textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes with this forecast update.

KEY MESSAGES

1. High pressure builds in with fair weather expected through Sunday. Overnight frost will be possible to the coast, especially Saturday night.

2. Rain chances increase late Sunday and persist into Monday as the remnants of Isaias cross the Northeast and a front approaches from the north. More showers are likely with a cold front on Tuesday that may last into part of Wednesday.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Secondary cold front will cross the region this evening. Another round or band of showers will cross the forecast area, then transitioning to upslope showers. Had to blend in a little CAM guidance to get shower coverage this evening. Overnight some light snow accumulation will be possible for the highest elevations. So there could be some snowliage in the morning. It will also be a race to see how quickly we can decouple the cold advection winds. NBM guidance is quite cold overnight, with MOS guidance warmer. I think the NBM is probably leaning a bit too hard on bias correction from past nights, so I brought lows up a bit. This keeps frost mostly out of the active zones. So no headlines currently, but if we do decouple early it may be something to consider for the evening shift.

Sat will bring shallow mixing and fair weather as the strong high settles overhead. This will set up Sat night to likely be the colder of the two nights. Despite high center drifting east and return flow developing aloft, the boundary layer should decouple quickly around sunset. I blended in MOS guidance which was cooler than NBM, so that areas of frost make it to the coast from Portland north. Headlines will likely be needed for the final night of the frost/freeze program.

KEY MESSAGE 2 DESCRIPTION...

Today's model guidance remains in good agreement in continuing to lift higher moisture associated with the remnants of Isaias toward the Mid-Atlantic and Northeast and a weak low developing near or just south of southern New England. This will bringing increasing rain chances Sunday evening into at least the first part of Monday, but there's still uncertainty in the northward extent of measurable rain. Southernmost NH remains on track to see the higher rain totals locally with box and whisker plots from ensembles favoring around 0.50" of rain possible, although there are still some juicier ensemble members suggesting upwards of an inch. Either way, hydro issues are not anticipated. Rainfall amounts diminish with northward extent, and then a drying trend should occur by Monday afternoon as the system pulls away to the east. A cold front approaching from the north could also bring some showers to the mountains late in the day or Monday night.

Turning to Tuesday, a low pressure and cold front is forecast to approach New England from the Great Lakes, bringing more showers Tuesday into at least Tuesday night. NBM PoP is already 60-80% NH and western ME, which is supported by the ensembles, but timing is the more uncertain aspect. The GFS is much faster in clearing the front, which would make Wednesday generally dry, cooler, and breezy while on the other hand the ECMWF camp is slower and keeps some rain into Wednesday. Regardless of the speed of the system, cooler air will be drawn in behind this system, which could bring in some higher elevation snow showers either Wednesday or Wednesday night.

High pressure is favored for Thursday to bring a dry day, and then global models are already in good agreement for this far out showing a low moving across Quebec and sending a cold front our way to bring the next chance of showers on Friday.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

Local MVFR or lower possible with showers late this afternoon and early this evening, then widespread VFR conditions expected thru Sun. Gusty northwest winds will diminish around sunset tonight along with showers, but a breeze overnight should be enough to stir the boundary layer and preclude any fog formation. Local IFR possible in valley fog Sat night.

Outlook:

Sunday night-Monday...IFR ceilings are looking increasing likely, especially south of a PWM-LEB line. This area is also most likely to see rain. To the north, MVFR ceilings and rain are possible but less certain. Some improvement is possible later in the day.

Tuesday-Wednesday: Rain showers and IFR to MVFR possible as a front/low pressure cross through on Tuesday and Tuesday night. Depending on how quickly the front moves through, Wednesday could be breezy and mainly VFR, but it's possible rain and some degree of restrictions linger into Wednesday.

MARINE

Gusty northwest winds are expected to stay below SCA thresholds this evening, then high pressure settles overhead and winds and seas diminish thru the weekend. Sunday night-Friday...Low pressure likely passes south of the waters Sunday night into Monday with winds turning east to northeast but currently expected to remain below SCA levels. However, rain is likely with fog also possible. SCA conditions are then possible Tuesday into Wednesday as low pressure and a frontal boundary approach from the west, although there is uncertainty if the low will be a northern or southern track or somewhere in between. High pressure should build in around Thursday, and then another cold front approaches late in the week, potentially bringing additional SCA conditions.

GYX WATCHES/WARNINGS/ADVISORIES

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