textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes to the forecast with this update.

KEY MESSAGES

1. High pressure slowly gives way to the remnants of Isaias through Sunday. Frost and freeze conditions are possible tonight.

2. Two shots of beneficial rain are likely early next week. The first arrives late Sunday into Monday morning and the second and better chance Tuesday, possibly lasting into Wednesday morning.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Strong high pressure remains in place tonight. It will be a tricky low temp forecast however as high cirrus outflow clouds from the remnants of Isaias are already spilling into the local area. I do not believe clouds will be thick enough to affect radiational cooling except for the southernmost counties in NH. I still think some upper 30s remain possible for those zones though, so no planned headline changes are anticipated. I did also add some patchy fog to the major river valleys, as warm waters should at least provide some steam fog if not radiation fog settling into the low lying areas.

KEY MESSAGE 2 DESCRIPTION...

Then most of Sun looks dry, but increasing cloud and chances for rain are expected thru the day. That moisture from Isaias will be pushing north but will meet pretty strong resistance from the building high pressure branching across the Great Lakes from central Canada. Rain will be most likely from the White Mtns to Penobscot Bay southward, and here NBM PoP looks pretty good. The QPF is a lower confidence forecast. Right now ensemble guidance is pretty bearish on significant rainfall. The probabilities for at least a half inch of rain are generally around 1 in 3 for southernmost NH, and this is driven primarily because of the GEPS with some support from the deterministic NAM. The probabilities for a wetting rain (at least a tenth of an inch) are at least 60 percent from a LEB to PWM line south to the Mass border. So this is looking like a fairly run of the mill high PoP/lower QPF event. The only unfortunate thing is that much of the rain will fall outside of the areas that need it the most. A frontal boundary will remain draped across the forecast area thru Mon and may provide a focus for some isolated showers during the day, but otherwise should hold off until the next shortwave arrives.

Some showers are possible Monday night as a front sags southward into the area, mainly in the mountains. Behind this front, high pressure builds in to our north with northeast winds bringing colder air into the region. This colder air may play a role on Tuesday but is uncertain and more on that below.

Overall, Tuesday is looking like a wet day. A wave of low pressure will ride along the boundary and allow mostly rain to overspread the area from west to east in the morning and afternoon that will likely stick around through Tuesday night. I say mostly rain as there indications of CAD for at least some of the day as the high shifts off to the north and east, which would keep temperatures colder for longer and could result in some wintry precipitation across the north, especially if the track of the low trends farther south (like the NAM solution and to some extent the CMC). If to occur, the better chances would be in the western ME mountains with the NAM suggesting as far south as the foothills. However, the NAM is the outlier at this point when compared with much of the ensemble guidance from global models.

Back to rainfall amounts: Ensemble means from the GFS/ECWMF are in pretty good agreement with rainfall amounts of 0.50" to 1.00" of rain possible for much of the area through Wednesday morning, except less amounts of 0.25" to 0.50" toward parts of central and southern NH.

Models are in a little better agreement than this time yesterday in regards to the timing of the cold front passing and the widespread precipitation winding down...generally sometime Wednesday morning. Behind the front, west to northwest winds will become breezy with guidance suggesting 25-35 mph possible during the day Wednesday, but additional showers will be possible as a 500mb shortwave swings through. Higher chances will be in the mountains through Wednesday night with upslope flow, where colder air moving in could also change these over to snow showers.

Thursday looks to remain breezy with good mixing and a tight gradient between low pressure in the Canadian Maritimes and high pressure building from the west. Upslope rain/snow showers may continue in the the mountains into the first part of the day, but elsewhere should remain precip-free as the airmass looks drier. Rain chances increase again Friday and Saturday as the next front approaches.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Largely VFR conditions expected thru Sun. Tonight some patchy valley fog is possible, most likely at LEB, but confidence is not really high enough to include in the TAF at this time. Rain begins to work into the forecast area Sun night with widespread MVFR south of the mtns and areas of IFR or lower developing, especially southwest of PWM.

Outlook:

Monday: Widespread MVFR/areas of IFR conditions improving to MVFR/VFR.

Monday night: MVFR to IFR ceilings possible with rain showers.

Tuesday-Tuesday night: IFR restrictions and rain showers are likely.

Wednesday: Restrictions possible early in the day, but most sites should return to VFR by the afternoon. That said, additional rain showers may produce instances of MVFR, especially at HIE. West to northwest winds may gust 25 to 30 kt.

Wednesday night: MVFR ceilings with possible rain and snow showers at HIE. VFR elsewhere.

Thursday: Mainly VFR with breezy northwest winds remaining possible. HIE may continue to see periods of MVFR and rain/snow showers.

MARINE

Winds and seas will remain below SCA thresholds thru Mon. Southerly winds Sun will allow seas to build, but only to 3 or 4 ft. Sun night into Mon most of the marine advection fog should remain south of the waters, but we will have to monitor how far north it creeps into Mon. Monday night-Saturday...Multiple periods of SCA conditions are possible this week. SCA conditions may occur as early as Monday night with increasing east to northeast winds between departing low pressure to the east and high pressure to the north. These conditions remain possible Tuesday as low pressure quickly approaches from the west bringing increasing south to southwest winds. This low will send a cold front through the waters around the early part of Wednesday with brisk west to northwest winds behind the front possibly prolonging SCA conditions through Thursday. Another front then approaches late week.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Freeze Warning from 2 AM to 8 AM EDT Sunday for MEZ019-020. Frost Advisory from 2 AM to 8 AM EDT Sunday for MEZ021>028. NH...Frost Advisory from 2 AM to 8 AM EDT Sunday for NHZ010-012-013. MARINE...None.