textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes since the previous forecast cycle.

KEY MESSAGES

1. Isolated to widely scattered showers and thunderstorms remain possible through this evening as a cold front approaches. Additional showers and gusty thunderstorms are possible on Friday afternoon.

2. High pressure brings fair weather with lower humidity over the weekend. Rain chances return late Sunday through mid week as a stationary front sets up near New England.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Satellite imagery this afternoon shows an advancing cirrus shield moving across the area from the west with some cu development as well. The latest SPC mesoanalysis shows up to around 500 J/KG of MU/MLCAPE along with mid-lvl lapse rates around 5.5 C/KM. Despite this, effective bulk shear is rather weak at around 25 kts and this combined with weak forcing for ascent should prohibit widespread shower and thunderstorm development through the remainder of the day. That being said, a few showers and storms remain possible through sunset, with perhaps some containing gusty winds.

Showers will wane tonight but clouds will largely remain in place with overnight low temperatures mainly into the 60s. Fog is likely to develop, bringing reduced visibility.

Fog will lift Friday morning with partly cloudy skies allowing high temperatures to reach the lower to middle 80s south of the mountains. A cold front will cross from the north during the afternoon and early evening, which combined with daytime instability and some shear will likely allow for showers and thunderstorms to develop. A few storms may contain gusty winds.

Cooler temperatures arrive behind the cold front with Friday night lows into the 40s and 50s.

KEY MESSAGE 2 DESCRIPTION...

Broad high pressure builds in from the west Saturday behind a departing cold front. A drier air mass will be in place with dew points in the 40s and 50s with mostly sunny skies and highs in the 70s to low 80s. High pressure sinks south Sunday as a trough swings across Atlantic Canada. This trough will send a cold front into northern areas Sunday afternoon and evening bring a chances for showers and thunderstorms. South of the mountains will remain dry will mostly sunny skies. Highs will range from the 70s north the mid 80s south with steady WSW winds allowing for mid 80s near the coast.

The cold front will push south through the rest of the area Sunday night and stall near southern New England Monday morning. Flow aloft will trend towards zonal through mid week allowing the cold front become stationary near New England. The position of this stationary front will effect temperatures with ensembles favoring the forecast area to remain on the cool side of the boundary. Latest deterministic and ensemble guidance suggest the boundary will be far enough south to limit shower chances on Monday. Model spread in precipitation increases Tuesday and Wednesday as waves of low pressure track west to east along the boundary. The latest NBM brings 40 to 60 percent PoPs during this time which seems reasonable with modest probabilities that portions of the forecast area will see an inch of QPF by the second half of next week.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Mainly VFR conditions through 04Z Fri, although widely scattered SHRA/-TSRA may bring TEMPO restrictions. FG will likely return tonight to most TAF sites, bringing IFR or lower restrictions. Conditions will improve back to VFR Fri morning through much of the day, although additional scattered SHRA/TSRA may bring TEMPO restrictions, with the highest chance across ME TAF sites. Winds will be predominantly out of the S through the period at 5-15 kts.

Outlook:

Friday night: Mainly VFR outside of valley fog.

Saturday-Sunday: Mainly VFR.

Sunday night-Tuesday: CIGs and periods of SHRA will likely bring at least MVFR conditions at times.

MARINE

S winds will prevail at 10-20 kts with seas of 2-4 ft, highest outside of the bays. Marine fog is possible tonight through Fri morning.

High pressure builds in from the west Saturday and sinks south of the waters Sunday. A cold front crosses the waters Sunday night and Monday and stalls south of the waters into Tuesday. Winds and seas will likely remain below SCA thresholds through the period.

GYX WATCHES/WARNINGS/ADVISORIES

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