textproduct: Gray - Portland

WHAT HAS CHANGED

Increased chances for rain showers 10 to 20 percent tonight for the northern half of the forecast area.

KEY MESSAGES

1. A weak cold front will brush the area overnight with a chance of showers mainly for the northern half of the forecast area.

2. A seasonably strong low pressure system will drag a cold front across the area late Wednesday into Thursday with showers becoming likely late Wednesday and Wednesday night.

3. A period of gusty winds will be possible on Thursday behind the front with dry weather returning late week through at least Saturday.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Just a perfect early fall day out there today. Diurnal cumulus will dissipate after sunset and I expect mostly clear skies at least during the first half of the overnight. A backdoor cold front will be sagging towards the region late this evening, but I do not think that significant prevents radiational cooling early. As such I expect some valley fog to develop in the usual locations prior to the clouds invading the area. Showers will become more likely just before sunrise mainly across the northern half of NH and the western Maine mtns into the Kennebec River Valley. NBM PoP is pretty low given the CAM depiction of precip and QPF, so I leaned on the HREF for timing and location and brought the values up to 30 or 40 percent.

Briefly high pressure will drift across the region Tue afternoon thru the overnight. That should allow for good radiational cooling conditions again, despite return flow starting towards daybreak Wed.

KEY MESSAGE 2 DESCRIPTION...

High pressure slides east Wednesday as low pressure moves slowly east toward James Bay and sends a cold front toward New England. Increasing southerly flow will advect higher moisture into the region, potentially enough for some light showers across the north, but mainly it will begin to feel more humid out as dewpoints are forecast to climb into the upper 50s to lower 60s with skies becoming mostly cloudy. Temperatures may be limited to the 60s for northern areas due to more clouds while southern areas still reach the 70s. With the slower approach of the front, prospects for thunder have lowered.

The better chances for showers arrive Wednesday evening and Wednesday night as the front enters the region and better support moves in aloft. PWATs increase to 1.70" to 2.00" which is well above the 90th percentile, meaning rain rates could be briefly heavy at times. But with the fast-moving nature of the system and drier antecedent conditions, not expecting any hydro issues. Models continue to favor the northern half of NH extending into the western ME mountains/foothills and Midcoast for the higher chances/more QPF with ensemble means in pretty good agreement of 0.25-0.50" although there are some probabilities exceeding 0.50". Chances/amounts decrease with southern extent, and there are hints that southern NH sees less 0.10", which is certainly not ideal.

KEY MESSAGE 3 DESCRIPTION... Any lingering showers will come to an end Thursday morning with much drier air moving in behind the initial front. West winds will also ramp up in the morning and afternoon as mixing brings down the higher winds from aloft and the surface low moves east toward the Maritimes. Based on forecast soundings, gusts of 25-35 mph appear likely, but there could be a few exceed 40 mph, per the top of the mixed layer. The cooler air doesn't move in until later in the day when a secondary front crosses, so it will be a pretty warm day with highs expected to be upper 70s to lower 80s south of the mountains.

Temperatures cool back down closer to normal for Friday and Saturday with highs mostly in the 70s and plenty of dry air from high pressure building into New England to keep skies mostly sunny with low humidity. Friday night may bring excellent radiational cooling to bring widespread lows in the 40s and some 30s across northern valleys.

The high shifts to the east by Sunday as another low pressure moves across the Hudson/James Bay region with its associated cold front approaching New England. This will bring another chance of showers either Sunday and/or Monday, which is supported by the 20-30% NBM PoPs in this time window.

AVIATION /18Z MONDAY THROUGH SATURDAY/

Through Tuesday...VFR conditions expected thru at least midnight. I do think skies will be clear enough around midnight for valley fog to form at both HIE and LEB. Eventually clouds move in and some showers are possible which will help the fog scatter out prior to sunrise. Showers will be possible mainly thru the mtns to the Midcoast, there PROB30 groups have been added to HIE, AUG, and RKD. Largely VFR conditions expected Tue, but isolated showers are possible in the afternoon.

Outlook:

Wednesday: Mainly VFR but MVFR possible later in the day as chances for showers begin to increase.

Wednesday night: A cold front brings widespread showers and restrictions. Highest chances for showers and IFR to even LIFR will be at HIE-AUG-RKD with lesser confidence in degree of restrictions toward PWM and the southern NH terminals. Gusts out of the S to SW around 20 kt possible.

Thursday: Mainly VFR with west winds gusting 25-30 kt, possibly as high as 35 kt.

Thursday night-Saturday: Mainly VFR. Valley fog possible at HIE and LEB by Friday night.

MARINE

Winds and seas are expected to remain below SCA thresholds thru Tue night. Then low pressure center moves east across Hudson Bay toward the Canadian Maritimes Wednesday into Thursday, which will send a cold front across the waters either late Wednesday night or early Thursday morning...and then a secondary front later in the day Thursday. Increasing south to southwest flow ahead of the fronts will likely reach SCA levels from late Wednesday into the first part of Thursday, and then possibly for a period behind the second front Thursday night when winds turn west to northwesterly. Conditions improve Friday and Saturday as high pressure builds across the waters. The high shifts east Sunday into Monday as another front approaches from the west.

GYX WATCHES/WARNINGS/ADVISORIES

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