Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 150138 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 938 PM EDT Fri Jul 14 2017 .SYNOPSIS... Hot and humid weather will continue through Saturday, but clouds and thunderstorms should increase with slightly cooler temperatures expected Sunday into Monday as a cold front approaches from the north. This front should dissipate to our north by Tuesday. Hot temperatures should redevelop for the second half of next week as high pressure expands eastward from the Central Plains states. && .NEAR TERM /THROUGH SATURDAY/...
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As of 900 PM Friday...The last evening without convection to deal with across the FA. Unusual but no unheard of for this time of the year. However, this makes it 3 days in a row without convection and that IS highly unusual for this time of the year. The ridging aloft and at the sfc will break down across the FA this period. A couple of sfc features to note that will aid in firing up thunderstorm activity across the ILM CWA Sat aftn and night. A sfc trof or cool front will drop to the central Carolinas during Sat. And the sea breeze will become the focus to help fire up widely scattered to scattered convection Sat aftn and Night. SW winds will remain active overnight, aided by a weak/modest low level SW jet. This will help prevent fog development overnight, This will also keep tonights lows on the hier side of guidance with widespread mid to upper 70s, to around 80 at the coast. Previous................................................. As of 300 PM Friday...Water vapor channel illustrates well the upper trough dipping south and east across the Great Lakes and Ohio Valley region that will bring a transition in our sensible weather regime. In the very near term a fair overnight period is anticipated with above normal minimums on tap with moderate SW breezes in the boundary layer. This will aid in keeping extensive fog formation at bay, and sub-saturation in the lowest couple hundred feet should preclude a widespread stratus outbreak. Meanwhile, a capped atmosphere aloft will keep the chances of measurable precipiation very low through Saturday morning. By late Saturday however, upper support, spiking precipitable H2Os and low level convergence along an inland surface trough will elevate convection and increase its coverage. This scenario is presently supported by satellite derived PWAT animations that show well the 1.9-2.0 inch values rising out of the Gulf states poised to be directed into the Carolinas ahead of the upper trough deepening upstream. Saturday will be hot with middle and low 90s and apparent temperatures of 100-104 degrees. Downdraft CAPES Saturday will support the possibility of locally strong downdraft winds in any deep convection even though the steering flow is relatively weak.
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&& .SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY NIGHT/...
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As of 300 PM Friday...An active period is expected as a cold front sags into the eastern Carolinas beneath a digging mid-level trough. Saturday night, the area will be entrenched in an extremely warm and unstable airmass thanks to Bermuda ridging and temps likely still close to 90 to begin the short term. Increasing column moisture will drive PWATs up to around 2.25 inches while height falls help drive MLCape to around 2000 J/kg even well after dark. This sets up what could be a widespread convective event Saturday night as the surface front dives into this air mass coincident with mid-level shortwave leading the deepening longwave trough. DCape values of 500-800 J/kg and 0-6km shear of 20-25 kts are enough to support multicell/small clusters with damaging wind gusts. These are most likely to occur in the form of wet microbursts thanks to the saturated atmosphere, despite ML lapse rates only near moist-adiabatic values. SPC has placed the area in a MRGL risk Saturday night for these reasons, and while a widespread severe event is not expected, isolated damaging wind gusts are possible within what should be numerous thunderstorms. Convection will wane very late as the atmosphere gets `worked over,` but the front will align parallel to the mid- level flow into Sunday, causing it to stall in the vicinity. Continued deep moist advection identified on soundings by persistent 700-500mb saturation and total PWATs well over 2 inches suggest the potential for scattered to numerous thunderstorms throughout Sunday with heavy rain the primary threat. MLCape is much lower Sunday, thanks to cooler temperatures and more widespread cloud cover, but enough instability is present in the charge separation zone for lightning, and updrafts will be aided by at least periphery lift within the RRQ of a 70 kt upper jet. WPC has placed the region in a MRGL risk for excessive rainfall on Sunday, and pockets of localized flooding are possible due to potentially very heavy rain rates and storm motion less than 10 kts. With the front remaining in place through Sunday night and relatively lowered thicknesses across the area, convection should persist in a weaker form into Monday morning. Temps this period will likely be warmest right at the beginning of Saturday night before falling into the mid 70s for lows both Saturday night and Sunday night. Highs on Sunday will be tempered considerably by clouds and convection, rising only into the mid 80s.
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&& .LONG TERM /MONDAY THROUGH FRIDAY/... As of 300 PM Friday...A relatively narrow upper trough will sit along the U.S. East Coast early next week, sandwiched between the Bermuda ridge offshore and an upper ridge over the Central Plains states. A weak surface low near Washington DC will anchor a front separating our maritime tropical airmass from cooler Canadian air to the north. This boundary will probably make its closest approach Monday night before washing out in the NC mountains on Tuesday. A humid and moderately unstable airmass Monday will probably yield showers and thunderstorms with locally heavy rainfall possible. Clouds and precip should hold temps in the 80s Monday, a welcome reprieve from the heat of the past several days. Upper level heights should bottom out on Tuesday, however drier air wrapping through the mid and upper level trough may reduce our potential of thunderstorms from 50-60 percent Monday to 30-50 percent Tuesday. By Wednesday, the upper trough should lift out to the northeast as the Central Plains upper ridge begins to build east into the Carolinas. This should allow heat to build Thursday and Friday in advance of this ridge, with deep offshore winds and 850 mb temps approaching +20C by Friday allowing temperatures to climb back into the 90s even down at the beaches. Model guidance is closely clustered with temps next week. && .AVIATION /01Z SATURDAY THROUGH WEDNESDAY/... As of 00Z Friday...Expect mainly VFR conditions this evening and overnight. SW-WSW winds in the lower levels will remain active enough to prevent fog development during the night time period. Look for increasing moisture thru the atm column as the ridging sfc and aloft for the past several days begins to break down across the Eastern Carolinas. Will see cu development by mid to late morning across all terminals. With NVA and subsidence aloft having weakened, the Cu field will become more expansive and likely grow to moderate and towering Cu from midday thru the end of this period. The forcing along the sea breeze and inland sfc trof across the central Carolinas, will further enhance the growing Cu field to CBs across the area Sat aftn and evening. At this point, will only go with Prob30s for convection at this time but will indicate possible convective wind gusts up to 35 kt. The gusty S to SSW winds will veer back to a more stable SW-WSW wind overnight. The sfc pg will remain tightened enough for winds to stay at or above 5 kt speeds overnight, eliminating any fog development. Will see a weak to modest low level SW jet overnight, further preventing fog development. Winds will increase to 10-15 kt later Sat morning thru the remainder of the Taf Issuance period. Winds will gust aoa 20 kt across the coastal terminals, aided by the sea breeze which will not progress as far inland compared to the previous several days. Extended Outlook...The threat for flight restrictions due to thunderstorms will increase late Saturday through early next week, as a frontal boundary drops southward and stalls across the forecast region. && .MARINE...
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NEAR TERM /THROUGH SATURDAY/... As of 915 PM Friday...Weak to modest sfc pg and low level SW jet respectively, will both aid in keeping a SW wind at 10 to 20 kt across the local waters overnight into daylight Sat. Significant seas will run 2 to 4 ft but would not be surprised to see a few 5 footers across the outer waters off Cape Fear and Romain respectively during Sat. The 8 to 9 second period ESE ground swell will barely remain the dominant feature of the significant seas spectrum Sat morning. Will need to keep an eye on possibly conditions reaching SCEC criteria Sat. Increasing convection will move off the mainland during Sat aftn/evening and across the local waters resulting with temporarily locally hier winds and seas. Previous................................................. As of 300 PM Friday...No advisories or caution statements are needed at this time, but seas will be a bit bumpy due to 20 KT gusts tonight through Saturday. Seas should hold around 3 feet, but up to 4 feet outer waters ought to be expected for mariners approaching the Gulf Stream. 2 foot S-SW wind waves of 3-5 sec will interacts with 2 foot ESE waves every 8-9 seconds to bring overall wave- heights of 3-4 feet. Showers and TSTMS will begin to increase over the waters by late Saturday afternoon, and mariners are advised to get a radar update if heading out Saturday evening as some storms could be strong to severe. SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY NIGHT/... As of 300 PM Friday...High pressure offshore will exert its influence across the waters this period, despite a cold front approaching from the NW. This front will reach nearby Saturday night, and then stall and waver in the vicinity during Sunday. The tightest gradient and highest winds will occur Sat night during the approach of this feature, when wind speeds will climb to around 15 kts from the SW. Much of Sunday and Sunday night, speeds will be 10- 15 kts, still from the SW, as the front weakens and stalls. A persistent 2ft/8sec SE swell will exist through the period, but will be masked Saturday night by a 3-4ft SW wind wave, producing seas of 3-4 ft. As this wind eases Sunday, predominant wave heights will fall to 2-3 ft, while the average period lengthens. Showers and thunderstorms will also be likely across the waters beginning Saturday night. Some of these will have the potential to create gusty and erratic winds, as well as cloud to sea lightning and very low visibility in torrential rainfall. LONG TERM /MONDAY THROUGH WEDNESDAY/... As of 300 PM Friday...Weak low pressure along a cold front near Washington DC Monday and Tuesday will dissipate Wednesday. The Bermuda high will remain well offshore of the Carolinas and should have only a peripheral influence on our weather. South to southwest winds should dominate Monday through Wednesday, but with wind speeds generally remaining 10 knots or less outside of thunderstorms. The best potential of thunderstorms should exist Monday and Tuesday, diminishing substantially by Wednesday as high pressure aloft begins to build in from the west.
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&& .ILM WATCHES/WARNINGS/ADVISORIES... SC...None. NC...None. MARINE...None. && $$ SYNOPSIS...TRA NEAR TERM...DCH/JDW SHORT TERM...MJC LONG TERM...TRA AVIATION...DCH

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