textproduct: Boston / Norton

WHAT HAS CHANGED

Growing risk for late evening and overnight showers and storms mainly south of the Mass Pike, but severe weather isn't anticipated from these storms.

KEY MESSAGES

- Showers and thunderstorms move east-southeast from NY after midnight and overnight, mainly for areas south of the Mass Pike. Turning more humid too.

- Early cloud cover Thursday dissipates, but isolated to widely scattered showers/weaker storms develop across northern MA.

- Becoming cooler and less humid Friday through Saturday night. A period of showers and possible thunderstorms Sunday afternoon into Monday Night. Dry weather should return Tuesday and Wednesday.

DISCUSSION

KEY MESSAGE 1...Showers and thunderstorms move east-southeast from NY after midnight and overnight, mainly for areas south of the Mass Pike. Turning more humid too.

Seasonably mild with tolerable humidity levels currently as of Wednesday afternoon, but we're looking at some changes to our airmass over the next 12-24 hours. Couple weak impulses in broad WNW flow aloft are contributing to scattered showers with a few thunderstorms. One is working through far northeast Ontario, responsible for the area of showers and storms over VT/northern NY. These should pass to our north but a couple could clip part of Essex County and the North Shore through sundown.

The more significant feature to Southern New England comes in the form of a weak impulse on water vapor imagery near the northern portion of Lake Michigan, which is currently helping to produce scattered showers and storms when coupled with isentropic ascent associated with a surface warm front/higher dewpoints in the mid 60s over Greater Toronto and Buffalo NY areas. NBM and CAMs were previously slow to resolve these, but the past few cycles have shown more of a risk for showers and storms more concentrated along and south of the Mass Pike and particularly for the Hartford-Providence- southeast MA area around/after midnight and into overnight. Mid- level lapse rates around 6 to 6.5 C/km but with dewpoints rising into the mid 60s, most-unstable CAPE values rise to about 1000-1200 J/kg in the CT-RI area. Expect a few thunderstorms to be around but should be elevated, with a couple stronger storms capable of frequent lightning, pea to dime size hail and wind gusts around 40 mph as they move off toward the southeast at a fairly good clip given fast WNW winds aloft. Not thinking storms rise to the level of severe weather. Otherwise, will turn a bit more humid than it's been but not oppressive, with lows in the mid to upper 60s.

KEY MESSAGE 2...Early cloud cover Thursday dissipates, but isolated to widely scattered showers/weaker storms develop across northern MA.

Showers/embedded storms from overnight should be peeling offshore early Thursday morning, with considerable cloudiness around during the morning hours. Gradual heating and mixing should allow for cloud cover to scatter out. While most of the day should be more-or-less dry, there is a risk for mid- afternoon showers or weak thunderstorms mainly north of the Mass Pike as a stronger shortwave trough aloft moves from northern Quebec into ME/NH and swings a cold front southward. I think the coverage and strength of showers or weak storms could be limited by drier air aloft, to go along with delayed heating/destabilization, so have capped PoPs to no more than 30%. So while Thursday ends up dry, there should be some showers or weak storms to dodge Thursday afternoon through sundown, but nothing that looks hazardous. Will begin to turn less humid late upon cold frontal passage.

KEY MESSAGE 3...Becoming cooler and less humid Friday through Saturday night. A period of showers and possible thunderstorms Sunday afternoon into Monday Night. Dry weather should return Tuesday and Wednesday.

Gaining some clarity that a cold front should be over the coastal waters friday morning and moving farther offshore during the day. High pressure then settles across southern New England for Saturday, before it too moves offshore Sunday. The main impact will be drier and cooler conditions during this time.

Anticipating a brief change back to more humid conditions late Sunday into Monday night. This should be when a low pressure passes by to our north, but drags its associated fronts through southern New England. The combination of the increased humidity and the lift from some front means at least a period of showers. Not overly concerned about thunderstorms just yet, but will need to keep this in mind with later forecasts. This should not be a washout this entire time, but the greatest risk for showers looks to be Sunday night.

A large high pressure over the Midwest should start nudging into our region Tuesday and be more of a factor in our weather during the middle of next week.

AVIATION /00Z THURSDAY THROUGH MONDAY/

Forecaster Confidence Levels...

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

Tonight: High confidence in trends, but moderate confidence in timing.

VFR initially, but SHRA/embedded weak TS coming in from the NW after 03z will deteriorate conditions into the VFR/MVFR range. BKN/OVC VFR/MVFR ceilings with SHRA more likely for BDL, but have left as PROB30s after 06z for BOS, PVD and ORH and the Cape airports. Winds become SW 5-10 kt.

Thursday: Moderate confidence.

Lingering SHRA/possible TS mainly PVD and Cape airports should move offshore by 14z. Leftover SCT/BKN VFR/MVFR decks should begin to scatter out by 15-17z, which will increase the risk for SHRA/TS mainly from the Mass Pike north as sfc cold front moves through. Coverage looks too spotty for a TAF mention but will need to be considered in later TAF issuances. Winds SW to W under 10 kt, then shift to WNW/NW 5-10k kt after 20z.

Thursday Night: High confidence.

SHRA dissipates early to VFR. NW winds 5-10 kt.

KBOS Terminal...High confidence.

KBDL Terminal...High confidence.

Outlook /Friday through Monday/...

Friday through Saturday Night: VFR.

Sunday: VFR. Breezy. Chance SHRA.

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

Monday: VFR. Breezy. Chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Through Thursday Night: High confidence.

Sub-SCA winds and seas through the period. N winds under 10 kt shift to SW early tonight around 10-15 kt, then remain so until frontal passage late Thurs/early Thurs night with windshift to NW around 10-15 kt. Seas 3 ft or less all waters. Possible thunderstorms over mainly southern waters overnight tonight, then more isolated SHRA Thurs afternoon over northeast waters.

Outlook /Friday through Monday/...

Friday through Saturday Night: Winds less than 25 kt.

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

Sunday Night: Winds less than 25 kt. Areas of rough seas. Chance of rain showers, slight chance of thunderstorms.

Monday: Winds less than 25 kt. Areas of rough seas. Chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...None.