textproduct: Gray - Portland

WHAT HAS CHANGED

Minor adjustments to rain amounts and timing through this evening.

KEY MESSAGES

1. Rain continues across southern New Hampshire through early evening. Fog develops across the interior overnight.

2. Return to above average temperatures during the latter half of the work week with increasing humidity. Thunderstorm chances increase Thursday afternoon onward.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Steady rain has redeveloped across much of far southern NH this afternoon. The parent low has begun to drift SE away from the southern New England coast, and this will soon influence the northeastward extent of precipitation in New Hampshire. Seeing as there is little instability to support deeper cores, have removed thunder chances for the rest of today/tonight.

Until the low exits, there will be pockets of moderate rainfall rates, but not expecting rates to be especially heavy or cause surface impacts. A result of the damp surface will be the development of fog overnight, especially if clouds thin to accelerate cooling. At this time, low stratus may simply thicken and lower through the night, but visibility should recover early Wednesday morning.

Dry as temperatures rebound Wednesday with highs pushing into the 80s. Southern areas may miss out on a full day of warming pending remaining clouds, with maxes for the forecast area towards the Kennebec Valley. With weak background winds, a seabreeze could push inland early afternoon to bring some relief to the warmer temps.

KEY MESSAGE 2 DESCRIPTION... Guidance has come into better agreement on Thursday being the warmer and more humid day of this upcoming stretch of above average temperatures. 850 mb temperatures climb to +16-18C under a low amplitude ridge with southwesterly flow providing a further bump to warming and moisture. Dewpoints are anticipated to climb into the upper 60s and low 70s giving the air a stickier feel and will probably push some location's heat index into the upper 90s, with ambient temperatures more in the upper 80s and to 90F south of the mountains. North of the mountains, clouds and maybe even some showers/storms are going to creep in during peak heating and likely cap afternoon temperatures in the low to mid 80s.

Confidence is increasing in Friday being much cooler than originally discussed with models now in good agreement with the frontal passage occuring and bringing cloudier and more showery conditions. 850 mb temperatures come down a bit (+14-17C) as the new airmass begins to move in, so thinking locations north of the mountains where this settles in first only reach the mid to upper 70s. Locations south of the mountains stand a better chance of climbing into the low to mid 80s, and with elevated dewpoints in place this would feel more like 90F. With these temperatures and dewpoints both Thursday and Friday will have a good shot at afternoon thunderstorm development, with Friday likely having the higher coverage due to the frontal boundary providing lift.

Cooler and drier air move in behind the front for the weekend as the 500 mb pattern transitions to a more amplified trough. Despite this the weekend is looking mostly dry as high pressure builds in at the surface. The effects of the trough are likely going to be seen as an increase in clouds during the afternoons and a northwesterly feed that keeps temperatures on the pleasant side. Precipitation chances return early next week as low pressure moves into the region with the upper trough still in place.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

Through 18Z Wednesday...Rain continues for southern NH terminals through early evening. Occasional MVFR cigs may accompany this, with rain rates light enough to limit widespread IFR vis. Tricky ceiling forecast tonight as near-term guidance presents split cases. IFR case has abundant low level moisture creating a stratus deck tonight that builds down with fog. Less impactful case is a MVFR deck that reinforces overnight across southern NH. I am a little more hesitant about the dense stratus/fog up the ME coast and interior as they lacked rain today, but continuing clouds over the south won't necessarily result in widespread fog/mist. Have taken both cases into account per TAF based on these thoughts, with either case resolving with VFR Wednesday morning. Towards LEB, clearing skies should create a window for valley fog tonight.

Outlook...

Wednesday: MVFR/IFR cig/vis possible in the morning, becoming VFR by noon local. A afternoon seabreeze could cause a wind shift at RKD/PWM/PSM.

Thursday-Sunday: VFR conditions expected for the latter half of the work week, with TEMPO MVFR conditions possible in afternoon showers and thunderstorms.

MARINE

Decided to keep the SCA up despite marginal conditions this afternoon. Do expect some 4 to 5 ft waves towards the outer portion of the zone, with gusts mainly around 20 kts. These should subside after midnight as the parent low sinks southward. Marine stratus/fog development is likely tonight over areas of the waters, thinning by Wednesday afternoon.

Generally sub-SCA conditions are expected into early next week as high pressure remains dominant over the Atlantic.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory until 2 AM EDT Wednesday for ANZ154.