textproduct: NWS Wilmington

WHAT HAS CHANGED

A Winter Storm Watch has been issued for all of Southeast NC and Northeast SC.

Based on the latest forecasts, a Cold Weather Advisory will not be required for the Bi-state region tonight.

KEY MESSAGES

1) Temperatures well below normal will result in periods of dangerously low wind chill temperatures below 16 degrees through at least Monday night.

2) Although the potential for winter weather and hazardous travel conditions this weekend are increasing, there remains some uncertainty with respect to key details.

DISCUSSION

KEY MESSAGE 1...Temperatures well below normal will result in periods of dangerously low wind chill temperatures below 16 degrees through at least Monday night.

A series of arctic air masses moving over the Southeast the next week will keep temperatures 10-20 degrees below normal. A Cold Weather Advisory may be needed for a portion of the area Fri night. Extreme Cold Warnings may be needed this weekend and potentially for the start of next week.

KEY MESSAGE 2:...Although the potential for winter weather and hazardous travel conditions this weekend are increasing, there remains a lot of uncertainty with respect to key details.

The large scale pattern and details have not changed. An unseasonably cold air mass will be in place over the Southeast this weekend (temperatures 20 degrees below normal at times). A southern stream shortwave/Gulf coast low crosses northern Florida then moves up the coast. Impressive 5h low/trough dropping south from the Great Lakes (originating around Hudson Bay) will enhance divergence aloft over the coastal low. A 150kt jet at 300mb moves across the low on Sat with the low in the favorable exit region, leading to impressive cyclogenesis off the Southeast coast. The guidance has come into somewhat better agreement with respect to the timing of the interaction between the surface and mid-level features and the location of the lows. Last night the surface low's location off the coast in much of the guidance ranged between Cape Fear and Cape Hatteras and anywhere from a hundred miles offshore to almost 500 miles offshore. There has been a shift with the position of the surface low to east of Cape Lookout and southeast of Cape Hatteras, around 200 miles from shore. This sets up the potential for significant snowfall across the area, although there are likely to be a sharp snowfall gradients.

Still a lot of uncertainty regarding specific forecast details. Anticipate a healthy dry slot developing, which will wrap into the 5h low and have at least some impact on the system. The exact location of the low and how much/quickly the dry slot wraps in will both be factors in where the heaviest precipitation bands set up. May see a bit of light rain or snow around daybreak Sat, but really think the precipitation will hold off until Sat. Much like the last event, the in situ air mass is very dry, it will take some time to moisten up and initially forcing is weak and the precipitation will not be very heavy.

Storm really starts to crank up later Sat into Sat night before moving off to the northeast Sun. While there is likely to be at least one area where total QPF exceeds 1", think these areas are more likely to be north and east of the forecast area (assuming the low location is remotely accurate). All indications are total QPF 0.30"-0.60" across the forecast area. The key to the forecast will be the snow ratio. While the air mass is cold, the coastal influence is strong and forecast soundings do show warming in the mid-levels as the flow aloft becomes more onshore and strengthens in response to the deepening low. Not enough for a change-over to freezing rain or sleet, but enough to affect the snow ratio. So while surface temperatures Sat night (the likely time of heaviest snowfall) suggest ratios of 15:1 or higher, not confident this will come to fruition.

For now thinking a good range across the forecast area is 3-6" with some higher pockets, especially along the northern border of the forecast area. However, there is potential for higher amounts (like double the forecast range). Somewhere there will be a band that sets up with double digit snowfall potential. Storm structure and location would favor north of the local area, but can't rule it out for the Cape Fear region. Another concern will be visibility restrictions due to falling/blowing snow. Wind gusts in excess of 25 mph will likely lead to white out conditions at times. Given the slight increase in confidence and the snowfall forecast a Winter Storm Watch has been issued for the area.

AVIATION /06Z THURSDAY THROUGH MONDAY/

VFR conditions are expected through the period. SKC into mid to late daytime morning hrs followed by opaque/thin cirrus thereafter. Weak CAA surge this morning will result in NW to N winds around 10 kt, with gusts up to 15+ kt possible between 14Z and 20Z. Center of bubble high pressure to temporarily park overhead late this aftn thru this evening. look for winds to diminish to less than 5 kt, possibly going calm at a few select terminals during this evening.

Extended Forecast...Another CAA surge to occur during the pre- dawn Fri hrs, followed by even colder temps. Flight restrictions are expected to develop with the next Winter storm system this weekend. Snow chances start late Fri night and continue thru Sunday morning, with heaviest rates and strongest winds late Sat and Sat night, potential producing brief whiteout conditions.

MARINE

Through Tonight...Weak CAA surge this morning will result in winds becoming NW-N 10-20 kt early this morning thru midday. There-after, weak bubble high moves overhead with the sfc pg relaxing. With NAA later this aftn into this evening combined with a relaxed sfc pg, N winds will further diminish to AOB 10 kt and continue thru sunrise Fri with hints of another CAA surge by or after sunrise Fri. Seas generally 2 to 4 ft early this morning, subsiding to 2 to 3 ft this afternoon and to around 2 ft tonight. Short period wind waves to dominate with an underlying 1 ft 10 second period E-SE swell.

Friday through Monday... Marine conditions will deteriorate late Fri with conditions meeting Small Craft Advisory thresholds Fri night. Winds continue to increase Sat into Sun with Gales all but certain and potential for Storm Force gusts. Conditions will start to improve late Sun as the still strengthening storm moves away from the region. However, treacherous marine conditions will continue through at least midday Mon. Potential for double digit seas late Sat into Sun after starting out 2-3 ft on Fri, even with a lot of offshore component. An easterly swell will be present through the weekend but the wind wave will be the bigger story and quite significant, running in excess of 5 or 6 ft at times.

ILM WATCHES/WARNINGS/ADVISORIES

NC...Winter Storm Watch late Friday night for NCZ087-096-099- 105>110. SC...Winter Storm Watch late Friday night for SCZ017-023-024-032- 033-039-054>056-058-059. MARINE...None.