Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 271314 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 914 AM EDT Sun May 27 2018 .SYNOPSIS...
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As Alberto moves into the northwest Gulf of Mexico tonight, an onshore push of tropical moisture from the Atlantic, will bring periods of heavy rain to the Carolinas late tonight and Monday. Because of already wet ground, a flood watch is in effect. Alberto is expected to drift north over Alabama and Tennessee early next week. This will maintain humid conditions, and a good chance of showers and thunderstorms much of the upcoming week, as tropical moisture remains across the region. A drying trend and less humid weather is expected into next weekend.
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&& .NEAR TERM /THROUGH TONIGHT/...
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As of 914 AM...A pleasant day in store today, overall minimal convective action, because a formidable dry-cap layer prevails generally between 700-500 mb, induced by adiabatic compression over the previous 24 hours. This should result in discreet low- topped showers only, in peak heating. The dry cap will erode in full later today. Meanwhile, a strengthening low-level upsurge in winds and moisture will pile deep PWATs onto shore, producing periods of downpours from around 4z onward. Because of sodden soils about the area, a flood watch has been posted. This indicates there is a potential for problematic flooding. If water breaches small streams, culverts, ditches, and begins moving, the hazard increases dramatically in flash flooding. This potential threat, will prevail through most of Monday. Primarily cirrus streams today and moderate cumulus. Visible imagery shows oceanic stratocumulus preceding deep Cb cores off the Georgia/SC coast, and this will trend north and west, approaching our coast this evening. As of 300 AM Sunday...The key message we want to get across this period is the heavy rain and flood threat that is expected to develop late tonight and through Mon. Widespread showers and some thunderstorms will arrive from S to N overnight. Average rainfall amounts through Mon could easily reach if not exceed 3 to 5 inches with the higher amounts favored nearer to the coast. Precipitable water values will climb to near or above 2.25 inches which per SPC Climatology is a record for late May at KCHS. The Excessive Rainfall Outlook produced by WPC has included the entire forecast area in a slight risk for excessive rainfall. Model soundings show the moisture deepening through the later portion of today and becoming very deep tonight as tropical plume sets up across the eastern Carolinas. A trough emanating from Alberto is expected to develop NE of the center and up across the offshore waters of the Carolinas today and then pivot onshore tonight. Models are also showing several shortwave troughs advancing N and then NW, well ahead of the subtropical system. Modest elevated instability coupled with strong vertical velocities and increased forcing strongly suggest heavy rain will advance N and into our FA this eve and overnight. This becomes problematic given the antecedent conditions as soils are wet to saturated from significant rainfall over the last week to 10 days. Significant ponding is likely to develop across some low-lying and poor drainage areas after dark with the possibility for more serious flooding toward morning and on Mon. Greater cloud cover today as compared to Sat should shave a degree or two off of the high temps, especially across South Carolina. We are forecasting mid and upper 80s, but can not rule out a couple 90 degree temps this afternoon. Highs near and at the beaches will be in the lower 80s due to a seabreeze circulation which will kick south winds up to 15 to 25 mph this afternoon. Lows tonight will be in the lower 70s.
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&& .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/... As of 300 AM Sunday...A wet short term period still in the cards. At the start of this period, pcpn will be ongoing ie Categorical POPs. A dry punch associated with Alberto may partially push across the FA Mon aftn and evening. This will help to lower the intensity of the pcpn or even temporarily reduce pcpn coverage to widely scattered. Nevertheless, the tropical moisture plume will continue to move across the FA Mon night into Tue before getting shunted east of the FA by late Tue and/or Tue night. At this point, WPC has the FA in a Slight Risk with respect to it`s Excessive Rainfall Outlook for Day2 and Day3 12z Sun thru 12z Tue. The FA will see widespread 2 to 3 inches this 2 -Day short term period with pockets of 3 to 5 inches possible. If the 3 to 5 inch totals were more widespread then would lean toward a FFA but at this point will continue to highlight the heavy rain in the local HWO for the time being. Overall, as the previous fcst alluded, Mon will basically be a washout with 100 POPs. POPs will slowly lower to the next categories, ie. Likely or good chance, come Tue into Tue night. This slow reduction is due to progged PWs still hanging around 2 inches and therefore not taking much to ignite the development of both showers and thunderstorms during this block of time. As for temps, a 10 degree or less diurnal range of temps expected Monday, followed by an improvement to 10 to 15 degrees on Tue. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... As of 300 PM Saturday...Alberto should begin tracking north from the Gulf Coast on Tues. The latest models are in fairly good agreement with keeping the center or remnant low moving north just east of the Mississippi River then northeast toward the Great Lakes and then toward the New England coast. This keeps forecast area in a deep stream of tropical moisture between the Atlantic ridge to the east and Alberto low to the distant west Tues through Wed night. The pcp water values remain well above normal, on the order of 2 to 2.3 inches, Tues through Thurs morning. Any drier air looks like it may hold off until Thurs aftn into Fri as winds veer to a more westerly direction in the low to mid levels as Alberto low reaches up near the Great Lakes. Even with the column drying out some on Fri, a very moist atmosphere will remain into Saturday. Small diurnal ranges expected in temps with low 80s most days and low 70s at night. Overall, expect a very warm and humid air mass in place with clouds and rain hanging on through at least Thurs with some breaks possible by Friday or Saturday. The most widespread rain should be through mid week but the timing and exact areas of heaviest rain will not be possible to say at this point, but expect periods of rain and locally heavy rain through Thurs. The rainfall should be diurnally enhanced, more widespread inland during the day and along the coast overnight. The local forecast area should not see any direct impacts from Alberto, just a continuous tropical feed of moisture for several days. && .AVIATION /13Z SUNDAY THROUGH THURSDAY/... As of 12Z...Fog will lift at KLBT bt 14Z...otherwise conditions at the terminals are starting off VFR this morning. Tropical airmass will remain in place with isolated to scattered showers from mid-morning through the evening hours, along with temporary MVFR conditions. Showers will become more numerous after midnight, with ceilings lowering to IFR. There is potential for a few lightning strikes this afternoon and overnight, but overall coverage of thunderstorms is expected to be isolated, so have included only showers for now. Extended outlook...On Monday, IFR with periods of heavy rain in showers and some thunderstorms. Tue-Thu, tempo MVFR/IFR in showers and thunderstorms. && .MARINE...
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NEAR TERM /THROUGH SUNDAY/... As of 914 AM Sunday...The ocean spectrum has changed to a more difficult one and will trend this way, as wave periods have transitioned a shorter wind-driven pitch. Seas will build rapidly later today and tonight as SE-SSE winds intensify, as Alberto moves into the NE Gulf of Mexico. SCA flags are flying. As of 300 AM Sunday...We have raised a Small Craft Advisory for the waters beginning tonight, first across the southern waters this eve and then expanding to include the northern waters overnight. S to SSE winds will prevail across the waters into this eve with S winds overnight. Wind speeds will be up to 15 to 20 kt beginning this afternoon. Seas will build from 3 to 4 ft today to 4 to 7 ft tonight. The Bermuda High will shift away from the area through tonight as Subtropical Storm Alberto moves slowly N across the eastern Gulf of Mexico. SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/... As of 300 AM Sunday...SCA will be ongoing at the start of this period and have it continuing thru Midday Tue. The local waters will be sandwiched between Alberto well west of the FA and Bermuda High pressure centered well east of the FA. As a result of this funneling effect, have leaned toward the hier side of guidance with respect to wind speeds ie within the 15 to 25 kt range with hier gusts. Looking at a S wind direction initially veering slightly to the SSW-SW Mon night thru Tue. WindSpeeds will be declining during the Tue time- frame. Significant seas will peak during Monday with all zones seeing 6 foot, or higher for the waters off Cape Fear and Romain respectively. The ongoing SE ground swell will get reinforced from upstream. At the same time, local wind waves Mon combined with the 7 to 9 second swell, will keep seas at SCA thresholds Mon thru Tue morning. Seas will fall below SCA thresholds midday Tue with winds having already dropped by this time frame. There will be widespread showers and embedded thunderstorms reducing VSBY to frequently to 1 to 3 nm and below 1 NM in heavy rains. LONG TERM /WEDNESDAY THROUGH THURSDAY/... As of 300 PM Saturday...Southerly winds will continue across the waters through Wed, mainly between 5 and 15 kts. the strongest winds may be early Tues and then again on Thurs as they veer to a more SW to W direction. The persistent southerly push will keep seas between 3 and 5 ft most of the period, with highest seas most likely to start Tues morning and then rising slightly again Thurs night. As winds lighten and veer Wed night into Thurs they should drop to the 2 to 4 ft range.
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&& .ILM WATCHES/WARNINGS/ADVISORIES... SC...Flash Flood Watch from late tonight through Monday afternoon for SCZ017-023-024-032-033-039-054>056-058-059. NC...Flash Flood Watch from late tonight through Monday afternoon for NCZ087-096-099-105>110. MARINE...Small Craft Advisory from 8 PM this evening to noon EDT Tuesday for AMZ254-256. Small Craft Advisory from 2 AM Monday to noon EDT Tuesday for AMZ250-252. && $$ SYNOPSIS...MJC NEAR TERM...MJC/RJD SHORT TERM...DCH LONG TERM...RGZ AVIATION...CRM

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