textproduct: Gray - Portland

WHAT HAS CHANGED

Minor update to the Aviation section for the 00Z TAFs.

KEY MESSAGES

1. Rapidly increasing moisture content may aid in local torrential downpours late Tuesday afternoon into Tuesday night.

2. A few strong to severe thunderstorms are possible Wednesday afternoon. High pressure then brings a period of mostly dry weather through the weekend.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A warm front will approach the region on Tuesday and generally remain mostly to our south through Tuesday evening before possible lifting northward into southern NH and perhaps coastal ME. Rapid moisture return is expected by Tuesday evening with forecast soundings showing PWATS rising to around 2 inches...and even higher in some spots. The main risk will be convection arriving late Tuesday afternoon or early evening in the western zones, then pushing eastward to the rest of the forecast area during the evening hours. There is a low risk for a few wind producing storms given the strong low level jet. However, the main threat will be torrential downpours with an attendant risk for isolated instances of flash flooding Tuesday evening into the overnight hours.

KEY MESSAGE 2 DESCRIPTION...

Wednesday, broad low pressure remains to the northwest of the forecast area, attempting to vertically stack across portions of western Quebec. Southern Maine and all of New Hampshire will reside in a warm sector, but primary moisture transport will have moved north or off into the Gulf of Maine. The trough axis nears later in the day, with a cold front forecast to move across much of the forecast area by mid evening.

This front will be the primary forcing agent for possible thunderstorm development. Amid a well sheared environment and ensembles supportive of up to 1000 j/kg of instability, some of these storms could be strong to severe. REFS hodographs are straight, with 35-40 kts of 0-6km shear and a IQR of 0-1km shear around 15 to 18kts. This is towards the end of the 12z REFS time range, so expect some changes in magnitude, but shear is the higher confidence variable at this range when comparing to instability. Given this setup, storms may congregate along the frontal forcing, but also can't rule out some isolated convection ahead of it. Shear would keep this organization possible as long as there is instability to muster. Any deeper storms would have the capability to produce gusty winds, with hail more likely in any discrete cells.

There are a few caveats to this setup:

Dry air will be more prevalent after the initial warm front surge on Tuesday. PWATs fall to a range of 0.75 inch to 1.5 inch across the forecast area Wednesday(the greater amounts reside towards the coast). This could keep coverage low, and perhaps only along the front.

Because air source region may be from the Upper Midwest, will need to factor in the potential for smoke to move in where the expected instability is to develop. HRRR Smoke model shows this possibility, although the concentration weakens towards the end of the range. This could be correct, or it could be a limitation of adding smoke to the model. Tend to trust the overall trajectory more at this time, so felt this possibility was worth mentioning as a caveat.

Into the evening, loss of instability is expected and will lead to a lower storm/precip threat. Winds gradually shift WNW overnight. A period of quieter weather is then expected Thursday and Friday as strong high pressure arrives from the Great Lakes.

AVIATION /00Z TUESDAY THROUGH SATURDAY/

Through Tonight...VFR with light southerly winds.

Tuesday: VFR with increasing clouds. MVFR cigs develop across southern NH early evening and spread northeastward reaching AUG and RKD by 00Z Wednesday.

Outlook...

Tuesday night: MVFR becoming IFR in showers and thunderstorms with low clouds and some fog. LLWS likely on the coastal plain and across southern ME during the evening hours.

Wednesday: SHRA/RA overspreads the area south to north. Widespread MVFR likely, IFR cigs/vis possible in periods of rainfall. This may linger for southern ME terminals into Wednesday afternoon.

Wednesday Night-Thursday: Improving to VFR Wed night with most rain exiting northeast. A round of SHRA/TS possible Thursday afternoon.

Friday-Saturday: VFR, no sig wx.

MARINE

Expect sub SCA conditions through Tuesday as high pressure moves south of New England. An advancing low pressure system through Ontario and Quebec will bring a cold front towards the coastal waters Wednesday. Expect increasing SE to S winds and waves Tuesday evening through Wednesday afternoon, with a SCA likely needed.

High pressure returns late week, leading to lighter winds and seas.

GYX WATCHES/WARNINGS/ADVISORIES

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