textproduct: NWS Wilmington

WHAT HAS CHANGED

Model ensembles are becoming more insistent the arctic front may push back onshore Sunday. This would imply warmer air temperatures across the area Sunday into Sunday night, adding additional uncertainty on how persistent sub-freezing air will be. Forecast confidence has not improved over the past 24 hours.

KEY MESSAGES

1) A winter storm is expected to bring accumulating freezing rain and sleet across portions of eastern North and South Carolina this weekend.

2) Well-below normal temperatures will continue for days after the winter storm, prolonging impacts to those without warm shelter next week.

DISCUSSION

KEY MESSAGE 1: A winter storm is expected to bring accumulating freezing rain and sleet across large portions of eastern North and South Carolina this weekend.

A cold and very dry arctic airmass will build southward across the eastern Carolinas Friday night into Saturday. GFS and NAM forecast soundings show the arctic air will only be 3000-4000 feet deep, topped off by a continued warm and humid southwest flow. Within the shallow arctic airmass dewpoints could drop into the teens or even single digits Saturday.

A strong shortwave over the northern Rockies will pick up a cutoff low across Baja California on Saturday. Although the two systems do not phase, troughing will move eastward across the United States early next week. Deep southwest flow ahead of these systems will draw Pacific and Gulf moisture northeastward and across the Carolinas.

Shallow isentropic lift occurring above the arctic air Saturday may result in areas of sleet or rain/freezing rain with generally minor accumulations expected. The depth and magnitude of lift should strengthen Saturday night into Sunday morning with increasing precipitation coverage expected, especially inland. This is the period where we have the greatest concern for impactful accumulations of freezing rain as arctic air thins to only 1500-2500 feet deep with continued warm/moist advection aloft. A quarter inch or more of freezing rain could accumulate across the area covered by the Winter Storm Watch. Closer to the coast, temperatures should remain below freezing long enough for freezing rain with forecast accumulations less than 0.25 inch.

Confidence in what happens Sunday remains exceptionally low as major, MAJOR differences exist between the colder 12z GFS and NAM versus the warmer 00/06z/12z ECMWF, Canadian, and latest multi-model ensemble blend. The arctic front may, if the warmer cluster of models is correct, push back onshore Sunday afternoon allowing air temps to spike into the 60s. On the other hand, the colder 12z GFS/NAM look more like the old-school conceptual idea of how Carolina ice storms operate with shallow but very dense and stubborn arctic air unwilling to yield even in the face of impressive warm advection overhead. The difference in weather impacts between these two model camps cannot be overstated. Until model agreement improves, Sunday's current forecast can be considered an unlikely midpoint between two opposing solutions.

KEY MESSAGE 2: Well-below normal temperatures will continue for days after the winter storm, prolonging impacts to those without warm shelter next week.

Once precipitation ends Monday, cold arctic air will either remain entrenched or build back across the area from the northwest as 1036 mb high pressure moves eastward across the Mid South region and toward the Carolinas.

Daytime temperatures in the 30s or 40s are expected to plunge into the teens Monday night with lower to middle 20s expected Tuesday and Wednesday nights. This long period of impressive cold may even linger into the second half of the week. If sufficient ice occurs to result in power outages this weekend, impacts from this magnitude of cold will be significant to those without warm shelter.

AVIATION /00Z FRIDAY THROUGH TUESDAY/

Overall VFR conditions will persist for the most part for all areas. Some MVFR ceilings enter the picture as do light showers late in the period inland.

Extended Forecast...A winter storm will impact the area this weekend with prolonged restrictions expected. KFLO and KLBT in particular stand the greatest chance at recording FZRA, especially late Saturday into Sunday. VFR should return by Monday afternoon or so, continuing through Tuesday.

MARINE

Through Friday...WNW winds at 5-10 kts become more light and variable late this afternoon into this evening. After a bit of a lull, winds start picking up out of the NNE at 5-10 kts prior to sunrise Friday morning. Gradient winds continue to pick up towards 10-15 kts by midday, and gusts up to 20 kts are possible by the late afternoon. Seas at 2-3 ft pick up slightly to 2-4 ft.

Friday Night through Tuesday...The arctic cold front should push off the Carolina coastline Friday evening. Northerly winds will increase to 25-30 knots by early Saturday morning and a Small Craft Advisory is being issued with this forecast update. Winds should veer northeasterly during the day Saturday, then may begin to diminish late Saturday night as a wave low pressure develops and pushes the arctic front back toward the coastline.

Uncertainty remains high, but it's possible the front could cross the coastal waters Sunday morning and move back onshore briefly, especially across the Cape Fear area, Sunday afternoon into Sunday evening. If this were to occur, wind directions would become southwesterly across the area until the front gets a kick back offshore late Sunday night or Monday morning. Strong offshore winds of 20-25 knots are expected Monday into Tuesday as arctic high pressure builds eastward across the Mid South.

ILM WATCHES/WARNINGS/ADVISORIES

NC...Winter Storm Watch from Saturday afternoon through Monday afternoon for NCZ087-096. SC...Winter Storm Watch from Saturday afternoon through Monday afternoon for SCZ017-023-024. MARINE...Small Craft Advisory from 1 AM Saturday to 7 AM EST Tuesday for AMZ250-252-254-256.