textproduct: Boston / Norton

WHAT HAS CHANGED

The slight risk (level 2 of 5) of severe weather today remains in effect for far western CT and MA. The chance of severe weather also remains low, but not zero, for eastern MA/RI.

KEY MESSAGES

- Isolated to scattered storms possible through this evening. A few may be strong to severe.

- Above normal temperatures continue this weekend, though humidity levels drop to more comfortable levels.

DISCUSSION

KEY MESSAGE 1...Isolated to scattered storms possible through this evening. A few may be strong to severe.

Under a moist and unstable airmass, a mid-level shortwave trough moves over Quebec helping trigger showers and thunderstorms this afternoon and evening. No major changes from the previous forecast. Modest shear paired with the daytime heating helping build instability (MLCAPE 1000-2000 J/kg). Recent runs of high-res guidance have shown a range of isolated to scattered storms. Some of the CAMs continue to show a few storms firing off a sea breeze boundary in east southern New England. There is also a general consensus across guidance for storms to develop in western MA/CT ahead of the cold front early this evening, moving eastward. Can't rule out a strong to severe storm or two with the greater risk in west and central portions of southern New England. Gusty, locally damaging winds and lightning are the main hazards. Brief downpours are also possible within any storms. Any remaining showers or storms push offshore by midnight.

KEY MESSAGE 2...Above normal temperatures continue this weekend, though humidity levels drop to more comfortable levels.

An anomalously warm airmass remains over the region this weekend with ridging aloft. This will support above normals highs well into the 80s with some low 90s. Despite the similar temperature readings from the last few days, it won't feel as sweltering as it has been Saturday. Dewpoints stay in the mid to upper 50s which will take out the humidity factor and provide some marginal relief. With ridging aloft, conditions stay dry for Saturday and a good portion of Sunday. Next chance for showers/storms returns later Sunday with moisture/humidity increasing later in the day. More on that in the next key message below.

KEY MESSAGE 3...Showers and thunderstorms Sunday evening/overnight, but the risk for severe weather is low.

After another very warm to hot Sunday, a cold front will be progressing into and through Southern New England Sunday night, which will increase the risk for showers and thunderstorms. Midlevel flow aloft increases and supports increasing deep-layer shear magnitudes of around 35 to 40 kt. However it still looks like instability looks on the low side, both due to the time of day post- sundown and lower dewpoints, which is a significant limiting factor to how strong any storms may get. Both CSU and NSSL machine-learning progs indicate low (< 10%) probs of severe weather for Sunday night. Still expect some thunderstorms to develop/move eastward into the night but the risk they produce severe weather seems low. Even how much rainfall we get from these storms is uncertain, despite elevated PWATs in the 1.5-1.8" range, as some of the global models (GFS/Canadian) simulate a convectively-driven subsynoptic low over the mid-Atlantic states which moves well offshore of Southern New England into the overnight. Were this to develop, it could greatly reduce how much rainfall we do get. Others, such as the ECMWF/its ensemble mean, generate about a quarter to half inch of rain areawide. Increased PoPs into the Categorical range between the 02- 10z timeframe from west to east before drying out as cold front moves through.

KEY MESSAGE 4...Return to more seasonable temperatures and lower humidity levels for the early to middle part of next week, then turning more active again into Thursday.

A cooler and much drier airmass then works into Southern New England Monday into Tuesday. Latest guidance shows 850 mb temps now dropping to around +8 to +10C, some 5 degrees C lower than today. Highs more into the 70s to low 80s with dewpoints plummeting into the mid 40s to mid 50s, so also substantially less humid compared to prior days with full sunshine. Early next week looks like a great stretch of summer weather; temperatures do start to warm up again as we move into Wednesday but are still seasonable in the low to mid 80s with still tolerable humidity levels.

More active weather looks to develop around Thursday or Thursday night, as a stronger disturbance aloft moves in and sweeps a cold front through Southern New England. This could bring a better shot at showers/storms, although it's still too early for specifics at this timeframe.

AVIATION /19Z FRIDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.

18Z TAF Update:

Today: Moderate confidence.

VFR. Light onshore winds for north coastal terminals, with light W winds away from the coastal TAFs. A few rounds of TSRA could develop, first in the 18-00Z time period over eastern MA and RI terminals along the seabreeze boundary, but confidence in coverage and placement is low. Then again a broken line of storms is possible in the 22z-04z period, diminishing as it moves east.

Tonight...High confidence.

SHRA/TSRA exiting by 04z. VFR except near the Cape/Islands where MVFR visbys under BR may creep in off the waters. NW winds 5-10 kts. Winds shift to the NW 00-04Z.

Saturday....High confidence.

VFR. NW winds around 8-12 kts.

KBOS Terminal...Moderate confidence in TAF. VFR. Could see a few TS develop 19-00z along the seabreeze boundary. Given the low confidence in placement, messaged as a VCTS given the boundary will be nearby. Low chance for another round of storms from western southern new england to reach the terminal. By the time it get east to the terminal, they are more likely to be SHRA.

KBDL Terminal...Moderate Confidence in TAF. VFR. Light W winds. Becoming NW after 00Z. Isolated thunderstorms possible 22-01Z.

Outlook /Saturday Night through Wednesday/...

Sunday: VFR. Breezy.

Sunday Night: Mainly VFR, with local MVFR possible. Breezy. SHRA, isolated TSRA.

Monday: VFR. Breezy.

Monday Night: VFR.

Tuesday: VFR. Breezy.

Tuesday Night: VFR.

Wednesday: VFR. Slight chance SHRA.

MARINE

Forecaster Confidence Levels...

Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.

Through Saturday Night...High confidence.

Seas generally 1-3 ft, except 3-4 ft across the far outer waters through Saturday morning. Isolated to scattered showers perhaps an isolated storm over the waters this evening. Otherwise, winds remain below SCA levels.

Outlook /Saturday Night through Wednesday/...

Sunday: Winds less than 25 kt.

Sunday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Rain showers, slight chance of thunderstorms.

Monday: Winds less than 25 kt. Slight chance of rain showers.

Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft.

Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft.

Tuesday Night: Winds less than 25 kt.

Wednesday: Winds less than 25 kt. Areas of seas approaching 5 ft.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Heat Advisory until 8 PM EDT this evening for CTZ002>004. MA...Heat Advisory until 8 PM EDT this evening for MAZ002>006- 008>014-017-018-026. Air Quality Alert until 11 PM EDT this evening for MAZ017>024. RI...Heat Advisory until 8 PM EDT this evening for RIZ001>004. Air Quality Alert until 11 PM EDT this evening for RIZ001>008. MARINE...None.