Area Forecast Discussion
Issued by NWS Wilmington, NC

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085 FXUS62 KILM 180600 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 159 AM EDT Fri May 18 2018 .SYNOPSIS... High pressure offshore will remain the dominant feature into the middle of next week. Southerly flow around this high will bring warm and humid conditions, along with deep tropical moisture, into the Carolinas. This will continue unsettled conditions with showers and thunderstorms possible each day. && .NEAR TERM /THROUGH FRIDAY/... As of 900 PM Thursday...The forecast is one of persistence as there has been little change in the overall airmass and weather features. The high resolution models are showing a lull or at least a decrease in showers during the late eve and into the overnight hours. However, even with this, a shower could develop anywhere and at anytime. Very late tonight and Fri morning, speed convergence may be enough to renew convection near the coast with an overall blossoming of convection during Fri morning throughout the Forecast Area. Otherwise clouds should hold across the area with a general lowering of ceilings. Given the elevated dewpoints and with winds not decoupling given the 15 to 20 kt just off the surface, will forecast lows in the lower 70s for the most part. Deep tropical moisture will remain over the Southeast through Fri (and beyond) with periods of showers and isolated thunderstorms across the area. Surface pattern remains mostly devoid of significant features, as does the pattern aloft. A series of weak shortwaves rounding the base of the 5h trough and moving up the coast could serve to enhance convection. Outside of shortwave influences, divergent flow aloft, surface based instability and deep moisture will support convection. Precipitable water values will touch 2 inches at times while CAPE exceeds 1000 J/kg on Fri and could be much higher if extended breaks of sunshine occur. In other words confidence in rainfall, which will have the potential to be very heavy at times, is high. However, confidence in favored locations during the next 24 to 36 hours is lower. Most areas are likely to see moderate to heavy rainfall. The coast is more likely to see showers and isolated thunderstorms overnight and during the morning hours while inland areas will see more convection in the afternoon and evening hours. Severe weather is very unlikely. The lack of strong forcing and marginal mid level lapse rates do not point to strong or severe storms. However, given the time of year a remote strong/severe storm cannot be totally ruled out. The potential for mainly minor flooding will continue to exist, mainly in low-lying and urbanized areas where persistent showers are able to train. Warm cloud depths are on the order of 11 kft so rainfall efficiency should be quite high. Temperatures will be near to slightly below climo during the day although highs well above forecast values will be possible should any extended breaks of sunshine develop. && .SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/... As of 300 PM Thursday...Nearly full column saturation to continue through the period as south-southwest flow will be found through the depth of the troposphere. This will lead to a continuation of nearly omnipresent rain chances, and any patch of showers will be capable of producing locally heavy rainfall. Thunder will be the exception and not the rule due to weak lapse rates through a deep layer and probably require pockets of solar insolation. Specificity regarding timing and placement of rainfall has proven tricky these last few days and will likely continue to be. However there should be a shift towards coastal regions vs. inland regions to be favored for slightly higher rain chances and moreso for higher rainfall amounts. && .LONG TERM /SUNDAY THROUGH THURSDAY/... As of 300 PM Thursday...More of this seemingly ever-persistent tropical airmass will exist much of the extended, although there may at least be breaks in the showers during the long term. Sunday looks to be another potentially very wet day as PWATs remain around 2 inches and southerly winds between offshore high pressure and weak trough to the west. However, late Sunday into early Tuesday the ridge to the east is still progged to expand to the west followed by some drying in the mid-level which will likely lead to reduced, but still non-zero POP. However, by late Tuesday and through Wednesday the ridge retreats in response to mid-level troughing digging down from the north, allowing for a return/surge of tropical moisture and once again periods of showers/tstms with heavy rainfall. There may finally be an end to this extended period of tropical air however, as this same trough that causes an uptick in rainfall potential, will drive a cold front into the area Thursday which should push through bringing some drier air late in the period. However, it is May, and this front could get hung up across the Carolinas, potentially producing even more convection chances as it serves as a focus in a high PWAT airmass. Will show lowered POP by Thursday, but cannot rule out tropical showers/tstms any day during the period. Temps will be in the low 80s for highs nearly every day, with mins right around 70, as the cloudy and humid airmass prevent large diurnal ranges. && .AVIATION /06Z FRIDAY THROUGH TUESDAY/...
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As of 06Z...Low confidence forecast. Overall will see periods of VFR with periods of MVFR/VCSH today into tonight in a deep tropical moisture feed from the Atlantic waters to the south. Models show fairly good coverage today into tonight with MVFR likely. Have not mentioned any thunderstorms as storms should be isolated. Expect gusty S-SE winds at coastal terminals through tonight. 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 FRIDAY/... As of 900 PM Thursday...Bermuda High will maintain southerly flow through the period. Gradient remains static and southerly flow will stay in the 15 to 20 kt range through Fri. Extended period of southerly winds will maintain seas of 4 to 7 ft through the period, thus the Small Craft Advisory remains in effect through and beyond the near term portion of the forecast. SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/... As of 300 PM Thursday...A mixed sea state of wind wave and very long fetch SE swell energy will keep Advisory flags in effect for the period. Not many features to our west on surface charts but the offshore high continues to be large and unmoving. The advisory will be wave-driven as opposed to wind. The former will run 4-6 ft, highest offshore while the latter will likely stay capped at 15-20kt. LONG TERM /SUNDAY THROUGH TUESDAY/... As of 300 PM Thursday...Bermuda type high pressure offshore will be the primary feature driving marine conditions into the middle of next week. S/SW winds around this feature will be 10-15 kts with little fluctuation through the period other than some slightly higher speeds on Sunday. This persistent SW flow and long fetch around the high will create a 7-8 sec dominant wave period, with amplitudes shrinking slightly into next week. Early Sunday, this will create 4-6 ft seas and an SCA may be required for the beginning of the period. Thereafter as the swell deamplifies, wave heights will as well, becoming 3-5 ft Monday, and 3-4 ft Tuesday. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...None. NC...None. MARINE...Small Craft Advisory until 6 PM EDT Saturday for AMZ250-252- 254-256. && $$ NEAR TERM...RJD/III SHORT TERM...MBB LONG TERM...JDW AVIATION...RGZ is the U.S. government's official web portal to all federal, state and local government web resources and services.