Area Forecast Discussion
Issued by NWS Wilmington, NC

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182 FXUS62 KILM 180832 AFDILM Area Forecast Discussion National Weather Service Wilmington NC 332 AM EST Sat Nov 18 2017 .SYNOPSIS...
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A breezy warmup today will be followed by a strong cold front late tonight and Sunday morning. The front may be accompanied by some gusty late night showers. The front will usher in much colder and drier air with Monday seeing the lowest daytime highs with the possibility for a freeze or frost late Sunday night and Monday night. Temperatures will moderate Tuesday and Wednesday. Forecast uncertainty increases mid to late next week as another cold front moves into the area. The weather could turn unsettled and will hinge on the track of developing low pressure in the Gulf of Mexico.
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&& .NEAR TERM /THROUGH TONIGHT/...
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As of 300 AM Saturday...Coldest temperatures of the season so far ongoing across the area this morning. Many locations have dropped to 35 or less, with a few, Marion, Lumberton, Kingstree, and Conway dropping to 32 or lower. The current 29 at Lumberton is the coldest anywhere in our CWA so far this fall. These numbers are all well below any available guidance, and while the LAV numbers and RAP profiles have tried to "catch up," they remain too warm and this has made the next few hours a difficult forecast. Temps are lower than guidance likely due to cloud cover eroding faster than forecast due to subtle ridging and downward motion aloft, combined with weak ageostrophic drainage into the area from the surface high centered just to the north allowing the coldest/dry air to bleed southward into the CWA. Although high level cloud cover is encroaching from the NW, expect this will have little impact on temps until just before daybreak when subtle rises may occur, and have expanded the FR.Y for nearly the entire area. It is possible some counties may reach freeze warning thresholds before daybreak, but confidence is low since temps should slowly start to creep upward after 5am, so have opted not to go with FZ.W, especially since it would be such a late decision. The aforementioned increased cloud cover is associated with a shortwave and vort max moving through NC this morning, which is also creating a few sprinkles/showers upstream. This feature is progged to weaken as it lifts ENE and do not expect any precip even in the far NW zones this morning - with moisture manifesting only as cloud cover instead. Despite the very cold start, temps will rebound quickly this morning. A warm front lifting through the Southeast ahead of a strong cold front will push the area into a brief but strong warm sector beginning this eve. 850mb temps are forecast to climb from around 7C this morning, to as high as 13C this evening. This will allow temps to soar into the low 70s most places, despite increasing clouds both at cirrus level, and some advective stratocu and diurnal cu in the lower levels of the column. Pushing these temperatures upward are increasingly strong SW winds typical in the warm sector, with forecast profiles suggesting speeds reaching 30 kt at the top of the aftn PBL. This will potentially mix down as wind gusts to 20-30 mph this evening, and at the same time preclude much of an inversion after sunset so winds will remain elevated through the end of the period. These strong winds will become even more important tonight as a strong cold front driven by a potent shortwave digs across the Carolinas, moving offshore Sunday morning. Along this front an area of convection is likely, and the question remains as to how strong this may be, and how much QPF will occur across the area. Guidance has slowed the FROPA, so any showers are not expected until well after midnight, and likely won`t exit the coast until Sunday morning. This front will encroach upon a region characterized by very little CAPE (the most I could find was just 60 J/kg of MUCape), but low-level winds of 50-60 kt at 2-3 kft, very strong 0-6km shear near 60 kt, and strong but weakening 925-700mb fgen. Parcels lifting along the front will be capped above 700mb, at which temperatures are too warm for charge separation, and the weak inversion above this layer will not be overcome due to the weak instability. For this reason have left out any mention of thunder for tonight. However, showers occurring within these very strong low-level wind fields will support strong wind gusts, possibly reaching 40-50 mph in any stronger echoes as the line shifts east towards daybreak Sunday. Rainfall will be limited across the area as showers likely weaken as they progress quickly to the east. With the slower FROPA, WAA will be ongoing much of tonight, so mins will be uncharacteristically warm, dropping only to around 60 at the coast, mid 50s inland, quite a change from the temperatures being experienced at present!
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&& .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
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As of 300 AM Saturday...Models have slowed and the passage of a strong cold front should take place a few hours later than previously thought. Current thinking is that the front will be moving through the inland zones near daybreak Sun and then offshore mid and late morning. The slower timing requires that we keep a low risk for showers in the forecast a little longer, but maintaining the theme of the last several cycles, QPF will be light and generally a tenth of an inch or less in any one location. Will end the risk for showers from W to E Sun morning as quick clearing surges across the eastern Carolinas. The coldest air will move across the area later in the afternoon and Sun eve with 850 mb temps dropping to near zero. The 50 kt low- level jet (2-3 kft off the surface) will be lifting out by/during Sun morning, but cold and dry air advection will allow for a well mixed environment and so it should be brisk on Sun with sustained winds in the 15 to 30 mph range. A slower timing, especially given the quick return to sunshine, should allow temps to reach within a few degrees of normal given lows Sat night will be well above normal. Canadian high pressure will build to the coast Sun night and then overhead Mon and Mon night. The strongest cold air advection will wane overnight Sun, however, radiational cooling will maximize as winds go light to calm. Precipitable water values drop to just a tenth of an inch and it does not get drier than that. Dewpoints will fall into the 20s overnight Sun and so there will be plenty of room for temps to drop. Much of the guidance has come in a little warmer for Sun night, but I think they will trend lower in later runs. Frost seems like a low risk given the magnitude of dry air, but a freeze is very much in play for much of our inland communities, with a much lower risk for freezing temps near the more immediate coast. Later packages will likely be raising Freeze Watches and Freeze Warnings for portions of the Forecast Area. Monday will be the coolest day of the week and after a chilly start, highs are only expected to recover into the mid to perhaps upper 50s, even with bright sunshine with afternoon relative humidity dropping through the 30s. Light winds on Mon will not add a significant sting to the chill. The center of the high will move offshore later Mon night, but will continue to ridge back across the eastern Carolinas, keeping the column exceedingly dry. Typically, models will show temps moderating too quickly in these situations. Thus, the low temp forecast undercuts model consensus in most locations. We are forecasting lower to mid 30s with upper 30s to 40 degrees at the immediate coast. Where the growing season is still ongoing, a widespread frost may occur with some inland locations dipping to freezing. The only limitation to frost development would be if dewpoints can not recover sufficiently to bring dewpoint depressions to within a degree or two of each other overnight. The risk for frost will exist as you near the coast as well, but beach communities should be spared except on an isolated basis.
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&& .LONG TERM /TUESDAY THROUGH FRIDAY/...
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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. 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.
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&& .AVIATION /06Z SATURDAY THROUGH WEDNESDAY/... As of 06Z...High confidence in primarily VFR this valid period although some light fog this morning may produce some short duration restrictions. High pressure overhead is creating nearly cloudless sky conditions this morning. Temp/dewpoint spreads are very low, but moisture is quite shallow so fog is not expected. Local UPS fog procedure did produce a few areas of patchy fog, but have kept out any mention in TAFs attm. Cannot rule out some short duration MVFR, especially at LBT or CRE, but will handle this with an AMD if it becomes necessary. Otherwise, VFR will be the rule through the valid period with only FEW/SCT diurnal CU this aftn beneath increasing cirrus. The biggest impact today will be increasing winds, especially this evening and tonight, as speeds rise to 10-15 kts with gusts of 20-25 kts from the SW. Winds are not likely to ease after dark as the gradient pinches ahead of a cold front. This front will bring gusty showers to the terminals Saturday night, but most of this activity will hold off until after 06Z Sunday, and have only a mention of VCSH at the inland terminals before that. Extended Outlook...Showers and possible MVFR Saturday night into Sunday along with gusty winds. VFR Monday and Tuesday. Showers possible Wednesday. && .MARINE...
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NEAR TERM /THROUGH TONIGHT/... As of 300 AM Saturday...Placid marine conditions early this morning will begin to deteriorate quickly this aftn. A warm front will lift north through the Southeast today, ahead of a cold front which will approach the waters late tonight. This leaves increasing SW winds across the area, with speeds rising from 10-15 kt late this morning, to 20-30 kt tonight. Gusts to gale force are also likely, and may approach 40 kts at times. These strong winds have necessitated the issuance of an SCA, which goes into effect at 7pm this evening for all waters. After a period of low- amplitude seas through this morning, wave heights will grow steadily, finally peaking at 5-9 ft tonight during the period of strongest winds. The cold front will likely be positioned just west of the waters at the end of the period, but scattered showers with locally higher and erratic winds are possible tonight anywhere in the local waters. SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... As of 300 AM Saturday...Strong Small Craft Advisory conditions will slowly wane through the day Sun. A strong cold front will be moving offshore Sun morning, sweeping in a chilly and very dry airmass. Canadian high pressure will slowly build from the west Sun afternoon and night and then overhead Mon. The center of the high will move offshore late Mon night. Sustained wind speeds will be up to 25 to 30 kt Sun morning with a few lingering gusts to gale force early, decreasing to about 20 kt by late Sun afternoon and 15 to 20 kt overnight Sun. A further reduction in wind speeds is expected Mon, from 10 to 15 kt to 10 kt or less and these light winds will hold Mon night. Seas will be highest Sun morning, up to 6 to 9 ft. The developing strong offshore flow Sun should knock seas below 6 ft throughout late in the day. Seas will subside to 3 to 4 ft Sun night and to 2 ft or less Mon afternoon and night. The wind direction will be shifting from SW and WSW to NW Sun morning. NW winds will veer to N Sun night. N winds Mon will veer to NE with easterly winds developing late Mon night. LONG TERM /TUESDAY THROUGH WEDNESDAY/... As of 300 PM Friday...Return flow behind the departing high Tuesday 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.
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&& .ILM WATCHES/WARNINGS/ADVISORIES...
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SC...Frost Advisory until 8 AM EST this morning for SCZ017-023-024- 032-033-039-053-055. NC...Frost Advisory until 8 AM EST this morning for NCZ087-096-099- 105>107-109. MARINE...Small Craft Advisory from 6 PM this evening to 6 PM EST Sunday for AMZ250-252-254-256.
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&& $$ NEAR TERM...JDW SHORT TERM...RJD LONG TERM...TRA AVIATION...JDW

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