Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 170534 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 1234 AM EST Fri Nov 17 2017 .SYNOPSIS... A slight cool down will come overnight tonight into Friday as Canadian high pressure builds in. The high will migrate east, moving offshore Friday night allowing temperatures to warm above normal on Saturday, the first time that has happened in about a week. Showers will accompany the passage of a strong cold front Saturday night. Much colder and drier air will be ushered in for Sunday and into early next week and we have a reasonable chance of a widespread freeze inland. Temperatures will moderate beginning Tuesday with dry weather expected next week. && .NEAR TERM /THROUGH TODAY/... As of 945 PM Thursday...Just some minor tweaks to hrly temps, dewpoints and winds mainly prior to the N to NE surge that takes place during the Friday predawn hours. Winds active, do not expect any radiational cooling conditions ie. winds decoupling, thus frost should not be a concern. Previous..................................................... As of 300 PM Thursday...Canadian high pressure will build down over the Carolinas through Friday as it migrates eastward. The mid to upper trough over New England will lift off to the northeast pushing cold front farther off to the east, allowing for a decent northerly surge of cool and dry Canadian air to move over the area. Initially, winds may drop off after sunset as atmosphere decouples, but after midnight, expect a decent surge moving in from the NW. Dewpoint temps will drop into the low to mid 30s most places by daybreak Fri. At the same time, winds will pick up keeping boundary layer mixed mainly after midnight. This should prevent problems with frost overnight. With that being said, any wind sheltered places could see some patchy frost but the surge of drier air and spikes in wind should prevent it. The best CAA will come after midnight with temps falling to 38 to 42 most places. The 850 temps up near 10c this aftn will fall close to 6c overnight in CAA. As Canadian high pressure to the north shifts east through Friday, winds will lighten and veer around to the N-NE. A cooler day overall on Friday with light northerly winds and temps reaching around 60 most places with plenty of sunshine. && .SHORT TERM /TONIGHT THROUGH SATURDAY NIGHT/... As of 300 PM Thursday...Zonal flow aloft will help push a surface high pressure speedily offshore Friday night into Saturday. As the high shifts offshore Friday night, weak warm advection will begin on developing return flow, so while temperatures will be cool and decoupling of winds are likely, mins should remain just warm enough to prevent any frost as lows drop into the mid/upr 30s. At the coast, some advective stratocu is possible noted on forecast profiles and time-heights showing saturation with weak omega, and this will keep mins along the coast from falling below 40. As the high retreats to the east further on Saturday, return flow increases and the Carolinas will enter a brief warm sector ahead of a sharp shortwave and associated cold front sweeping in from the west. 850mb temps climb quickly to 10-12C Saturday evening as PWATs climb towards 1.25 inches on a strong and moist LLJ reaching 40-50 kts. While this LLJ and strong WAA will allow temps to climb into the low 70s during Saturday aftn, it also sets the stage for convection Saturday night. The actual surface front will cross the CWA after midnight to be offshore by Sunday morning, and will likely be accompanied by a band of showers and potentially isolated tstms. However, instability is limited, reaching only up to 200 J/kg of MUCape, with the unstable region confined beneath 700mb. This will occur in an environment with strengthening low-level winds, all guidance has 50+ kts at 850mb coincident with FROPA driving 0-6 km shear of 50-60 kts, combined with weakening but present sloped FGen between 925-800mb driving enhanced omega beneath the approach of the RFQ of a 130 kt upper jet. The upper dynamics appear to lag at least slightly behind those in the low-level, but expect enough forcing that iso TRW has been added to the forecast. Gusty winds and briefly heavy rain are possible in any showers before the front pushes offshore at the end of the period. The late FROPA and strong pre-frontal WAA will keep mins around 50 Saturday night. && .LONG TERM /SUNDAY THROUGH THURSDAY/... As of 300 PM Thursday...An amplified upper pattern will exist across North America next week. The forecast is fairly high confidence with two exceptions, owing to increasing model differences in the handling of significant shortwave energy into the southern branch of the jet Tuesday through Thursday. WPC prefers the ECMWF handling of this scenario next week which features a strong slow moving upper level low across the northern Gulf of Mexico by next Thursday, compared to the weaker and more progressive GFS pattern. The surface cold front Sunday morning should already be offshore by daybreak, taking any lingering showers quickly offshore before noon. Cool, northwest breezes could reach 20-25 mph Sunday as Canadian high pressure over the Plains states moves eastward. This high should be over the southern Appalachians by Monday morning, close enough to provide a night of clear skies and reasonably good radiational cooling. Lows should fall into the lower 30s inland and mid 30s at the coast. The hazardous weather outlook will continue to mention the potential of freezing temperatures. The high will slip offshore Tuesday, with return flow buckling off the Southeast as a weak coastal trough develops offshore of the FL/GA/SC coast. The ECMWF shows this feature close enough to the coast that perhaps some showers could push onshore Tuesday. Wednesday and Thursday a strong shortwave should dive into the Gulf. Differences in the handling of this feature between the GFS and ECMWF will determine the position of surface low pressure development and subsequent flux of moisture northward from the Gulf and/or Atlantic. There`s enough uncertainty at this point, and the feature may be far enough south, that differences are relatively minor. && .AVIATION /06Z FRIDAY THROUGH TUESDAY/...
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As of 06Z...Light and variable to calm winds across the area will become northerly and increase during the next few hours as a dry cold front moves south across the area and cold advection develops. A few areas of short duration patchy fog across the area will dissipate once the northerly winds pick up. Might see a brief period with gusty winds as the front passes, but upstream the gusts have decreased in both coverage and strength. This could be down to increasing low level stability and decided to pull gusts from all terminals. VFR conditions will persist through the period outside of the potential for brief MVFR or IFR in the next hour or two, mainly along the SC coast. High pressure moves overhead later today into tonight and winds will become calm around sunset. Extended Outlook...VFR with a chance of showers and possible MVFR/SHRA Saturday night into Sunday.
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&& .MARINE... NEAR TERM /THROUGH FRIDAY/... As of 1000 PM Thursday...Looking at the N to NE surge dropping across the area late this evening thru several hrs after midnight. SCEC conditions for the NC Waters still aok but will have to watch the southernmost SC Waters for possible SCEC conditions, with significant seas being the parameter in question. For now, will peak seas in the 4 foot range South of Murrells Inlet. All zones will see N to NE winds peak in the 15 to 20 kt range from the pre-dawn Fri hrs thru Fri early afternoon b4 subsiding as the center of high pressure migrates overhead late in the day and Fri night. Previous................................................... As of 300 PM Thursday...High pressure building in from the northwest tonight behind cold front will produce a decent northerly surge. Have include precautionary headlines, SCEC, after midnight tonight ending by noon on Friday. Off shore winds generally 10 kts or less through this evening will pick up after midnight as surge of dry and cool air rush in from the NW as cold Canadian high pressure builds in behind cold front. WNA shows an area of seas up to 6 to 8 ft as cooler air rushes over warmer gulf stream waters heading into tomorrow morning, but there remains a tight gradient with lower seas closer to the coast in shallow cooler waters. Overall, expect a short duration of SCEC to even borderline SCA right near the outer limits of local forecast area waters, mainly north of Cape Fear, confined to northern two zones. For now will go with SCEC for seas up to 4 to 5 ft in northerly surge entering the waters basically between 09z and 12z, toward daybreak. As high pressure reaching down from the north migrates eastward through Friday,winds will veer around to the N-NE and diminish by noon. SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/... As of 300 PM Thursday...NE winds around 10 kts will start the period as surface high pressure pushes towards the waters. This high will push offshore Saturday morning allowing winds to shift to the S/SW as return flow begins ahead of an approaching cold front. The gradient tightens in this warm sector, and winds will gradually increase becoming 15-20 kts Saturday aftn, and then 20-25 kts Saturday evening. A small craft advisory will almost certainly be needed Saturday night due to these wind speeds, and wind driven waves becoming 4-7 ft. Seas otherwise will be 2-3 ft Friday night, before slowly climbing through Saturday to the SCA thresholds mentioned late. At the very end of the period the cold front will swing across the waters with showers and locally higher winds, followed by winds shifting to the west by Sunday morning. LONG TERM /SUNDAY THROUGH TUESDAY/... As of 300 PM Thursday...A cold front should be offshore early Sunday morning. Northwest winds behind the front of 15-20 knots are expected with gusts around 25 kt during the day Sunday. Some residual 6-foot seas also will be possible near Cape Fear which may necessitate keeping a Small Craft Advisory posted for a portion of the day. Breezy north winds will continue Sunday night before the incoming Canadian High arrives into the Carolinas on Monday. This high should provide much lighter wind speeds as it moves overhead Monday afternoon and evening. The new GFS has come into better agreement with the ECMWF model about a faster veering around to E and SE winds offshore Tuesday as a weak coastal trough develops along the FL/GA/SC coastline. It remains to be seen how well-defined this coastal trough becomes, and how far west is can move. Our latest forecast does bring a period of E/SE winds into the coastal waters Tuesday, but with winds veering back to the north late Tuesday night as the trough is shown by most models to retreat offshore then. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...None. NC...None. MARINE...None. && $$ SYNOPSIS... NEAR TERM...DCH/RGZ SHORT TERM...JDW LONG TERM...TRA AVIATION...III MARINE...

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