Marine Weather and Tide Forecast for South Portland, ME

KEY MESSAGES
1. A distant coastal low will bring rain to portions of western ME today. Winter-like conditions continue in the higher elevations. Windy conditions today in the wake of the low.

2. Warm temperatures arrive to the region Monday-Tuesday.

3. An unsettled pattern with daily precipitation chances all week, however it will not be raining the whole time. The best chance for a long-duration soaking rain will be Wednesday-Thursday in two separate waves.

DISCUSSION
KEY MESSAGE 1 DESCRIPTION...

Deepening low pressure travels just east of the Gulf of Maine today. The broad precipitation shield is forecast to bring rain to portions of the forecast area, mainly the southern ME coast up through the lower Kennebec Valley.

Because of the low's gradual NE track, the western flank will see a tight gradient of dry air to the west and column saturated air to the east. As has been the topic of many AFDs this past week, the gradient between this humid and dry air will determine how far inland rain falls. There has been some persistence in guidance, increasing confidence that the bulk of measurable rainfall will occur along the coast from Casco Bay north... and then inland to the I-95 corridor through the lower Kennebec Valley. Here rainfall of 0.25" to 0.50" and possibly more in the 0.50" to 0.75" range closer to Penobscot Bay is forecast through late afternoon. NW Wind gusts 20 to 30 MPH are likely today.

There will be the chance of some showers elsewhere in southern ME and the Whites this afternoon. However, these will likely be progressive, end through the morning, and not produce near the same amnts to the east.

KEY MESSAGE 2 DESCRIPTION...

Height rises will start the week ahead of a deepening trough. This looks like it will open up the region to southwest WAA starting Monday and lasting through Tuesday. The airmass will quickly warm, with 850mb temperatures peaking in the 11-14C range on Tuesday. In addition, a tightening pressure gradient between high pressure to the southeast and low pressure to the northwest should allow for moderate southwest breezes, which will preclude sea breezes and act as a land breeze from Casco Bay points south.

The amount of clearing each day will dictate exactly how warm it will get. Monday's highs should solidly be in the 60s. On Tuesday, the combination of gusty southwest winds and mild temperatures aloft suggest the potential for temperatures to reach well into the 70s, which will likely be the "warmest" day of 2026 so far for a good portion of the forecast area.

KEY MESSAGE 3 DESCRIPTION...

Multiple chances for precipitation this week, as a trough approaches the area than stalls out overhead. 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. POPs are quite low as some model output keep us completely dry on Monday, but have noticed a bit of an uptrend on recent CAM guidance for some showers breaking out, focused from 10AM-6PM. Could see a few rumbles of thunder with ~500 J/Kg MUCAPE and mid-level lapse rates ~7 C/Km.

On Tuesday a cold front approaches from the northwest late in the day. Expecting thunderstorms to initially develop from Upstate NY to VT, then it will be a race against time to see if they can survive into our forecast area before the loss of daytime heating and instability by evening. Latest available NWP guidance suggest that upwards of 1000 J/kg of MU CAPE and deep layer shear around 35 kts will be possible Tuesday afternoon from the CT Valley through central NH and into western Maine mountains. This would support some organized updrafts while the limiting potential for strong to severe storms will be forcing remaining to the west. Machine Learning Guidance out of CSU does show a modest signal for severe storms Tuesday so will have to watch trends as we get into the window of CAM guidance in the next 24 to 36 hours.

We then see the front stall out overhead, with daily chances for rain as two separate surface lows ride along a trough axis that will be taking on an increasingly negative tilt. The first surface low will impact the region on Wednesday with primarily soaking steady rainfall. The pattern becomes rather amplified by late week as a second low pressure moves through on Thursday, likely the stronger of the two. The meridional flow suggests a tropical connection leading to locally heavy rainfall potential, with PWATs rising to near an inch. NBM mean has a strong signal at this point even with the long lead time of >1 inch of rain.
Will have to watch how this trends, but precipitation will generally be stratiform and steady, easing flood concerns.
Combined with the current rainfall deficits, this will be much needed and welcomed precipitation.

AVIATION /03Z SUNDAY THROUGH THURSDAY/
Through 06Z Monday...IFR to MVFR ceilings are most likely for the Midcoast ME terminals along with into the morning hours.
MVFR ceilings are possible for northern NH terminals. Northwest gusts 20-25 kt today for those that see higher ceilings, mainly in NH and then into southern ME.

All terminals trend VFR by 00z Mon.

Outlook:

Sunday night: Mainly VFR expected, but there is a chance of showers at HIE late.

Monday-Monday Night: Mainly VFR expected, with gusty southwest winds. Rain showers could bring MVFR restrictions at HIE.

Tuesday-Tuesday Night: Mainly VFR with gusty southwest winds during the day Tuesday. 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.

Wednesday through Friday: MVFR cigs likely with rain showers.
IFR possible Wednesday night through Thursday night in low CIGs and widespread rain. Drying trend Friday will bring improving conditions.

MARINE
Winds become northerly today with SCA conditions expected. Have not included the bays in the SCA but occasional periods of 25 kt gusts will be possible there.

Wave heights increase late Sunday night, 4 to 6 ft, and continue through much of Monday night amid increasing SW flow. Gales possible late Tuesday into Wednesday, but strong surface inversion may diminish gust strength to 25 to 30 kts for the coastal waters. The greater chances for gusts to gale through this period would fall on the outer coastal zones. South to southwest winds relax some Wednesday night into Thursday while SCAs will likely still be needed into Friday morning.

GYX WATCHES/WARNINGS/ADVISORIES
ME...None.
NH...None.
MARINE...Small Craft Advisory from 11 PM this evening to 6 AM EDT Tuesday for ANZ150-152-154.