textproduct: Gray - Portland

WHAT HAS CHANGED

Latest hi-res forecast guidance has continued to trend wetter with QPF of generally between 1/3rd to 1/2 inch. This results in a more widespread 3-5" snowfall and given that it will impact this evening commute, went ahead and issued a winter weather advisory for portions of the region. A glaze of ice remains possible across southern NH as well.

KEY MESSAGES

1. Some slick travel is likely late this afternoon through this evening as light snow overspreads south-central New Hampshire into far southwest Maine. Light snow expands northeastward tonight with snow ending as a light wintry mix across southern New Hampshire.

2. Warmer temps return on Wednesday, before a cool down on Thursday.

3. Another system is then possible on Friday, but much uncertainty remains.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

High pressure drifts east this morning as a weak short wave approaches from the west. This short wave combined with weak jet streak coupling will spawn a wave of low pressure that tracks across southern New England this evening. This low will close off in the Gulf of Maine late this evening bringing a brief wind shift out of the north.

Precipitation will push in from the southwest this afternoon while surface temperatures creep into the low to mid 30s. Low level dry air will aid in cooling surface temperatures through wet bulbing while it may take time for snow to accumulate across southern New Hampshire during the afternoon. Snowfall rates will increase across south-central New Hampshire into far southwest Maine going into the evening commute with rates around 0.5 inches per hour. This will likely produce some slick travel for the evening commute. Snowfall with continue to expand northeastward late this evening through the first half of tonight. Latest available 00Z guidance has shown slight increase in QPF with localized amounts approaching 0.5 inches. This has pushed snowfall amounts up slightly with a swath of 2-4 inches along and south of the mountains with locally higher amounts in the higher terrain of New Hampshire. Model soundings continue to suggest that a warm nose aloft will creep into southern NH and portions of York County while the northerly shift in surface winds could keep temperatures near or below freezing. This may allow for snow to change to a light wintry mix as precipitation comes to an end here. This transition will occur late this evening after the evening commute with little in the way of impacts anticipated from light ice accretion. Precipitation will end NW to SE after midnight followed by temperatures quickly rising above freezing Wednesday morning.

KEY MESSAGE 2 DESCRIPTION...

Warmer temps make an appearance on Wednesday as mixing on a westerly wind allows temps to warm into the 40s to near 50 degrees downwind of the mountains. High pressure slips through during the daytime on Wednesday, moving offshore late and bringing in a sea breeze along the MidCoast by the late afternoon hours. Mainly clear skies then allow for radiational cooling Wednesday night.

A strong temperature gradient is possible during the day on Thursday. A back door cold front likely drops temps for at least Maine during the day, and then New Hampshire Thursday night. Models still aren't defining this feature well, but it is more likely to be better resolved as it comes into view of the high res guidance today. If the front comes through during the morning hours, highs would likely struggle to make it out the mid 30s across much of Maine on the heels of the radiational cooling Wednesday night. We'll continue to monitor this feature over the coming days. Meanwhile, New Hampshire stands the best chance to see highs warm into the upper 40s to low 50s again on Thursday before the front comes through at night.

KEY MESSAGE 3 DESCRIPTION...

With cooler air returning, attention then turns toward the next system for Friday. For at least three days now, models have been back and forth on whether this system brings moisture into our area, or if it mostly gets suppressed to the south. At this point the answer remains unclear. But with the colder air back in place, whatever does arrive is likely to be in the form of either snow or a wintry mix, with snow more likely on the northeastern side of the precip shield, and a mix more likely further toward the southwest.

However, these details remain moot if the precip fails to reach New Hampshire or Maine, so we'll continue to wait for a better consensus amongst the models. With some recent events trending wetter, and others drier in the last few weeks, the pattern doesn't really offer any better clues against the models either.

AVIATION /09Z TUESDAY THROUGH SATURDAY/

Through 00Z Wednesday: Winds remain generally light and variable through this morning with VFR conditions continuing. We could see some breezier southwest winds at AUG and RKD by mid to late this morning or early afternoon. Clouds thicken and lower from SW to NE during the afternoon. Snow overspreads from SW to NE towards the end of the period with conditions deteriorating to at least IFR between 21Z and 00Z.

Outlook:

Tonight: IFR to LIFR in low ceilings and continued light snow. A brief period of sleet or freezing rain cannot be ruled out along and south of a line from KCON to KPWM. Precipitation ends after 06Z Wednesday while low ceilings may linger into Wednesday morning.

Wednesday: VFR is likely for all TAF sites during the day

Wednesday night - Thursday: Mainly VFR, but valley fog possible at night. MVFR ceilings are then possible through Maine and PSM by Thursday afternoon.

Thursday night and Friday: IFR or lower possible due to snow, rain and wintry mix.

Friday night and Saturday: IFR possibly lingering with showery mixed precip.

MARINE

A wave of low pressure will cross southern New England today and into the Gulf of Maine tonight. Southwest to south winds increase this morning between departing high pressure and incoming low pressure. This will bring SCA conditions this morning and last through the evening in the southerly flow regime. Conditions ease as high pressure crosses the waters on Wednesday.

SCA conditions in northeasterly flow are possible by late Thursday as a backdoor cold front crosses the waters. Low pressure then rides south of the waters Friday, enhancing the northeasterly flow into late Friday. Conditions ease Friday night, but then southerly winds freshen on Saturday as a cold front approaches from the west.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Winter Weather Advisory from 1 PM this afternoon to 4 AM EST Wednesday for MEZ018>020-023>025. NH...Winter Weather Advisory from 1 PM this afternoon to 4 AM EST Wednesday for NHZ003>015. MARINE...Small Craft Advisory until 1 AM EST Wednesday for ANZ150>154.