Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 291651 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 1251 PM EDT Thu Sep 29 2016 .SYNOPSIS... A cold front approaching from the west will slow down on Friday and linger near or along the Carolina coast through this weekend. Periods of showers and thunderstorms are possible along this front. High pressure will build in behind the front through early next week. && .NEAR TERM /THROUGH TONIGHT/... As of 1000 AM Thursday...Very active morning is expected to transition to a busy afternoon as thunderstorms continue to develop and expand across the CWA. A Flash Flood Watch has been issued for the ILM NC counties, and is in effect until 10 PM. Deep cutoff upper low across IN/OH continues to spin mostly in place, while pieces of vort energy, "spokes," rotate around the main cutoff. At the same time, much drier air is evident on WV imagery rotating through GA and into SC, but this will be impeded in much eastern progress, so the eastern half of the CWA will remain quite wet today. Thunderstorms with very heavy rainfall have been ongoing since daybreak, and have expanded back to the SW through late morning while motion remains quick to the NE. While individual cells move quickly NE, backbuilding and training is forecast, hence the recently issued FFA. Freezing levels near 14 kft combined with moderate MUCape is creating efficient warm rain processes, and with PWATS around 1.7 inches, this is coalescing as very heavy rainfall. SPC Mesoanalysis shows good wind/moisture convergence at 850mb, which combined with moderate diffluence aloft on the lee side of the upper low, is allowing strong lift, which will only increase this aftn with heating and increasing instability. While periods of heavy rainfall leading to flash flooding is likely, and is supported by recent ARW and RAP guidance, the exact location for any significant rain may be difficult to pinpoint. Expect a rain axis to continue where it currently exists along the best low-level wind convergence and along the moisture axis from east to west, but otherwise areas of thunderstorms with heavy rainfall will develop across eastern portions of the CWA through this evening. The "fly in the ointment" to this is a clearly evident hybrid outflow/cold front sinking southward across the Pee Dee this morning. While this boundary may initially force updrafts in the warm sector due to surface convergence, air behind it is cooler and more stable than to the SE of it, and may be limiting both to temperatures and rainfall potential. However, deep southerly flow through the column should eventually slow and stall the progress of this boundary, causing it to lift back to the north this aftn. A very complex forecast however, and bust potential even in the near term is high for temps and precip near this wavering boundary. Highs today are expected to rise into the low to mid 80s, but may fluctuate quite a bit in heavy rainfall and depending on where that front wavers this aftn. While convection is forecast to wane after dark, warm southerly flow will persist, keeping mins from falling out of the low 70s near the coast, upper 60s inland in the drier air. && .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/... As of 345 AM Thursday...To no surprise, the cutoff upper low and mid-level vorts, like spokes on a bicycle tire, rotating around it will be the driver of the sensible weather across the ILM CWA this period. The good news, models have this cutoff/closed low slowly opening up and lifting northward by the end of this period as a result of the strong westerlies now able to move this upper feature. At the sfc, the cold front will slowly make some progress to the coast on Friday and on Saturday it basically stalls either along the immediate coast or just offshore due to it becoming parallel with the flow aloft. Therefore POPs will be confined along the eastern 1/3rd of the ILM CWA and the adjacent Atlantic waters thruout this period. The dry slot or tongue will affect mainly the western 2/3 of the ILM CWA helping to eliminate POPs in that area. Could see hier POPs during the pre- dawn hrs along the immediate coast during the pre- dawn Sat and Sun hours due to the combined nocturnal development over the waters and any mid- level vorts having either rotated around the cutoff low or picked up off the Florida west coast or the northern Bahamas. Max/Min temps to remain above normal, 1 to 2 categories, thruout this period. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... As of 3 AM Thursday...Large upper low finally lifting out to the northeast early in the period. In its wake will be a weak pressure gradient and a dissipating frontal boundary just offshore. A few showers along this front may affect coastal locales but suspect that most areas stay rain-free and seasonable. As the upper low begins to interact with troughiness east of Canada rising surface pressures advect into the eastern U.S. Tuesday and Wednesday. Should this flow be NE then some lower dewpoints may creep into the Carolinas but current progs show more ENE and likely preclude that. Building heights aloft argue for not much in the way of meaningful precip though a few sprinkles tough to rule out in the onshore low level flow. This gradient could become pinched by Matthew and it may become breezy locally by Wednesday if the quick GFS is correct. && .AVIATION /18Z THURSDAY THROUGH TUESDAY/...
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As of 18Z...SCT +SHRA/TSRA from KILM to KLBT, isolated elsewhere through 21Z. AFT 21Z mainly VCSH all terminals, light south to southwest wind overnight. Within strong isolated SHRA this TAF cycle, expect localized surface wind gusts to 30 KT. Mainly VFR this period with occasional MVFR ceiling in and near convection and patch VSBY restrictions of 1-3 SM between 9z-12z Friday. 12z- 18z Z VCSH continues across all terminals. EXTENDED OUTLOOK...Primarily VFR. Isolated to scattered afternoon showers through much the period, although limited in strength and coverage due to dry air aloft.
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&& .MARINE... NEAR TERM /THROUGH TONIGHT/... As of 1000 AM Thursday...Surface cold front will remain inland today, keeping predominantly S/SW winds across the waters. Wind speeds have been 10-15 kts this morning, and little change in speed is forecast, although occasional gusts up to 20kts are possible, especially this evening and tonight. Seas will remain a nearly uniform 3 ft, thanks to a 10-11 sec SE swell and 5-6 sec southerly wind wave. SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/... As of 400 AM Thursday...The movement of the cold front at a snail`s pace will push it to the coast late Friday and possibly either across the coastal waters or just offshore from Carolina coastlines. The sfc pressure pattern will result in a S to SW wind direction thruout this period. The sfc pg will remain semi-tightened east of the cold front, with 10 to 15 kt speeds, and somewhat relaxed in the vicinity of the cold front, with 10 kt or less speeds. Significant seas to continue at 2 to 3 ft thruout this period. The ese ground swell at 7 to 9 second periods will mainly dominate the seas with local wind driven waves at 3 to 4 seconds on top. Expect convection thruout this period, with the pre-dawn hrs thru daytime morning hrs of each day exhibiting the hier pops during this period. LONG TERM/SUNDAY THROUGH MONDAY/... As of 3 AM Thursday...Weakening front along the coast Sunday will keep winds very light though there is some uncertainty regarding direction-though primarily onshore. Small wind chop combine w ESE swell for a 2 to 3 ft dominant wave height. No significant changes noted heading into Monday. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...None. NC...Flash Flood Watch until 10 PM EDT this evening for NCZ087-096- 099-105>110. MARINE...None. && $$ SYNOPSIS...Bacon NEAR TERM...JDW SHORT TERM...DOUGH LONG TERM...Bacon AVIATION...Colby

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