textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes with this forecast update.

KEY MESSAGES

1. High pressure over SE Canada will keep the area mostly dry today.

2. Low pressure tracks SE across the area Wednesday night through Thursday bringing rain and perhaps a few thunderstorms.

3. A gradual drying trend follows for Friday and Labor Day weekend, although there will be chances for scattered showers Saturday afternoon.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Low clouds will likely be in place through day break across much of NH and portions of western Maine south of the mountains. High pressure pressing southwestward from Canada will allow ceilings to lift and any lingering fog to dissipate through the morning. Skies will likely remain mostly cloudy through the day as low pressure approaching from the northwest will spread additional clouds into the area. Highs will be in the low to mid 70s with steady southeast winds.

KEY MESSAGE 2 DESCRIPTION...

Low pressure near James Bay this evening will send a warm front towards the forecast area tonight. Rain chances will increase west to east overnight along with deep moisture increasing as PWATs climb to 1.5 inches or more. These high PWATs combined with a strong southerly low level jet, some instability, and deep warm clouds depths will allow for bouts of moderate rain to localized heavy rain late tonight into Thursday morning. These bouts of heavy rain will be scattered in coverage and short enough to preclude and flood concerns while the timing will coincide with the Thursday morning commute that could lead to reduced visibility and ponding on roadways.

The low will occlude through Thursday morning as the cold front catches the warm front that makes limited progress into Maine. The main area of rainfall will exit through central Maine around mid day with CAMs suggesting portions of southern NH will get into the warm sector of a secondary low forming near the triple point. CAMs suggest MU CAPE will build to around 1500 J/kg across southern NH early Thursday afternoon that could lead to a few thunderstorms. Available forcing and dry air aloft seem to be limiting coverage from latest CAMs. Will have to watch trends in the positioning of the warm sector as there appears to be enough shear for organized updrafts and potential for strong storms Thursday afternoon across southern NH. Otherwise, much of the area will see a drying trend Thursday afternoon with skies trending towards mostly sunny by the evening.

KEY MESSAGE 3 DESCRIPTION...

An upper low over Quebec Friday will slowly cross northern Maine through Saturday. Latest model guidance is in reasonable agreement that Friday will be mostly dry outside of some showers in the mountains. Colder air aloft will move overhead Saturday that will aid in destabilizing the atmosphere Saturday afternoon. This will allow for some instability showers to develop while PoPs generally top out around 40 percent across the interior with lower chances near the coast. The upper low exits east Saturday night with ridging aloft building in Sunday. This will bring mostly fair weather Sunday into Monday.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

Through Thursday...Moist onshore flow has brought an area of IFR or lower CIGs into parts of the area this morning. Over the next few hours that CIG will lift and scatter out. Some isolated showers may linger across central NH, but generally VFR conditions expected this afternoon and evening. Conditions deteriorate again around sunset at the coast, working inland overnight. An area of rain will cross the area and widespread IFR or lower conditions are anticipated. This continues thru Thu morning. LLWS will also be likely Thu morning as a strong low level jet translates east across the area. Improvement is likely by early Thu afternoon, but a few thunderstorms may be possible in the afternoon across southern NH.

Outlook...

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

High pressure remains centered northeast of the waters through today. A warm front will approach the waters tonight as an area of low pressure tracks SE across New England through Thursday. A period of SCA conditions are possible in southerly flow ahead of this system on Thursday along with periods of rain and fog. 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.