Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 170244 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 1044 PM EDT Wed May 16 2018 .SYNOPSIS... Bermuda high pressure will remain well offshore into the weekend. Tropical moisture will continue to flow around this high and into the Carolinas bringing periods of showers and thunderstorms with heavy rain. Some reduction in shower and thunderstorm coverage is possible late in the weekend as the Bermuda high strengthens. A return to unsettled weather is likely early next week. && .NEAR TERM /THROUGH THURSDAY/...
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As of 1000 PM Wednesday...Showers and some thunderstorms were moving N ahead of weak area of low pressure and this activity is expected to move into northeast South Carolina after midnight and then into southeast North Carolina after about 3-4 am. Still could be a rumble of thunder overnight, although the brunt of the activity will be in the form of showers. Brief heavy downpours remain possible as the airmass will continue to be very moist through a deep layer. Both GFS and WRF show an increase in divergence tonight from roughly 03Z to 09Z, along an axis extending almost due north from where the NC/SC state line meets the coast. High resolution guidance is also hinting at this, although not as strongly. Southerly flow, cloud cover and moisture will keep lows well above climo. Mid-level trough axis remains west of the area through the period while 5h ridge sits off the Southeast coast, pumping tropical moisture into the region. Several weak shortwaves will rotate around the base of the trough tonight and Thu. None of these look particularly impressive and on their own probably would not generate much convection. However, precipitable water values remain high for mid May, ranging from 1.8 to almost 2 inches through the period. Low level convergence along the coast along with the presence of these weak waves in flow that is weakly divergent aloft suggests at least some potential for showers and thunderstorms overnight. Same ingredients remain in place on Thu. Abundant moisture, limited surfaced based instability, and weak forcing aloft. Showers and isolated thunderstorms will develop with coastal areas favored earlier in the day before transitioning inland. Not really much to hang one`s hat on in terms of forcing, other than the weak sea breeze. Could end up with differential heating boundaries, but trying to predict these a day in advance is next to impossible. Will continue to carry a likely pop Thu but confidence in location of heaviest showers and thunderstorms is low. Temperatures will be near to above climo, with potential for highs several degrees warmer than currently forecast if an area experiences an extended period of sunshine.
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&& .SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/... As of 300 PM Wednesday...Deep layer south to southwesterly flow in place through the period a result of large Atlantic high pressure paired with not only Atlantic mid level ridge but also healthy trough to our west. This will keep precipitable water near 2.00" through the period; very near the maximum observed values for the time of year. Mid level flow should contain enough small scale vorticity maxima to provide ascent for lift, and LCLs will remain lower enough for fairly widespread shower activity. Heavy rain if it occurs will be quite short-lived in any given location. Temperatures will be near normal by day and elevated above climatology at night. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... As of 300 PM Wednesday...Unsettled and tropical airmass remains in place at least on Saturday despite the weak upper low/surface reflection in the Gulf lifting away. Guidance is indicating that Saturday will be a continuation of much of the week with humid conditions and the potential for widespread showers/tstms with heavy rainfall. In fact, Saturday may be one of the wetter days of the entire week with PWATs over 2 inches forecast and a deep moisture tap directly into the Gulf. There is some indication thereafter that a break in the pattern may develop, albeit briefly, Sun/Mon as the mid-level ridge strengthens and retrogrades to the west. Although this will not bring an end to the convective coverage, less widespread activity is possible into early next week despite a weakness in the ridge remaining to the west and deep tropical moisture persisting to the South. However, by Tuesday and into the middle of next week a secondary cutoff may develop in the Gulf returning our area to a very similar pattern with better rainfall chances and continued humid conditions. Although no individual day appears to be a washout, each day is expected to feature periods of showers and tstms with the potential for heavy downpours. Temps through the period will remain around to slightly above climo for highs, and well above climo for lows with minimal diurnal ranges due to clouds and humidity. && .AVIATION /00Z THURSDAY THROUGH MONDAY/...
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As of 00Z...Thunderstorms will to the N of KFLO by 01Z, otherwise should see a decrease in mid/upper clouds and precip S-N at all terminals during the evening. Winds will continue SE-S at 5 to 10 kt, lightest at KFLO/KLBT. MVFR cigs are possible at any time, but will side with VFR for now. MOS guidance is suggesting IFR cigs KFLO/KLBT overnight but believe this is overdone due to the tropical airmass. However, due to rain today and scattered skies, periods of MVFR not out of the question. At the coastal terminals there is moderate confidence another impulse will move up the coast overnight spreading MVFR cigs and showers into the coastal terminals by 09-11Z. Most precipitation should remain offshore however. MVFR cigs/showers coastal terminals should become mainly VFR/VCSH by mid morning as S winds increase to 10-15 kt. Showers/isolated tstms move in to the KFLO/KLBT terminals by 19 and persist through the afternoon. Extended Outlook...Periods of MVFR/IFR cigs/vsbys associated with tropical type showers and tstorms. 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...
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NEAR TERM /THROUGH THURSDAY/... As of 1000 PM Wednesday...Southerly flow will continue through the period, increasing to 15 to 20 kt overnight and Thu. Gradient tightens up as weak surface low over the southern Gulf of Mexico moves onshore overnight. Combination of increasing wind speeds and extended period of southerly flow will build seas to 6 ft or so as early as Thu morning. A Small Craft Advisory has been issued for Thu. SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/... As of 300 PM Wednesday...A fairly long fused Small Craft Advisory remains in effect for most if not all of the period. Wind speeds will tend to stay sub-advisory but an atypically large area of high pressure over the west Atlantic will generate healthy swell energy into the entire Southeast Coast. LONG TERM /SATURDAY THROUGH MONDAY/... As of 300 PM Wednesday...High pressure offshore will remain the dominant feature into early next week, with southerly flow creating periods of showers and thunderstorms across the waters. Winds Saturday will be nearly uniformly from the south at 15-20 kts, before veering to the SW Sun/Mon along with slightly lower speeds as the ridge re-asserts itself and ridges to the west. The gusty winds Saturday combined with long duration fetch around the offshore high will produce a 6-7 ft 7-8 sec S swell combined with a shorter period SW wind wave to produce 4-6 ft seas. An extension of the SCA currently in effect for Friday will likely be extended into Saturday. By Sun, the swell de-amplifies a bit and the winds ease, so seas become 3-5 ft Sunday, and 3-4 ft Monday.
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&& .ILM WATCHES/WARNINGS/ADVISORIES...
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SC...None. NC...None. MARINE...Small Craft Advisory from noon Thursday to 9 PM EDT Friday for AMZ254-256. Small Craft Advisory from 6 AM Thursday to 9 PM EDT Friday for AMZ250-252.
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&& $$ NEAR TERM...RJD/III SHORT TERM...MBB LONG TERM...JDW AVIATION...MRR

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