textproduct: Gray - Portland

WHAT HAS CHANGED

Added mention of some isolated showers across western central Maine and into northern New Hampshire through this afternoon. Given thin high clouds, enough instability should build to develop a few short-lived showers through daytime.

KEY MESSAGES

1. A heavy rain event will impact the Northeast U.S. today and tonight with New Hampshire/western Maine on the northern periphery of it. There remains uncertainty in rainfall amounts as high pressure will also be pushing south through northern New England.

2. Temperatures warm during the latter part of the work week with elevated humidity returning Friday into the weekend as well. Chances for showers and thunderstorms will increase on Thursday into Saturday.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A stationary front currently situated over the Mid Atlantic will lift slightly north, with an attendant low pressure that will ride along it. This surface low will track near or south of the Southern New England coast. Such a track will result in us staying on the "cool" side of the system with the warm sector well to the south. One important player on the field will be Canadian high pressure to our north. With 90th percentile 500mb heights to the north and similar climo for surface high pressure to the northeast, there will be a strong opposition to northern moisture advection out of central New England. This is paired with marginal IVT through southern New England.

Overall mixed signals, as despite plenty of synoptic-level suppression from the north, the environment across southern NH does look favorable for efficient rainfall; PWATs near the 90th percentile of climatology, warm cloud depths of 12-13 kft, and some signals for FGEN banding. Localized heavy rainfall remains possible over the southern tier of the CWA.

WPC has maintained a marginal risk for excessive rainfall today across southern NH. And on Tuesday, the Seacoast is clipped by another marginal risk. Speaking probabilities, ensemble guidance indicates a 20% chance to exceed an inch along and south of a Portsmouth-Concord-Claremont line. For 2", that number drops to just 5%. So the broad idea is the "chance" for a soaking rainfall starting this morning, and continuing into Tuesday, focused across far southern NH, but also the potential that rainfall almost entirely winds up south of our forecast area. Northern NH and western Maine are likely to see limited precipitation if not fully dry conditions.

KEY MESSAGE 2 DESCRIPTION...

Fair weather returns for Wednesday. Warmer weather with increasing humidity continues to be in store for the end of the week. However, signals are that it will not be anywhere near the level that we experienced last week. Various pieces of guidance indicate that a cold front may attempt to drop in from the north later Thursday or Friday. Uncertainty exists in timing but chances for showers and thunderstorms should increase at least in the north on Thursday, and then for the rest of the area Friday and perhaps Saturday.

AVIATION /13Z MONDAY THROUGH FRIDAY/

Through 12Z Tuesday...VFR expected through today with clouds thickening and lowering south to north. MVFR is likely at MHT late tonight in low cigs and -RA with MVFR becoming increasingly likely at CON and PSM toward Tuesday morning. Marine stratus and fog will bring potential for LIFR at RKD tonight. Elsewhere mainly VFR overnight.

Outlook...

Tuesday: Cigs continue to lower Tuesday morning with IFR cigs likely for CON/MHT/PSM, with chances for PWM into mid day as well. Cigs gradually improve west to east Tues night.

Wednesday-Friday: VFR conditions expected for the latter half of the work week, with TEMPO MVFR conditions possible in afternoon showers and thunderstorms.

MARINE

Low pressure is expected pass south of the waters through Tues. Will see some increase in winds and seas tonight/Tues associated with this. Will monitor the need for a SCA Tuesday for the waters off the NH Seacoast as wave heights build towards 4 ft. If SCA conditions are met Tues/Wed it would be marginal and likely over our southern zones.

Generally sub-SCA conditions are expected from midweek through the weekend as high pressure remains dominant over the Atlantic.

GYX WATCHES/WARNINGS/ADVISORIES

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