Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 292318 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 718 PM EDT Thu Sep 29 2016 .SYNOPSIS... A stationary front will waver along the coast through this weekend. Periods of showers with an isolated thunderstorm are possible along this front. Weak high pressure will build in early next week. Hurricane Matthew may bring increasing waves by the middle of next week. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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As of 730 PM Thursday...Radar loops show a marked decrease in coverage and intensity of showers/t-storms across the area over the past couple of hours. We have dropped the Flash Flood Watch since widespread heavy rainfall no longer appears likely. Scattered showers and t-storms will continue for the next couple of hours but should reach a minimum in coverage sometime around midnight, with the bulk of the remaining activity likely offshore then. It remains to be seen if we see a rebound in coverage late tonight as a weak vorticity lobe pushes northward across the coastal waters. Latest RAP model suggests it`s wise to show a secondary peak in forecast coverage focused around 3-6 AM. Only small changes made to temperatures. Discussion from 330 PM follows... WV imagery paints the picture this aftn with a massive bowling ball of an upper low spinning across IN/KY, and our sensible weather is almost exclusively developed in response to this feature. The southern side of this low features very dry air moving across the Gulf Coast states and pushing into SC/NC, while on the eastern side moisture remains deep, and lift is being aided by diffluence at 500mb and above, creating the tremendous rain seen across portions of eastern NC today. The dividing line between these two features is a nearly stationary cold front, recognized more by a dewpoint gradient than any significant temperature advection, and while this front will try to push slowly east, it is aligned nearly parallel to the mid- level flow and will struggle to make much longitudinal gain through tonight. The combination of this cold front, significant diffluence aloft, and many mesoscale outflow boundaries has allowed convection to become deep and heavy, and a flash flood watch remains in place for all of SE NC until 10pm. A line of storms has developed along a 925- 850mb confluence boundary which is somewhat being aided by a weak sea breeze as well. Convergence along this line combined with diffluence at 300mb is driving updrafts upward, but with only marginal MLCape in an environment of high freezing levels and high PWATs, warm rain processes are likely dominating any Bergeron processes, and hence rain rates are very impressive - 2 to 3 inches per hour at times. While the mean flow is from the SW, driving storms to the NE, they continue to redevelop to the SW, so more flash flooding is possible through this evening, especially in places that haven`t been worked over already. High res ARW and RAP continue to model this pretty well, and this suggests convection will wane with loss of heating this evening. Another round of showers/storms may approach the coast overnight as another vort lobe rotates northward near the area, but most of this precip is forecast to remain just offshore. Another thing to note is that, while SPC has removed the MRGL risk locally, a hybrid outflow/warm front exists across NE SC. As cells move across this boundary they are ingesting enhanced helicity and latest SPC mesoanalysis has 100-150 m2/s2 of 0-1 km SRH. This is not abnormally strong, but have noticed several weakly rotating cells, and this will likely continue for several more hours. Local hodographs are not very supportive for any tornadoes due to primarily unidirectional flow, but local backing along this boundary is enough to keep cells rotating, with just a very minimal chance for a tornado this aftn. After convection wanes tonight, southerly winds will persist, mainly near the coast, with some light W/SW winds developing far western zones near/behind the cold front. This creates a challenging temperature forecast with a significant gradient from west to east, but expect lows near the coast to remain warm, low 70s, with some mid 60s possible well west of i-95.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH SATURDAY NIGHT/... As of 345 PM Thursday...Little change in the synoptic setup into the weekend as bowling ball of the upper low spins slowly across IN/KY before lifting into OH late in the period. At the surface, a very slow moving cold front will push eastward, likely finally crossing offshore Friday night before stalling once again. With diffluence remaining east of the upper low, and low level confluence likely persisting along and east of the boundary, more convection with periods of heavy rain is forecast Friday. The best chance for rainfall will again be the eastern third of the CWA where mid-level RH remains elevated despite a very dry punch within the dry slot working into the Pee Dee. This creates 2 distinct air-masses across just the local CWA, with PWATS forecast to drop below 1 inch well inland, while remaining at 1.5-1.75 near the coast, and inland counties may finally have a dry day Friday. By Saturday, most of the CWA is forecast to get into the drier air, but once again the immediate coast may still see no relief from this humid airmass with more showers/tstms possible along the coast. Highs both Friday and Saturday are forecast to be slightly above climo, low 80s, but will be heavily impacted by clouds and rainfall. Mins inland will drop into the upper 50s/low 60s both nights within that drier air mass, but will remain near 70 at the coast. && .LONG TERM /SUNDAY THROUGH THURSDAY/... As of 3 PM Thursday...Incredibly the relatively sharp west to east precipitable water gradient remains in place across NE SC/SE NC much of the extended period before the potential of tropical moisture late next week. In essence this brings a sensible weather forecast of chances for showers near the coast and very little along the far interior zones. This in part due to the dry mid-level air wrapped around the large upper low to the west, and the remnant presence of dry air aloft across the area even as the low pulls away from the region on Sunday. Even so a chance of showers will prevail along the eastern 1/3 of the forecast zones days 4-7 with isolated TSTMS favored nocturnally over the coastal waters, or a sea breeze pop up in the middle afternoon. Temperatures to run near to a bit above normal, especially for the minimums this period. Tempering of maximum temperatures days 6 and 7 is attributed to increasing NE flow across the area, but about normal max temp readings for early OCT. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/...
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As of 00Z...Upper low still nearly stationary, with a weak surface front over our CWA. Look for dying convection over the next several hours. Showers could briefly affect the coastal terminals, with VCSH sufficing for now. Overnight, some light fog is possible, briefly IFR at LBT. A coastal shower cannot be ruled out along the coast toward morning. Friday, more of the same with convection becoming scattered, most numerous around or just after max heating. 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...
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NEAR TERM /THROUGH TONIGHT/... As of 730 PM Thursday...No substantial changes are needed to the forecast this evening. Scattered showers and storms remain the largest weather concern to mariners. Shower activity should reach a minimum in coverage around midnight but should grow in coverage late tonight as an upper level disturbance approaches from the south. Discussion from 330 PM follows... Surface cold front will remain well inland through tonight, and winds over the waters of 10-15 kts will persist from the south through the near term. With these winds creating a 2ft/4-5 sec southerly wave, and a low-amplitude 10-11 sec SE swell also existing, seas will remain 3-4 ft through tonight. Showers and thunderstorms may develop late in the period as well, most likely affecting the outer portions of the NC waters. SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/... As of 345 PM Thursday...A cold front will be west of the waters Friday before wavering across or potentially still just inland on Saturday. This creates predominantly S/SW winds of 10-15 kts on Friday, backing to the SE around 10 kts on Saturday, again dependent on where the front stalls. A period of variable winds is possible late Friday into Saturday as the front drifts into the vicinity, but speeds will be light during that time. Seas will hover around 3 ft both Friday and Saturday with a low amplitude 9-10 sec SE swell and 2-3 ft 5 sec southerly wind wave comprising the spectrum. LONG TERM/SUNDAY THROUGH TUESDAY/... As of 3 PM Thursday...This period will be marked by 3-5 foot seas and highest seas offshore, but elevated wave heights may gradually arrive after Tuesday due to increasing swell from Hurricane Matthew. No advisories expected through Tuesday. Isolated showers and TSTMS can be expected this period and radar updates are encouraged.
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&& .ILM WATCHES/WARNINGS/ADVISORIES...
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SC...None. NC...None. MARINE...None.
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&& $$ SYNOPSIS...MJC NEAR TERM...TRA/JDW SHORT TERM...JDW LONG TERM...MJC AVIATION...DL

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