textproduct: Gray - Portland

WHAT HAS CHANGED

Some of the short term guidance has slowed the progression of rain this morning and continued a more southerly trend.

KEY MESSAGES

1. Low pressure tracking along a stationary front will provide a focus for clouds and rain today, mainly across south-central NH and SW Maine.

2. An unsettled pattern brings the best chance for widespread rainfall late Wednesday and Thursday.

3. A gradual drying trend follows for Friday and Labor Day weekend.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Expect a crisp morning with lows mainly in the mid 40s north to the mid 50s elsewhere. The one exception will be southern New Hampshire and the far southern tip of Maine where we may only dip into the lower 60s. Valley fog will be possible north as light winds and clear skies promote efficient radiational cooling.

A modest upper-level ridge axis will move slowly towards the region today with strands of vorticity forecast to ripple through northwest flow aloft. At the surface, a stationary front remains in place, extending from around Lake Ontario to Long Island, NY. A low is forecast to ride east southeast along this boundary, which will lead to some rain across southern New Hampshire and portions of western Maine. The northern extent of the rain remains a bit uncertain as moisture will be fighting against dry high pressure to the north. Some of the latest short term guidance has trended a touch later with the precipitation onset and there has been a slight nudge south the past few runs of the HRRR. These trends could suggest high pressure winning out versus more widespread precipitation over the forecast area.

All of that said, confidence remains high that we should see some steady rainfall over much of southern New Hampshire and portions of the western Maine, most likely the southwest. 24-hour NBM QPF probabilities suggest that the highest totals will be maximized over southwest New Hampshire with amounts most likely in the half inch to one inch range. Totals will diminish rather quickly moving northeast.

We should see plenty of cloud cover today which will keep temperatures on the cooler side. Afternoon highs are forecast to range from the upper 60s to mid 70s. The coolest temperatures will likely be across southern New Hampshire and southwest Maine where we will see the most rainy/cloudy conditions.

KEY MESSAGE 2 DESCRIPTION...

We find ourselves between shortwaves on Wednesday, which should lead to rain-free conditions during the day. Likely will still have to contend with mostly cloudy skies and onshore flow, which suggests another day of cooler highs, primarily in the 60s-near 70, but a bit of a muggy feel persisting, which will also lead to another mild night of lows in the mid 50s-low 60s.

The second wave moves in for Thursday. Rain chances start as early as Wednesday night, but higher PoPs, and the chance for heavier rain, will likely be on Thursday when the best upper-level support and forcing arrives locally. Timing of precipitation remains to be ironed out, with outcomes ranging from a soaking rain all day, to a few rounds of more convective precipitation with some dry periods. Storm motions on the slower side, combined with a rather soupy airmass, suggests locally heavy rain may be the primary threat. Dew points reach the 60s-near 70, and PWATs approaching 1.7-1.8" in our southern zones. Around 30 kts of deep-layer shear suggest storms could organize a bit, but guidance varies significantly at this time on if instability will be able to build or not.

KEY MESSAGE 3 DESCRIPTION...

Thursday's wave closes off and deepens over Nova Scotia Friday into early weekend, and is slow to depart. Drying northerly flow out of Canada should flush out with humidity with an overall cooler airmass. This will lead to a taste of early fall especially at night, but winds out of the north should also serve to downslope/act as an offshore wind, which will keep afternoons mild with 70s in the lower elevations.

With us still on the edge of the trough and cold core aloft, could see some diurnal clouds and showers each afternoon, mostly focused in the mountains/interior, but not expecting anything high-impact or widespread. Finally, it will be breezy near the coast with a rather tight pressure gradient setting up.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

Through tonight...Any remaining valley fog will lift as daytime heating commences and clouds move into the area. Most of the area cloud cover will remain mid to high level, but areas of MVFR CIGs will be likely across southern NH. Rain will also move into that same area, generally along and south of a LEB to PSM line. To the north, VFR conditions prevail. This will be the rule until tonight, when some fog/stratus may develop north of the warm front. Areas of IFR conditions are possible, but low confidence especially to the north. Some improvement is expected by sunrise Wed morning.

Outlook...

Wednesday-Thursday: MVFR or lower possible by late Wednesday at most terminals with showers.

Thursday night-Friday: Improving conditions, with MVFR still possible in brief showers. Valley fog is possible at LEB and HIE.

Friday night - Sunday: Mainly VFR during the daytime, but brief MVFR ceilings remain possible in afternoon showers. Nighttime valley fog is likely at HIE, LEB, and CON, and still possible at all other terminals.

MARINE

Winds and seas remain below SCA thresholds through tonight. Low pressure will track along a front south of the waters today bringing steady SE to E flow and periods of rain. Chances for rain diminish tonight as the low tracks southeast of Cape Cod.

A broad developing low pressure system approaches from the west Wednesday night, crossing the waters on Thursday. A period of SCA conditions are possible in southerly flow ahead of this system on Thursday. Northerly flow brings a return to fair conditions Thursday night and Friday. Marginal SCA conditions are then possible across the outer waters this weekend in northerly flow as low pressure deepens across Atlantic Canada.

GYX WATCHES/WARNINGS/ADVISORIES

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