Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 231939 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 339 PM EDT Mon Apr 23 2018 .SYNOPSIS...
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An approaching storm system will bring wet weather into Tuesday, along with windy coastal conditions, dangerous surf, and high beach tides. Easing winds and a warming trend will follow into mid-week in wake of this system. Another low pressure emerging from the Gulf states, will bring rain chances Thursday into Friday. Drying and seasonable temperatures are expected Saturday into early next week.
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&& NEAR TERM /THROUGH TUESDAY/... As of 300 PM Monday...A messy and complex forecast developing for the near term and the entire area is expected to see heavy rainfall and gusty winds this evening through early on Tuesday. Bowling ball type upper low clearly evident across on GOES-E WV imagery across the TN VLY is above an occluded surface low. An occluded front extends from the low towards secondary low- development in eastern GA, from which a triple point exists with a warm front and cold front into the Atlantic Ocean and Gulf of Mexico, respectively. Ahead of this warm front, moisture is pooling and increasing across the Atlantic, and as flow becomes more strongly onshore this evening and convergence becomes maximized, a period of potentially very heavy rainfall is likely. Locally, the setup looks similar to October, 2015, of course that was a 4 day event and this will last 12 hours. Light rain has overspread the entire CWA and will continue periodically through the evening. This rainfall is due to a combination of isentropic lift atop the surface high extending down from New England, the edge of the 850mb theta-e ridge, and weak forcing due to mid-level disturbances embedded within the flow. The next several hours will feature periods of rainfall with QPF less than one-quarter inch. However, this is truly the `appetizer` before the main event which is progged to occur overnight. As the warm front lifts northward, moisture will surge into the Carolinas in response to a strengthening gradient driving better moist advection onshore, and an anomalously strong LLJ of 50+ kts at 850mb surging PWATs towards 1.7 inches. These winds are forecast to reach nearly 4 standard deviations above the mean, which when combined with +2 sigma PWAT, and warm cloud depth of over 3km suggest an environment which will support very heavy rainfall. Additionally, strong diffluence both in the mid levels thanks to the closed nature of the low to the west, and within the LFQ of an upper jet, as well as strong FGEN within the 850mb-700mb layer along the warm front, and low-level convergence both along the front and due to frictional effects onshore, help drive lift and strong UVVs despite weak to none instability. As the warm front pivots NE overnight, all of these other ingredients will accompany this motion, and it is expected that a band of very heavy rainfall will lift SW to NE across the CWA overnight. While lack of instability is a mitigating factor, the other parameters all point towards widespread 1-3" of rainfall with locally higher amounts, especially closer to the coast. Fortunately, we have been dry, and 12-hr FFG guidance is 5-6", so no Flash Flood Watch is needed and flooding should be confined to short term poor drainage type. The main axis of rainfall will lift NE out of the area by late morning, and the region will briefly be within the warm sector ahead of Tuesday`s cold front which will lift NE through the aftn. Guidance is hinting that enough forcing and residual moisture will re-spawn some showers and maybe a tstm Tuesday aftn, but very dry air above 700mb by that time will limit the overall coverage and threat. By Tuesday evening the rainfall should be ending across the CWA. Additionally, a wind advisory is in effect for the coastal counties as winds will steadily increase this evening and even overnight as the LLJ impinges on the region. Attm do not expect true advisory criteria to be met (sustained 31-39 mph or gusts 46-57 mph), but enough mixing within the PBL should create wind gusts of 30-40 mph, with higher gusts possible near the coast. These winds are likely to occur during the heaviest rain as well, so for the potential for some downed trees in saturated soils /public impact/ have extended the wind advisory for all coastal counties through 5am. While typically winds ease at night, progs suggest the highest wind gusts are actually expected overnight thanks to the LLJ, and have added gusty winds into the wx grids as well. Winds will ease slowly on Tuesday, but will remain gusty through the aftn. Temps have not warmed as much as expected today due to enhanced cloud cover and onshore flow, with most places stalling in the low to mid 60s. For this same reason, temps will be slow to fall tonight with only a few degrees of cooling into the upper 50s/low 60s expected. Increasing sunshine and the aftn warm sector will drive highs on Tuesday to much warmer values more typical of late April, into the mid 70s away from the coast. && .SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/...
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As of 300 PM Monday...Upper level low pressure across the southern Appalachians Tuesday night will open up and lift northeastward Wednesday as a pair of strong shortwaves approach from the north and west. Surface low pressure will move northeastward and away from the area in tandem with the weakening upper low, allowing sunshine to return on Wednesday. Highs on Wednesday should reach the upper 70s to near 80 degrees away from the beaches. These warm temperatures in combination with dewpoints hanging in the 50s should develop a substantial field of cumulus clouds during the day. While across central and northern North Carolina colder mid-level temperatures should allow these cumulus to grow deeper and produce showers, a subsidence inversion between 7000-10000 feet AGL that becomes stronger the farther south you go should prevent showers from developing locally. I`ve got 10% PoPs across Lumberton, Elizabethtown and Wilmington during the day Wednesday but honestly expect most convective showers to remain farther north. As the departing surface low deepens as it moves from NJ into New England Wednesday night, a cold front will get a renewed shove southward and should move through the Carolinas and off the coast overnight. The atmosphere should remain stable for this frontal passage with insufficient moisture for any showers.
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&& .LONG TERM /THURSDAY THROUGH MONDAY/...
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As of 3 PM Monday...Another area of low pressure may bring showers to the area Thursday into Friday. Otherwise, the long term looks dry with temperatures near or slightly below seasonal normals. Normals for this time of year run mid to upper 70s by day and mid 50s overnight.
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&& .AVIATION /18Z MONDAY THROUGH SATURDAY/... As of 18Z...Conditions will slowly deteriorate through the afternoon hours as low pressure and associated front move northeast. Light rain will overspread the CWA as isentropic lift increases. Ceilings will hang on to VFR until this afternoon, except FLO as they are expected to lower to MVFR soon. Tonight, heavy rain expected with ceilings and visibilities eventually becoming IFR. Easterly winds will be quite strong, with gusts to 30 kts possible. Tuesday, continued heavy rain in the morning with IFR conditions. We should get dry slotted between 10-13Z from southwest to northeast. It is possible that we may get enough cape to kick off a brief round of convection around midday. Extended outlook...Tue morning MVFR/IFR in heavy rain. late Tue night-Fri VFR, except for possible MVFR Thu night. && .MARINE...
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NEAR TERM /THROUGH TUESDAY/... As of 300 PM Monday...Poor marine conditions forecast through the period as a strong low pressure system impacts the waters. Surface low developing across GA is draping a warm front and cold front into the SE. This entire system will lift NE overnight into Tuesday, while high pressure near New England slowly retreats. The gradient between the two becomes increasingly pinched, and winds will climb steadily this evening and overnight, becoming 20-30 kts from the E/SE with gusts to 40 kts. A Gale Warning is in effect and will persist into Tuesday aftn. During Tuesday conditions will slowly improve with winds veering from SE to SW by the end of the period as the cold front lifts across the waters. During this wind shift speeds will gradually lower as well, becoming 15-20 kts with higher gusts, and an SCA will likely be needed once the GLW drops. In addition to these strong winds, a rapidly amplifying sea state is forecast as waves build to as high as 13ft/9sec at Frying Pan Shoals overnight. This type of swell will push coastal seas from the current 4-8 ft, to 8-12 ft overnight, before slowly easing back to 4- 8 ft by the end of the period. Additionally, a period of very heavy rainfall is likely across the waters overnight. This rain will produce severely reduced visibility to 1NM or less at times, in addition to chaotic and stronger winds. The most likely time for these poor conditions will be overnight, improving on Tuesday, and although lightning should not be widespread, isolated cloud to sea lightning strikes are also possible overnight. SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/... As of 300 PM Monday...Low pressure centered near Raleigh Tuesday evening should drift northeastward across the Chesapeake Bay Wednesday morning, then into New England Wednesday night. A cold front behind the low will initially be very slow to move south, but should get a renewed kick Wednesday night as the low deepens. Compared to winds coming up tonight and Tuesday, winds Tuesday night through Wednesday night should be a piece of cake: 15 knots from the southwest Tuesday evening, then around 15 knots again with the seabreeze on Wednesday afternoon. Wind directions should veer northerly by late Wednesday night as the cold front moves across the area. Despite diminishing wind speeds, large seas will remain with us through at least Wednesday morning, a consequence of the storminess along the Southeast coast today through Tuesday. Nine-second southeast swell as large as 8 feet will move through the coastal waters Tuesday evening before diminishing to 3-6 feet Wednesday morning. We`ll likely need to keep a Small Craft Advisory running through Wednesday morning. LONG TERM /THURSDAY THROUGH SATURDAY/... As of 300 PM Monday...Expect shifting winds over the waters Thursday and Thursday night with low pressure moving through. NE winds Thu AM will become SE by Thu night and SW on Friday. Speeds currently look to run about 15 KT but this could change depending on how this next low progresses. W to SW winds of 10 to 15 KT are expected for the remainder of the period. Seas of 3 to 4 FT THU will subside to 2 to 3 FT Friday and Saturday.
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&& .ILM WATCHES/WARNINGS/ADVISORIES... SC...Wind Advisory until 5 AM EDT Tuesday for SCZ054>056-058-059. High Rip Current Risk until 8 PM EDT this evening for SCZ054- 056. NC...Wind Advisory until 5 AM EDT Tuesday for NCZ105>110. High Rip Current Risk until 8 PM EDT this evening for NCZ106- 108-110. Beach Hazards Statement until 8 PM EDT this evening for NCZ110. MARINE...Gale Warning until noon EDT Tuesday for AMZ250-252-254-256. && $$ SYNOPSIS... NEAR TERM...JDW SHORT TERM...TRA LONG TERM...RAN AVIATION...DL/JDW

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