textproduct: Gray - Portland

WHAT HAS CHANGED

A Frost Advisory has been issued for the New Hampshire and far western Maine foothills where temperatures will fall into the mid-30s tonight.

KEY MESSAGES

1. Showers and storms weaken as they push off the coast this evening. A cooler airmass arrives tonight, with frost across the north and into the foothills of New Hampshire and far western Maine.

2. Drier and cooler conditions are expected towards the middle of this week, with chances for frost possible Tuesday night.

3. Low pressure appears to dominate the northeast in the 2nd half of this week, bring multiple days of potentially widespread steady rain through Friday.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A cold front crosses the forecast area through this evening. Along with the front, a line of showers and thunderstorms will move through. Stronger showers/storms may contain gusty winds and even small hail before weakening early this evening.

After a cloudy start, daytime heating has destabilized much of the forecast area. Timing between the frontal passage and when clouds dissipated this morning was short, and this has largely limited how much CAPE could build through the early afternoon. As a result, shower/storm coverage and potency is expected to be limited through early evening even amid a great sheared environment.

Onshore wind component beginning from the Seacoast of NH through the southern ME coast should act as a stable environment, elevating convection enroute to the coast. Think this should limit gust potential for coastal ME as these showers and storms arrive, before background winds shift W later this evening.

Into this evening, a light W/SW breeze should remain. This will keep temps from decreasing rapidly in most locations in addition to continuing clouds near the coast. Passing disturbance to the south and off the coast will thicken these clouds, with some making their way across the interior. Points towards the foothills and mountains will see clearer skies as winds fall off later. This should allow for temps to fall into the low to mid 30s as long as the cirrus deck remains SE. Issued a Frost Advisory tonight along the foothills of NH and far western ME, but could need extension a bit south should clouds remain very thin and allow radiational cooling to be more continuous.

Continued to mention some isolated showers for Monday as cold air aloft and remaining moisture pivots through. Will continue to monitor temperatures Mon night as another chance for frost. A larger footprint will be possible through the interior and portions of the coast.

KEY MESSAGE 2 DESCRIPTION...

Mid-level troughing will allow for cold air to filter into the area early Tuesday morning. Widespread frost is likely north of the mountains, though the potential exists for frost to form even as far south as Concord and Auburn / Lewiston as light winds and mostly clear skies pave the way for radiational cooling. However, conditions look even more favorable for radiational cooling Tuesday night into Wednesday morning and low temperatures have the potential to run even cooler Tuesday night. This will likely result in much more widespread frost north of the mountains, and areas of frost across the foothills.

KEY MESSAGE 3 DESCRIPTION...

By Wednesday night, low pressure will likely arrive from the west. The low is likely to bring prolonged rainfall as its warm front lifts across the region overnight Wednesday. By Thursday morning, the low pivots and may allow for a storm track that favors an even more soaking rain across southern New Hampshire and western Maine. However, the kicker is that this system probably won't be very efficient at producing rainfall given PWATs are quite low (around 1 inch) for an event like this in the late Spring. So while the storm track itself allows for the opportunity to see prolonged, heavier rainfall, there simply just isn't enough moisture for this system to produce a more impactful rainfall. This does make this storm ideal to slowly restore groundwater levels and improve the severe drought conditions still being experienced across New Hampshire and western Maine.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

Through 18z Monday...SHRA with embedded TS passes through coastal ME/NH terminals through 23z. Lightning has been most common with discrete single cells with sustained reflectivity cores. The low freezing level allows these to produce lightning fairly quickly. Best chance for TS through early evening will be PWM to MHT/PSM.

A cold front will provide a westerly wind shift, already through LEB/HIE, this evening. Low clouds and possibly BR/FG may advect onshore for IWI/RKD through mid evening before suppressing back to the coast late. Otherwise, VFR expected tonight into Monday morning.

Outlook:

Monday: Cu field returns, but expect VFR. Occasional light SHRA possible across the mountains to interior terminals. W gusts increase 15 to 20 kts.

Tuesday: VFR Expected.

Wednesday: AM VFR, MVFR in the afternoon due to rain.

Thursday and Friday: MVFR due to showers and thunderstorms.

MARINE

A cold front will cross the waters tonight, shifting winds offshore. Did opt to extend the SCA through much of Mon as swell builds due to passing offshore disturbance tonight and into Mon. This could keep 4 to 6 ft waves towards the northern coastal waters into Monday afternoon.

Light and variable winds and 2-4ft seas are expected Tuesday through Wednesday evening. Winds pick up from the south Wednesday night and may reach SCA-levels briefly during the day on Thursday. Winds diminish and flip to light northerlies by Friday morning, with seas continuing to remain at 2-4ft.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Frost Advisory from 2 AM to 9 AM EDT Monday for MEZ012-013-033. NH...Frost Advisory from 2 AM to 9 AM EDT Monday for NHZ003>006. MARINE...Small Craft Advisory until 6 PM EDT Monday for ANZ150-152.