textproduct: Raleigh

WHAT HAS CHANGED

As of 325 PM Friday....

Only slight adjustments were made to the snowfall forecast for this weekend's winter storm, but the current spread in high-resolution model guidance is high, which reduces confidence.

KEY MESSAGES

As of 325 PM Friday...

1) A winter storm is expected to affect central NC from this evening into Saturday night and Sunday morning. Precipitation will be all snow, with a general 4 to 8 inches expected across most of the area, but locally higher amounts of 8 to 12 inches will be possible, with the best chance along and east of I-95. However, there is still plenty of uncertainty on exact amounts.

2) A prolonged Arctic air mass in place will result in well below normal temperatures across central NC from today through much of next week.

3) There is a chance for precipitation mid-week, but the timing, temperatures, and overall amounts remain uncertain at this time.

DISCUSSION

As of 325 PM Friday...

KEY MESSAGE 1... A winter storm is expected to affect central NC from this evening into Saturday night and Sunday morning. Precipitation will be all snow, with a general 4 to 8 inches expected across most of the area, but locally higher amounts of 8 to 12 inches will be possible, with the best chance along and east of I- 95. However, there is still plenty of uncertainty on exact amounts.

Current water vapor imagery depicts the northern stream mid/upper trough that will be the main driver of our winter storm currently diving south into the eastern Dakotas/MN/IA. This trough will close into a mid/upper low as it swings over the mid MS and TN Valleys tonight, eventually becoming negatively tilted as it moves across and offshore the Carolinas on Saturday night and Sunday morning. Meanwhile a surface low will develop over the northern Gulf today, taking a classic "Miller A" track as it tracks NE off the coast of the Carolinas along a baroclinic zone on Saturday. As it interacts with the mid/upper low and reaches the Gulf Stream on Saturday night, it will rapidly deepen and slow down east of the Outer Banks, becoming ~970 mb as it finally pushes NE out to sea on Sunday afternoon. The atmosphere is sufficiently cold that all snow is expected with this event everywhere across central NC. The main question is how much precipitation falls and where the banding sets up.

Light snow is already being reported in eastern KY and western VA associated with the mid/upper low to our NW. Latest high-resolution models suggest that it will increase in coverage to include the far northern Piedmont of NC by early this evening. Another area of snow associated with the coastal low is expected to develop overnight tonight and spread into at least our far SE zones. Any snowfall amounts through sunrise Saturday are likely to be less than an inch.

Snow should really increase in coverage and intensity beginning Saturday morning, with the event likely peaking in the afternoon and evening as the mid/upper low interacts with the deepening coastal low and a band of 850 mb frontogenesis sets up. With very cold temperatures in place, the snow will be of the dry and fluffy variety as snow to liquid ratios through the event generally range from 12:1 to 20:1, lowest in the SE and initially, highest in the NW and by Saturday evening/night. Liquid equivalent QPF for the event is generally expected to range from a third to three quarters of an inch, lowest N and highest SE. This generally translates to 4 to 8 inches across most of central NC, but a band of 8 to 12 inches is likely to set up somewhere over the state with the best chance along and east of I-95. There is also a scenario where many spots receive lower amounts that will have to be monitored, which is explained below.

Despite only being 24 hours from the event, there is still quite a bit of discrepancy among the models on where exactly the band of heavier snow will be, and it will depend on where exactly the elevated frontal zone sets up, as well as how far south the mid/upper low tracks. This is a complex setup and small shifts in track and intensity can have a huge effect on how much snow falls. If the mid/upper low tracks farther south, it will take longer to interact with the coastal low, keeping the 8+ inch amounts to our east, and potentially resulting in lower than expected snowfall totals across a good portion of central NC. This is the scenario depicted by the 12z NAMNest and 18z HRRR, which depict very light amounts stretching from the northern Coastal Plain into the NE Piedmont due to a gap in forcing between the mid/upper low to our west and the coastal low to our east, a signal that has shown up on some model runs since yesterday. This is a concerning trend that will have to be monitored to see if it continues. But until there is more consistency from run to run, it's hard to take this one scenario with too much weight. High-res models have been waffling back and forth, with a notable shift east in the axis of heaviest snow in some of the 12z and 18z runs compared to 06z. However, if the mid/upper low and coastal low interact sooner, as depicted by models like the 12z GFS and lower-resolution NAM, a larger area of moderate to heavy snow will develop over central NC. Bust potential is fairly high and this may be a nowcasting situation where we don't know exactly how far inland the heaviest snow will get. For now, a Winter Storm Warning continues from this evening through Sunday morning.

Where moderate to heavy snow does fall, travel will likely be very difficult if not impossible at times from Saturday into Sunday morning, as temperatures will be cold enough for snow to stick quite efficiently. Winds will gust to 25 to 35 mph, perhaps gusting as high as 40 mph, particularly in the Coastal Plain. This could cause blowing and drifting of snow, reducing visibilities to less than a mile, especially considering it will be light and easily blown around. There could also be isolated power outages, but the lightweight variety of snow and lack of ice should keep trees and power lines from being weighed down too much and prevent widespread outages from taking place. Snow will end from west to east late Saturday night and early Sunday morning. But gusty NW winds behind the deepening low could keep some blowing snow through the day on Sunday.

KEY MESSAGE 2... A prolonged Arctic air mass in place will result in well below normal temperatures across central NC from today through much of next week.

Cold weather headlines are in place tonight (Cold Weather Advisory) through Saturday night (Extreme Cold Watch). The Extreme Cold Watch for Saturday night will likely need to be upgraded to either a Cold Weather Advisory or Extreme Cold Warning with subsequent forecast updates.

As the strong 1045 mb Arctic high tracks south into the central US tonight into Saturday morning, overnight lows tonight will drop into the mid to upper teens across central NC. The sfc pressure gradient will start to strengthen some overnight as the offshore low begins to develop. Forecast soundings indicate mixing potential overnight with possible gusts of upwards to 10 to 20 mph. As such, wind chills will likely hit upper single digits tonight/early Saturday across most of central NC.

With persistent nly flow and a highly anomalous temperature profile (925-850 mb temps bottom out in the 1st to 5th percentile compared to climo), daytime highs will only reach the lower to mid 20s Saturday afternoon (~30 degrees below normal). As we pivot to Saturday night, wind gusts via the deepening offshore low will increase to 25 to 35 mph (higher gusts possible across the Coastal Plain). As such, minimum wind chills Saturday night into early Sunday will likely reach dangerous levels (0 to -5 for most of central NC). Blustery conditions will persist early Sunday, but winds should lax some in the afternoon as the low pulls further offshore.

Snow will have ended by Sunday evening. Given the lingering snowpack, overnight lows will easily drop into the upper single digits to lower teens everywhere Sunday night. Winds should largely diminish overnight Sunday as the low pulls further offshore, but general minimum wind chills will likely reach Cold Weather Advisory criteria for much of central NC Sunday night.

Nwly flow aloft behind the departing low keeps conditions cold Monday as the arctic high transitions across the Chesapeake. Given the cold airmass, and likely remnants of lingering snowpack, highs will struggle to reach the mid to upper 30s Monday afternoon. Overnight lows Monday will again be quite cold in the mid teens, however calm winds should preclude the need for cold weather advisories.

Flow aloft turns zonal Tuesday with some moderation in the low-level thicknesses. Daytime highs will reach the lower to mid 40s both Tuesday and Wednesday afternoons. A brief shot of colder air may be possible behind our next potential system Thursday.

KEY MESSAGE 3... There is a chance for precipitation mid-week, but the timing, temperatures, and overall amounts remain uncertain at this time.

There continues to be a signal for potential precipitation middle of next week but details in timing, ptype, and potential impacts are vague at this point. It does appear that guidance is at least in some agreement that this may be more in line with a classic Miller B setup, which could lead to mixed ptypes. For now, given the uncertainty this far out, decided to keep in just rain/snow mix at times Wednesday into Thursday.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

As of 1225 PM Friday...

TAF period: VFR conditions are expected through the afternoon into the evening, but ceilings will decrease as more moisture moves into the region with snow developing late this evening into the overnight hours. It's still a little early to determine where banding may set up, although there is definitely potential for that to occur overnight into Saturday. Widespread IFR visibilities are expected by sunrise, although this will likely be delayed until after the TAF period at RWI. The wind will be out of the east-northeast around 5 kt through the afternoon, with 20-25 kt gusts developing this evening into the overnight hours at all sites.

Outlook: Widespread light to moderate snow will continue through Saturday and Saturday night, tapering off by Sunday morning. IFR/LIFR conditions will continue in snow, and gusts could reach as high as 30-35 kt Saturday night and Sunday. VFR conditions should return on Sunday, although with gusty wind continuing, cannot rule out restrictions due to blowing snow with the fresh snowpack. The next chance for precipitation will be on Wednesday, with temperatures likely warm enough for any precipitation to fall as rain.

CLIMATE

Record Low Temperatures:

January 31: KGSO: 3/1966, KRDU: 3/1966, KFAY: 9/1934 February 1: KGSO:-4/1936, KRDU: 8/1981, KFAY: 1/1936

Record Low Maximum Temperatures:

February 1: KGSO: 27/1971, KRDU: 28/1900, KFAY: 32/1981 February 2: KGSO: 30/1951, KRDU: 31/1994, KFAY: 31/1948 February 3: KGSO: 23/1996, KRDU: 24/1996, KFAY: 30/1961

RAH WATCHES/WARNINGS/ADVISORIES

Winter Storm Warning from 4 PM this afternoon to 7 AM EST Sunday for NCZ007>010-021>026-038>041-073>076-083-084. Cold Weather Advisory from 1 AM Saturday to 1 AM EST Sunday for NCZ007>011-021>028-038>043-073>078-083>086-088-089. Extreme Cold Watch from late Saturday night through Sunday morning for NCZ007>011-021>028-038>043-073>078-083>086-088-089. Winter Storm Warning from 10 PM this evening to 1 PM EST Sunday for NCZ011-027-028-042-043-077-078-085-086-088-089.