textproduct: Gray - Portland

WHAT HAS CHANGED

There are no significant changes to the forecast at this time.

KEY MESSAGES

1. High pressure slides south of New England tonight, bringing warmer air northward for Tuesday.

2. Humidity increases on Wednesday and will remain elevated through the end of the week with daily chances for showers and thunderstorms. Highs in the 80s to low 90s combined with the humidity will pose a heat risk.

3. Summer warmth hangs on into early next week, but a cold front passing over the weekend looks to break the humidity.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... High pressure slides south of New England tonight and into the day on Tuesday. The dry and seasonable airmass in place today begins to modify tonight as return flow around the high brings in warmer air and higher dew points begin to work in overnight and on Tuesday. With this set up, lows aren't as cool tonight, but radiational cooling does bring lows into the mid 40s across the northern valleys, to mid 50s elsewhere. Valley fog is also likely again tonight across the northern valleys and through the CT River Valley.

Tuesday brings warmer temps and an uptick in humidity. NBM temps appear to be disconnected from reality with NBM forecast highs running +17-19C 850mb temps. So highs were brought down by a few degrees overall to keep them closer to modeled 850mb temps +16C. This results in highs in the upper 70s across the north, to mid and upper 80s across the south and along the coast. Similar to today, a sea breeze likely develops by the early afternoon, but mainly hugs the immediate coastline with broad southwest flow over the mainland.

Clouds begin to filter in overnight tomorrow night, helping to keep lows on the warmer side and lessening the extent of the valley fog across the north. Lows generally range from the mid 50s across the north, to low 60s through the Seacoast of NH.

KEY MESSAGE 2 DESCRIPTION... The ridge axis moves overhead Wednesday with global ensemble means suggesting 850 mb temperatures rising into the +15-17C range. This should be good for surface temperatures to mix into the low to mid-80s, but with southwesterly flow ramping up dewpoints are going to be rising into the low to mid-60s so it may feel more like the upper 80s to near 90 in spots. A shortwave attempts to move through the ridge Wednesday evening which will provide the forcing for some afternoon/evening showers and thunderstorms.

This pattern continues through at least Friday, with Thursday looking like our best shot at seeing ambient temperatures reach into the 90s, but it also looks like this may end up confined to the Merrimack and Connecticut River Valleys. Diurnal convection is potentially going to be a limiting factor in regards to how high temperatures can get over this time period which is causing uncertainty in what degree of a heat risk we may be looking at. Thursday would be the day to watch for any potential Heat Advisories as it looks like that's when the heat and humidity peak for us, but again it looks like it may not end up being a widespread threat.

KEY MESSAGE 3 DESCRIPTION... Late in the week the ridge moves over the Atlantic and an upper low takes residency over Ontario. The resulting troughing over the Northern Plains is going to try to push a cold front through New England, but the models are uncertain about how far along it is going to get (currently they are all around the Friday night/Saturday time range). The reason this bears watching is because this is going to bring the relief from the humidity and also a chance for some stronger storms. This signal is beginning to show up on CSU's Machine Learning guidance, but is also spread out over several days owing to the uncertainty of the frontal passage timing.

Regardless, the summer warmth sticks around into early next week with continued chances for showers as the ridging over the Atlantic keeps the aforementioned upper trough stalled just to our west.

AVIATION /18Z MONDAY THROUGH SATURDAY/

Through 18Z Tuesday...

Mainly VFR prevails through tomorrow night at most terminals. However, nighttime valley fog with LIFR conditions is expected again after midnight tonight at HIE and LEB, and is then possible again tomorrow night.

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

MARINE

Fair conditions continue through tomorrow night as high pressure gradually move south of New England through Tuesday. Quiet conditions are expected on the waters with afternoon seabreezes into next week as high pressure settles south of the Gulf of Maine.

GYX WATCHES/WARNINGS/ADVISORIES

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