textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes with this forecast update.

KEY MESSAGES

1. Temperatures continue an upward trend today into Thursday with high pressure providing fair weather.

2. Strong cold fronts Friday then again Monday cross the region, bringing brief rain chances and bursts of crisp fall weather.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Broad high pressure along the East Coast will become centered south of New England today through Thursday. This will provide fair weather while parts of today will feature more clouds than sun with more in the way of sunshine Thursday. As high pressure will be centered south of the region, temperatures at 925 mb will climb to around +15C today and +19C Thursday. While winds will be light, model soundings show mixing to at least 925 mb will be possible both days which will translate to highs in the low 70s today and upper 70s Thursday. Light winds and skies trending mostly clear tonight will likely lead to valley fog. Steady SW winds and a warm air mass in place will lead to a mild night Thursday night with lows failing to drop below 60 degrees across much of the area.

KEY MESSAGE 2 DESCRIPTION...

A shortwave and attendant cold front will cross the region Friday afternoon-night. Temperatures are more uncertain owing to potential cloud cover and precipitation, but guidance has slowed down the front enough to suggest one more warm day in store, with 850 mb temps pushing 13-15 C along with warm SW surface winds, which supports highs well into the mid-upper 70s. With a frontal passage around/just after peak heating, cannot rule out some heavier downpours and convection with around 500 J/Kg of CAPE indicated on multiple sources. PWATs also surge to 1.5-1.8"+ depending on model of choice, which is 2-3 SD above climatology, indicating the potential for heavy downpours. A very progressive system looks to cap precipitation totals however, with only a 30% of exceeding 0.5" of rain for the forecast area on the ECMWF ensemble. Overall does not look like a particularly high impact event as of now, but sometimes stronger dynamics this time of year can compensate for more marginal instability/moisture.

Speaking of, very strong CAA behind the cold front with 850 mb temps nosediving to 2-5 C by Saturday evening, which will be quite a airmass change. Highs in the 50s-60s this weekend will be representative of a cooler shift from this week.

Very strong agreement on global guidance in another dynamic trough and attendant cold front approaching the region late Sunday then crossing through Monday. Similar to the first wave, a bit of a glancing blow with the surface low and attendant vort max well to the north up in Quebec. This keeps the system progressive with only a weak signal for quick-hitting precipitation along the cold front, but this may even be a mostly dry frontal passage depending on how it all shakes out, as ensemble support for rainfall is not high currently. An even stronger cold punch is possible behind this second front with 850 mb temps going negative, which at a glance could spell quite chilly and crisp conditions for a few days.

AVIATION /01Z WEDNESDAY THROUGH SUNDAY/

Through 12Z Thursday...MVFR cigs likely across western ME into through this morning while southern NH terminals could be VFR or MVFR. Fog remains possible at all sites through day break but is mostly likely to bring IFR to LIFR at LEB, HIE, and CON. Fog should lift by mid- morning today with mostly VFR through 00Z. Mainly VFR tonight with the exception being valley fog at LEB, HIE, and possibly CON.

Outlook:

Thursday: Mainly VFR.

Friday: VFR likely in the morning with MVFR possible late due to showers.

Saturday: VFR expected.

Sunday: Mainly VFR with chances for showers increasing Sunday night.

MARINE

Low pressure will be moving east of Nova Scotia this morning with high pressure building along the East Coast today and settling south of the waters Thursday. This will allow for winds and seas to remain below SCA thresholds.

SW winds will freshen Thursday night as a cold front approaches from the NW. The front crosses Friday night shifting winds out of the NW with high pressure building in over the weekend. Winds and seas will continue to remain below SCA thresholds while some gusts to 20 kts are likely in SW flow.

GYX WATCHES/WARNINGS/ADVISORIES

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