Area Forecast Discussion
Issued by NWS Charleston, SC

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000 FXUS62 KCHS 050844 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 344 AM EST Fri Mar 5 2021 .SYNOPSIS... High pressure will prevail into next week. && .NEAR TERM /THROUGH TONIGHT/... Pre-dawn: The backdoor cold front was seen on Nighttime Microphysics satellite imagery near the Santee River and becoming more diffuse with time. However, strong high pressure building out of south-central Canada and the western Great Lakes appears to be bridging across the front, as winds have turned northerly even out ahead of the front. Aside from this wind shift, the front is coming in void of any clouds, but is altering the hourly temperature curve. Even so, expect temperatures at the beginning of the day to be very close to climatological values. For today: Very quiet conditions will prevail, with a dry northwest flow in wake of the deep trough moving into the western Atlantic, and giving way to flat ridging or a zonal flow that arrives from the west. At the surface, a meso-high develops over the Carolina`s this afternoon, while a second meso-high is found in the upper Midwest, and low pressure over eastern Texas weakens as it approaches the northwest Gulf of Mexico. Given the lack of moisture with PWat down under 1/4 to 1/2 inch, and large scale subsidence, plenty of sunshine will prevail, until some mid and high level clouds arrive from the west in association with a short wave dropping across Oklahoma and Texas toward the Arklatex by 00Z Saturday. Since there is cold advection this morning, and 850 mb temperatures of no more than about 3-5C, we didn`t go quite as warm as the low level thickness forecast. But we did lean toward the consensus of the NAM, GFS and ECMWF MOS which provided excellent guidance yesterday. This equates to maximum temperatures ranging from near 60F close to the Santee River, the lower and middle 60s most elsewhere, except for some upper 60s south of I-16 in Georgia. Winds over the coastal corridor will be breezy this morning due to the best push of cold advection in conjunction with modest isallobaric pressure climbs and a fairly tight gradient. Tonight: The meso-high initially over the region becomes absorbed within the circulation of the parent high in the upper Midwest and southern Canada. At the same time, a short wave moving toward the lower Mississippi Valley generates weak cyclogenesis in the Gulf of Mexico. This short wave begins to phase with the larger trough covering the northeast quadrant of the country. The short does dampen some as it draws closer to the local district, but the approach of this feature and the sub-tropical jet overhead will produce an increase in mid and high level clouds, that will lower and thicken through the night. There is subtle isentropic ascent that occurs overnight, but given limited moisture below 700 mb, the risk for anything more than sprinkles (if that) is less than 15%. So this doesn`t require any mention in the forecast. Temperatures will be dictated by how fast clouds thicken up into an overcast deck, but do look to be similar to or a tad colder than early this morning for actual minimums. && .SHORT TERM /SATURDAY THROUGH MONDAY/... Saturday: The mid-levels will consist of a long-wave trough over the East Coast. A short-wave will be over the Southern Plains in the morning. It will move over the Southeast overnight, as it starts to round the base of the trough. At the surface, a weak system in the Gulf of Mexico in the morning will pass to our south during the afternoon, then move offshore in the evening, and away overnight. Minimal impacts are expected from this system across our area. The reason is broad high pressure to our northwest will provide enough dry air to deflect the system away from us. Both synoptic models and the CAMS keep the measurable rainfall either to our south or over our GA coastal waters during the daylight hours. Since the models trended drier, our forecasted trended that way as well. The only POPs we have are a slight chance for McIntosh County during the day. The rest of our area will remain dry. The evening and overnight will be dry everywhere. The combination of northeast winds and cold 850 mb temperatures will cause high temperatures to be below normal. Most locations will be in the 50s with it struggling to reach the 60 degree mark across portions of GA. Overnight will be even colder. The combination of light northwest winds, cold air advection, and clearing skies will yield temperatures in the 30s for most of the area, and near 40 degrees at the beaches. If winds ease a bit and RH values creep higher, we could have quite a bit of frost across the area. For now, we introduced patchy frost. This will need to be adjusted with future forecasts. Sunday and Monday: The mid-levels will consist of a long-wave trough over the East Coast Sunday morning. The trough will move offshore Sunday night, pushed away by ridging approaching from the west. Heights will rise over the Southeast on Monday. At the surface, an elongated area of high pressure will be to the west of the Appalachian Sunday morning. The high will slowly shift to the southeast, becoming located over the Southeast U.S. by late Monday. The high will dominate our weather both days, bringing dry conditions and sunny/clear skies. Despite all of the sun, temperatures will remain below normal. North to northwest winds and cold 850 mb temperatures will yield highs in the 50s for most of our area on Sunday. The cold temperatures will carry over into Sunday night, with lows bottoming out in the 30s for most locations, and near 40 degrees at the beaches. Once again, frost formation is a concern, so we have patchy frost in the forecast. Though, it`s not out of the question some locations far inland could have freezing temperatures. High temperatures on Monday are expected to be a few degrees warmer than on Sunday, but still below normal for this time of year. && .LONG TERM /MONDAY NIGHT THROUGH THURSDAY/... Models are in excellent agreement for the long term forecast. They keep high pressure off the coast, with it`s periphery stretching into the Southeast U.S. Dry conditions with moderating temperatures are expected. && .AVIATION /08Z FRIDAY THROUGH TUESDAY/... VFR. Extended Aviation Outlook: A weak system passing to our southeast will bring a low probability of flight restrictions on Saturday. Otherwise, VFR will prevail. && .MARINE... Today: A decent N-NE surge will occur across the local waters this morning, as strong high pressure builds atop a diffuse backdoor cold front. Cold advection, a tightening of the gradient and steady isallobaric pressure rises will push winds close to Small Craft Advisory levels. However, any winds of 25 kt or greater will be too short-lived where we opted not to hoist the advisory with our latest forecasts. Winds will veer around 20-30 degrees this afternoon as they drop to around 12-18 kt by days end. Seas will build as large as 3-4 feet within 20 nm of the coast, and up to 4 to 5 feet further offshore. Tonight: High pressure will be the dominant surface feature, while weak low pressure forms in the north-northwest Gulf of Mexico. Even though there is a gradual tightening of the gradient, NE winds will hold below 15 or 20 kt, while seas average 2-4 feet. Saturday: A weak system will pass to our south during the day, then move offshore overnight. High pressure will then approach from the northwest overnight. The interaction between these two features along with cold air advection will cause winds to increase during the evening and overnight. It`s borderline whether we`ll need a Small Craft Advisory or not for the GA waters beyond 20 nm. If so, it would be late at night for wind gusts ~25 kt. Sunday: The combination of pressure rises from broad High pressure building towards the region and cold air advection will be causing elevated winds and seas in the morning. As the day progresses, the combo will lessen, causing winds to ease. If there are any Small Craft Advisories up, they`ll be taken down in the afternoon. Conditions will continue to improve into the evening and overnight. Monday and Beyond: Broad high pressure will bring tranquil marine conditions. No marine headlines are expected. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...None. && $$ NEAR TERM... SHORT TERM... LONG TERM... AVIATION... MARINE...

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