textproduct: Boston / Norton

WHAT HAS CHANGED

Widely scattered thunderstorms remain possible this evening, generally in the 5 pm to midnight time frame. Greatest risk for any thunderstorms will be across northeast MA in the 6 to 10 pm window.

KEY MESSAGES

- Widely scattered thunderstorms possible between 5 pm and midnight, with the greatest risk in northeast MA between 6 and 10 pm. Mainly dry and seasonable Sat outside a few brief spot showers.

- A few showers possible Sunday. Start of a warming trend.

- Warm and humid midweek, along with a risk of showers towards the second half of next week.

DISCUSSION

KEY MESSAGE 1...Widely scattered thunderstorms possible between 5 pm and midnight, with the greatest risk in northeast MA between 6 and 10 pm. Mainly dry and seasonable Sat outside a few brief spot showers.

The main forecast concern continues to revolve around the potential for widely scattered thunderstorms this evening.

The earlier showers this morning have pretty much departed the region. Otherwise...we were beginning to some breaks in the clouds and some partial sunshine trying to overspread the region this afternoon. This should result in the development of some instability...surface capes should reach between 1000 and 1500 J/KG across most of the region. Jet dynamics are quite favorable and the SPC meso-analysis indicates effective bulk shear increasing to over 50 knots this evening. When these two ingredients overlap each other conditions are often favorable for thunderstorm development and severe weather. However...the big limiting factors in this case are the subsidence behind the round of showers that crossed the region earlier this morning and an abundance of mid level dry air. While mid level dry air can also be favorable for convection...too much of it can cut off developing updrafts. This is likely the main reason that a lot of the high resolution guidance is rather limited in its areal coverage of potential thunderstorm development.

So in a nutshell, a few thunderstorms are possible in the 5 pm to midnight time frame this evening. Greatest risk though will be between 6 and 10 pm near the NH border and particularly in northeast MA. The reason for that is that there is bit better mid level moisture in that region. This is also indicated by much of our machine learning guidance etc. So to sum up...a few thunderstorms will be possible anywhere across southern New England but the greatest risk will be across northeast MA and near the NH border. Cloud to ground lightning is the main risk from any thunderstorms...but localized strong/damaging wind gusts are certainly possible. Confidence on thunderstorm development will increase across northeast MA if we see activity pushing into southeast NH...so this will be a nowcast type of forecast this evening.

Otherwise...generally dry weather after midnight into Saturday outside a few brief spot showers with the greatest risk near the south coast. Partial sunshine on Sat should yield highs mainly in the upper 70s to the lower 80s...although might be a few degrees cooler along portions of the immediate eastern MA coast.

KEY MESSAGE 2...A few showers possible Sunday. Start of a warming trend.

Somewhat of an uncertain forecast for Sunday. Southern New England is sort of stuck in between a high pressure north of the Great Lakes and a nearly stationary front just south of New England. The mid level flow still anticipated to be more on the troughy side. Overall, looking like at least a risk for a few afternoon showers. Not impossible for a few thunderstorms, but the lift looks rather weak, with CAPE values between 200-500 J/kg. Confidence in the detail Sunday is lower than normal for this time range. Additional forecast changes for Sunday are possible with later issuances.

KEY MESSAGE 3...Warm and humid midweek, along with a risk of showers towards the second half of next week.

The main concern for next week remains the threat of hazardous heat. NBM 90th percentile high temperatures showed reasonable worst-case scenario high temperatures above 90F for Monday and Tuesday, with high temperatures over 100F Wednesday through Friday. The spread in the temperatures within the NBM percentiles is rather broad. Current forecast highs are more conservative and possibly underdone. Future upward adjustments are likely as we get closer in time.

Increased humidity likely as well. Our region should be more along the periphery of the anticipated heat dome. A shortwave or two may "roll" along the eastern side of the upper level ridge, which may bring some unsettled weather for midweek. Location-specific and refined timing details are not yet known, though latest guidance is pointing towards sometime between Tuesday and Thursday.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

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

Through This Afternoon...Moderate confidence.

The risk for isolated thunderstorms, some of which may turn severe, returns in the afternoon primarily after 21z. PROB30s were included at most terminals to account for this risk. Highest probs E of ORH. Periods of MVFR visibilities and ceilings possible with any afternoon showers and storms. Uncertainty remains in exact location/overall coverage.

Tonight...Moderate confidence.

Showers and storms should clear the region by midnight. Patchy fog leading to areas of MVFR/IFR may develop overnight across much of SE MA, Cape Cod, and the Islands. Low stratus also a possibility, particularly towards the Cape and Islands. Winds shift more W and go light to calm overnight.

Saturday...Moderate confidence.

Mostly lower end VFR with areas of MVFR. Showers mostly scraping along the south coast are favored; lower confidence in those going much further north than that. These may also approach later in the day than expected. Light winds, with a sea breeze developing by 14z.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Sunday through Wednesday/...

Sunday: VFR. Slight chance SHRA.

Sunday Night through Monday Night: VFR.

Tuesday: VFR. Breezy. Slight chance SHRA, slight chance TSRA.

Tuesday Night: VFR. Chance SHRA, slight chance TSRA.

Wednesday: VFR. Breezy. Slight chance SHRA, slight chance TSRA.

MARINE

Forecaster Confidence Levels...

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

Tonight through Saturday night...High confidence on winds/seas. Moderate confidence on thunderstorm risk in our northeast MA waters this evening.

A weak pressure gradient will keep winds/seas below small craft advisory thresholds through Saturday night. The main concern will be for scattered thunderstorms potentially impact our northeast MA adjacent coastal waters this evening. Dangerous cloud to ground lightning and locally strong wind gusts would be possible in this region. The greatest risk in the 6 to 10 pm time frame. Otherwise...areas of fog patches may impact the waters at times mainly during the overnight and early morning hours.

Outlook /Sunday through Wednesday/...

Sunday through Tuesday: Winds less than 25 kt.

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

Wednesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

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