Scientific Forecaster Discussion

000 fxus62 kchs 180857 afdchs Area forecast discussion National Weather Service Charleston SC 357 am EST Mon Feb 18 2019 Synopsis... an unsettled pattern is expected as a series of frontal systems affect the area through next weekend. && Near term /through tonight/... today: the flow aloft will feature deep layer, nearly zonal flow through the day. At the surface, we will start the day with a cold front situated directly across the forecast area that will quickly move offshore by the early afternoon. This front will bring with it a band of weak isolated to scattered showers, and rain chances this morning will top out in the 30-40 percent range. Rain chances will quickly come to an end this afternoon, and total rainfall amounts should be very unimpressive with a few hundredths of an inch at the most. Overcast skies should prevail for most of the day across most of the forecast area. The main exception will be the northern tier of the forecast area where skies will clear for a brief period from the north later in the afternoon. Temperatures will be tricky today. Values have surged overnight with the passage of the warm front, and many places will start off the morning in the mid to upper 60s. A decrease will take place with the passage of the front, but then areas that get some insolation this afternoon will see temperatures rise again. Forecast highs are in the mid to upper 60s in most areas, with some low 70s possible along the far southern Georgia coast. Tonight: clouds will increase through the overnight and high pressure will steadily build in from the north. This will result in an increasingly tight pressure gradient, and northeast winds will increase late with some gusts to around 20 mph possible along the coast. The forecast is expected to be dry. Temperatures will drop from the northeast late in the night as the high builds in. Lows are forecast to bottom out in the mid to upper 50s across the northern half, with low 50s across the southern half including coastal southeast Georgia. && Short term /Tuesday through Thursday/... a pronounced wedge pattern will exist on Tuesday as high pressure to the north builds down the eastern Seaboard in the Lee of the Appalachians. Meanwhile weak shortwave energy will rotate around the broad Southeast Ridge, aiding the development of a coastal trough, along with an inland surface wave. Isolated to scattered showers are expected to develop on Tuesday as isentropic ascent gradually strengthens. Better shower coverage is expected Tuesday night through Wednesday morning as the coastal trough strengthens and shifts northwest. Once the coastal trough is out of the picture, we could still see occasional rain showers through Thursday due to a series of weak disturbances moving through. Tuesday will be cooler due to the cold air damming regime and evaporational cooling from rain showers. Highs will range from lower 50s north to upper 50s south. Most of the area should end up in the warm sector by the end of Wednesday afternoon, allowing highs to reach the mid 60s inland SC and middle 70s far South. Strong warm air advection will yield well above normal temperatures in the upper 70s to lower 80s on Thursday. && Long term /Thursday night through Sunday/... unseasonably warm temperatures will continue through Sunday with only scattered showers due to occasional upper shortwaves. A strong cold front will move through late Sunday, bringing somewhat greater rain chances, followed by more normal temperatures early next week. && Aviation /09z Monday through Friday/... a distinct area of clearing has worked its way through ksav and will soon clear kchs as well. This will result in several hours of VFR conditions before clouds increase from the west with the approach of a cold front. Closer to sunrise, the forecast gets tricky as the chance for ceilings increases. For now, the thinking is that low-end MVFR conditions are most likely, but there will be a chance for IFR ceilings as well. The forecast features MVFR, but we may need to amend as trends become more apparent. A period of light showers will be possible at both sites with the passage of the front later this morning. Both sites should see a return to prevailing VFR conditions by the late morning/midday time period which should continue through the end of the forecast period. Extended aviation outlook: flight restrictions likely Tuesday night through Wednesday night due to a stalled front and scattered to numerous showers. && Marine... today: a cold front will move across the waters today, shifting low level winds from southwesterly to northwesterly late today. Wind speeds should top out in the southwesterly flow this morning, mainly in the 15 knot range. Seas should average 2-4 feet. Any lingering marine/sea fog should be patchy at best this morning. Then, once the front moves offshore, fog has been removed from the forecast starting at midday. Tonight: high pressure will steadily build in from the north overnight and the pressure gradient will increasingly tighten. This will result in an impressive northeast surge after midnight. Winds will increase into the 20-25 knot range in the early morning hours and small craft advisories will almost certainly be needed for all zones including the Charleston Harbor late. Seas will increase in response to the strengthening winds, becoming 3-5 feet late. A tight northeast gradient will likely produce Small Craft Advisory conditions over most of the waters Tuesday through Wednesday. Seas will probably linger at or above 6 ft over the offshore Georgia waters through Thursday despite winds having shifted southerly by then. Dewpoints surging into the lower 60s Wednesday night may support sea fog development. If it develops, the fog could persist into late week. && Tides/coastal flooding... the upcoming high tide will occur around 630-645 am and it looks like both Charleston Harbor and fort Pulaski will reach coastal flood advisory thresholds (7 ft MLLW in Charleston Harbor and 9.2 ft MLLW at fort pulaski). Despite the low level flow not being optimal (southwesterly) current tidal departures are running at almost 1.7 ft in Charleston Harbor and 1.6 ft at fort Pulaski. Even if this departure decreases a bit at high tide, we should solidly exceed advisory criteria at both sites. A coastal flood advisory is in effect for the entire coast until 9 am. Tides will likely reach coastal flood advisory thresholds with the high tides Tuesday through Wednesday due to strengthening northeast flow and increasing astronomical factors. At this time it appears that the highest high tide will occur Wednesday morning. Tide levels could approach warning thresholds (8 ft MLLW in Charleston Harbor and 10 ft MLLW at fort pulaski) and a coastal Flood Watch and/or warning could be needed for that particular high tide cycle. && Chs watches/warnings/advisories... Georgia...coastal flood advisory until 9 am EST this morning for gaz117- 119-139-141. SC...coastal flood advisory until 9 am EST this morning for scz048>051. Marine...none. && $$ Near term...bsh short term...jrl

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