Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 160029 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 829 PM EDT Sun Apr 15 2018 .SYNOPSIS... Strong to severe thunderstorms this evening could bring damaging winds and heavy rain. A strong cold front will move overhead late tonight bringing dry and cooler weather Monday. As high pressure to our south moves offshore Tuesday, temperatures will warm with above normal temperatures expected Wednesday and Thursday. A cold front will move across the area Thursday night. High pressure will follow Friday and Saturday. Another frontal system will approach from the west Sunday and bring an small chance of showers and thunderstorms. && .NEAR TERM /THROUGH MONDAY/... As of 645 PM Sunday...The first leading squall line has pushed into the Pee Dee counties, but has since stalled its eastward motion. This is due to the atmospheric steering flow backing around to due south in advance of a strong jet streak digging across Georgia and into the base of the upper level trough. The entire upper system is taking on a negative tilt which is excellent for producing wide-open divergence aloft. This should help sustain a couple lines of strong to severe thunderstorms over the next 6 hours that should push across the eastern Carolinas and drop a widespread 1 to 2 inches of rain with locally higher amounts. The low-level winds have also backed around slightly east of due south which is advecting a relatively stable marine boundary layer onshore. The last of the `large` surface instability (CAPE 1000 J/kg or more) exists where temperatures are still in the mid-upper 70s inland ahead of the squall line. With the loss of daytime heating look for surface-based instability to drop to less than 500 J/kg, but with elevated CAPE remaining a bit larger. This instability coupled with excellent upper dynamics should be sufficient to sustain storms as they move across the area. PoPs remain at 100 percent area wide. One change to the forecast was to raise a wind advisory this evening for areas within 20-30 miles of the coast. Most model guidance shows a powerful southerly low level jet developing with 50 knot winds dipping as low as 1000 feet AGL. Even outside of convection look for surface winds near the coast to increase to 25-35 mph with gusts as high as 45 mph as the LLJ moves overhead. This may occur literally just ahead the main convective line along the coast late this evening. Convective downdrafts could easily tap into this wind energy and produce additional strong wind gusts, and we`re maintaining severe wording in the forecast through midnight. Discussion from 300 PM follows... A Tornado Watch is in effect for portions of our inland areas of northeast South Carolina and southeast North Carolina and will likely be expanded east later this afternoon or early eve. The watch is currently valid til 11p. The severe threat continues to be late day and this eve across the Interstate 95 corridor and points west in association with a bowing squall line. This squall line will then continue to move east across the remainder of the area, moving offshore within an hour or two either side of midnight. Given the 0-6 km storm motion, individual cells will be moving to the N at more than 50 mph. The primary risk continues to be damaging thunderstorm winds and there have been some gusts upstream in excess of 60 and 70 mph. However, there also remains a smaller risk for tornadoes, especially with cells that develop near a break in the squall line or out ahead of it. Some large hail can not be ruled out with the strongest updrafts. The mid to upper trough will become negatively tilted as it moves east with strong upper level support. Strong upward vertical motion with sharp mid-level cooling and diurnal heating will help to destabilize the atmosphere. Sounding data shows moisture and instability increasing in warm air mass ahead of the cold front with mixed layer CAPE around 1100 J/kg and precipitable water values around 1.80 inches. Dewpoints have surged into the upper 60s and lower 70s this afternoon. A low level jet of 60 kt within 2 kft of the surface was just upstream and will impinge on the area later this afternoon and tonight. Wind speeds to 75 kt were being sampled by upstream radars 5-10 kft above the surface. The wind profile does veer slightly from the surface through 5 kft, from SSE to SSW. Deep moisture with a Gulf of Mexico/Atlantic tap will help to produce very high rainfall rates. This suggests ponding water in low spots will be quite common. At this time, we are forecasting near 1 inch to 2 inches of rain across the area. However, we fully expect that some locations will tally more than 2 inches and isolated 3 inch amounts are certainly possible. In the wake of a strong cold front, plenty of cool and dry air will advect in from WSW to ENE between midnight and sunrise Monday. The column dries out quickly with precipitable water values dropping to less than a half inch by 12z Mon. Temps will drop down into the 50s. Monday will be dry with cold air advection underway. A weak mid level trough will skirt the area to the N, bringing some clouds later Mon and Mon eve, but it appears too dry to support any precipitation. Highs will be in the lower to mid 60s. .SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/... As of 300 PM Sunday...Behind secondary surge of cold and dry air, skies will clear Mon night and we are setting up for a cold night. Winds will have a difficult time completely decoupling given the 20- 30 kt winds above the boundary layer. No freeze is expected, but lows in the upper 30s and lower 40s are forecast. Our traditionally cold spots will likely dip to the mid 30s. Frost is not expected, given moist ground temps mainly in the mid 50s and lower 60s by that time and dewpoint depressions slow to narrow. High pressure to our S will move offshore Tue afternoon commencing a period of warm air advection. We should see temps recover to at least the mid and upper 60s Tue afternoon despite the abnormally cold start. As winds turn SW late day, a few lower 70s will be possible inland. Minimum relative humidity will drop as low as 25 to 30 percent with a few afternoon wind gusts of 15 to 20 mph likely. Winds will be very light to calm Tue night, but the overall flow will be SW which should keep temps from dropping below the mid and upper 40s and closer to 50 at the immediate coast. .LONG TERM /WEDNESDAY THROUGH SUNDAY/... As of 300 PM Sunday...Fair weather is expected Wednesday through Saturday, despite the passage of a cold front on Thursday as moisture is forecast to be limited with the front. Cool high pressure will then control area weather into Sunday when another frontal system approaching from the west will bring a small chance for showers and thunderstorms. Max temperatures Wed and Thu will reach the upper 70s to lower 80s with upper 60s to lower 70s expected Fri through Sun. Mins in the upper 50s to lower 60s Wed night will cool to the upper 40s to lower 50s for the remainder of the long term period. && .SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/... As of 300 PM Sunday...Behind secondary surge of cold and dry air, skies will clear Mon night and we are setting up for a cold night. Winds will have a difficult time completely decoupling given the 20- 30 kt winds above the boundary layer. No freeze is expected, but lows in the upper 30s and lower 40s are forecast. Our traditionally cold spots will likely dip to the mid 30s. Frost is not expected, given moist ground temps mainly in the mid 50s and lower 60s by that time and dewpoint depressions slow to narrow. High pressure to our S will move offshore Tue afternoon commencing a period of warm air advection. We should see temps recover to at least the mid and upper 60s Tue afternoon despite the abnormally cold start. As winds turn SW late day, a few lower 70s will be possible inland. Minimum relative humidity will drop as low as 25 to 30 percent with a few afternoon wind gusts of 15 to 20 mph likely. Winds will be very light to calm Tue night, but the overall flow will be SW which should keep temps from dropping below the mid and upper 40s and closer to 50 at the immediate coast. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... Fair weather is expected Wednesday through Saturday, despite the passage of a cold front on Thursday as moisture is forecast to be limited with the front. Cool high pressure will then control area weather into Sunday when another frontal system approaching from the west will bring a small chance for showers and thunderstorms. Max temperatures Wed and Thu will reach the upper 70s to lower 80s with upper 60s to lower 70s expected Fri through Sun. Mins in the upper 50s to lower 60s Wed night will cool to the upper 40s to lower 50s for the remainder of the long term period. && .AVIATION /00Z MONDAY THROUGH FRIDAY/...
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As of 00Z...Expect an array of flight categories due to convection, strong to severe, that will affect the area terminals for the majority of this evening and into the pre-dawn Mon hours. The line of strong to severe thunderstorms extends from Red Springs south across Sellers to Georgetown. The line is moving to the east at 15 to 25 kt whereas individual storms along this line are racing to the north upwards to 50 kt. This convection will roughly reach a line running from Jerome south to North Myrtle Beach by 930 PM...and a line running from Wallace south across Wilmington to Bald Head Island by 1100 PM. After the line of convection pushes thru, expect leftover showers with improving flight categories to MVFR to eventually VFR towards daybreak Mon. Enough moisture in the low levels will be left as a mid-level vort rotates thru during daylight Mon for a SCT to BKN Cumulus field in the 4k to 6k ft level. Expect strong synoptic south winds at 15 to 25 kt sustained with gusts up to 35 kt. After the convection pushes thru winds will veer to the SW initially around 10 kt during the pre-dawn hrs but then pick back up to 15 to 20 kt gusts to 25 kt by daybreak. The winds will further veer to the W during Mon aftn at the same speeds. Extended outlook...VFR.
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&& .MARINE... NEAR TERM /THROUGH MONDAY/... As of 630 PM Sunday...The squall line is still inland across the eastern Carolinas but should maintain itself quite well as it moves across the coastal waters late this evening and overnight. A power low level jet as strong as 50 knots just 1000 feet aloft should still bring gale-force winds tonight, even outside of thunderstorm activity. Latest buoy reports show wave heights are rising, and we still anticipate they could reach 10 feet near 20 miles distance from shore during the worst of the event tonight. Discussion from 300 PM follows... A Gale Warning is in effect for all waters through tonight. Numerous to widespread thunderstorms are expected tonight, ending before daybreak Mon with the passage of a cold front. Some thunderstorms later this eve and into the first part of the overnight will likely be severe. Gradient winds will increase to 30 to 35 kt, the strongest winds are expected this eve. Gusts to around 40 kt are expected tonight. S winds will veer to SW and then W behind the cold front. Seas will build to 7 to 11 ft, highest this eve and into the overnight before beginning a slow recession as offshore winds take hold. Small Craft Advisory conditions are expected Mon for winds of 20 to 25 kt and gusts to around 30 kt. Sustained winds early Mon morning will be near 30 kt. Seas will be subsiding from 6 to 9 ft early in the morning to 5 to 7 ft late in the afternoon. SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/... As of 300 PM Sunday...A secondary boundary will move across the waters Mon night, shifting the wind direction from W to NW. Winds will then back to W and SW Tue afternoon as high pressure to our S moves offshore. Wind speeds should still be 20 to 25 kt Mon night, diminishing to 15 to 20 kt late. Wind speeds of 15 to 20 kt Tue will decrease to 10 to 15 kt overnight Tue. Offshore flow Mon night will drive seas from 5 to 7 ft during the eve to 3 to 5 ft by Tue morning. Seas will be 3 to 4 ft for the remainder of the period, although 5 ft seas may linger across the outer northern waters Tue. LONG TERM /WEDNESDAY THROUGH FRIDAY/... As of 300 PM Sunday...High pressure will give way to a quick moving front that will cross the waters late on thursday. Southwest winds will increase from 10 to 15 knots early Wednesday to 15 to 20 KT Wed night and Thu. Winds will become W to NW after the frontal passage Thu night before shifting to the N and decreasing to 10 to 15 KT on Friday. There is a chance winds could reach 25 KT Wed night and Thu and a small craft advisory may be required Wednesday night into Thursday. Seas of 2 to 3 FT early Wed will build to 3 to 5 FT by evening and continue into Thu before subsiding to 2 to 3 FT by late Fri. && .ILM WATCHES/WARNINGS/ADVISORIES...
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SC...Wind Advisory until 1 AM EDT Monday for SCZ054>056-058. NC...Wind Advisory until 1 AM EDT Monday for NCZ105>110. Coastal Flood Advisory until midnight EDT tonight for NCZ107. MARINE...Gale Warning until 6 AM EDT Monday for AMZ250-252-254-256.
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&& $$ SYNOPSIS...RJD NEAR TERM...RJD/TRA/RGZ SHORT TERM...RJD LONG TERM...RAN AVIATION...DCH MARINE...

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