Area Forecast Discussion
Issued by NWS Charleston, SC

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789 FXUS62 KCHS 242151 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 551 PM EDT Sat Mar 24 2018 .SYNOPSIS... A weak warm front will drift north across the forecast area through this evening. A strong backdoor cold front will push southward through the entire area on Sunday, followed by cool high pressure into mid week. Warmer conditions arrive towards the end of the week before another cold front pushes through the area Friday. Dry high pressure will then return over the weekend. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... Although the sea breeze is obscuring the exact position on the warm front, it appears to be located north of a line from Beaufort to Allendale as of early evening. There has been a few sprinkles or virga near the front along the CSRA, but given such large dew point depressions the risk of anything measurable this evening is close to zero. The latest Hi-Res guidance suggests that the onset of the showers will be delayed by about 1-3 hours from the previous forecast, and we have thus slowed down the beginning of any rains until after 1 or 2 am, perhaps even a little later if the slower trend continues. However, we did not alter the late night/p[re-dawn coverage of showers, still showing 12 hour PoP as high as 60-80% north of I-16 in GA, with 50% PoP so of there through 12Z Sunday. Probability of thunder/lightning is still low enough where we have not added to the forecast, but given Showalter values less than zero and decent mid level lapse rates around 6C/km, the risk is non-zero. Previous discussion... As of 330 PM: Latest sfc analysis indicated that a warm front was lifting north across the Savannah River Valley. Near term guidance shows the warm front lifting north of Berkeley County by late this evening. NAM12 shows the H85 temperature over KCHS warming from 5C at 21Z to 11C by 9Z Sunday. Conditions across the CWA this evening should see slowly cooling temperatures and isolated showers across the CHS Tri-County. Tonight: GFS1deg shows a large field of mid level Q-vector convergence passing over the Carolinas and Georgia between 6Z-12Z. At the sfc, a frontal wave is expected to ripple along the old warm front late tonight, likely off the Grand Strand between 9Z-12Z. NAM12 shows strong isentropic lift along the front along and ahead of the frontal wave between 295K-305K. Circulation around the sfc low should aid in driving the wedge front southward through the rest of the night. HRRR simulated radar times a band of showers southward across the SC Lowcountry through 8Z, reaching SE GA after 8Z. I will increase PoPs to categorical from Jenkins County east across Charleston County. QPF tonight should range from around 0.30" over the CHS Tri-County to a tenth or less south of the Savannah River. Using a blend of MOS, low temperatures are forecast to cool to the low 50s across Berkeley County to near 60 along the Altamaha River && .SHORT TERM /6 AM SUNDAY MORNING THROUGH TUESDAY/... Sunday: The mid/upper levels will consist of a trough off the Northeast Coast and a ridge with it`s axis stretching from the Great Lakes Region into the Lower MS Valley. Both features will be amplifying during the day and night. Generally, flow over our area will be from the northwest. At the surface in the morning, there will be two lows, one roughly over TN and the second off near the Outer Banks of NC. Attached to these lows will be a cold front. As time progresses, the TN low will move southward and dissipate into the evening. The other low will rapidly move to the ENE. Additionally, the cold front will move southward, passing through our area during the day, then becoming positioned to our south by the evening. Very strong high pressure north of New England. It`s southern periphery will help to push the aforementioned front southward. There will be plenty of moisture in place with the front, with PWATs ~1.4". Lift from the front is decent, enhanced a bit by waves moving near or overhead and some jet stream support. The result will be showers for the entire area, moving southward with the front in time. Though, models indicate they may hang around the southern portion of our forecast area into the overnight period. Hence we have higher POPs persisting here into the night. Measurable rainfall is expected across the entire area. There is very little, if any instability with the front, so we`re not forecasting any thunderstorms. Temperatures will be highly dependent on the position and southward movement of the front. The bust potential is high. The current thinking is highs in the far northwest tier in the mid/upper 50s, mainly on the back end of the front. Further south, highs could be well into the 60s with maybe even a few 70s near the Altamaha. These will probably be morning highs, with temperatures falling in the afternoon. Lows will generally be in the 40s, lowest in the far NW tier and highest at the beaches and near the Altamaha. Monday: The mid/upper levels in the morning will consist of a trough off the Northeast Coast and a ridge with it`s axis stretching from the Great Lakes Region into the Lower MS Valley. By night, the trough will move be pushed offshore as the ridge moves eastward. The ridge will be amplifying, with it`s axis generally located from the northeast Gulf of Mexico stretching northward, in lee of the Appalachians, all the way into New England. The flow over our area will be from the northwest, shifting to the north northwest. At the surface, very strong high pressure will be moving SSE from Canada, making it into New England by night. A weak high may try to form in the Mid- Atlantic region, but the southern periphery of the initial high will stretch into our forecast and impact the weather. Models indicate >1" PWATs across our southernmost forecast area in the morning, getting pushed southward by drier air building in from the north. Models indicate some showers sticking around in the southern portion of our forecast area, despite the front being far away and no impressive forcing nearby. Hence we have elevated POPs here tapering northward. POPs also decrease into the afternoon. Highs will be more comparable to January due to the cold air advection within northeast surface winds, struggling to reach the upper 50s in most locations. Lows will be a bit closer to normal, aided by the mostly cloudy skies. Tuesday: The mid/upper levels will consist of an trough off the East Coast and a ridge with it`s axis stretching from the northeast Gulf of Mexico stretching northward, in lee of the Appalachians, all the way into New England. The flow over our area will be from the north northwest. At the surface, very strong high pressure over New England will move out to sea. However, high pressure in the Mid-Atlantic region will strengthen, with it`s southern periphery stretching into our forecast area. Subsidence and plenty of dry air will lead to a dry forecast. Highs will be warmer as winds shift to the east, but still several degrees below normal. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... A wedge of high pressure will slowly weaken and shift offshore Wednesday, supporting southerly winds well in advance of low pressure developing over the South-central United States through Thursday. The pattern will likely result in a noticeable warmup Wednesday and Thursday, with highs approaching the mid/upper 70s Wednesday afternoon and upper 70s/lower 80s Thursday afternoon. Overnight lows will also favor a warming trend, initially dipping into the upper 40s/lower 50s Tuesday night, then mid/upper 50s Wednesday night, followed by upper 50s/lower 60s Thursday night. The next chance of precip, including a few thunderstorms should come on Friday as a southerly wind advects moisture over the Southeast in advance of a cold front shifting through the region. Temps should peak in the mid 70s for most areas. Dry and slightly cooler weather should then return on Saturday as high pressure builds into the area behind the departing front. && .AVIATION /22Z SATURDAY THROUGH THURSDAY/... A weak warm front will drift northward across the forecast area through early this evening. A frontal wave is forecast to track east along the front late this evening. The low will shift off the coast of NC by late tonight, resulting in a backdoor cold front to drive southward across the forecast area through the rest of the TAF period. Winds are forecast to remain from the SSE through this evening, turning from the N shortly after sunrise Sunday, then from the NE by the end of the 18Z TAF period. Forecast soundings indicate that MVFR stratus will develop during the predawn hours, then lowering to IFR during the mid to late morning. Showers should become widespread around sunrise Sunday, especially across KCHS. Extended Aviation Outlook: Flight restrictions are likely at both CHS and SAV terminals through Monday as a backdoor cold front pushes through the area with low clouds and rain/showers. VFR conditions should return at both terminals Tuesday and persist through Wednesday. && .MARINE... Tonight: Latest sfc analysis indicated that a warm front was lifting north across the SC waters as of early evening. Near term guidance shows the warm front lifting north of Charleston County waters by late this evening. SE and S winds will gradually strengthen to around 15 kt through this evening, a bit stronger across AMZ374. Low pressure will slip off the Grand Strand late tonight, resulting in a backdoor cold front to surge southward during the pre dawn hours. Winds will veer to the SW at 15 or 15-20 kt late tonight, gusts reaching into the low 20 kts. Wave heights are expected to build from 2-3 ft this evening up to 3-4 ft across the near shore waters (a few 5 footers over AMZ350) and generally 4-5 ft across the outer GA waters. Sunday through Thursday: A backdoor cold front will quickly shift over the waters on Sunday, supporting conditions that deteriorate from north to south as strong cold air advection and a pinched pressure gradient develop over the Southeast. Small Craft Advisories are likely for all waters by Sunday night, which should then persist into Monday night/early Tuesday. North/northeast winds should gust as up to 25-30 kt while seas build to 5-7 ft in nearshore waters and 7-10 ft in offshore Georgia waters. A period of Gales is possible for some of the waters Sunday night into early Monday. Conditions are expected to improve Tuesday as high pressure expands over the area and the pressure gradient relaxes, but high seas could linger in offshore Georgia waters through midweek. Elsewhere, winds/seas should then remain below Small Craft Advisory levels Wednesday and Thursday well ahead of a cold front advancing toward the region early next weekend. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...Small Craft Advisory from 2 PM Sunday to noon EDT Tuesday for AMZ350. && $$ NEAR TERM... SHORT TERM... LONG TERM... AVIATION... MARINE... is the U.S. government's official web portal to all federal, state and local government web resources and services.