Area Forecast Discussion
Issued by NWS Wilmington, NC

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000 FXUS62 KILM 180216 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 916 PM EST Fri Nov 17 2017 .SYNOPSIS... A cool night tonight will be followed by a breezy warmup on 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 with the possibility for Freeze warnings and/or Frost Advisories for portions of the forecast area during Sunday and Monday nights. Temperatures will moderate Tuesday and Wednesday. Uncertainty increases by the mid to late week period of next week as another cold front moves into the area and the forecast will hinge on the track of developing low pressure in the Gulf of Mexico. && .NEAR TERM /THROUGH SATURDAY/... As of 845 PM Friday...Center of high pressure is nearly overhead. Clear skies, subsidence and a decent sfc based inversion after sunset, has resulted in excellent radiational cooling conditions with winds having decoupled and temps dropping quickly into the low 40s with upper 30s inland by 8 PM. With sfc dewpoints in the 30s, temperatures for the remainder of this evening and overnight will either hold steady or fall a few more degrees as it converges toward the sfc dewpoint. Enough moisture at ground level combined with temps dropping into the 35 to 39 degree range, patchy frost away from the immediate coast will occur. Frost could develop as early as late this evening but expect it during the pre-dawn Sat hours. Mid to upper level clouds toward sunrise Sat may hamper the rad cooling conditions enough for temps to rise a few degrees above Frost threshold occurrence. Basically, frost is not expected to be widespread and therefore an advisory is not applicable at this time. Previous.................................................... As of 300 PM Friday...Cool and dry airmass will remain in place through tonight as Canadian high pressure migrates south and east. The center of the high will settle almost directly overhead tonight before shifting off the coast by daybreak Saturday. This will produce ideal radiational cooling with near very light to calm winds and clear skies. Once the atmosphere decouples after sunset temps should drop off quite rapidly. The very dry air mass in place will produce a large diurnal swing with temps dropping into the mid to upper 30s most places just inland. Inland Soundings show temps dropping just enough to produce a very shallow saturated layer. There may be just enough very shallow moisture to produce some patchy frost which I have included in the grids inland. Places along the coast may see some marine moisture produce some fog or a few lower clouds advecting on shore under subsidence inversion toward morning. Heading through Saturday, the winds will shift to the south in a deeper increasing low level return flow as a cold front begins to make its way toward the Carolinas from the northwest later in the day. Pcp water values down near a quarter of inch through this evening race back up as high shifts off the coast into Sat. Moisture profiles show potential for some mainly lower and high clouds through the day on Saturday, but plenty of mid level dry air and subsidence will keep any pcp out of the forecast until after the near term period when cold front moves in. May see decent cloud layer under subsidence inversion just under 6k ft by 21z. Overall expect a breezy and warm day as temps recover nicely in WAA. The 850 temps jump back up tao near 12c through Sat aftn. Temps will reach up into the 70s most places, but an increase in clouds may hold back temps a bit. && .SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY NIGHT/... As of 300 PM Friday...Impressive cold front due Saturday night. And though the baroclinicity is considerable and wind fields quite strong guidance continues to insist on a low QPF event. Given the quick movement of the boundary and its associated forcing this seems very plausible. Forecast soundings show enough lack of instability to keep thunder out of the forecast...but a deep enough convective shower may be able to mix down 40-50kt winds to the surface as such wind speeds may be lurking a mere 2000-2500ft up. Cold advection Sunday drops 850mb temps from 8C to 2C from 12Z to 00Z. NW flow continues Sunday night but warm advection kicks in as low level thermal ridge builds in from the west. This advection will occur above the surface-based boundary layer however and Sunday night lows will be in the low to mid 30s and a freeze warning may be needed for northwestern zones. && .LONG TERM /MONDAY THROUGH FRIDAY/... As of 300 PM Friday...A split upper pattern will exist next week across North America. A series of shortwaves moving across southeastern Canada in the northern stream will have relatively little impact on the Carolinas, however a pair of shortwaves in the southern stream will have impacts on our sensible weather Tuesday and Thursday Night into Friday. Canadian high pressure at the surface Monday will move eastward and off the coast late Monday night. Model support for this idea is quite high. Veering low-level winds and a warm advection pattern developing Tuesday and Tuesday night should support the development of a coastal trough as a weak shortwave approaches from the southwest. There are significant model differences in the track the shortwave takes (ECMWF from south GA to Cape Fear; GFS from central GA to near Norfolk) which then determine how well-developed the surface system can become. A compromise solution should advect enough shallow Atlantic moisture back onshore for a 20-30 percent chance of showers Tuesday night, clearing out by Wednesday morning as the shortwave moves offshore. Cold air will sneak south as another Canadian high moves across the Ohio Valley. This should keep highs on Thanksgiving Day only in the upper 50s to around 60, a good 6-8 degrees below normal. Attention then turns to a much stronger shortwave diving into the western and central Gulf of Mexico Wednesday night and Thursday. Just like yesterday there are still model differences here, with the ECMWF slower than the GFS with the eastward movement of the trough across the Gulf of Mexico Thursday and Friday. A compromise solution of the operational GFS, ECMWF, and their ensembles brings rain chances into the area Thursday night through Friday Night, but given all the uncertainty (and to better blend with surrounding NWS offices) I`m capping PoPs no higher than 30 percent for now. && .AVIATION /01Z SATURDAY THROUGH WEDNESDAY/... As of 23Z...High pressure remainS in control across the region but will give way to an advancing cold front reaching the region towards the end of the period. atmosphere has decoupled with winds becoming mainly clam across the terminals and staying that way for much of the night...maybe a 203kt winds across the myrtles. Some light fog is possible at all terminals after 08Z, with moderately favorable moisture profiles. On Saturday, clouds will increase in advance of a cold front. Winds will shift to the south as high pressure moves offshore and will increase to around 13kts. Extended Outlook...VFR with a chance of showers and possible MVFR/SHRA Saturday night into Sunday. && .MARINE... NEAR TERM /THROUGH SATURDAY/... As of 845 PM Friday...The center of sfc high pressure will basically be overhead this evening and overnight before progressing off the coast and offashore during Sat. Winds will drop to NE at 5 to 10 kt with possible calm conditions across portions of the local waters due to a well relaxed sfc pg. The progged sfc pressure pattern indicates once the high moves off the coast, return flow from off the Atlantic will become common across all waters after sunrise Sat. Significant seas initially will run 2 to 3 ft except up to 4 ft off Cape Fear. Seas will drop to their lowest Sat morning before picking back up during Sat afternoon and night. A pseudo Easterly ground swell will dominate the local seas initially given the absence of any notable wind driven waves. This will change during Sat after the sfc high moves offshore. With increasing wind driven waves becoming more dominant as morning SE-S winds increase that eventually veer to the SW and W ahead of the approaching cold front late Sat thru Sat night. Previous....................................................... As of 300 PM Friday...Winds will diminish further as high pressure shifts closer overhead through tonight. This will also allow for a veering of winds from NE to E by midnight, shifting further around to the SE and S through Sat morning as the center of the high shifts off the coast of Hatteras. Overall, winds will drop off into tonight to under 10 kts before picking up again through Saturday. Gradient will tighten through Saturday aftn between high pressure to the east and a cold front approaching the Carolinas from the NW. Expect winds increasing up to 15 kts by end of period with gusts up to 20 kts. Seas will remain less than 3 ft through Sat morning, but increasing southerly winds Sat aftn, will push seas up to 3 to 4 ft by Sat eve. SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY NIGHT/... As of 300 PM Friday...Powerful cold front approaching Saturday night. A small craft advisory will be issued next forecast cycle or thereabouts. Both the pre-frontal and post-frontal flow regimes will support advisory-worthy wind and seas. The actual FROPA and wind shift will come right around daybreak Sunday. At this time it seems that the SSTs being much cooler than the airmass will preclude pre-frontal Gale conditions and later in the period as the airmass cools the wind fields will weaken sufficiently to do the same. LONG TERM /MONDAY THROUGH WEDNESDAY/... As of 300 PM Friday...High pressure will move across the Carolinas Monday, then offshore late Monday night. By Tuesday return flow behind the departing high will buckle as a coastal trough develops along the Southeast coast. Winds should turn easterly on Tuesday, then perhaps southeasterly for a time Tuesday night as the trough makes its closest approach to shore, but by Wednesday morning the feature should retreat back offshore as high pressure advances across the Ohio Valley and shifts our winds to a more northerly direction. && .ILM WATCHES/WARNINGS/ADVISORIES... SC...None. NC...None. MARINE...None. && $$ SYNOPSIS...TRA NEAR TERM...DCH/RGZ SHORT TERM...MBB LONG TERM...TRA AVIATION...MAC is the U.S. government's official web portal to all federal, state and local government web resources and services.