Area Forecast Discussion
Issued by NWS Wilmington, NC

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905 FXUS62 KILM 211119 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 621 AM EST Tue Nov 21 2017 .SYNOPSIS... A coastal trough just off the Carolina coasts and an approaching cold front from the northwest will combine to produce light pcpn late this afternoon thru tonight...with the coastal areas seeing the best shot of seeing measurable rainfall. High pressure will follow and slowly build in from the west Wednesday thru Friday. At the same time, low pressure will pass just south and east of the area Wed night thru Thu with the immediate coast again seeing the best chance for rainfall. A strong cold front will approach from the west during Sat and will sweep across the forecast area and off the coast to well offshore by late Sat night. Strong, dry and cold Canadian high pressure will follow Sunday thru early next week. && .NEAR TERM /THROUGH TONIGHT/... As of 300 AM Tuesday...Yet another cold night ongoing across the area, but with the growing season ended in most of the CWA, no frost/freeze products are in effect or needed. Temps will continue to fall slowly overnight, but should level off in the few hours before dawn as high level cloudiness increases from the SW. Big story today will be the development of a coastal trough currently beginning to sharpen offshore. This is noted by a weakness in the pressure field, as well as slowly increasing low/mid level cloudiness advecting NW just offshore. This 12 UTC guidance has jogged eastward. I did trend the forecast in feature will strengthen and try to advect westward as the surface high across eastern NC retreats to the East, but will likely remain just offshore through tonight. As this coastal trough sharpens, SW flow aloft will increase and pieces of vorticity energy will lift overhead downstream of a trough digging through the MS VLY. This causes slowly increasing isentropic lift, as well spawning a weak area of low pressure which will move NE offshore tonight. The NAM is an outlier being aggressive with saturation and rainfall today, while there is good agreement between much of the other guidance. It takes a long time for the mid-levels to saturate, but do expect light rain showers to develop from SW to NE this evening and tonight, with the heaviest rain likely along the immediate coast. QPF should be light, but over a tenth of an inch is possible across the Grand Strand and into Cape Fear. As the surface low pulls away tonight, a cold front will cross the Carolinas to be offshore by Wednesday morning, causing rapid drying and bringing an end to any rainfall chances. Highs today will climb well into the 60s, but have forecast slightly below MOS numbers to account for cloud cover and onshore flow off cool ocean waters. Despite the FROPA overnight, cold advection is weak and lags, so much warmer mins are forecast for tonight, 12 UTC guidance has jogged eastward. I did trend the forecast in falling only into the upper 40s NW of i-95, low 50s near the coast. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... As of 330 AM Tuesday...First thing Wednesday morning, weak low pressure accelerating ENE, will be pulling away from the FA taking any patchy pcpn and cloudiness with it. A coastal trof along the immediate coast, will get pulled back over the Atl waters during the morning. A cold front will drop from the NW, and sweep across the FA by midday and off Carolina coasts before stalling offshore and parallel to the coastline Wed night. Will observe partial clearing during Wed aftn and evening as weak CAA occurs via partial thicknesses schemes ie. 1000-850mb along with 850mb temps dropping. Max temps will run in the mid 60s slightly cooler than MOS guidance but normal for this time of the year. Min temps Wed night will also run about normal with lows in the upper 30s to low 40s. For Thu thru Thu night, models are defying unity with respect to the final wx outcome of this time period. This is in reference to a northern stream 5H s/w trof that drops to the SE States during Thu and progresses eastward Thu night. Sfc low pressure will develop along the stalled front offshore from SC late Thu and accelerates to the ENE Thu night, pulling away from the ILM CWA. The European and to a degree the NAM are both drier than the GFS fcst. This likely due to a less amplified or a flatter 5H s/w trof when both compared to the GFS. Will stay closer 12 UTC guidance has jogged eastward. I did trend the forecast into a dry bias, where the sfc low with best POPs and QPF staying off the ILM CWA Coast. Will still exhibit low chance POPs, mainly well east of the I-95 corridor to the immediate coast. By daybreak Fri, any pcpn and cloudiness across the FA will be accelerating off to the ENE, away from the FA. For Max temps Thu, will see widespread 50s via European MOS due to clouds and low chance pcpn. On a side note, the wetter GFS MOS Guidance indicates upper 40s for Thu highs well inland, ie. along and west of the I-95 corridor. Friday morning lows will exhibit climo norms, with upper 30s to near 40 covering it. && .LONG TERM /FRIDAY THROUGH MONDAY/... As of 245 PM Monday...The extended forecast will be mostly influenced by an almost standing wave trough across the eastern third of the conus. Ridging will remain focused across the four corners region. There remains some indication of a couple of systems moving across the baroclinic zone to the east but the 12 UTC guidance has jogged eastward. I did trend the forecast in the early periods a little drier so its essentially a dry forecast throughout. Overall temperatures will run a little below normal starting out on the cooler side of climatology with a brief warmup for the weekend. This will be followed by another cold shot early next week. && .AVIATION /12Z TUESDAY THROUGH SATURDAY/... As of 12Z...Mainly jet cirrus today. A developing low pressure system near Florida will track up the eastern seaboard this evening, bringing a chance for rain mainly at the coastal terminals. After the low passes to the northeast, winds will swing around to northwest, with a brief period of IFR ceilings/vis expected. Conditions should improve after daybreak on Wednesday. Extended Outlook...MVFR conditions may develop in low clouds and light rain early Wednesday morning,. VFR Thursday. MVFR to IFR conditions are possible late Friday into Saturday in low clouds and rain. && .MARINE... NEAR TERM /THROUGH TONIGHT/... As of 300 AM Tuesday...Light E/NE winds this morning will gradually transition to E/SE as a coastal trough sharpens and tries to lift NW towards the coast. Do not expect this feature to make it onshore, but it will cause the subtle wind direction change through this evening. The gradient will remain weak, so wind speeds will be around 10 kts regardless of direction. Tonight, a wave of low pressure offshore will pull the trough back to the east with winds becoming SW, followed by the rapid passage of a cold front by Wednesday morning shifting winds to the NW. The strongest winds are expected post-FROPA, reaching 10-15 kts at that time. Despite these light winds, seas will gradually rise through tonight as a SE swell around the offshore high pressure begins to amplify. Seas this morning are just 1-2 ft, and although the wind will do little to increase the seas, an 7-8 sec SE swell will grow to 4-5 ft, pushing wave heights to 3-4 ft tonight. SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... As of 330 AM Tuesday...Wednesday will be highlighted with CAA under northerly winds in the morning veering to the NE in the aftn and night. Wind speeds will run 10 to 20 kt Wed into Wed night as CAA and a tightened sfc pg contribute to the winds. With significant seas building to 3 to possibly as high as 6 ft during Wed night into Thu. Majority of the seas will come from short period, 5 to 6 second wind waves. For Thu thru Thu night, depending where the sfc low develops and intensifies relative to the ILM Coastal Waters, will determine both winds and locally produced seas. GFS again wants to intensify a low that is just short of a Gale late Thu. Will follow this trend but keep the low further offshore along with the best sfc PG. Will definitely see SCEC and possible SCA conditions Thu into Thu night and will highlight in the Hazardous Wx Outlook. Sig. seas will hold in the 3 to 6 ft range Thu into Thu night with a few 7 footers possible. LONG TERM /FRIDAY THROUGH SATURDAY/... As of 245 PM Monday...Northeast winds of 10-15 knots will prevail through the first two days of the extended marine forecast. There may be a few hours of 15-20 early on but overall 10-15 should do it. By Saturday a modest southwest flow will develop ahead of another cold front which should move across later in the weekend. Significant seas will trend downward from 3-5 feet early to 2-4 and possibly 1-3 feet by Saturday. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...None. NC...None. MARINE...None. && $$ SYNOPSIS...DCH NEAR TERM...JDW SHORT TERM...DCH LONG TERM...SHK AVIATION...43

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