textproduct: Gray - Portland

WHAT HAS CHANGED

Winter Weather Advisories have been expanded across the southern tier of Counties in New Hampshire.

The start time to the Small Craft Advisory has sped up to now start at 12Z this morning.

Temperatures have trended colder for Saturday.

KEY MESSAGES

1. A wave of low pressure brings a mix of snow, sleet, and freezing rain across southern New Hampshire with a narrow area of accumulating snow across south-central New Hampshire and southwest Maine. Difficult travel will be likely across these areas tonight into the Friday morning commute.

2. Cool conditions and an extended period of freezing drizzle look increasingly likely for Friday into Saturday.

3. A warm airmass makes multiple pushes toward Northern New England from late this weekend through midweek next week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A short wave crossing Atlantic Canada this morning will send a cold front southwestward through the area today with high pressure building in from the north. Colder air will drain into the region from the northeast with temperatures falling during the afternoon. The cold front will stall over southern New England this evening while a low amplitude short wave approaches from the west. This short wave will bring moisture into the region with a wave of low pressure forming along the stalled front.

Latest available 00Z guidance shows the wave of low pressure will bring a narrow axis of precipitation across the southern half of the forecast area tonight through Friday morning. Precipitation types will be tricky as low level cold air will undercut warm air aloft this evening with surface temperatures dropping below freezing by the time precipitation starts across southern New Hampshire around 6 PM. A mixture of snow, sleet, and freezing rain is likely across south-central New Hampshire into far southwestern Maine at precipitation onset from 6 PM into the first half of tonight. Several CAM solutions suggest that cold air will win out with the snow/mix line pushing back south after midnight tonight with snow continuing into Friday morning. High pressure holding firm to the north will create a sharp cutoff in accumulating snow with the going forecast keeping accumulations confined along and south of a line from the White Mountains to KRKD.

Winter Weather Advisories have been expanded across all southern counties in New Hampshire for the combination of light icing and snow amounts pushing 4 inches across southeast New Hampshire. North of the Advisory precipitation will mainly be snow with amounts of 1-3 inches with amounts decreasing to the north. Precipitation likely continues across south-central NH and far SW Maine into Friday morning with slick travel likely on untreated surfaces. Precipitation tapers off north to south through the morning into early afternoon.

KEY MESSAGE 2 DESCRIPTION...

Expecting cold air damming to hold strong across the entire region through Saturday as low level cold and cloudy conditions limit mixing potential. Feel the NAM has a decent handle on the cold air holding firm while other guidance is well too warm. As has been observed already this week, the models have trended colder getting closer to the event, well after the pattern suggested that warmth was not favored. So with that in mind, temperatures were adjusted well lower than NBM guidance as a result utilizing the coldest available guidance. This keeps temperatures near or below freezing into at least the first half of Saturday.

This set up also favors a period of freezing drizzle Friday night- Saturday as long as surface temperatures remain below freezing. Soundings show a saturated profile up through 850 mb with a wedge of dry air aloft, supporting the potential. Worth mentioning most guidance in range is showing a few hundredths of ice accretion, while the ECMWF and RRFS are offering even higher amounts with some scattered steadier precipitation moving through. Temperatures are likely to moderate some later into the day Saturday especially across S NH, but even there it will be a raw day in the 30s/near 40. There is a chance northern portions of the region stays near/below freezing through the day, prolonging the threat for freezing drizzle. This period will continue to be monitored for the threat of slick travel. These conditions persist into Saturday night before the passage of a cold front overnight.

KEY MESSAGE 3 DESCRIPTION...

Following the passage of a cold front Saturday night, better mixing on westerly flow allows temps to warm into the 40s to low 50s on Sunday with mostly sunny skies. This marks our first real chance for our next seasonably warm day.

By early next week the pattern looks to favor a highly amplified ridge building into the Northeast. This would support warm air pushing into northern New England, and bringing significant snow melt. Indeed, most global guidance continues to show temperatures pushing into the 60s and even 70s by midweek next week. A ridge as amplified as the models are currently showing would support this, but there area cracks starting to show up in this warm scenario.

As has been observed with Todays's forecast, and now Saturday's forecast, the cold has a way of winning out in these borderline spring warm up events. The ridge next week is still showing up as being highly amplified, but a notable change since last night's runs has been toward a less amplified ridge. Temperatures still look warm in New England, but there's been a notable trend southward with the colder air across Quebec. Should this trend continue, it will once again bring the cooler air into New England and risk thwarting the warm up in both amplitude and duration. Again, this still has not shown up on the models, but it's a potential outcome to continue to monitor for over the next few days given the pattern and recent experience.

AVIATION /06Z THURSDAY THROUGH MONDAY/

From 18Z WEDNESDAY THROUGH THURSDAY NIGHT:

Mainly VFR through this morning. Clouds thicken and lower south to north late morning into the afternoon with increasing ENE winds. MVFR will develop at KLEB, KMHT, KCON, and KPSM by late morning into early afternoon. By this evening, snow, sleet and freezing rain are all expected across the area and will lead to IFR at KMHT, KCON, and KPSM. North and east of these sites will see MVFR and possibly IFR in any snow, mainly at KLEB and KPWM. Low ceilings will likely bring IFR to MVFR for all sites Thursday night. Wintry precipitation will likely continue into Friday morning.

Outlook:

Friday Night: Areas of IFR or lower in low cloud cover. Freezing drizzle is also increasing likely at most terminals.

Saturday: Areas of IFR to MVFR in low cloud cover. Morning freezing drizzle, then afternoon drizzle are increasingly likely.

Saturday Night: Areas of MVFR or lower possible with precip changing to rain showers.

Sunday: Improving to VFR during the day.

Sunday Night - Monday Night: VFR conditions expected.

MARINE

A backdoor cold front crosses this morning with NE winds bringing SCA conditions outside the bays by 12Z this morning. ENE winds continue SCA conditions through Friday. Another round of SCA conditions in southwesterly flow are possible late Saturday and Saturday night ahead of a cold front. The front passes early Sunday, brining a brief period of fair conditions before another possible period of southwesterly SCA conditions develops by late Monday.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...Winter Weather Advisory from 6 PM Thursday to 1 PM EST Friday for NHZ011>015. MARINE...Small Craft Advisory from 7 AM Thursday to 7 PM EST Friday for ANZ150-152-154.