Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 230711 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 311 AM EDT Sun Jul 23 2017 .SYNOPSIS...
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A Heat Advisory remains in effect today. A cold front will approach from the north and may slip into the area mid week before stalling and then dissipating during Thursday. Showers and thunderstorms with heavy rain will increase in coverage Monday through Wednesday and this will serve to knock down the heat although the high humidity will remain. Drier air may briefly work into the area Thursday, but the arrival of a stronger cold front will bring more thunderstorms by the weekend.
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&& .NEAR TERM /THROUGH TONIGHT/...
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As of 300 AM Sunday...Mid level pattern starts to change today as the 5h ridge weakens and is replaced by subtle 5h troughing. Minimal amount of mid level subsidence present yesterday is gone today while precipitable water values exceed 2.25 inches. Mid level forcing will be weak to non-existent with only a weak shortwave passing north of the area around peak heating. Main forcing today will be from diurnal heating and the sea breeze/Piedmont trough. Given a more favorable environment aloft anticipate some deeper convection along the sea breeze with slightly higher coverage across western areas, associated with the trough. Activity will be diurnally driven with storms weakening in the evening and dissipating within an hour or so of sunset. Nocturnal convection seems like a good bet overnight, although coverage is likely to be limited given a lack of forcing. Some of this activity may drift onshore and will carry a slight chc pop along the coast overnight. 00Z GFS does show some weak PVA pushing offshore late in the period but this could be feedback. The 18Z version had the same feature but in a much stronger state. The latest Canadian, ECMWF, and NAM do not have this feature. Temperatures will once again run above climo with highs ranging from lower 90s along the coast to mid 90s inland. Cannot rule out an isolated upper 90 degree reading in far western locations but these should be isolated. Continued southerly flow will maintain dew points in the low to mid 70s. This combination will push heat index values above 105 this afternoon and require continuation of the heat advisory. Temps well above climo will continue overnight with lingering debris cloud and southerly flow keeping lows in the mid to upper 70s.
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&& .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
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As of 300 AM Sunday...This forecast period will be marked by increasing shower and thunderstorm coverage. The hot ridge will have retrograded back to the Southern Plains while the Bermuda ridge remains offshore and suppressed to our S. This will allow an east coast trough to dig which will coax a surface cold front slowly southward. The front should be in close proximity by Tue morning and will likely stall overhead by Wed. In addition, the Piedmont Trough is expected to be displaced further E. This scenario should allow for a likelihood of showers and thunderstorms. These thunderstorms will be enhanced by mid-level short wave troughs. Currently models are showing one of these upper level impulses will move across the area Mon afternoon and eve and another Tue afternoon and night. The later impulse may actually close off across the Carolinas and linger. Rainfall rates should be very high given precipitable water values will be on the order of 2.25 inches or higher for much of the period. Excessive rainfall is certainly possible given slow storm motions and the expectation that cells will train along the front. Rainfall amounts could easily reach 2 to 3 inches across a majority of the area during this time with more rainfall possible on Wed as well. The clouds and widespread and significant rainfall will knock down high temps, but the high humidity will remain. Highs both days will be around 90, averaging just above 90 on Mon and just below 90 on Tue. Heat index values will be in the 100 to 105 degree range Mon and around 100 Tue. Lows will be in the muggy lower to mid 70s.
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&& .LONG TERM /WEDNESDAY THROUGH SATURDAY/...
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As of 300 PM Saturday...The upper air pattern will transition to a cooler pattern for the eastern U.S. next week. The upper ridge will retrograde to the Four Corners region by Saturday. A downstream trough will develop along the East Coast, enhanced by a shortwave that almost cuts off across the Carolinas mid week, and a progressive shortwave that moves through New England Friday. At the surface, low pressure should sink down to coastal South Carolina by Wednesday night. This will drag a weak cold front down from the north. While yesterday`s ECMWF kept the front across central North Carolina, today`s run is in agreement with the GFS that the front will sink all the way down into the Charleston, SC area. With low-level convergence focused along the front and cooler upper-air conditions with the disturbance aloft, expect thunderstorms to be fairly widespread into Wed. There is the potential we`ll see 1-3 inches of rain area-wide. As the upper disturbance kicks offshore Thursday afternoon and drier mid-level air bleeds in from the west, the surface front should dissipate. Friday should be the warmest day in the extended period with highs popping back into the lower 90s and heat indices over 100 degrees again. By Saturday, the GFS and ECMWF are in surprisingly good agreement that a stronger cold front will dive southward and into area, with another enhancement in the potential of thunderstorms.
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&& .AVIATION /06Z SUNDAY THROUGH THURSDAY/... As of 06Z...VFR conditions will dominate the period with brief period of MVFR or even IFR possible from afternoon and evening showers and thunderstorms. Both the sea breeze and the Piedmont trough should be more active than in past days but coverage will still be limited. Storm coverage and activity will diminish quickly with the loss of heating as the sun sets. Bermuda High will maintain southerly flow through the valid TAF period. Sea breeze will bring gusts to coastal sites with potential for speeds in excess of 20 kt from the southeast. Speeds tonight will be around 10 kt, preventing any significant fog development. Extended Outlook...Possible MVFR from patchy stratus near sunrise Mon. Brief MVFR/IFR possible in showers and thunderstorms Mon thru Thu. && .MARINE...
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NEAR TERM /THROUGH TONIGHT/... As of 300 AM Sunday...Pinched gradient this afternoon and evening will push speeds to 15 to 20 kt with potential for solid 20 kt at times. Prolonged and increasing south to southwest flow will help build seas to 3 to 5 ft with widespread 5 ft likely well away from shore. Will maintain inherited SCEC for all zones for today continuing into the overnight hours. SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/... As of 300 AM Sunday...A cold front will sag south and be in close proximity Tue and may stall overhead Tue night or Wed. The eastward displaced Piedmont Trough will serve to keep a rather tight pressure gradient in place and this will keep sustained southwest winds near 20 kt Mon and Mon eve. As the cold front gains proximity late Mon night and through the remainder of the period, wind speeds will decrease to no higher than 10 to 15 kt with 10 kt or less by late Tue night as the wind direction becomes variable. Seas will be 4 to 5 ft through the first half of the forecast period, subsiding to 3 ft or less during Tue and Tue night. Thunderstorms will be increasing across the waters during this time and mariners should expect poor visibility in very heavy rain along with locally higher winds and seas. LONG TERM /WEDNESDAY THROUGH THURSDAY/... As of 300 PM Saturday...A cold front will become increasingly ill-defined on Wednesday, and by Thursday afternoon should wash out as southwesterly winds increase ahead of a stronger front that should affect the area next weekend.
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&& .ILM WATCHES/WARNINGS/ADVISORIES... SC...Heat Advisory from 11 AM this morning to 7 PM EDT this evening for SCZ017-023-024-032-033-039-053>056. NC...Heat Advisory from 11 AM this morning to 7 PM EDT this evening for NCZ087-096-099-105>110. MARINE...None. && $$ NEAR TERM...III SHORT TERM...RJD LONG TERM...TRA AVIATION...III

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