textproduct: Gray - Portland

WHAT HAS CHANGED

Shifted rainfall amounts south for Monday night through Tuesday. Temperatures may be seasonably cool for early July across southern New Hampshire and far southern Maine Tuesday.

KEY MESSAGES

1. Fair weather continues tonight and Monday. The next chance for rain arrives Monday evening through Tuesday, mainly across the southern half of New Hampshire and far southern Maine.

2. Temperatures warm during the latter part of the work week with elevated humidity returning Friday into the weekend as well.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Outside of a few sprinkles later this afternoon towards central Maine, fair weather continues into the evening and overnight hours with high pressure to the north. Cumulus humilis has been the predominant cloud type today with a notable subsidence inversion preventing further deepening. Increasing cirrus is expected this evening, building NE overnight. This should make for overcast skies for much of the southern CWA Monday with gradual lowering. This will bring another comfortable day to the area, albeit a couple degrees cooler in the south than north.

Next chance of rain arrives late Monday, continuing overnight and at least through early afternoon Tuesday. While much of guidance has gravitated towards a more southern solution, there continues to be some opportunities for rain to push further north. With 90th percentile 500mb heights to the north and similar climo for surface high pressure to the northeast, there will be a strong opposition to northern moisture advection out of central New England. This is paired with marginal IVT through southern New England.

With these factors in mind, have refined QPF a further south, especially for amounts greater than a quarter-inch. Dry air will likely keep a tighter gradient on the northern periphery, but some showers will still be possible through the central portion of the CWA. Where ingredients are a little more in line for precip efficiency: FGEN banding in southern NH amid 90th percentile PWAT values and warm cloud depths of 12 to 13 kft, locally higher amounts around 1 inch can't be ruled out through Tuesday evening.

The system is expected to slowly exit east through the day Tuesday, with gradually improving conditions Tuesday afternoon. There remains a wide spread in temperatures Tuesday, but this likely hinges on the presence of rain/thicker clouds. Should the northern track be completely ruled out, think we see the chance for temps to bump up further across western ME and north NH where clouds will undoubtedly be thinner. This would pin the cooler temps for the day in southern NH and far southern ME where 850mb temps will be seasonably cool and result in sfc values in the mid 60 to around 70.

KEY MESSAGE 2 DESCRIPTION...

A trough moves off the coast early Wednesday with a low amplitude ridge passing through behind it. This will be enough to dry things out and kick off a warming trend with highs climbing back into the upper 70s to low 80s in most locations. By Thursday the ridge axis shifts eastward and high pressure takes up residency in the Atlantic which starts up southwest flow into the Northeast. Global ensemble spreads are small in the 25-75th percentile window for inland locations Thursday and Friday with moderate confidence in temperatures climbing into the upper 80s to near 90F. For coastal areas the spread is a bit larger on Thursday as there is potential for westerly winds to fight off the seabreeze, and that would make the difference between mid 70s and mid 80s. The ensembles are more confident in the seabreeze failing Friday with the westerly winds as the 25-75th spread is centered around 80F, however with westerly winds we usually get a little bit of a warming boost so temperatures in the upper 80s to near 90 are not out of the question.

Trends for temperatures on Friday are going to have to be watched closely as continued southerly flow will also be increasing dewpoints into the upper 60s and low 70s which means some locations could be pushing a 95F+ heat index. Saturday will have to be watched closely as well as currently models are suggesting a frontal passage cools temperatures and dewpoints right back down, but as usual timing is everything and the Euro camp is notably slower with it. These are details that will be fleshed out later, but right now there is moderate confidence in Friday being the peak of the heat and humidity.

The other factor worth watching is that the 500 mb pattern becomes more zonal and even troughy late week which usually lends its self to cloudier and showerier/stormier conditions as shortwaves move through the broader trough. This would act to put a damper on hotter temperatures if they materialize and I think what the NBM is currently advertising with 20-40% PoPs most days is a good representation of that uncertainty, especially coverage-wise.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

Through 18Z Monday...VFR. Onshore seabreeze weakens this evening with VRB winds overnight. Will see cu thin but cirrus thicken overnight. This should prevent valley fog formation, but if it thins can't rule out a brief return at LEB/HIE early Mon morning. Could see FEW or SCT MVFR early Mon, but bases rise through the morning hours.

Outlook...

Monday Night-Tuesday: Clouds thicken and lower Mon evening with SHRA and RA across southern NH terminals. IFR cigs likely for CON/MHT/PSM, with chances for PWM into Tues morning as well. Cigs gradually improve west to east Tues night.

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

MARINE

Strong high pressure resides to the NE through tonight, with low pressure expected pass south of the waters through Mon-Tues. Will see some increase in winds and seas Mon/Tues associated with this. Will monitor the need for a SCA Tuesday for the waters off the NH Seacoast as wave heights build towards 4 ft.

Generally sub-SCA conditions are expected from midweek through the weekend as high pressure remains dominant over the Atlantic.

GYX WATCHES/WARNINGS/ADVISORIES

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