textproduct: Gray - Portland

WHAT HAS CHANGED

Cold and winter weather headlines remain unchanged this morning. Snowfall amounts are largely unchanged and a significant winter storm is still expected for at least parts of the forecast area.

KEY MESSAGES

1. Bitterly cold air mass will remain in place thru Sat night. Dangerous wind chills are anticipated for swaths of the area for much of that time beginning this morning.

2. A significant snow storm will bring widespread accumulating snowfall to the region Sunday afternoon through Monday. All snow is expected, which will bring hazardous travel conditions through the Monday morning commute across the area.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION: The Arctic cold front has arrived and strong cold advection is underway. In the largely unblocked flow over the mtns scattered snow showers may continue for much of the night into the morning hours. But the main story is the bitterly cold air mass taking up residence thru Sat night. Temps will steadily fall behind the front thru sunrise but coupled with gusty northwesterly winds it will feel much colder. Cold weather advisories or extreme cold warnings remain in place for all zones. Wind chills tonight are forecast largely between 15 and 40 below at their coldest. There will not be much relief during the daylight either. High temps may not get out of the single digits, at least not for very long, and may not go above 0 in the mtns at least until Sun. Once the extreme cold warning expires I anticipate that a cold weather advisory will be necessary for much of the White Mtns and western Maine mtns. The only notable deviation from the NBM guidance will be Sat night. High pressure builds in and settles over the area. Surface ridging over fresh snows should allow temps to go colder than the NBM forecast, and indeed the median is already colder than the mean. For starters I will blend in some of the 25th percentile, which brings the northern valleys down between 20 and 25 below. So even without any appreciable wind this would require a cold weather advisory.

KEY MESSAGE 2 DESCRIPTION: Forecast is still on track for a significant winter storm Sunday and Monday across the forecast area. We're entering the time range where we can start picking out smaller features that can make a difference in snow rates and totals for the event. The primary messaging stands firm: significant travel impacts can be expected Sunday afternoon/evening and persisting through at least Monday evening. Increasing winds through the period will easily blow and drift the fluffier snow character, challenging snow removal in open and unprotected stretches of roadway.

One smaller scale feature to watch amid the larger system will be ocean effect enhancement along the southern ME coast and NH Seacoast. Given the very cold airmass preceding the storm, the relatively warmer waters will provide some low level instability harboring locally stronger lift. Onshore flow Sunday evening through much of the night would bring some snow enhancement along the I95 corridor in York and Rockingham Co. Flow becomes more parallel to the coast come Monday morning, reducing this added effect, but greater storm dynamics will still be in play.

Mesoscale banding will be another feature to watch amid a very expansive precip shield through Monday. 850/700mb frontogenesis depicted in NAM/GFS runs is fairly progressive as it lifts through New England Sunday evening. While deterministic and not set in stone, this can provide good insight into how quickly a stronger band of snow could transit the region. The current timeline overlaps with ensemble guidance suggesting heavy snow rates of 1 to 2 inches per hour during the overnight hours.

Finally, a factor that could hamper snow accumulations. Overall trend in the storm has been an earlier onset and slow northern crawl. Despite the abundant and anomalous moisture content with this system, the lifecycle of low pressure systems usually features a dry slot contingent on the low's maturity. This northern crawl the past few days has brought this feature closer to the forecast area, nosing in from southern New England. This would bring a period of less efficient snowfall. Still early to factor this in effectively for the forecast, but an item to watch.

AVIATION /04Z SATURDAY THROUGH WEDNESDAY/

Generally VFR conditions prevail thru morning, but some local MVFR or lower is possible in widely scattered snow showers spilling over the mtns. Gusty northwesterly winds will be more widespread, with surface gusts around 25 kt possible thru most of the day. Clouds will begin to lower and thicken Sat night, but VFR conditions will prevail with CIGs staying above 5000 ft.

Outlook:

Sunday: Clouds thicken and lower. MVFR cigs with IFR vis arrives during the afternoon SW to NE with SN.

Sunday Night and Monday: IFR in widespread moderate to heavy snow. LIFR vis possible in passing snow bands Sunday night. Snow intensity drops Monday, perhaps some improvement in vis. N winds gusting 15 to 20 kts for most terminals S and E of the ME/NH mtns.

Monday Night: Trending VFR. Winds shift NW 15 to 20 kts. HIE may keep MVFR ceilings with light SN.

Tuesday and Wednesday: Mostly VFR with no sig wx. W winds 15 to 20 kts. MVFR ceilings may remain NW of the Whites.

MARINE

Gale force wind gusts continue outside the bays behind a strong cold front. Those winds are generally expected to continue thru sunrise when they will begin to gradually diminish thru the day. Some marginal SCA conditions may continue over parts of the waters north of Cape Elizabeth into Sat evening before high pressure builds in and allows any lingering gustiness to drop.

Low pressure will exit the Mid-Atlantic coast Sunday, tracking NE just outside of Nova Scotia by Monday evening. Continued strengthening is expected as this low tracks outside the coastal waters. This will bring periods of visibility reducing snow and building Gale conditions Sunday evening through Monday. Gusts to 35 kts may continue on the outer coastal zones into Tuesday morning. Waves will also rapidly build 6 to 10 ft through Monday. Conditions subside to SCA Tuesday. Also expect periods of light to moderate freezing spray Sunday night through Tuesday given cold air mass approaching.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Extreme Cold Warning until 10 AM EST Saturday for MEZ007>009- 012>014-033. Winter Storm Watch from Sunday afternoon through Monday evening for MEZ007>009-012>014-018>028-033. Cold Weather Advisory until 10 AM EST Saturday for MEZ018>028. NH...Extreme Cold Warning until 10 AM EST Saturday for NHZ001>006- 009. Winter Storm Watch from Sunday afternoon through Monday evening for NHZ001>015. Cold Weather Advisory until 10 AM EST Saturday for NHZ007-008- 010>015. MARINE...Gale Warning until 7 AM EST Saturday for ANZ150-152-154. Heavy Freezing Spray Warning until 11 AM EST Saturday for ANZ150>153. Small Craft Advisory until 7 AM EST Saturday for ANZ151-153. Freezing Spray Advisory until 10 AM EST Sunday for ANZ154.