Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 231628 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 1228 PM EDT Fri Sep 23 2016 .SYNOPSIS... Weak low pressure will circulate near the South Carolina coast today. The low will drift offshore Saturday and allow high pressure and drier air to build in. A modest cold front will drop south across the area Sunday, followed by high pressure through Monday. Rain chances will increase by midweek ahead of another cold front. && .NEAR TERM /THROUGH TONIGHT/... As of 1230 PM Friday...Surface low pressure is located on satellite and surface observations just west of Georgetown, SC. This swirl has been one of the most persistent weather features we`ve had in ages! Models suggest this low should slide southward tonight, eventually allowing some drier air to punch in from the north. A trough axis just off the coast separates east and southeast winds observed offshore from the more northeasterly winds nearshore and inland. Light showers inland have been decreasing most of the morning. Showers developing along the trough offshore are the most impressive ones currently. Models suggest daytime heating should be sufficient to fire new convection today given surface-based CAPE of 500-1000 J/kg and a moist, uncapped atmosphere. Very slow cell motion is indicated today, only 3-7 knots in most cases. Locally heavy rainfall is possible, especially across coastal SE North Carolina where the best low-level convergence may develop due to more insolation from breaks in the cloud cover. Given how much rain fell yesterday across coastal New Hanover and Pender counties, we`ve issued a short-term Flash Flood Watch for this area, mainly because soils are saturated due to the 5-8 inches of rain that fell yesterday near and east of Highway 17. It might only take 1-2 inches to create flash flooding today. No significant changes have been made to forecast high temps today: 81 to 84 for most areas. I have bumped up evening PoPs along the coast as models suggest showers and storms may linger past sunset before the drier air begins to bleed in from the north overnight. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... As of 300 AM Friday...Some of the drier air seen on water vapor imagery should be able to work its way into the area by Sat. However, there still will be some troffiness across the area. A backdoor cold front will approach from the N Sat night and should move across the forecast area during Sun. Will keep POPs largely below threshold on Sat with a small POP across southern areas where moisture will be slightly greater. Then will include a small POP for Sun with the front moving across the area. Precipitable water values will be dropping through the period resulting in much lower rainfall rates and we are not expecting significant rainfall during the weekend. More sunshine on Sat should bring highs to the mid and upper 80s. Highs on Sun will still be above normal, lower to mid 80s. Lows will be in the upper 60s to lower 70s Sat night and mid and upper 60s Sun night with near 70 at the beaches. && .LONG TERM /MONDAY THROUGH THURSDAY/... As of 300 AM Friday...A mid and upper level ridge axis will transition over the area Monday, ahead of a deep upper low that will be dropping across the Great Lakes. Surface high pressure build in from the north on Monday but shift offshore Tuesday. This will result in an increase of moisture off the Atlantic as winds become southeast, and expect rain chances to increase Tuesday and Wednesday ahead of a cold front which is expected to move off the coast by early Thursday. && .AVIATION /12Z FRIDAY THROUGH WEDNESDAY/... As of 12Z...A surface low is S of KMYR with a weak trough along the coast. Flight conditions will be a general repeat of the previous 24 hours. There is high confidence LIFR will lift to 14-15Z, after which MVFR will occur most of the morning. Cigs will improve to VFR by this afternoon outside of precipitation. Precipitation is currently curring in widely scattered bands rotating slowly around low pressure to the south. Satellite picture suggest enhanced clouds not as prevalent as yesterday morning so this mornings precipitation may be less. Convection will increase as the afternoon approaches along the coast and in bands rotating around the low. Confidence is low as to where exactly bands of precipitation will set up. If a band sets up directly over a terminal there could be an extended period of SHRA/IFR/VCTS. Due to low confidence of timing this is not indicated in TAFs. Have included TEMPO groups indicating MVFR and will amd as precipiation develops as necessary for IFR. Models suggest the surface low will fill and weak high pressure will build into the terminals overnight. With moist low levels, light winds and sct skies there is increasing confidence of dense fog development. EXTENDED OUTLOOK...MVFR/IFR stratus/fog possible late Friday night and Saturday morning. Expect mainly VFR Sunday and Monday, but chances for showers and tstms will begin to increase Tuesday and Wednesday. && .MARINE... NEAR TERM /THROUGH TONIGHT/... As of 1230 PM Friday...Satellite data and surface observations show the weak low pressure area centered just west of Georgetown, SC. A trough axis extending along the coast of the Carolina separates east or even southeast winds offshore from northeast winds at the beaches. The low should weaken further and sink southward later today, with mainly northeasterly wind persisting across the 0-20 nm coastal waters. Scattered showers and t-storms may diminish in coverage near Cape Fear as the main focus for storms shifts inland. Perhaps of equal importance, the atmosphere is favorable for waterspout development today. Seas are currently 2-3 feet. Long-period swells from growing Tropical Storm Karl just south of Bermuda, already showing up in spectral wave data from Frying Pan Shoals, will increase this afternoon and tonight. Combined seas could reach 4 feet in some of the outer portion of the coastal waters, mainly near and north of Cape Fear, tonight. SHORT TERM /Saturday FRIDAY THROUGH SATURDAY NIGHT/... As of 300 AM Friday...Weak surface low pressure should move further offshore. A backdoor cold front will move across the waters on Sun. The strongest wind speeds this period will be in the wake of the front, Sun and Sun night. Winds speeds will be up to 15 to 20 kt, otherwise sustained winds will be 10 to 15 kt. The wind direction will be mainly NE, becoming ENE Sun night. Seas will build to 4 to 5 ft Sun and Sun night as a robust easterly swell remains. LONG TERM /MONDAY THROUGH TUESDAY/... As of 300 AM Friday...The period will begin with northeast flow as high pressure ridges in behind Sundays cold frontal passage. As the high transitions off the mid-Atlantic coast Monday afternoon, winds will veer to the east, then south behind an inverted trough that will lift north across the waters Tuesday. Onshore flow and a moderate pressure gradient will support 15 knots of wind and 3-4 ft seas on Monday, and the continued onshore trajectory Tuesday should see seas build to 4 ft throughout, with perhaps some 5 footers in the vicinity of Frying Pan Shoals, and near 20 nm out. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...None. NC...Flash Flood Watch until 9 PM EDT this evening for NCZ105>108. MARINE...None. && $$ SYNOPSIS... NEAR TERM...TRA SHORT TERM...RJD LONG TERM...CRM AVIATION...MRR

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