textproduct: Gray - Portland

WHAT HAS CHANGED

Mainly refined temps, which have been much colder than guidance so far this event. For the most part I expect temps to remain in the single digits thru morning.

Otherwise I adjusted PoP to match radar trends. Snowfall locally is largely being driven by intense warm advection and lift around 700 mb. This will continue to translate northward providing a fairly uniform 6 to 10 inches of snow. The 850 mb frontogenesis is forecast to lift towards then stall near the Mass/NH border then slide eastward. This is where the highest totals remain in the forecast. I feel pretty comfortable with southern NH looking at snowfall rates around 1 inch per hour thru midnight or 1 am, which should add an additional 4 to 5 inches. Given the light snow expected Mon, storm totals around 18 inches still seem reasonable, but 2 feet may be off the table unless some higher reports start to trickle in shortly.

KEY MESSAGES

1. A major winter storm will bring significant snowfall to the region tonight through Monday evening. Widespread double digit snowfall is expected with localized amounts nearing 2 feet possible across portions of coastal SW ME and SE NH. This will result in significant travel impacts.

2. Below normal temperatures continue Tuesday through the end of the work week. No significant precipitation is expected, but an upper low will lead to periodic low chances for light snow showers.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... As of 18Z Sun sfc low pressure (~1010 hpa) is located offshore of the Delmarva with an H5 positively tilted trof axis over the Upper Midwest. Northeast radar mosaic shows a precipitation shield stretching from Mid-Atlantic northward towards southern NH. ASOS/AWOS reports upstream indicate moderate to heavy snowfall now occurring across much of southern New England and entering southern NH with lighter snow now developing further to the north. This area of low pressure will continue to move northeastward before arriving near Cape Cod around 12Z Mon and in doing so, will spread widespread snowfall across our local region.

Latest forecast guidance continues to indicate that a region of strong H7 FGEN forcing will move across the area from southwest to northeast this evening before exiting to our east early Mon morning. This strong forcing for ascent combined with a saturated DGZ will result in increasing snowfall rates with the latest WPC snowband probability tracker (SPT) showing peak rates between 1-3"/hr likely. The time window of greatest concern for these high snowfall rates will be generally between 4 PM to 4 AM with coastal and southern locations the most likely to experience these snowfall rates. This will result in rapidly deteriorating travel conditions along with poor visibility. Further to the north, snowfall rates of generally 0.5-1.0"/hr are likely.

Snowfall by around dawn on Mon is expected to range between generally 12-18" south of the mtns with locally higher amounts likely towards coastal SW ME and SE NH. Some drier air is then likely to move in aloft later Mon morning, with decreasing snowfall rates. This will then be followed by another period of light to moderate snowfall Mon afternoon and evening as forcing for ascent increases again from the trof axis that will be arriving from the west. An additional couple of inches of snow is expected through the day Mon into Mon evening. Snow will then taper off from west to east laster Mon evening and night. Total snowfall of 12-18" is likely for most locations south of the mtns with amounts potentially nearing 2 ft, especially towards coastal SW ME and SE NH. Lesser amounts of snow are expected towards the Canadian border with 6-12" expected. Another thing to mention is that winds will increase some tonight through Mon night with gusts up to 25-30 mph. While not strong enough for power outage concerns, it will likely result in blowing and drifting snow.

KEY MESSAGE 2 DESCRIPTION...

Behind the exiting shortwave, we will see progressive upper level ridging moving across the region on Tuesday. Ridging will be short- lived, however, as a stubborn upper low meanders around the Ontario/Quebec/Maine region. This low will periodically rotate weak shortwaves into the northeast CONUS through the end of the week before it finally starts to move out of the area by the weekend. While no significant precipitation is anticipated in the long term, the general unsettled pattern will lead to plenty of cloud cover and periodic chances of light snow showers, especially in the mountains.

The stubborn low will also keep letting the arctic air filter in so we should remain chilly through the work week with highs mainly ranging from the upper single digits and lower teens north, to the upper teens and lower 20s south. Ensemble guidance does suggest we may start a gradual warming trend on Saturday. While it is still seven days out, NBM high temperature spread is generally small so there maybe a bit more confidence than usual this far out for a modest warmup. However, spread is still rather large for lows Thursday, Friday, and Saturday mornings. Even with some more uncertainty regarding the lows, the coldest mornings of the long term are likely to be Thursday and Friday mornings when the NBM is suggesting lows ranging from near 10 below zero to around zero. Widespread wind chills in the double digits below zero are also possible each morning.

AVIATION /01Z MONDAY THROUGH FRIDAY/

Widespread SN will result in IFR-LIFR restrictions through Mon evening. Periods of +SN are expected, especially at KMHT, KCON, KPSM, KPWM, and KRKD with vis down to 1/4SM or less. NE wind gusts of 20-25 kts will develop this evening before becoming N on Mon. SN will gradually end from W-E Mon evening and night with gradual improvements expected in flight restrictions.

Outlook:

Tuesday Through Friday: Mostly VFR with no sig wx. W or NW winds 15 to 20 kts. MVFR ceilings with periodic light snow showers remain possible NW of the Whites.

MARINE

Gale force NE winds are expected late tonight through Mon afternoon outside of the bays with gusts up to around 40 kts and seas of 6-12 ft. Across the bays, SCA level winds are expected with gusts up to 30 kts and seas of 2-5 ft. Light freezing spray is also possible along with low vis due to snow. SCA to near gale force N-NW winds will then persist Mon night into Tues.

Winds and seas should lower to SCA levels by the end of the day Tuesday, with 3-6ft seas expected Tuesday afternoon through Wednesday. Beyond Wednesday, expect winds and seas to go back and forth between normal and SCA conditions.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Winter Storm Warning until 1 AM EST Tuesday for MEZ007>009- 012>014-019>022-024>028-033. Winter Storm Warning until 7 PM EST Monday for MEZ018-023. NH...Winter Storm Warning until 7 PM EST Monday for NHZ001>015. MARINE...Gale Warning from midnight tonight to 4 PM EST Monday for ANZ150-152-154. Small Craft Advisory from 1 AM to 7 PM EST Monday for ANZ151- 153.