textproduct: Gray - Portland

WHAT HAS CHANGED

A Coastal Flood Statement has been issued for Portland and points south for tonight's high tide cycle.

KEY MESSAGES

1. Seasonable temperatures will persist into Tuesday with comfortable overnight lows. High astronomical tides will continue into the middle of the week and minor flooding remains possible around the times of high tide.

2. An unsettled second half of the week is likely with multiple rounds of showers and thunderstorms in the forecast.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Diurnally driven cumulus will dissipate this evening with the loss of daytime heating. Skies overnight will be clear to partly cloudy, which combined with weakening winds will allow for some radiational cooling. Overnight lows will range from the 40s across the north and interior valleys with 50s elsewhere. Some valley fog is possible after midnight.

Tuesday will feature dry conditions with mainly sunny skies and high temperatures into the 70s to near 80 degrees from north to south. Tuesday night will be another comfortable night with low temperatures falling into the 40s and 50s.

High astronomical tides will persist through the middle of this week with minor coastal flooding possible around the times of high tide each night, especially from Portland and points south. A Coastal Flood Statement has been issued for Portland and points south for tonight.

KEY MESSAGE 2 DESCRIPTION...

Surface high pressure will move east and out to sea on Wednesday as a shortwave trough starts to approach from the west. Warm air advection could lead to some showers with a slight chance of thunderstorms, especially over the mountains. This activity should be very hit or miss and will likely diminish in the evening with the loss of daytime heating.

A stronger upper low then moves from Ontario into southern Quebec on Thursday, which should lead to widespread high precipitation chances (greater than 70 percent) across pretty much all of New Hampshire and western Maine. Isolated to scattered thunderstorms will also be likely as this system moves into the region and a surface low deepens over Quebec with a cold front trailing to the south/southwest. While the system has trended a bit faster than in previous days, strong forcing, very strong shear, and weak to moderate instability could lead to some early to late afternoon strong to severe storms. The best environment will likely be over the southern half of New Hampshire, where NAM soundings suggest we could see dewpoints into the mid to upper 60s, MLCAPE values around 500 J/kg, and MUCAPE around 1000 J/kg. Additionally, shear profiles look more like something we would usually see from a cool season setup with 0-1 km values up to 40 knots and 0-6 km values well over 60 knots. At face value, these shear values would be very concerning for severe weather but plenty of flies remain in the ointment this far out. For one, we will likely see quite a bit of early day convection/rain/cloud cover which could limit the already marginal instability that models are forecasting. The best kinematic environment will be in the potential dry slotting behind this early day convection which may not clear out in time for enough destabilization. The ECMWF and it's ensemble keep temperatures much cooler with lower dewpoints than the NBM which would also reduce the threat significantly. Secondly, forecast mid-level lapse rates are unimpressive which is likely a result of the early day convection/clouds. All that said, it will be interesting to see the forecast trends as we move into the range of the CAMs over the next couple of days and get a better idea of timing and finer mesoscale features. A few fast moving strong to severe thunderstorms will be possible depending on the evolution of said features. Locally heavy rainfall will also be possible, but modeled precipitable water is forecast to be around average for this time of year and the system should quickly move out Thursday night.

Ensembles favor troughing sticking around through the weekend so the pattern may remain unsettled with near daily chances of showers and storms, especially across the higher terrain.

Regarding temperatures, Wednesday should be a little bit warmer than on Tuesday with highs mainly in the lower 70s to lower 80s. Temperatures will then be fairly similar through the rest of the work week and through the weekend, but maybe a little bit cooler in the mountains and a touch warmer along the coast at times.

AVIATION /18Z MONDAY THROUGH SATURDAY/

Through 18Z Tuesday: VFR conditions expected. The exception may be at KHIE where valley FG is possible late tonight into early Tuesday morning, although forecast confidence is low. NW wind gusts up to 25 kts will remain possible through 00Z before becoming light and variable overnight and then NW at 5-15 kts on Tuesday. No LLWS is anticipated.

Tuesday Night: VFR conditions with light and variable winds.

Outlook:

Wednesday: VFR conditions generally expected. Local MVFR or lower possible in the afternoon or early evening under isolated showers and thunderstorms.

Thursday - Thursday night: Areas of MVFR or lower possible under showers and thunderstorms and low CIGs.

Friday: Low CIGs may linger into Fri before clearing during the afternoon.

Friday night - Saturday: VFR conditions generally expected. Local MVFR or lower possible under isolated showers and thunderstorms.

MARINE

Winds will remain offshore through Tuesday night with gusts remaining largely below 20 kts and seas of 1-3 ft.

Winds and seas remain below SCA thresholds until the next system arrives Thursday. Southeasterly winds turning southwesterly may gust up to 30 kt with seas building above 5 ft outside of the bays.

GYX WATCHES/WARNINGS/ADVISORIES

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