Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 212347 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 647 PM EST Tue Nov 21 2017 .SYNOPSIS... A coastal wave will move northward overnight, bringing a chance of showers. A cold front will press southward across the Carolinas on Wednesday. A few coastal showers are possible Thanksgiving Day, as a low pressure wave passes offshore. Seasonably cool and dry conditions will prevail into the weekend and early next week, as a series cold fronts sweep off the coast, and low pressure areas remain offshore. Strong Canadian high pressure will pass across the area early next week. && .NEAR TERM /THROUGH WEDNESDAY/... As of 630 PM Tuesday...Isolated light showers moving ashore from the south this evening. HRRR handling situation quite well so far and has rain coverage expanding through the evening hours from S to N, peaking around midnight or so with a focus on the coast, then out of the area by around daybreak. This scenario is close to what we already have in the forecast so if any changes are made they will be minor tweaks to coverage and timing. Still looks like a low QPF/stratiform event. Previous discussion from this afternoon follows: Coastal trough sitting offshore will attempt to move onshore this evening and confidence is low on whether or not this will happen. Should it move onshore the most noticeable difference would be increased rainfall rates within showers east of the trough as opposed to lighter rain on the west side. In the grand scheme of things the increase in rainfall would not be significant, a couple tenths of an inch at most as opposed to around a tenth of an inch. Temperatures would also briefly spike up as the trough moves onshore. Regardless of where the trough ends up this evening it will be lifted north and then pushed east later tonight. A shortwave rounding the base of the high amplitude 5h trough over the eastern CONUS will move northeast just off the Southeast coast. Surface wave/low associated with the wave will lift northeast along the coastal trough, helping move the feature out of the region. Light rain will spread over the area this evening as isentropic lift on the 295k surface increases and deeper moisture arrives. Precipitable water increases from half an inch around midday to around 1.3 inches overnight which should allow for measurable rainfall across much of the area. Rainfall amounts and coverage will be highest closer to the coast. Isentropic lift starts to wind down around midnight with any rainfall expected to come to an end shortly thereafter. Westerly flow below 7k ft as the surface and mid-level wave exit help dry the region out during the second half of the overnight period. Lack of any significant cold advection, combined with lingering cloud cover and mixing helps to keep lows above climo. Cold front trailing northern stream low moving into eastern Canada late tonight moves across the area Wed. The front is starved of moisture by the time it reaches the area and it will pass dry while helping to clear out any lingering clouds. Some modest cold advection behind the front but the air arrives too late to have any real impact on highs, which will end up a couple degrees above climo. Winds will become breezy following the passage of the front in the afternoon, especially as sunshine increases and stronger winds aloft start mixing to the surface. && .SHORT TERM /WEDNESDAY NIGHT THROUGH THURSDAY NIGHT/... As of 230 PM Tuesday...H5 ridge across the Desert SW and a trough will be positioned across the Eastern U.S. through the short term period. At the surface, high pressure will prevail as it builds across the Carolinas, possibly supplanting the areas of low pressure far enough offshore to limit rainfall chances during Thanksgiving. Isentropic lift is shown to be lacking as well. Currently, we will maintain the highest POPs across the coastal zones during Thanksgiving and keep QPF low unless the low can evolve farther west. Maintained a blend of MET/MAV temperatures through the period. && .LONG TERM /FRIDAY THROUGH TUESDAY/... As of 230 PM Tuesday...A progressive yet overall dry extended forecast prevails with seasonably cool temperatures, and dry largely in part, because the atmosphere remains dry above 8000 feet. A moderately strong cold front Saturday night may bring a few clouds but column moisture appears too scarce for rain. The dynamics aloft will be running strong, so a brief shower over SE NC cannot be ruled out but no QPF expectations at this time. Sunday should bring brisk north wind, as high pressure builds from the west. The core of the high, and clear skies will team up overhead daybreak on Monday, and may greet some inland areas with sub-freezing air. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/... As of 23Z...Latest radar and satellite pics showing area of light to some patches of moderate rain off the GA coast moving to the NNE. Developing low pressure system near Florida will track up the coast this evening, bringing the chance for rain. The rain will mainly be at the coastal terminals at least in the prevailing group and in the tempo group across the inland terminals. After the low passes to the northeast, winds will swing around to northwest, there will be a brief window of IFR ceilings/vis expected. Conditions are expected to improve within a few hours after daybreak as a cold front pushes across the region and helps to clear out the fog and low ceilings. Extended Outlook...VFR conditions expected by Wednesday afternoon. VFR Thursday. Low confidence for MVFR to IFR conditions near the coast late Thursday through Friday night in low clouds and rain. && .MARINE... NEAR TERM /THROUGH WEDNESDAY/... As of 630 PM Tuesday...Winds are in the 10 to 15 kt range this evening with a variable direction due to the proximity of the coastal trough. Seas range right around 3 ft. May update the forecast again for wind direction once we get a better sense as to the progression, or lack thereof, of the coastal trough. Given low wind speeds though, any changes will prove to be minor. We do expect increasing rain coverage through the evening hours, with the rain being mainly on the light, drizzly side. Previous discussion from this afternoon follows: Coastal trough continues to linger in the region with light southeast winds over the eastern waters and northeast along the coast. The trough will try to move onshore, and may briefly do so this evening. This would result in east to southeast winds across all of the waters but the weak gradient will keep speeds 10 kt or less. Surface wave moves northeast along the trough tonight, with winds backing to southwest and then west as night turns into day. Offshore flow following the wave/trough is not particularly strong, 10 to 15 kt, but a dry cold front will cross the waters late in the day. This feature is likely to be followed by a stronger surge of cold air, albeit after the end of the period. Seas will steadily build through the period with 2 to 3 ft this afternoon increasing to 2 to 4 ft this evening. Seas continue to build as the wave approaches and then, with the switch to southwest and then west, keep increasing, peaking at 3 to 5 ft during Wed. SHORT TERM /WEDNESDAY NIGHT THROUGH THURSDAY NIGHT/... As of 230 PM Tuesday...High pressure building across the waters from the north Wednesday night will maintain a northeasterly fetch and choppy seas. During Thursday, the area of high pressure will likely suppress areas of low pressure farther off the Southeast U.S. coast. This pattern will shift the stronger pressure gradient offshore by Thursday night allowing winds to gradually diminish across the coastal waters. At this time think seas will remain just below Small Craft thresholds across the outer waters but will need to monitor. LONG TERM /FRIDAY THROUGH SUNDAY/... As of 230 PM Tuesday...An easing trend in NE winds Friday will begin to back to the NW and increase on Saturday as deepening low pressure passes well offshore. The low will pull well away to the NE Sunday, with moderate N-NNW winds as high pressure builds from the west. An exercise caution period is possible Sunday for N wind, and elevated seas offshore. No 0-20 NM TSTMS this period but a few showers possible late SAT/early SUN as a cold front moves through. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...None. NC...None. MARINE...None. && $$ SYNOPSIS...99 NEAR TERM...REK/III SHORT TERM...SRP LONG TERM...MJC AVIATION...MAC MARINE...MJC/REK/III/SRP

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