textproduct: Gray - Portland

WHAT HAS CHANGED

Confidence is increasing in some degree of a heat risk during the second half of next week. However, uncertainty remains in regards to how high we could see temperatures get.

KEY MESSAGES

1. A cold front continues to bring showers and storms into this evening, followed by dry and seasonably warm conditions for Monday.

2. Heat and humidity build the second half of this week posing a heat risk, as well as chances for showers and thunderstorms.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

A cold front continues to make slow progress through northern New England this afternoon as low pressure tracks from Quebec through eastern Maine. Scattered showers continue into the evening hours, with isolated thunderstorms possible across southern Maine and NH into the early evening hours. The front then slides offshore by late this evening, with clearing behind the front for overnight. With the recent rainfall and clear, calm conditions overnight, valley fog is likely north of the mountains tonight where the longest period of radiational cooling is expected.

High pressure follows behind the front for tomorrow, with a much drier airmass and seasonably warm temperatures. Near full sunshine overcomes the cooler airmass to push highs into the mid 70s across the north, to low 80s downwind of the mountains tomorrow. With light northwesterly flow, a light sea breeze likely develops in the early afternoon, but is most likely to remain confined to the immediate coastline.

The high shifts south of New England overnight tomorrow night, setting up a warming trend going into midweek. The warming airmass will already be evident by tomorrow night as lows generally only fall into the upper 40s across the north, to mid 50s along the coast.

KEY MESSAGE 2 DESCRIPTION... The forecast for this week is going to be dominated by ridging over the Eastern Seaboard which is going to allow heat and humidity to build over several days. As the ridge begins to take hold in the first half of the week, global models are in good agreement 850 mb temperatures quickly build to +12 to 14C, which should result in low to mid 80s north of the mountains and mid to upper 80s to the south. With northerly flow downstream of the trough, dewpoints remain bearable in the 50s which should make it feel pleasant. However, the ridge axis shifts east of the area on Wednesday turning flow more southwesterly and driving dewpoints up. This, combined with the building heat, is going to make conditions feel more oppressive as the week goes on. There is still some decent spread in the models, but the middle of the road solution suggests 850 mb temperatures climb to +16-18C (which is in the 90th percentile of climatology) by the end of the work week, with dewpoints climbing into the upper 60s. The result would be a toasty second half of the week with widespread ambient air temperatures in the mid to upper 80s and near 90 in some of the normally warmer locations in the Merrimack Valley. This of course means apparent temperatures would be a little higher with those sticky dewpoints. I don't have the confidence to throw around the word heat wave yet (our local definition requires 3 days with greater than 90F degree temperatures), but I could see Heat Advisories eventually being needed for parts of the CWA especially when considering heat accumulation with nighttime temperatures potentially only being able to drop into the mid-60s. As usual the higher terrain should stay a bit cooler, as well as the coast where seabreezes are all but guaranteed with little in the way of opposing flow. Lastly, the increasing moisture means an increase in chances for afternoon showers and thunderstorms. Coverage at this time range is obviously uncertain, but there is currently nothing that suggests it would be any more than the normal summertime isolated to scattered pop-ups.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

Showers continue to bring MVFR to IFR conditions into early this evening, with VFR returning to most terminals by midnight. VFR then prevails through tomorrow night, but some valley fog is possible at HIE tonight, and then possible at HIE and LEB Monday night.

Outlook... Tuesday: VFR conditions expected under mostly clear skies.

Wednesday-Friday: Generally VFR conditions expected, but afternoon showers and/or thunderstorms may bring about TEMPO MVFR conditions.

MARINE

Conditions remain below SCA levels as a cold front slowly crosses the waters this evening. High pressure moves in behind the front for Monday, and then slides south of New England Monday night.

Quite conditions are expected on the waters with afternoon seabreezes through next week as high pressure builds over the Gulf of Maine and only slowly moves eastward.

GYX WATCHES/WARNINGS/ADVISORIES

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