Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 171534 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 1035 AM EST Fri Nov 17 2017 .SYNOPSIS... A cooler day today will be followed by a breezy warmup Saturday ahead of a strong cold front. The front may be accompanied by some gusty showers as it sweeps across the area and offshore late Saturday night and early Sunday morning. Much colder and drier air will be ushered in for Sunday and into early next week and we have a reasonable chance of a widespread freeze. Temperatures will moderate Tuesday and Wednesday. Uncertainty increases mid and late next week as another cold front moves into the area and forecast will hinge on the track of developing low pressure in the Gulf of Mexico. && .NEAR TERM /THROUGH TONIGHT/... As of 300 AM Friday...Canadian High building in from the north this morning will shift southeast later today. Cold advection following the dry cold front, which is just about to cross the area, will continue through mid to late morning before starting to weaken. Incoming air mass is cool and dry with precipitable water at or below 3 tenths of an inch through the period. Will see plenty of sunshine today but the northerly flow and the airmass`s Canadian origin will contribute to temperatures below climo. High will end up around 10 degrees cooler than highs seen on Thu. The center of the high settles south this afternoon and overnight, allowing for good radiational cooling conditions. Lows will be below climo with light to calm winds and very dry air allowing temps to drop into the mid to upper 30s. Forecast soundings show an increase in moisture above 20 kft late in the period which could lead to some patches of cirrus spreading over the area a little before daybreak. These clouds could potentially reduce the effectiveness of radiational cooling late in the period. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... As of 300 AM Friday...A breezy warmup is in store ahead of a strong cold front which will sweep across the area late Sat night and then offshore around sunrise Sun. Canadian high pressure will then slowly build from the W through Sun night. Wind fields remain impressive with this system. The low-level jet will be on the order of 50 kt overnight Sat, at about 2-2.5 kft and not quite as low to the ground as was indicated 24 hours ago. The environment remains virtually stable during this time with CAPE values barely registering. However, bulk shear values are high and a strong upper level jet favorably, although briefly aligns. Moisture return is also brief and shallow and we are not expecting much in the way of rainfall, a tenth of an inch or less on average still looks in the ballpark. The showers will have the potential to drag down some of the stronger winds aloft, but even without this, it will become breezy later Sat and through Sat night with wind speeds on the order of 15 to 30 mph with higher gusts into early Sun. After collaborating with adjacent weather offices, decided to remove thunder given the almost total lack of instability, brief and shallow moisture return and the best forcing missing our FA to the north. Attention then turns to the magnitude of cold air advection with 850 mb temps flirting with zero by Sun night. The cold air will be enhanced as the pressure gradient weakens and radiational cooling maximizes. Temps will drop well down into the 30s during the wee hours of Mon morning and a freeze is becoming increasingly likely except perhaps along the immediate coast. Highs Sat will be in the lower 70s with the beaches staying near or just under 70 due to the cooling marine influence. We will lob off a good 10 degrees for highs on Sun and a brisk NW wind will make it feel as if it were in the 50s for a good portion of the day. Lows will be above normal Sat night, lower to mid 50s. However, watch for that freeze Sun night as the dry and chilly air maximizes. Did want to briefly highlight the elevated water levels at the beaches and along portions of the lower Cape Fear River with the Sat morning high tide. The new moon is on Sat and with that, we may be near or at minor coastal flood thresholds. The highest risk for reaching these thresholds will be across the Pender and New Hanover County beaches and in downtown Wilmington. && .LONG TERM /MONDAY THROUGH THURSDAY/... As of 300 PM Thursday...An amplified upper pattern will exist across North America next week. The forecast is fairly high confidence with two exceptions, owing to increasing model differences in the handling of significant shortwave energy into the southern branch of the jet Tuesday through Thursday. WPC prefers the ECMWF handling of this scenario next week which features a strong slow moving upper level low across the northern Gulf of Mexico by next Thursday, compared to the weaker and more progressive GFS pattern. Canadian high pressure overhead Monday will slip offshore Tuesday, with return flow buckling off the Southeast states as a weak coastal trough develops offshore of the FL/GA/SC coast. The ECMWF shows this feature close enough to the coast that perhaps some showers could push onshore Tuesday. Wednesday and Thursday a strong shortwave should dive into the Gulf. Differences in the handling of this feature between the GFS and ECMWF will determine the position of surface low pressure development and subsequent flux of moisture northward from the Gulf and/or Atlantic. There`s enough uncertainty at this point, and the feature may be far enough south, that differences are relatively minor. && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
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As of 18Z...High pressure will remain in control for one more day. Winds are a bit more gusty than anticipated, but they will diminish diurnally this evening. Some light fog is possible at all terminals after 08Z, with moderately favorable moisture profiles. Saturday, increasing cloudiness in advance of the next system. Winds will shift to the south as high pressure moves offshore. Extended Outlook...VFR with a chance of showers and possible MVFR/SHRA Saturday night into Sunday.
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&& .MARINE... NEAR TERM /THROUGH TONIGHT/... As of 300 AM Friday...Small Craft Exercise Caution headline remains in effect for NC waters this morning. Northerly surge will develop during the next few hours as dry cold front pushes south of the area and Canadian high starts to build in from the northwest. Pinched gradient will push speeds to a solid 20 kt through the morning hours before the gradient starts to relax and wind speeds decrease. Northerly wind this morning will veer to northeast in the afternoon with speeds dropping to 10 to 15 kt. Northeast winds further decrease overnight and the center of the high shifts south. Speeds will drop under 10 kt with direction becoming variable after midnight. Seas this morning will range from 4 to 5 ft off the NC coast to 3 to 4 ft with isolated 5 ft off the SC coast. Weakening northeast flow today will drop seas to 3 to 4 ft by midday and 2 to 3 ft by evening. Seas will be 2 ft or less by the end of the period. SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... As of 300 AM Friday...A strong cold front will sweep offshore around sunrise Sun with Canadian high pressure to slowly follow Sun night and Mon. The pressure gradient will tighten significantly with a low level jet on the order of 50 kt impinging on the area overnight Sat into early Sun. Water temps have cooled to within a couple degrees of 60 across the near shore waters. Still, such an energetic system will bring strong Small Craft Advisory conditions with SW winds increasing across the waters Sat and peaking in the 25 to 30 kt range late Sat night. Gusts to gale force are possible for about 6 hours late Sat night and it is possible marginal Gale conditions will materialize across the outer northern waters for a short time. In the wake of the front Sun, winds will shift to the W and then NW and then N Sun night as the center of the high nears from the W. Wind speeds will decrease to 15 to 20 kt late on Sun. An increasing cold and dry surge will allow for sustained winds near 20 kt Sun night. Seas will ramp higher late Sat and then peak in the 5 to 8 ft range late Sat night. Developing offshore flow Sun morning will quickly knock seas back to 3 to 5 ft Sun afternoon with similar seas into Sun night. LONG TERM /MONDAY THROUGH TUESDAY/... As of 300 PM Thursday...Canadian high pressure will build strongly across the waters on Mon and will provide much lighter winds as it moves overhead Mon afternoon and evening. The new GFS has come into better agreement with the ECMWF model about a faster veering around to E and SE winds offshore Tue as a weak coastal trough develops along the FL/GA/SC coastline. It remains to be seen how well-defined this coastal trough becomes, and how far west is can move. Our latest forecast does bring a period of E/SE winds into the coastal waters Tue, but with winds veering back to the north late Tue night as the trough is shown by most models to retreat offshore then. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...None. NC...None. MARINE...None. && $$ NEAR TERM...III SHORT TERM...RJD LONG TERM...TRA AVIATION...43

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