textproduct: Gray - Portland

WHAT HAS CHANGED

Little change to the going forecast at this time, although there remains some uncertainty in timing of PoPs this weekend.

KEY MESSAGES

1. Warm and mostly dry conditions will continue through today and tonight with cooler and more unsettled weather returning for Thursday.

2. A cooler and unsettled pattern develops the second half of this week, peaking Friday-Saturday.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A cold front will move southward across the forecast area this morning with little fanfare. An isolated shower can't be ruled out, but most locations should stay dry. Highs in the 80s for today still looks good, with 70s in the far north. A few showers will be possible across far northern zones late this afternoon and tonight with the passage of a short wave trough.

Thursday looks to be a little more unsettled and quite a bit cooler. However, it will not be a washout by any means. Upper level low pressure will be placed just to east of Maine on Thursday with spokes of vorticity and moisture rotating around it. With cold air aloft, at least scattered showers look to be the theme during the day perhaps along with isolated thunderstorms.

KEY MESSAGE 2 DESCRIPTION...

After a couple of warm days, longwave troughing develops over the eastern CONUS, cooling temperatures down to near or below seasonal normals for highs through at least this weekend. Ensembles are suggesting a rather anomalous blocking pattern developing by this weekend. Upstream ridging over Hudson Bay, combined with troughing over the SW CONUS, will resemble an omega block pattern. This is likely to result in persistent troughing over the Northeast U.S. for the foreseeable future.

While daily diurnal shower chances exist due to a persistent cold pool aloft, the Friday-Saturday timeframe has emerged as an area of focus for some higher-imapct weather. With a trough axis sitting overhead, cyclonic upper-level flow will swing a shortwave originating from the Arctic Circle due south into New England. Given this origin location, temperatures aloft will be anomalously cold for late May: freezing levels will drop below 850 mb. This suggests mountain snow will be likely with this system, with a cold rain below the mountains. The cold air aloft may also provide some marginal instability, which could fuel some heavier downpours that may produce graupel or a few rumbles of thunder.

There remain some timing differences with the GEFS faster to arrive and quicker to exit while the CMC/ECMWF ensembles are slower to arrive and slower to exit. Most solutions show the upper low spawning a surface low in Gulf of Maine. The GEFS camp suggests that steady precipitation will end Saturday afternoon with the NBM tapering PoPs from 70 percent Saturday morning to around 50 percent Saturday evening.

AVIATION /10Z WEDNESDAY THROUGH SUNDAY/

Through 12z Thursday...VFR today as winds increase this morning, with a NW shift and gusts to 20 kt for this afternoon. Passing SHRA possible for HIE and terminals near the US/CAN border, sprinkles into interior ME/NH.

Light winds tonight as MVFR cigs invade from the NE. These seem most likely for the mountains and Kennebec Valley (AUG/RKD) by early Thurs morning.

Outlook:

Thursday: Scattered showers and isolated thunder bring potential for brief MVFR on Thursday. This may be more continuous across the mountains.

Thursday night-Friday: Mainly VFR. Chances for showers Friday afternoon bring potential for restrictions, particularly at HIE.

Saturday-Sunday: Periods of rain likely bring MVFR to IFR with ENE winds gusting 25 kts. Some improvement possible Saturday night into Sunday.

MARINE

Winds become northerly later today and tonight and continue through Thursday night with conditions below SCA.

Fri-Monday...Winds and seas remain below SCA thresholds into Saturday morning. Low pressure develops in the Gulf of Maine Saturday with ENE winds shifting N bringing SCA conditions into Sunday morning. Winds drop below SCA thresholds Sunday.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...None.