textproduct: Gray - Portland

WHAT HAS CHANGED

For Saturday, an Extreme Cold Warning has been issued for northern, central NH and northwestern Maine. A Cold Weather Advisory has been issued for southern NH and coastal Maine. For Sunday/Monday, A Winter Storm Watch has been issued for portions of southern NH and southwestern ME.

KEY MESSAGES

1. Snow showers and squalls will move through northern New England this afternoon, allowing for some areas of reduced visibility and locally slippery roads.

2. An Arctic blast will bring dangerously cold weather to the region this weekend. Sub-zero wind chills begin Friday night and last through the day Saturday. It will still feel below zero come Sunday morning despite lighter winds. Frostbite and hypothermia can onset quickly at these temperatures for anyone with exposed skin or inadequate shelter.

3. Increasing confidence that low pressure exiting the Mid- Atlantic coast and towards Cape Cod will bring widespread accumulating snowfall to the region late Sunday through Monday. The highest snowfall amounts are likely across southern locations.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION: A cold front will quickly approach from the west this afternoon. Instability ahead of the cold front will allow for the development of snow squalls across the region. The front has triggered a few squalls over upstate New York and expect these to arrive in New Hampshire by this afternoon. Snow squalls will make roads slippery quickly and brief bouts of heavy snow could obscure visibility on roadways. Snow squalls look to taper off through the evening.

Some flurries may accompany arctic front passage on Friday.

KEY MESSAGE 2 DESCRIPTION: Broad and strong high pressure pushes into the Upper Midwest and Great Lakes Friday night. Northern New England will be squarely downstream of a frigid airmass advecting across continental Canada into the weekend. While temperatures will be decreasing, winds will be increasing as the pressure gradient between the aforementioned high pressure and low pressure over the Canadian Maritimes tightens.

The combination of cold temperatures and winds will create a period of dangerously low temperatures for the forecast area. The main concern will be over northern NH and the western ME mountains where Friday night winds chills plummet to 30 to 40 below zero. Here, frostbite can occur in as little as 10 minutes for exposed skin. Elsewhere, values fall to the teens and 20s below zero when factoring in wind. Frequent gusts of 20 to 30 mph are likely overnight.

While these cold conditions look to bottom out Friday night/Saturday morning, wind chill values remain below zero through the daytime Saturday. Daytime high temperatures in the single digits above and below zero are likely for the area. Winds lighten as the day goes on and pressure gradient relaxes, but the cold air mass settles nearby Saturday night. What we lose in winds in proximity to the high, we gain in potential radiational cooling for cloud-free portions of the area. Right now, there is uncertainty exactly how much cloud cover there may be, but do expect a consecutive night of temperatures below zero for the entire forecast area.

KEY MESSAGE 3 DESCRIPTION:

There continues to be increasing confidence that low pressure exiting the Mid-Atlantic coastline and towards Cape Cod Sunday into Monday will bring widespread accumulating snowfall to the region. Latest model guidance continues to show a northward trend in the track of this low, which paired with an approaching sharp trof axis that will be moving in from the west and favorable jet dynamics, a moderate to heavy snowfall is becoming increasingly likely. Latest ensemble and AI based guidance shows probabilities for total QPF of at least 0.50" greater than 70% across much of the region south of the mountains with lower probabilities further to the north. A strong area of high pressure (~1020 hpa) will be located to our north over Quebec, which will aide in a supply of very cold air along with some potentially drier air over at least northern zones. Snow to liquid ratios (SLRs) will be rather high given this cold airmass with values likely around 15:1 based on latest guidance. These ratios combined with QPF of at least 0.50" will likely yield snowfall totals into warning criteria for at least southern zones but potentially across much of the CWA.

Given this latest guidance, went ahead and issued a Winter Storm Watch for southern NH and extending into York and Cumberland counties in ME. This watch may need to be later expanded but since we are still 5+ periods out and there remains some uncertainty about potential drier air, kept the watch to where confidence is highest for now. As mentioned previously, snow character is expected to be dry, and therefore power outages should not be an issue. Significant travel impacts are possible though late Sunday through Monday. The other thing to mention is that gusty winds are possible as the system departs Monday into Tuesday and this may result in some blowing and drifting snow.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Thursday Afternoon: VFR, with localized MVFR likely due to scattered snow showers and squalls.

Thursday night: VFR expected.

Friday: VFR expected, low chance of MVFR due to snow shower at KHIE.

Friday night: VFR expected.

Saturday: VFR prevails at all terminals. WNW winds at 15-20 kts. Forecast confidence is high.

Sunday and Sunday Night: SN likely, especially late day through the night. IFR to LIFR restrictions are possible. N winds at 10-15 kts. Forecast confidence is moderate.

Monday and Monday Night: SN likely with IFR to LIFR restrictions possible through 00Z Tuesday. NNW winds at 10-20 kts. Forecast confidence is moderate.

Tuesday: VFR conditions likely with W winds at 15-20 kts. Forecast confidence is moderate.

MARINE

SCA to near gale force westerly winds are expected this afternoon through Saturday morning, with 5-9ft seas expected through this time period. Winds and seas slacken and briefly go below SCA thresholds Saturday evening and into Sunday morning. SCA to near gale force winds are likely to return late Sunday through Tuesday as low pressure crosses near the waters. Seas of 6-10 ft are possible outside of the bays along with freezing spray and snow.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Extreme Cold Warning from 1 AM to 10 AM EST Saturday for MEZ007>009-012>014-033. Cold Weather Advisory from 1 AM to 10 AM EST Saturday for MEZ018>028. Winter Storm Watch from Sunday afternoon through Monday evening for MEZ018-019-023-024. NH...Extreme Cold Warning from 1 AM to 10 AM EST Saturday for NHZ001>006-009. Cold Weather Advisory from 1 AM to 10 AM EST Saturday for NHZ007-008-010>015. Winter Storm Watch from Sunday afternoon through Monday evening for NHZ005>015. MARINE...Small Craft Advisory until 3 PM EST this afternoon for ANZ150- 152. Gale Warning until 7 AM EST Saturday for ANZ150-152. Freezing Spray Advisory from 10 PM Friday to 10 AM EST Sunday for ANZ150>154. Small Craft Advisory until 7 AM EST Saturday for ANZ151-153. Small Craft Advisory until 1 PM EST Friday for ANZ154. Gale Warning from 1 PM Friday to 7 AM EST Saturday for ANZ154.