Area Forecast Discussion
Issued by NWS Charleston, SC

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000 FXUS62 KCHS 231141 AFDCHS Area Forecast Discussion National Weather Service Charleston SC 641 AM EST Thu Nov 23 2017 .SYNOPSIS... A series of low pressure systems will move up the coast today through Friday. A cold front will shift through the area Saturday night, followed by dry high pressure through the middle of next week. && .NEAR TERM /THROUGH TONIGHT/...
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As of 635 AM: Composite radar images indicate that light rainfall was spreading north across SE GA and the lower Savannah River Valley. Observations indicated that the low values of dBZ were associated with very light rain to sprinkles. I will update the forecast to adjust PoPs slightly higher and inland. In addition, I will increase sky and tweak hourly temperatures to align with observations. As of 340 AM: Regional radar composite indicated low values of dBZ spreading north across SE GA. The activity was developing north of sfc low pressure developing near the Big Bend of Florida early this morning. Recent surface observations indicated that drizzle to light rain was occurring under the radar returns. I will indicate schc to chc values of PoP gradually increasing across SE GA early this morning, then increasing across SE SC after sunrise. The axis of a mid level trough is forecast to slide east through the day, reaching the SC/GA coast late this evening. As the trough pushes east, low pressure is expected to track northeast off the Southeast coast. It appears that a compact rain shield will exist northwest of the low center as high pressure remains centered over the DelMarVa. In fact, near term guidance indicates that the H850-700 frontogenesis will track over the adjacent Atlantic waters. Given the passage of the deeper forcing and instability, the greatest rainfall rates will remain off shore today. It appears that rainfall totals from today through tonight will peak around a quarter of an inch across the coastal counties, further inland will struggle to see measurable rainfall. Steady NE winds and thick cloud cover should limit high temperatures in the upper 50s. Tonight: Low pressure will slowly track northeast, leaving at least partly cloudy conditions across the CWA through the night. Steady NE winds should strengthen slightly late tonight as the pressure gradient increases in the wake of the low. However, the GFS and NAM indicate very weak H85 CAA late tonight, generally cooling around a degree during the pre-dawn hours. A few weak showers will likely linger near the coast for most of the night, but conditions should become dry toward dawn Friday. Using a blend of MOS, low temperatures should range from the low 40s inland to around 50 across the beaches.
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&& .SHORT TERM /FRIDAY THROUGH SUNDAY/... The latest model suite is in better agreement on a less amplified upper shortwave moving through on Friday which translates to a slightly weaker and farther east coastal low track. We held onto slight chance showers along the immediate coast Friday with greater coverage over the waters, though even this might be a tad overdone if the 00Z GFS verifies. Cold advection will yield highs in the low to mid 60s. Dry weather is anticipated Saturday through Sunday. A broad upper trough pattern will persist, with weak surface high pressure moving east of the area on Saturday, allowing a southwest flow to bring warmer temperatures in the upper 60s. A potent northern stream shortwave will drop through the area Saturday night, pushing a dry cold front past the area. Expansive dry high pressure will spread over the southeast United States on Sunday, dropping high temps a few degrees. && .LONG TERM /SUNDAY NIGHT THROUGH WEDNESDAY/... Dry high pressure will prevail Sunday night through Wednesday. A cold front may approach late Wednesday or on Thursday, potentially bringing scattered showers to the area. Sunday and Monday will be slightly cooler due to the influence of cold high pressure. Temps warm up during mid week as the high shifts east and southerly flow develops. && .AVIATION /12Z THURSDAY THROUGH MONDAY/...
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Periods of light rain will pass over the terminals today into this evening, the greatest rates should occur between 20-24Z. Cloud bases should lower from around 10 kft around TAF onset to around 3.5 kft by midday. MVFR ceilings should accompany the heavier rainfall rates late this afternoon and early evening. In addition, the center of sfc low pressure is forecast to pass off the GA/SC coast between 21-03Z. Tonight, VFR conditions with steady NE winds is expected at KCHS and KSAV. Extended Aviation Outlook: MVFR ceilings could continue into Friday at both terminals, then VFR returns.
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&& .MARINE... Sfc low pressure is expected to develop near the Big Bend of Florida early this morning, as high pressure remains centered over the DelMarVa region. The high is expected to move very little through tonight. The sfc low is forecast to track NE off the GA and SC coast today into tonight. This pattern will support gusty NE winds across the marine zones today and tonight. Wave guidance indicates that wave heights will increase through this morning, with 6 foot seas spreading into the nearshore zones this afternoon. Winds are forecast to increase this evening and tonight, with gusts around 25 kts common. A Small Craft Advisory will be in effect outside the CHS Harbor afternoon noon for the nearshore waters, outer GA waters will remain in effect through the period. Marginal Small Craft conditions will extend into Friday over the nearshore waters and into Friday night for the offshore GA waters, mainly due to some residual 6 ft seas. Then, sub- advisory conditions will prevail through at least the middle of next week as high pressure takes hold. && .CHS WATCHES/WARNINGS/ADVISORIES... GA...None. SC...None. MARINE...Small Craft Advisory from noon today to noon EST Friday for AMZ350-352-354. Small Craft Advisory until 3 AM EST Saturday for AMZ374. && $$ NEAR TERM...NED SHORT TERM...JRL LONG TERM...JRL AVIATION...JRL/NED MARINE...JRL/NED

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