textproduct: Gray - Portland

WHAT HAS CHANGED

Increased winds today and Tuesday in anticipation of deeper mixing while an active jet resides aloft. Can't rule out some gusts to 40 mph either day, but winds will decrease come evening unless conflicting with convection.

KEY MESSAGES

1. Temperatures rapidly warm early this week, accompanied by gusty south to southwest winds during the daytime.

2. Several chances for instability-driven downpours and thunderstorms Monday-Wednesday

3. A slow moving front will bring chances for beneficial rain Tuesday night through Thursday night areawide, with the highest chances centered on Wednesday.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

The forecast area is set to see three straight days of low level warm air advection through Wednesday. A broad region of SW flow stretches from the Gulf coast to New England, in between a low to the west and Atlantic high pressure to the east. The quick warmup will be most noticeable today and Tuesday, where high temperatures look to improve some 10 degrees from each previous day.

The warmest days of the week will likely be today and Tuesday with daytime highs pushing into the mid to upper 60s today and widespread 70s Tuesday. Ample mixing in the low levels should tap into WAA aloft where +2 to +5C 850mb temps will be advancing this afternoon. This increases to around +9C come Tuesday as approaching mid level jet and expanding low level jet aid in further SW advection.

Overnight lows also increase, and with some background winds and cloud cover, radiational cooling will be limited. This will lead to values bottoming out in the 40s to around 50 Monday night, and widespread 50s Tuesday night.

The expanding low level jet will also bring breezy conditions to the region. For Monday, accelerating LLJ into the exiting low pressure in the Canadian Maritimes will coincide with morning mixing. This could lead to a window of SW gusts around 40 mph, before slackening into a range of 25 to 35 mph through the afternoon. Forecast soundings develop a surface inversion into the evening and overnight hours, leading to a return to light winds overnight.

Jet dynamics only increase heading into Tuesday as a 500mb jet impinges on broadening LLJ below it. A second day of mixing towards 700mb will attempt to mix 40 kt winds aloft. Will continue to monitor gusts on Tuesday, but can't rule out a slight chance of gusts near Wind Advisory criteria inland based on model soundings. There will be the complicating factor of increasing cloud cover w/ showers/storms from the west during the afternoon. This could also limit mixing depth and how well daytime gusts perform to the west.

KEY MESSAGE 2 DESCRIPTION...

Kicking things off on Monday, a conditional chance for some scattered showers/downpours, fueled by warm air advection aloft and a lobe of vorticity moving through the flow. Timing focused on the midday-afternoon hours, generally along and north of a line extending from Whitefield to Lewiston. Cannot rule out a rumble or two of thunder given rather steep mid-level lapse rates at or above 7 C/km.

On Tuesday a cold front approaches from the northwest late in the day. This will be the focus for the greatest thunderstorm threat of the week. Rather deep mixing suggests an environment with steep low- level lapse rates of 8-9 C/km, and high LCLs above 1500 m. Moisture will be modest, with surface dew points at best in the low-mid 50s. Not hard to think there will at least be a low risk for strong to damaging winds in any thunderstorms given the favorable environment for mixing winds downward. 0-6 km BWD approaching 40 kts suggests some storm organization/longevity is possible. Weak CAPE generally at or below 1000 J/Kg will put a ceiling on the severe threat. Similar to Monday, thinking the best chance for activity will be across the northern half of the forecast area, but there remains at least an isolated chance for thunderstorms to make it farther southeast and towards the coast. The threat looks to be focused in the 3-9 PM window.

By Wednesday we'll see the front stall out overhead, with a trough axis that will be taking on an increasingly negative tilt. Depending on if any of the region can stay in the warm sector, another chance exists for downpours and thunderstorms. Given only a few hundred J/Kg of CAPE at best, not expecting severe storms at this time despite impressive dynamics and shear, but cannot rule out some rumbles of thunder. Downpours are likely however; it will be a juicy environment as dew points surge to near 60 in the Manchester-Portland corridor.

KEY MESSAGE 3 DESCRIPTION...

No significant changes in regards to a round of soaking rain midweek. There remains uncertainty on the pace of the front, and therefore how many embedded waves train over the forecast area. Present forecast supports a slow arrival Tuesday evening, lingering for the daytime Wednesday (greatest rainfall amounts fall), and a quicker departure Wednesday evening into Thursday.

For about 24 hours, Maine and New Hampshire will be within a IVT corridor stretching all the way from the southern Plains. While not an ideal moisture source region, broad and strong LLJ should support the quick transport towards New England. Will have to monitor how the wave train progresses east as well, should more undulation join the otherwise flat boundary, a second wave could continue rain chances into late week.

AVIATION /10Z MONDAY THROUGH FRIDAY/

Through 06z Tuesday...MVFR ceilings possible at HIE. Breezy with westerly/southwesterly winds gusting as high as 25 to 30 kt. Can't rule out a couple of showers near AUG later. LLWS likely today and Tuesday most terminals.

Surface winds subside this evening, but jet aloft will be moving in. LLWS likely in the evening as a 35-40kt jet passes above surface inversion.

Outlook:

Tuesday-Tuesday Night: During the day mainly VFR with southwest winds gusting 25 to 35 kts. Chances for showers and thunderstorms increase NW to SE Tuesday afternoon, although storms are unlikely from CON to AUG and points south. Chances for showers Tuesday night bring some potential for MVFR. LLWS possible overnight.

Wednesday through Friday: MVFR cigs likely with rain. Periods of IFR possible Wednesday into Wednesday night. Shower chances will bring potential for at least MVFR Thursday with a drying trend Friday improving conditions.

MARINE

SCA conditions continue over the waters through Tuesday with gales possible across our far outer zones Tue afternoon and evening.

A cold front sinks through the waters Thursday into Friday shifting winds offshore.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory until 2 PM EDT Tuesday for ANZ150-152-154. Gale Watch from Tuesday afternoon through late Tuesday night for ANZ180-182-184.