textproduct: Gray - Portland

WHAT HAS CHANGED

Snow will likely be falling for the morning commute Wednesday morning. A period of greater snow rates will push across southern NH and then interior/coastal Maine mid to late morning.

There remains some uncertainty on coverage of another round of late afternoon/evening snow showers along a cold front Wednesday.

Late week storm has trended farther out to sea and less of a threat to impact the area.

KEY MESSAGES

1. Cool and quiet tonight, but a period of snow is expected to move through the forecast area, west to east, Wednesday morning. Snowfall rates around a half inch per hour will impact the morning commute from southern New Hampshire into south interior and coastal Maine, creating slick road conditions.

2. A cold front approaches the area Wednesday evening. This front may bring a heavy snow showers to NH and far western ME. These could cause sudden visibility reductions and additional slick spots due to light accumulations.

3. Coastal storm continues to trend south Thursday night into Friday and impacts are not currently anticipated locally.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... Skies remain mostly clear into the evening and overnight hours tonight. Combined with winds becoming lighter, temperatures should quickly cool into the teens and single digits...where afternoon dewpoints sit. Void of much low level mixing, temperatures should continue to cool overnight into the single digits for much of the area. Clouds will begin advancing from the west late, which will tend to slow temp flux.

Snow arrives from the west in the early morning hours Wednesday. This will first cross NH and then into ME. The most impactful aspect of this will be the potential for greater snow rates, around half an inch per hour, embedded in the broader snow shield. This moves across southern NH between 7 and 10am, and then up the Maine coast and interior 8am and 1pm. High res guidance has been increasing QPF through the NH and ME Lakes Region and southern ME coast, indicative of some additional strengthening of the wave as it passes into the Gulf of Maine.

Resulting snow amounts are forecast to remain a broad 1-3 inch snowfall, but some amounts closer to 4 or 5 inches cannot be ruled out in the increased QPF area mentioned above. Despite the cool overnight temperatures, sfc temps increase quickly with the arrival of the snow, this puts the DGZ aloft, while the best lift is below. Thus snow ratios trend towards 12:1 vs 18:1 earlier in the night.

End of the snow should come as a fine edge as deeper moisture pushes east this is largely just before noontime for much of NH, and then 2 to 3 pm for Maine.

KEY MESSAGE 2 DESCRIPTION... Associated cold front with the clipper will near in the late afternoon and evening. High res guidance like the HRRR and NAMnest depict snow showers accompanying the front, and low level moisture and instability support cellular development. The question is intensity, and current timing brings the front into NH late. Low level lapse rates may begin weakening at this time, reducing coverage and intensity. In addition, frontogenesis is stronger to the south of the forecast area. Some of these may contain locally gusty winds which could blow around fresh snow from the morning.

Will continue to message the chance for sudden reduced visibility and localized slick spots due to the snow showers alone, but remains uncertainty on timing and intensity.

KEY MESSAGE 3 DESCRIPTION... Clear trends from ensemble guidance over the last 24 hours regarding the potential late week storm system. Now roughly two thirds of ensemble members are misses altogether and more than 95 percent of members less than a tenth of QPF reaching southern zones. Guidance has trended the entire pattern flatter, and therefore the shortwave escapes east before amplifying enough to expand QPF shield north into the forecast area. DESI cluster analysis shows that off the various clusters, the most amplified cluster is the most likely to bring precip to the forecast area. Even then that accounts for less than 20 percent of member. NBM PoP is slight chance with the latest run and that seems fair, but still may be too high. Beyond this time period the pattern remains fairly active and there will be plenty of opportunities for additional precip.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

Through 18z Wednesday...VFR tonight with NW gusts subsiding late this afternoon. Clouds thicken and lower quickly Wed morning with SN pushing across NH and then ME after 12z. This will likely result in MVFR ceilings and IFR to LIFR vis. Lowest vis is expected to be across EEN, CON, MHT, PSM, and then up the ME coast where periods of 1/2SM is possible. Vis should improve after noon local, but ceilings may remain down. There may be a period of LLWS between 12z and 18z pushing across the forecast area, generally 30 to 40 kts.

Outlook:

Wednesday Night: A cold front will bring SHSN in the late afternoon/evening. These could bring erratic gusts and locally lower vis.

Thursday: VFR conditions expected.

Thursday Night: VFR conditions expected.

Friday: VFR conditions expected.

Friday Night: VFR conditions expected.

Saturday: VFR conditions expected.

Saturday Night: VFR conditions expected.

MARINE

Waves will be slow to decrease this evening, but should fall below SCA criteria after midnight tonight. Winds become southerly Wed morning, increasing towards 30 kts through the day. These should remain below Gale given the warm air over cool waters.

Any lingering SCA winds and seas will diminish Thu. Then the approaching upper trof will allow winds to turn southwesterly heading into Sat. While winds will increase, warmer air moving over the coldest waters of the season will have trouble mixing down. While guidance suggests SCA conditions are possible, I am skeptical that it will be anything more than marginally 25 kt gusts unless the frontal system strengthens considerably. However sustained southwesterly winds thru the weekend may allow seas to build to around 5 ft outside of the bays, especially north of Cape Elizabeth.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory until 1 AM EST Wednesday for ANZ150- 152>154.