textproduct: Gray - Portland

WHAT HAS CHANGED

Rainfall amounts have trended upward for Sunday night into Monday morning, but there remains differences on intensity and coverage.

KEY MESSAGES

1. A strong storm or two will be possible over portions of southern NH later this afternoon and early evening. Otherwise, high pressure will bring dry weather to start the weekend with seasonably cool nights.

2. Rain will accompany a passing system Sunday night into Monday morning. A period of heavy rainfall will be possible which may result in nuisance flooding or ponding in poor drainage areas for the morning commute.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Most of the area will remain dry the rest of today, but a shortwave trough will quickly move in from the northwest and may bring a few showers and storms to southwest NH and possibly farther east toward Concord, Manchester, and Nashua through early this evening. Environmental soundings across this corridor show a few hundred J/kg of instability and sufficient wind shear that a storm or two could be strong with gusty winds thanks to high dewpoint depressions in the low levels. Freezing levels remain around 9000 ft, so small hail will also be possible. This threat should end by sunset.

For tonight through Saturday night, Canadian high pressure will build toward the region tonight and then become centered over New England through Saturday night. Plenty of subsidence and dry air will provide mostly sunny and dry conditions with highs in the 70s to low 80s Saturday along with comfortable humidity. We'll have also have a couple of seasonably cool nights tonight and Saturday night with good radiational cooling. Lows in the 40s are likely in the northern valleys and typically cooler spots across the interior...with lower 50s elsewhere. The light winds and mostly clear skies will also promote patchy valley fog each of these nights, and it will probably be cool enough for steam fog over many area lakes and ponds.

KEY MESSAGE 2 DESCRIPTION...

Sunday begins with high pressure overhead, quickly exiting east into the open Atlantic. Surface return flow combined with a WSW low level flow will begin to saturate the low levels while high clouds overrun from the west. Right now, mid level saturation isn't likely until the late afternoon or evening, and this keeps rain chances lower through the day until the evening and overnight hours.

Forecast ensemble IVT plumes differ in magnitude and trajectory, with a more northeastward segment from the ECMWF ENS than the GEFS (a northward suppression, more progressive through the Mid Atlantic). In line with this vapor transport in the ENS, EFI QPF fields have also jumped comparing guidance from Wednesday evening to Thursday evening's runs indicating some potential for an above average precipitation event. Shift of tails however remains muted, with an extreme precipitation event unlikely.

Warm cloud depth in deterministic models is impressive, with runs of the GFS, ECMWF, NAM, and Canadian all depicting depths greater than 12kft for a period late Sunday night and early Monday morning. Coinciding with above normal PWATs around 2 inches, a period of heavy rain is becoming more likely, but exact location and radar presentation will need to be resolved with time.

One of the larger differences in guidance, and therefore an important distinction between outcomes, is how far south the trough or low cuts into southern New England. A track further into southern New England could kink vapor transport, limiting moisture depth and duration, while a flatter path across northern New England keeps this SW moisture flux open, resulting in more efficient rain processes.

Remaining rain tapers off through the day Monday, with drier weather and a bit of a warmup through midweek as highs climb into the lower to mid 80s. The next chance for rain then looks to be Wednesday into Thursday as an elongated cold front approaches from Quebec. It is yet to be determined if a wave develops out of this front, but could offer some rain enhancement depending on location through northern New England.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

Through 18Z Saturday...VFR with NW gusts to around 20 kt through 23Z- 00Z this evening. Can't completely rule out a shower or storm approaching MHT or CON later this afternoon, but chances are too low for TAF. VFR tonight with valley fog likely to produce LIFR or IFR at HIE and LEB. VFR Saturday morning.

Outlook:

Rest of Saturday-Saturday night: VFR, except IFR/LIFR possible at HIE and LEB with more valley fog Saturday night.

Sunday: VFR, increasing clouds with SHRA arriving west to east towards the evening.

Sunday Night: MVFR/IFR cigs in RA, RA may be heavy at times limiting vis.

Monday: Decreasing RA coverage, improvement towards VFR likely for points outside of the mountains.

Monday night and Tuesday: VFR. No sig wx.

Wednesday: VFR. SHRA possible late ahead of an incoming cold front.

MARINE

Through Saturday night...SCA conditions are not expected as high pressure builds toward the waters from the northwest. Winds will be mainly out of the northwest through Saturday morning but could be briefly from the west or southwest this afternoon. Flow may turn more onshore Saturday afternoon as the seabreeze tries to move onshore but then will return to more light out of the north or northwest Saturday night.

Southerly winds increase Sunday, with gusts 25 to 30 kt overnight into Monday morning. A low will cross northern New England and then the coastal waters Monday. It is possible a low level inversion keeps these gusts below SCA criteria. High pressure passes through the Mid Atlantic into midweek, before another cold front approaches from the west.

GYX WATCHES/WARNINGS/ADVISORIES

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