lower 80s most places. The warmest will be parts of the Charleston tri-county district, where 84F will occur. This will easily exceed the daily record at KCHS, and will also approach the February highest on record. (See CLIMATE section below). The cool SST`s of the ocean will limit max temps along coastal sections to the upper 60s/lower 70s north, lower-mid 70s south. In addition to the record warmth, gusty SW-W breezes will occur during the afternoon with deep mixing allowing for speeds to reach 15-25 mph, with gusts as high as near 30 mph in parts of the CWFA. Tonight: Cold advection quickly fills in behind the departing cold front in the Atlantic, as skies become clear. A gigantic diurnal swing in temps will transpire, with lows by Sunday morning down some 40F colder than they will be today. Tonight will be the coldest night for many of us in about a week, as high pressure builds in from the W. Lake Winds: Deep mixing this afternoon, followed by CAA late today into tonight, will lead to marginal Lake Wind Advisory conditions on Lake Moultrie this afternoon into early tonight. && .SHORT TERM /SUNDAY THROUGH TUESDAY/... Sunday: High pressure will become centered over the Southeast, leading to all day sunshine and light wind. Overall, temps will be closer to normal for this time of year. In general, high temps will range in the low/mid 60s, warmest away from the coast in Southeast Georgia. Monday: Dry high pressure centered over the Southeast will slide offshore, allowing a south/southeast wind to return moisture and warmer temps to the region. We think the morning should be dry but conditions will be changing rather rapidly Mon afternoon as deep layered moisture builds in from the Atlc and Gulf of Mexico ahead of an advancing baroclinic zone along the Gulf Coast and northern GA. Warmer temps will be accompanied by increasing chances for showers over southeast GA and perhaps isolated thunderstorms over south GA. Scattered showers could even reach southeast SC later in the day around sunset. Monday Night: Unsettled weather becoming more likely each forecast package. A series of mid level impulses will ride over a zone of deeper moisture and low level moisture convergence producing scattered to locally numerous showers spreading northward along a warm frontal feature. We bumped pops up to 50 percent many areas with some likely POPs along coastal SC to trend. Lows will be quite mild in the 57-60 degree range most areas. Tuesday: The rains on Monday night should have moved off to the northeast, setting the stage for a warm day. Warmest temps will be over southeast GA with lower to mid 80s common under partly sunny skies in the afternoon. There should be a bit more cloudiness over northern SC zones but highs will still reach the upper 70s to around 80. Most of the convective precip will be west of the region but an isolated shower or late day tstm is possible to the north of I-16. && .LONG TERM /TUESDAY NIGHT THROUGH FRIDAY/... A long wave upper trough will advance from the Great Plains to the Tennessee Valley Tuesday night through Thursday. There is decent model agreement that an advancing cold front will reach the forecast area either late Wednesday night or early Thursday with the GFS running just a tad faster than the ECMWF. March will come in with potential record warmth given 13C-15C 850 MB temps in southwest synoptic flow. High temps could soar to the mid 80s with periodic breaks in the clouds. Higher rain chances should come ahead of the cold front either Wednesday night or early Thursday. Despite increasing deep layered shear on Wednesday and Wednesday night, severe potential still looks rather low-end due to model differences in frontal timing and marginal instability. In the wake of the front, cooler and closer to climo temps are expected Friday and Saturday with dry weather both days. && .AVIATION /16Z SATURDAY THROUGH WEDNESDAY/... A cold front could produce a brief light shower at KCHS or KSAV between about 17Z and 20Z, but no significant impacts are expected. Of more concern will be gusty W-SW winds that occur both in advance of a cold front, but more especially behind the front through sunset. Speeds will reach 15-20 kt with gusts around 25 kt during the afternoon hours. Winds are expected to veer to the NW and ease overnight. Otherwise, VFR will prevail. Extended Aviation Outlook: VFR conditions are expected into Monday morning. Another risk of flight restrictions is possible late Monday and Monday night with low clouds/showers. Low clouds are also possible Tuesday and Wednesday Mornings with both afternoon likely VFR. A cold front will move through Thursday, brief restrictions possible in clouds/showers. && .MARINE... Visible satellite imagery shows an area of low stratus and/or fog across a portion of AMZ350. With no observations within the area at this typing, it`s hard to know the impacts. Regardless, it is moving to the NE and should gradually move out of the area over the next hour or two as a cold front approaches from the west. The front will move through around 4-9 pm. Warm advection out ahead of the front limits mixing, but the land/sea interface of Charleston Harbor will give a boost to SW and W winds this afternoon. We`ll be close to requiring a Small Craft Advisory, but for now have limited winds to 15-20 kt. Seas for AMZ374 are still ~6 ft this morning, so the advisory continues. But the advisory for AMZ350 was allowed to expire as seas appear to now be maxing out at 5 ft. Cold advection behind the front tonight and decent isallobaric pressure rises will could lead to advisories for all of the coastal waters, except the Charleston Harbor. With new model data coming in shortly, opted to wait to hoist the next round of advisories until the afternoon update. At this time, AMZ350 is the most likely candidate to need one, due to wind gusts. Sunday through Wednesday: Marine conditions will improve early Sunday as high pressure builds over the area. The SCA over outer GA waters will likely come down by mid morning. Return flow on the back-side of Atlc high pressure will result in a southeast to southwest flow regime during the early to middle part of next week, advisory conditions are unlikely. Patchy sea fog is possible Tuesday into mid week but it`s too early for any mentions in the forecast just yet. A cold front moving into the waters on Thursday should result in an uptick in wind and seas with unfavorable mixing profiles until cold air advection ensues post front. Advisory conditions may be possible. && .CLIMATE... High temperatures this afternoon could approach record levels for the month of February: 86F at KCHS (Feb 28 1962) and KSAV (last set Feb 24 2012) and 83F in downtown Charleston (Feb 27 1962). Record Highs for Feb 25: KCHS: 80 set in 1949. KCXM: 80 set in 1930. KSAV: 82 set in 1985/1930. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...Lake Wind Advisory until 11 PM EST this evening for SCZ045. MARINE...Small Craft Advisory until 9 AM EST Sunday for AMZ374. && $$ NEAR TERM... SHORT TERM... LONG TERM... AVIATION... MARINE... CLIMATE...

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