Area Forecast Discussion
Issued by NWS Charleston, SC

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000 FXUS62 KCHS 202350 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 650 PM EST Thu Feb 20 2020 .SYNOPSIS... Developing low pressure offshore will move northeast tonight. High pressure will then move into the area on Friday, bringing cold, drier air through the weekend. Another warm front will move into the area early next week, bringing more rain to the area. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... The much anticipated cyclogenesis off the Southeast U.S. coast is well underway with 20/23z surface objective analysis placing the low about 170 miles east of Tybee Island, GA. Widespread light moderate rains associated with the low with support from a shortwave digging across the Ohio Valley and a powerful polar jet across the Deep South will persist across southern South Carolina into Southeast Georgia through at least midnight before slowly winding down from west-east through the overnight as the surface low pulls propagates farther offshore. The heaviest of the rains will fall over the next 3-4 hours before tapering off. High resolution guidance has trended slightly lower with the ending of the precipitation across the Charleston Tri-County area late tonight. With this slower movement, there is some concern with p-type as temperatures fall into the lower-mid 30s north of the I-26 corridor. High resolution soundings with support from both the 20/18z NAM3km and GFS show there should be just barely enough moisture left over in the dendritic growth zone (DGZ) to support some ice nucleation. Soundings at KCHS, KMKS and Jamestown in the 09-13z time frame show a sub-freezing layer above the boundary layer so expect hydrometers to remain frozen before melting just before reaching the ground. This scenario will support either a rain-snow mix or possibly a brief burst of very light snow before as the precipitation moves offshore, mainly along/north of the I-26 corridor. Sleet appears unlikely given the depth of the cold air. No impacts are expected given the warm ground temperatures and this should be more of a novelty event this far to the south. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH SUNDAY/... High pressure will continue to build into the area through Friday. A strong pressure gradient will support gusty winds up to 25 kts through Friday night into Saturday morning over Lake Moultrie. Temperatures will continue to be below normal, with lows in the upper 20`s and low 30`s through the weekend. High pressure will then prevail through the weekend, bringing quiet weather. Sunday will see temperatures in the upper 50s/low 60s. && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... A warm front is forecast to lift north through the forecast area on Monday as low pressure lifts into the Ohio River Valley. Some rain could accompany the front, but the best rain chances will come late Monday night and Tuesday as a cold front passes through. Another stronger front could approach on Wednesday as a deep upper trough moves into the eastern U.S. Temperatures through the period are expected to be near to above normal. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/... High-end IFR to low-end MVFR cigs will linger through the evening before lifting to more solid MVFR after midnight as rain begins to diminish. VSbys of 2-4 miles will persist before returning to P6SM as the rain wanes. There is a slight chance rain could mix with or briefly change to snow at KCHS just before daybreak as the precipitation ends, but there will be no impacts. VFR will return by mid-morning as skies begin to clear. Gusty winds will persist through the 00z TAF period with gusts 20-25 kt at both KSAV and KCHS. Extended Aviation Outlook: VFR will prevail through the weekend with gusty winds through Friday night. Some restrictions likely starting Monday. && .MARINE... Tonight: Solid Small Craft Advisory conditions will give way to Gale Warning conditions outside Charleston Harbor as the pressure gradient intensifies due to low pressure strengthening just east of the waters. We do not have a Gale Warning in Charleston Harbor at this point but certainly can`t rule out a few gale force gusts near the mouth of the harbor. Waves will build substantially with 6-9 foot seas beyond the pilot buoys and 10-12 ft out closer to the warm Gulf Stream waters. Friday through Tuesday: Winds will remain quite gusty through Saturday evening. Gale warnings will be in effect through Friday evening for all Charleston waters except the Charleston Harbor, which has a small craft advisory. As the pressure gradient relaxes, small craft advisories will be needed to replace the gale warnings which should remain in effect until Saturday afternoon/evening for all waters. No concerns expected Sunday into Monday, however an uptick in winds and seas is possible on Tuesday ahead of the next low pressure system. && .TIDES/COASTAL FLOODING... Strong northeast winds are likely to cause minor coastal flooding with the Friday morning high tide along the SC coast, possibly Saturday morning as well. && .EQUIPMENT... There will be a 03Z special sounding released in support of the NASA IMPACTS winter storm project. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...Lake Wind Advisory until 7 PM EST Friday for SCZ045. MARINE...Gale Warning until 5 PM EST Friday for AMZ350-352-354. Gale Warning until 10 PM EST Friday for AMZ374. Small Craft Advisory until 7 PM EST Friday for AMZ330. && $$

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