textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes with this forecast update.

KEY MESSAGES

1. A cold front will bring the chance for showers and thunderstorms through this evening followed by fair weather on Monday as high pressure builds in

2. A cooler and unsettled pattern sets in for the middle and later portion of the week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Latest surface analysis places a cold front across southern Quebec with a pre-frontal trough draped across the forecast area. This trough has allowed for mostly cloudy skies across much of the area along with areas of showers moving across central NH into SW Maine. These clouds and showers have put a halt on destabilization while areas near the Canadian Border has seen some filtered sunshine and MU CAPE build to around 500 J/kg. Latest radar shows thunderstorms along and ahead of the cold front with these forecast to move into areas north of the mountains through early this evening. These storms will track towards the Kennebec Valley through sunset. Limited CAPE will likely keep storms sub-severe while some may contain gusty winds up to 40 mph.

The cold front will progress southward tonight with showers weakening as they reach the coast. Much of tonight will be dry with patchy fog possible, especially in valley locations and areas that see some rainfall. Lows tonight with range from the 40s north to 50s south.

High pressure will be in control Monday for fair weather and temperatures ranging from the 70s north to low 80s south. Light and variable winds will turn onshore along the coast.

KEY MESSAGE 2 DESCRIPTION...

A ridge builds into the Great Lakes through the week, while New England looks to remain on the eastern periphery of the ridge. The ridge continues to amplify across Hudson Bay by late in the week, then folds over across northern Quebec. While remaining on the edge of the ridge midweek, New England looks to become enveloped in a deepening trough across Atlantic Canada by late in the week. This brings a cooler and unsettled pattern, that may extend into Labor Day weekend as well.

The first system arrives in New England late Monday night and Tuesday. Low pressure tracks through southern Canada, while a warm front looks likely to stall across New England and serve to focus rounds of showers and storms. The first round looks to miss most of the area to the south and west, leaving southwest New Hampshire the most likely spot to see showers and storms with the first round.

The front drifts north a bit into central New England by late Wednesday and Thursday. A fresh wave of moisture drifts northward into the area at the same time, while the low pressure center makes its closest pass through southern Quebec. This brings the best chance for the most widespread showers and storms to much of the area.

By late Thursday and Friday, the trough looks to deepen south of Nova Scotia. With blocking off to north, any moisture or showers would have to track southeastward to exit New England. This keeps the chance for showers ongoing into at least Saturday. The best chance for New England to dry out for Labor Day weekend would be for the trough to deepen more south of Nova Scotia, resulting in a more consolidated trough and low pressure system. There is some support for this, mainly from the Euro AIFS. New England would still mostly remain in a cooler troughing pattern, but with a brighter and drier northwesterly flow pattern. Much uncertainty remains in the evolution of this feature from Friday onward, but its worth noting that a wet weekend is far from a lock at this point.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

Through Monday...-SHRA will be possible along a line from LEB to PWM into this evening with SHRA/TSRA possible north of this line. Mainly VFR conditions will prevail while local MVFR or lower conditions are possible in any convection. VFR likely prevails tonight while some fog will be possible in the valleys and anywhere that receives rain. VFR conditions will prevail on Monday with light offshore winds giving way to a sea breeze in the afternoon.

Outlook...

Monday night: Clouds thicken and lower from SW to NE with -SHRA bringing potential for MVFR across southern NH.

Tuesday-Tuesday Night: VFR more likely across northern and eastern terminals. MVFR and showers possible, across southwestern terminals.

Wednesday-Thursday: MVFR more likely at most terminals with showers.

Friday: Some improvement possible while chances for showers remain.

MARINE

Winds and seas remain below SCA thresholds through Monday. SW winds will shift NW tonight as a cold front crosses the waters. Winds become light and variable Monday morning with a sea breeze developing in the afternoon. High pressure prevails across the waters to start the week with fair conditions. A warm front then stalls across the waters Wednesday through Thursday, with marginal SCA conditions possible in freshening southeasterly flow.

GYX WATCHES/WARNINGS/ADVISORIES

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