Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 190537 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 127 AM EDT Sat May 19 2018 .SYNOPSIS... Tropical moisture will continue to stream north across the Carolinas for much of the period. Significant rainfall is expected. Temperatures will be near to slightly above normal into late next week. && .NEAR TERM /THROUGH SATURDAY/... As of 1000 PM Friday...Persistent convection to our W this afternoon and eve has been slowly shifting E as it moves north. This activity continues to wane with the loss of heating and should largely stay W of the I-95 corridor overnight. A plume of deeper moisture offshore per water vapor imagery has been lifting N with a slow westward drift. It is these showers that will impact portions of the Forecast Area overnight as the activity continues to drift onshore and very slowly westward. Still, rainfall rates associated with this activity are not significant as of late this eve. However, frictional convergence may be enough to wring out higher rainfall rates overnight as depicted by some of the higher resolution models. Given the deep and persistent tropical airmass, this scenario is hard to ignore and opted for mainly likely POPs overnight, increasing to mainly categorical Sat morning. Tropical moisture plume will continue to spread over the region from the south through Saturday. Periods of heavy showers along with isolated thunderstorms are expected during this time. Precipitable water values will hover around 2 inches overnight and tomorrow. Deep moisture and high freezing level/low LCL will keep rainfall efficiencies very high and flooding, particularly urban and small stream, may be an issue Sat. Overall synoptic pattern has not changed a lot during the last few days. Moisture is being drawn north by the combination of 5h ridge over the western Atlantic and a 5h trough well west of the area. A series of shortwaves will move across the area from the south, with the strongest expected later this evening followed by another around midday Sat. These features will be able to enhance convection already occurring due to weak low level convergence, surface based instability, and divergent flow aloft. Guidance is showing some low level jet convergence moving into the area Sat which could further enhance convection. Other than being able to pick out times that seem a little more favorable for convection, we can`t say with any certainty which areas will see rain at any given time. Temperatures will be near to slightly below climo on Sat while overnight temps will be above to well above climo. After coordination with neighboring offices have decided to hold off on a Flash Flood Watch (FFA). Areas to the north have received more rain recently than most of the ILM area of responsibility. In fact the 14 day percent of normal rainfall for the ILM area ranges from 25-50% while areas to the north range from near normal to almost 200% of normal. Flash flood guidance also highlights this with much of the guidance for the local area running around 3 inches per hour and around 6 inches over a 24 hour period. Compare this to areas to the north which are showing FFG values closer to 1.5 inches per hour and around 3.5 inches for 24 hours. Do think there will be a few isolated pockets of flooding during the next 24 to 36 hours, but confidence in significant flash flooding remains low. We will continue to refine our QPF and the need for a Flash Flood Watch. && .SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY NIGHT/... As of 300 PM Friday...Some changes with the short term forecast can be expected. A decent mid level trough will move across the Ohio Valley and the result will be a subtle but important shifting of the deep tropical plume to just offshore. The forecast reflects this with a slight decrease in pops for daytime Sunday and especially for overnight Sunday into early Monday. Pops remain somewhat elevated overnight Saturday into Sunday morning as the shift will not have taken place as of yet. Certainly no air mass change at the surface thus temperatures are essentially unchanged with highs in the lower to middle 80s and lows a couple of degrees either side of 70. && .LONG TERM /MONDAY THROUGH FRIDAY/... As of 300 PM Friday...A brief reprieve in total shower and thunderstorm activity is still forecast for Mon/Tue, but a return to this humid, unsettled, tropical airmass is forecast by the middle of next week. Ridge of high pressure offshore should expand westward Monday into Tuesday bringing a decrease in shower activity as well as slightly warmer temperature and lower humidity. PWATs are forecast to fall to around 1 inch, down from the near-record 2 inches this weekend, which combined with weak subsidence and lack of any notable forcing is likely to create a break in convection early next week. While scattered showers/tstms cannot be ruled out, total coverage will be much less than nearly every day of the past 7, and more diurnally driven as well. More sunshine, a drier column, and slightly greater thicknesses will allow temps to climb into the mid 80s Mon/Tue. By Wednesday the upper mid-level ridge begins to break down in response to a trough digging from the north. This will push a cold front towards the area, and cause the surface high to retreat back to the SE. Although guidance begins to differ by mid-week with the upper pattern, it appears that pinched flow between a weak mid-level low developing to the west, and the high to the east, will again begin to transport tropical moisture back into the Carolinas Wed/Thu. This combined with a stalling surface front, especially Thursday, will regenerate scattered to widespread showers/tstms with periods of heavy rainfall possible. It is possible that on Friday another reduction in coverage could occur as the front wavers back to the north, but guidance suggesting renewed tropical moisture pooling to the south suggests lowered confidence and will maintain at least CHC POP through the end of the period. The more significant cloud/precip coverage and lowered heights will keep highs Wed-Fri more towards seasonable norms. Mins through the week will remain a degree or two either side of 70. && .AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
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As of 06Z...Cloud bases continue to fluctuate leaving mainly tempo ceilings of IFR to MVFR as tropical moisture continues to stream northward into the area in light to moderate S-SE winds. Models continue to show fairly widespread coverage of shwrs with only iso tstms as plume of moisture shifts east through the day. Heading into this eve, main plume of moisture aligns closer to the coast. Inland TAFs have better chc of longer duration VFR overnight while coastal terminals have greater chc of MVFR to IFR ceilings and vsbys in SHRA. Although, MOS guidance suggests possibility of MVFR cigs overnight KFLO/KLBT with periods of IFR. Extended Outlook...Periods of MVFR/IFR cigs/vsbys associated with tropical type showers and tstorms through the period. Activity will be more numerous along the coast pre-dawn hours thru mid-morning hours each day, then translate to the inland terminals midday-early evening.
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&& .MARINE... NEAR TERM /THROUGH SATURDAY/... As of 1000 PM Friday...A Small Craft Advisory continues through Sat and into Sat night. The gradient between the Bermuda High and low pressure moving north through the Mississippi Valley will remain tight through the period. South winds will remain in the 15 to 20 kt range with gusts up to 25 kt. The persistent southerly flow will keep seas in the 4 to 7 ft range with a healthy 6 to 8 second SSE swell. SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY NIGHT/... As of 300 PM Friday...The pressure pattern that has been in place for some time will weaken somewhat with flag or headline conditions coming to an end by/during Sunday. There will be some lingering six foot seas into Saturday evening, but by Sunday afternoon 2-4 feet should be in place and continue through Monday morning. In between these times some five footers will likely remain especially across the outer waters. Winds will be southwest at 10-15 knots for the most part. LONG TERM /MONDAY THROUGH WEDNESDAY/... As of 300 PM Friday...Little change through the middle of next week other than subtle variations in wind trajectories as offshore high pressure remains in place. This will continue the S/SW winds of 10- 15 kts each day through the period. The long fetch around the high will persist a 2-3ft/8sec SE swell, which will be present in the spectrum in addition to a 2-3ft/5sec SW wind wave. These two wave groups together will produce seas of 3-4 ft with little change each day. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...None. NC...None. MARINE...Small Craft Advisory until midnight EDT Saturday night for AMZ250-252-254-256. && $$ NEAR TERM...RJD/III SHORT TERM...SHK LONG TERM...JDW AVIATION...RGZ

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