Area Forecast Discussion
Issued by NWS Charleston, SC

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000 FXUS62 KCHS 091435 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 1035 AM EDT Sun May 9 2021 .SYNOPSIS... High pressure will prevail through early week. A cold front will drop south through the region Tuesday and linger through the end of the week as high pressure remains to the north. && .NEAR TERM /THROUGH TONIGHT/...
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Update: Observations indicate temperatures are within a few degrees of the hourly forecast. However, dew points are much higher than anticipated. Similar to the past few days, we anticipate the dew points to mix out and lower rapidly in the afternoon as winds increase. We adjusted the dew point trends to account for this, and still have the afternoon dew points as advertised, mainly to keep the fire weather forecast on track. Today: A zonal flow will prevail aloft, while an expansive region of sub-tropical high pressure centered near Bermuda maintains its hold on the local area. While scattered to broken cirrus clouds will traverse the region through the day, these clouds will have no bearing on insolation, which will be in full force. That along with 850 mb temperatures climbing to 14-16C and 1000-850 mb thickness as high as 1400-1425 meters, will lead to a warm day with above climo temperatures. Inland from the coast we look for afternoon highs to peak in the mid and upper 80s, or about 2-3 degrees above normal. Shoreline communities will be a bit cooler, with highs in the upper 70s and lower 80s, the result of a fairly strong sea breeze that develops. That sea circulation, plus the tightening of the pressure gradient between the oceanic high and a frontal system from the Ohio valley to Texas, will result in breezy conditions for the afternoon. Southerly winds will reach 15-20 mph, with gusts of 25 or even 30 mph to occur. The strongest winds through the afternoon will occur over the coastal counties, including the Charleston, Beaufort and Savannah areas. Tonight: While the anti-cyclonic circulation around the Atlantic high weakens a bit, it is still strong enough to hold off a cold front to the northwest. The boundary layer will never decouple, as a 25-35 kt low level jet moves through in advance of the front. This will keep conditions quite warm, especially at the coast where nocturnal mixing should be the greatest. Lows will only reach the mid and upper 60s, with even some spots near the coast to not fall below 70F. Some of the guidance hints at a few showers making a run for the Charleston County coast late at night. But this solution is often overdone by the models, as the synoptic flow is either offshore or at best parallel to the coast. In addition, moisture is still lacking, so we have maintained a rainfree forecast everywhere, with any showers that form to remain out near the Gulf Stream.
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&& .SHORT TERM /MONDAY THROUGH WEDNESDAY/... Monday: A potent shortwave will approach from the west before pushing offshore late in the day. All the while, a cold front will shift closer to the region. Showers associated with the shortwave could reach our interior SE GA counties by daybreak, though accumulation should be minimal. While there are still inconsistencies within the models of timing and rain coverage, light showers will be possible throughout SE GA and SE SC by afternoon. Although the instability is lacking, the area will remain comfortably in the warm section and we could see isolated thunderstorms, mainly in the afternoon. Highs will be in the 80s. Lows will be in the 60s. Tuesday: The cold front situated just to the north will edge its way toward the area as high pressure helps push it southward. Unsettled weather is expected and showers could persist throughout the day, though the precipitation coverage will greatly depend on the timing of the cold front. The biggest challenge of the day will be the extent of thunderstorms and the severe weather risk. While some model guidance suggests surface-based instability values of 2000 J/kg, others have instability maxing out around 800 J/kg. Though, a stronger storm or two cannot be ruled out. Highs will be in the mid to upper 70s across SE SC and low to mid 80s in SE GA. Lows will be mid to upper 50s inland and low to mid 60s along the coast. Wednesday: Once the aforementioned front sags just south across Florida and the waters, unsettled weather will continue for much of the day. Shortwave energy will be best on Wednesday and with sufficient isentropic lift, showers and isolated thunderstorms are expected. Though, thunderstorms should be limited to far SE GA and over the waters after daybreak. Temperatures could be a bit tricky and cooler than currently forecast as a wedge pattern could set up to the north. Highs will be in the 60s to low 70s. Lows will be in the 50s. && .LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/... Wednesday night through Friday: An unsettled weather pattern will continue throughout mid to late week as the stationary front sags further south and high pressure pushes into the area from the north. PWATs will remain above an inch and shortwave energy will be present. Showers will be possible overnight Wednesday. The best chance for widespread precip will be Thursday afternoon based on 00Z model runs. The instability is again lacking so have left out any mention of thunder. While the risk of severe weather is quite low, there could be heavy rainfall. By daybreak Friday, the majority of the moisture looks to be offshore, though there could be some lingering showers so have maintained slight chance POPs. Then, Friday night should be rain-free. Thursday high temps will be in the mid to upper 60s to low 70s. For Friday, temps will warm slightly and be in the 70s. Lows each night will be in the low to mid 50s inland and upper 50s to low 60s along the coast. Saturday and Sunday: High pressure will prevail throughout the weekend and sunshine will return. High temperatures will generally be in the low to mid 80s inland with mid to upper 70s just along the coast. Lows will be in the 50s to 60s. && .AVIATION /15Z SUNDAY THROUGH THURSDAY/... VFR conditions will prevail through 12z Monday. South winds will become gusty in the afternoon and continue into tonight. Sustained winds will average 15-18 kt, with gusts around 22-25 kt after 17z. Extended Aviation Outlook: Flight restrictions will be possible during showers/thunderstorms and low clouds as a cold front impacts the area through late week. && .MARINE...
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Wind observations for the Charleston Harbor are already running a few kt higher than anticipated, so we adjusted the forecast to account for this. If this trend keeps up, then we may need to start the Small Craft Advisory earlier than advertised. Today and tonight: A huge region of high pressure that covers much of the west and southwest Atlantic will maintain its influence across the local waters. The combination of a tightening of the gradient between the high and an upstream cold front, the afternoon and evening sea breeze circulation, plus a 25-35 kt low level jet that moves through tonight, will lead to strengthening winds all waters. Utilizing a mixture of geostrophic winds of 30-35 kt and forecast soundings, we have opted to hoist Small Craft Advisories for all waters, including the Charleston Harbor starting this afternoon. S and SW winds will be as high as 20 to 25 kt, with seas peaking at 3-6 feet. Monday through Friday: A southerly flow will persist on Monday with winds around 10-15 knots with gusts to 20 knots ahead of an approaching cold front. Seas are expected to remain below 6 ft. On Tuesday into early Wednesday, a wind shift will occur, though wind gusts should be less than 15 knots and seas 2-3 feet. Then on Wednesday, an uptick in winds will occur and Small Craft Advisories could be needed for both northeast winds and seas for all marine areas, including the Charleston Harbor. On Thursday, winds and seas should fall below SCA criteria, though winds could still gust to 20 knots through Friday. Rip Currents: There is a moderate risk of rip currents in effect for all beaches today, mainly due to the stronger winds, the warmer temperatures and astronomical influences. There will also be a fairly strong south to north moving longshore current.
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&& .FIRE WEATHER... Relative humidity values are expected to drop into the 25 to 30 percent range across much of the inland sections of southeast South Carolina and southeast Georgia this afternoon into the early evening. At the same time south winds will increase to 15 to 20 mph, with gusts around 25 mph. These conditions along with marginal fuel moisture has prompted us to issue a Fire Danger Statement, with an enhanced fire danger to exist. Closer to the coast the winds will actually be stronger, but relative humidity levels will be higher. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...Small Craft Advisory from 4 PM this afternoon to 11 PM EDT this evening for AMZ330. Small Craft Advisory from 4 PM this afternoon to 2 AM EDT Monday for AMZ352-354. Small Craft Advisory from 4 PM this afternoon to 5 AM EDT Monday for AMZ350-374. && $$ NEAR TERM... SHORT TERM... LONG TERM... AVIATION... MARINE... FIRE WEATHER...

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