textproduct: Boston / Norton

WHAT HAS CHANGED

Both the marginal risk (level 1 of 5) and slight risk (level 2 of 5) of severe weather on Friday were pulled back a bit further west; chance of severe weather is low but not zero for eastern MA/RI.

KEY MESSAGES

- Heat and humidity continues through Friday before we get a break in the humidity, even if not so much from the heat.

- Afternoon and evening showers and thunderstorms are possible both Thursday and Friday, a few of which could be severe.

- Above normal temperatures continue this weekend but with a little less humidity. Dry weather prevails until a risk for showers and thunderstorms develops Sunday night, and a couple could become strong.

- Cooldown to more seasonable temperatures with drier weather for early next week.

DISCUSSION

KEY MESSAGE 1...Heat and humidity continues through Friday before we get a break in the humidity, even if not so much from the heat.

Temperatures in the 80s to low 90s and dewpoints in the upper 60s to low 70s have brought a muggy feel to the air today under a very moist airmass (PWATs near 1.75"). 850 mb temps surging to near 20C today will persist through Friday under a building mid level ridge and this prolongs the bout of above normal heat/humidity through Friday. Those elevated dewpoints will make temps in the upper 80s and 90s feel more like the 90s to near 100 (in the CT valley). The exception will be along the coast, especially the east coast of MA where onshore easterly flow keeps temps more comfortably in the 70s to mid 80s. The Heat Advisory continues through Friday with low temperatures limited by the elevated dewpoints to the upper 60s/low 70s.

KEY MESSAGE 2...Afternoon and evening showers and thunderstorms are possible both Thursday and Friday, a few of which could be severe.

The same heat and humidity mentioned above are contributing to 2,000+ J/kg of CAPE across SNE this afternoon and that will be conducive to scattered thunderstorms this afternoon owing to a few mid level shortwaves providing enough lift as they slide overhead. Ample moisture and instability are available, but a limiting factor is minimal 0-6km bulk shear 20-25 kts in the area where the best forcing and instability are. That being said, given the magnitude of instability it should be enough to overcome the limiting shear values to produce a few strong storms, one or two of which could be severe. The main threat will be damaging winds with a batch of storms moving in from NY; this is where the greatest threat is. Another batch of showers/storms likely initialize along a seabreeze boundary over eastern MA this evening as well with a lesser severe risk.

Friday we'll have a similar setup with not much change in the airmass (instability and shear specifically) as the broader mid level shortwave approaches so modest height falls as well as an approaching cold front and quasi- seabreeze boundary will provide some lift to kick off a few clusters of thunderstorms. The first to see storms will likely be eastern MA and RI where that quasi- seabreeze boundary/onshore flow will likely help initialize some storms by mid afternoon. An approaching cold front from NY will also be moving through midnight which will lead to a broken line of storms from west to east 7pm-midnight; however, we will be losing our diurnal heating early in that period so instability and severe potential will be decreasing quickly; best chance of severe weather is in western MA and CT where we have placed a Slight (level 2 of 5) severe risk.

KEY MESSAGE 3...Above normal temperatures continue this weekend but with a little less humidity. Dry weather prevails until a risk for showers and thunderstorms develops Sunday night, and a couple could become strong.

While still very warm to hot on Saturday, we should see a bit less humidity compared to the past couple days. Dry weather should prevail as shortwave ridging governs, with areas along the MA east coast and around Narragansett Bay. Highs should again reach well into the 80s to low 90s but with dewpoint temperatures in the upper 50s/near 60 compared to the upper 60s/low 70s, it should be a somewhat drier heat. Under shortwave ridging, the risk for afternoon storms also looks to be mitigated with dry weather anticipated.

Southwest flow resumes on Sunday and with it comes a bit of an increase to humidity level but not to the extent of today or Friday. We'll also be waiting on an approaching cold front to trigger showers and thunderstorms, and while the exact timing varies, there's some general agreement on it being a later arrival (after 5 PM) into western New England. Midlevel flow increases Sunday in response to the approaching shortwave aloft with WSWly 0-6 km shear vector magnitudes around 35 to 45 kt, supportive of severe weather potential. The issue is more tempered instability with the later-day timing and dewpoints being on the lower side, with better instability more restricted to the northern mid-Atlantic region where SPC has delineated a 15% probability for severe storms. PoPs increase into the Likely range for showers and thunderstorms Sunday evening, but if storms can produce severe weather is still a question mark, which will hinge on either an earlier timing or higher forecast dewpoint temperatures.

KEY MESSAGE 4...Cooldown to more seasonable temperatures with drier weather for early next week.

Cooldown to temperatures for early next week, as the offshore passage of the cold front allows more of Southern New England to become governed by cooler cylonic flow aloft. 850 mb temps drop to around the +10 to +12C range versus the +16 to +18C range, supporting highs in the 70s to low 80s under sunny conditions to go along with comfortable humidities. Dry weather also prevails through Tuesday until a wave of low pressure in SW flow brings a risk for showers and thunderstorms around midweek.

AVIATION /00Z FRIDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

00Z TAF Update:

Tonight: Moderate confidence.

An ongoing cluster of TSRA over central NY near BGM/SYR offers best chance at TSRA between 01-04z, with best chance at BDL and and into ORH/PVD; used PROB30s given the late time of day. A low risk exists elsewhere but too low confidence to include in the TAF. These showers/storms also could cast question when/if IFR stratus develops over the Cape and Islands; offered 07-08z for development but this is more uncertain and it could remain VFR most of the night. Light west winds, with seabreeze at BOS persisting thru 03z.

Friday: Moderate confidence.

VFR. Light onshore winds developing, with light W winds away from the coastal TAFs. A few rounds of TSRA developing, first in the 18-00Z time period over eastern MA and RI terminals, then again a broken line of storms is possible in the 23z-06z period. Both are too low-confidence to include in TAFs.

Friday night...High confidence.

SHRA/TSRA exiting by 06z. VFR. NW winds 5-10 kts.

KBOS Terminal...Moderate confidence in TAF. E/NE winds should persist thru 03z Friday before turning light westerly. Two periods of possible TS: one after 03z until 07z Fri, with another around 16-23z Fri as seabreeze develops. Neither of these periods offer high enough confidence to include in the TAF as yet.

KBDL Terminal...Moderate Confidence in TAF. Better chance at TSRA around 01-03z as line of storms moves in from the Berkshires. Should be VFR thereafter with light W winds but turn light southerly tomorrow, and another round of late-day TS possible.

Outlook /Friday Night through Tuesday/...

Saturday: VFR. Breezy.

Saturday Night: VFR.

Sunday: VFR. Breezy.

Sunday Night: VFR. Breezy. Chance SHRA, slight chance TSRA.

Monday: VFR. Chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Through Friday night...High confidence.

Seas 2-4 feet for the majority of the waters except 3-5 feet across the southern outer waters tonight. SW winds increase to 15-25 kt with gusts as high as 30 kts. SCA conditions continue for the southern waters today before gradually subsiding by the late evening. Seas decrease to 1-3 ft by Friday and continue into Friday night. Isolated to scattered showers and thunderstorms are possible Thursday night into Friday morning then again Friday evening into Friday night.

Outlook /Friday Night through Tuesday/...

Friday Night: Winds less than 25 kt. Chance of rain showers.

Saturday through Sunday: Winds less than 25 kt.

Sunday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Rain showers likely, slight chance of thunderstorms.

Monday: Winds less than 25 kt. Chance of rain showers, slight chance of thunderstorms.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Heat Advisory until 8 PM EDT Friday for CTZ002>004. MA...Heat Advisory until 8 PM EDT Friday for MAZ002>006-008>014-017- 018-026. Air Quality Alert until 11 PM EDT this evening for MAZ013- 016>024. Air Quality Alert from 11 AM to 11 PM EDT Friday for MAZ017>024. RI...Heat Advisory until 8 PM EDT Friday for RIZ001>004. Air Quality Alert until 11 PM EDT this evening for RIZ001>008. Air Quality Alert from 11 AM to 11 PM EDT Friday for RIZ001>008. MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ254>256.