Marine Weather and Tide Forecast for South Portland, ME

KEY MESSAGES
1. A potent low pressure system will continue to bring multiple waves of rain to nearly all of New Hampshire and western Maine through Tuesday morning. The heaviest rain will mainly fall late this afternoon and into the overnight hours. Some localized flooding is possible as a result of significant runoff and snowmelt. The risk of flooding as a result of ice jams remains low, but non- zero as some ice does remain in the mountains and northern basins.

2. This system will also lead to strong winds this evening and overnight as a cold front approaches and moves through the region. The strongest wind gusts will mainly be in the 40 to 50 mph range over southern New Hampshire, coastal areas, the Interior of Maine, and the southern Kennebec River Valley. Any convection in the vicinity could lead to some higher gusts.

3. Colder air briefly returns midweek with quieter weather before temperatures trend back toward normal for the second half of the week.

DISCUSSION
KEY MESSAGE 1 DESCRIPTION...

The well advertised low continues to approach early this afternoon. Now that the initial slug of warm air advection rain has generally pushed through, we are seeing some scattered showers in it's wake. The expectation is that the radar will fill in through the rest of the afternoon as the better forcing for ascent arrives, which is already starting to be observed over southern and central New Hampshire. The heaviest rain is still anticipated late this afternoon through late tonight. Rain will then gradually taper off from west to east early on Tuesday.
Storm total 1 to 2 inch values with some locally higher amounts to 3 inches still seem reasonable. For more details on the flooding potential, please see the Hydrology section below.

Finally, most of the CAMs suggest the potential for at least isolated thunderstorms with this activity later this afternoon and into the evening. RAP forecast soundings are advertising around 100 to 300 J/kg of MUCAPE through the afternoon into the early evening. While the best chances for some thunder look to be along coastal areas, the Interior of Maine and up into the southern Kennebec Valley, some rumbles cannot be ruled out elsewhere given the strong forcing and fairly steep mid- level lapse rates. We also can't rule out a strong to marginally severe storm given the strong wind fields right off the surface (more about that next).

KEY MESSAGE 2 DESCRIPTION...

As eluded to in the previous section, this system will also likely bring strong winds to much of the forecast area this evening and overnight. Forecast soundings continue to advertise a highly anamolous low-level jet in the 70 to 90+ knot range ahead of the approaching cold front. The magnitude of surface wind gusts will greatly depend on how much momentum can mix down which is highly uncertain given a strong temperature inversion in the lower levels. Even with this inversion, widespread 40 to 50 mph gusts seem reasonable, especially over coastal areas, the Interior of Maine, and the southern Kennebec River Valley. If any deeper convection moves overhead, there will be a chance for some stronger winds to mix down so it will be something to watch closely. For now, the Wind Advisory continues to be in good shape but a short fused High Wind Warning or convective warnings could be needed this evening depending on how the system evolves. After the cold front moves though late tonight, winds will shift to out of the west and remain fairly gusty through the day on Tuesday. While winds will not be as strong as this evening and tonight, we will likely see widespread gusts up to 35 mph through the afternoon. Afternoon temperatures will also only be in the upper 20s to upper 30s so it will be a raw blustery day with wind chills in the lower teens north, to the upper 20s and lower 30s south.

KEY MESSAGE 3 DESCRIPTION...

A period of more tranquil but cold weather is still expected after the departure of the early week system. Northwesterly winds and very cold temperatures will allow for a bitterly cold morning on Wednesday. Wind chills are expected to be in the single digits below zero in the mountains, with single-digits above elsewhere.

High pressure continues through the rest of the week, with temperatures gradually warming as the mid-level trough makes a slight realignment to the west. The next chance for precipitation looks to be next weekend, though models remain very uncertain on timing and storm track.

AVIATION /21Z MONDAY THROUGH SATURDAY/
Through 18Z Tuesday...IFR to LIFR conditions will continue through the rest the day under low ceilings and periodic rain and/or fog. Enhanced low- level wind shear around 35-40 kts will also be likely at almost all sites through Tuesday morning.
High confidence in widespread IFR/LIFR cigs continues this evening as a second round of moderate to heavy rain arrives after 00Z Tuesday. LLWS continues with many terminals seeing an increase to 40-60kts, with strong SE to S surface gusts, 30-40 kts, along at least coastal terminals. Conditions will gradually improve Tuesday morning.

Outlook...

Tuesday afternoon: Trending VFR with MVFR ceilings possibly hanging on at HIE. Gusty W winds to 30 kts.

Tuesday night-Friday: VFR expected, with a slight chance of lower CIGs at HIE due to a stray snow shower on Friday.

Saturday: VFR expected, though restrictions can not be ruled out for light rain and snow.

MARINE
Expect frequent gales through the rest of today and frequent storm force gusts by tonight. Storm force gusts taper off early Tuesday morning with gales tapering off by Tuesday afternoon.
Wind gusts 25-30 kts continue through Tuesday evening. This wind will build wave heights to a peak of 12-15ft early Tuesday morning, and then they will very slowly start to fall through the day Tuesday. This strong system will bring heavy rain an possibly thunder to the waters through tonight.

Seas of 8-12ft and westerly gales are expected Tuesday night.
Conditions start to improve by Wednesday morning, with sub-SCA level westerlies and 6-10ft seas expected by then. Sub-SCA winds and seas of 3-7ft are expected through the remainder of the week.

HYDROLOGY
A Flood Watch continues into Tuesday as an anomalously moist airmass interacts with the ripe snowpack across the area. The main focus is on excessive runoff when the heaviest rainfall tonight coincides with rapid melt, leading to dramatic rises in area streams and rivers. There is high confidence that we will see 1 to 2 inches of rain, with some ensembles targeting the White Mountains and the Midcoast of Maine with up to 3 inches.
Given the anamolous moisture, expected PWATs should be in the 1.2 to 1.4 inch range (near the 99th percentile for mid- March).
Thus, the environment is primed for heavy rainfall.

The late moisture surge tonight will be the real nudge needed to push water out of the already ripened snowpack. The expected snowmelt water loss of 1 to 1.5 inches will mostly occur between this evening and early Tuesday morning, about 6 to 12 hours.
The coinciding of heaviest rainfall with rapid snowmelt will lead to swift rises on small streams and rivers. The mainstream rivers have a lot of channel capacity due to lingering drought, but more confined channels in the mountains or small streams could easily be overwhelmed resulting in localized minor flooding. Deep frost will limit infiltration, so for some areas the sheer volume of water could overwhelm culverts, low-lying areas, and low-water crossings. Expect urban street flooding and drainage issues. While many southern rivers are ice-free, some ice remains in the mountains and northern basins. There remains a non-zero risk for mechanical break-up and unpredictable jams as flows increase.

GYX WATCHES/WARNINGS/ADVISORIES
ME...Flood Watch through Tuesday afternoon for MEZ007>009-012>014- 018>028-033.
Wind Advisory until 8 AM EDT Tuesday for MEZ014-021>028.
Wind Advisory until 5 AM EDT Tuesday for MEZ018>020.
NH...Flood Watch through Tuesday afternoon for NHZ001>015.
Wind Advisory until 8 AM EDT Tuesday for NHZ014.
Wind Advisory until 5 AM EDT Tuesday for NHZ007>013-015.
MARINE...Storm Warning until 8 AM EDT Tuesday for ANZ150>152-154.
Gale Warning until 8 AM EDT Tuesday for ANZ153.