textproduct: Boston / Norton

WHAT HAS CHANGED

No major changes to the forecast. Trends remain relatively consistent.

KEY MESSAGES

- A few showers possible along the southern coastline this afternoon.

- Scattered showers possible Sunday, then the start of a warming trend.

- Warmth and humidity increase next week with the potential for dangerous heat Wednesday through Friday. Risk for showers and thunderstorms mid to late week.

DISCUSSION

KEY MESSAGE 1...A few showers possible along the southern coastline this afternoon.

As a weak low pressure meanders nearby, a front remains stalled out offshore. This will yield showers across the southern coastline with a couple isolated thunderstorms possible across the southeastern tier of MA. These showers have a very low chance of being impactful as the environment remains fairly unfavorable as CAPE < 600 J/kg. Temperatures will climb into the upper 70s to low 80s across most of the region, except for along the eastern MA immediate coastline as fog and/or low stratus has kept the temperatures in the upper 60s to low 70s.

KEY MESSAGE 2...Scattered showers possible Sunday, then the start of a warming trend.

Ahead of the ridge building from the west, the region situates itself under weak troughing on Sunday with surface high pressure extending into southern New England. The chance for showers and thunderstorms returns on Sunday afternoon with the latest guidance hinting at showers initiated across northwestern MA and shifting southeastward across the region. Similar to the previous day, the severe risk remains low given the lack of forcing and MLCAPE ~500 J/kg.

Thereafter, drier weather is expected to return briefly as high pressure shifts into southern New England. This ridge will be a key feature to monitor for next week, as it is the anticipated heat dome that will impact much of the eastern US. A warming trend also kicks off, continuing and intensifying into mid-week.

KEY MESSAGE 3...Warmth and humidity increase next week with the potential for dangerous heat Wednesday through Friday. Risk for showers and thunderstorms mid to late week.

Confidence continues to increase for a period of hot to dangerously hot temperatures next week. Ensembles show a strong upper level ridge building to 594dm over the Ohio Valley before moving east over much of the NE CONUS. Still see some impressive signals for intense heat from ensemble and deterministic guidance. Namely, the GFS and ECMWF indicate 925 mb temperatures soaring from +25C on Tuesday to +29C Wednesday through Friday! The mean NBM probabilities for temperatures equal to or greater than 100 increase markedly starting Wednesday with widespread 35-50% probs in the Connecticut and Merrimack River Valleys. Probabilities increase another 10-15 percent on Thursday with what will likely be the day of most intense and widespread heat of the event. Again, the strongest signal for >100 degree temps lies in interior river valleys and larger urban areas. Equally concerning is the signal for limited cooling at night mid week. The NBM shows a widespread 60-80% chance for low temperatures above 70 degrees Wednesday to Friday, with a 50-70% chance for lows greater than 75 degrees for much of Southern New England! Seeing signals for extreme heat impacts appear in Heat Risk guidance with widespread Major (3/4) impacts expanding over much of the CWA on Wednesday. By Thursday, several urban areas including Boston and portions of the CT River Valley may see Extreme (4/4) heat impacts. Levels like these are typically reserved for rare and/or long duration heat with little overnight relief.

Limited relief in the form of thunderstorm activity may come as shortwave troughs known as "ridge rollers" rotate around the northern periphery of the developing ridge. While there will be a daily chance for thunderstorms during the upcoming week, the greatest chance will be on either side of the worst heat impacts. This means Tuesday as the ridge builds into New England and Friday as it begins to breakdown. Thunderstorm chances, while non-zero, drop considerably Wednesday and Thursday.

AVIATION /19Z SATURDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

18z TAF Update...

This Afternoon through Sunday Afternoon...Moderate confidence.

Most locations will remain in VFR conditions through the period with light winds, except for the terminals along the immediate coastline where fog and/or stratus persists. This fog bank continues to wane back and forth along the coastline yielding LIFR/IFR conditions for KBOS. As for KHYA, it has been going back and forth between VFR and MVFR conditions over the last couple hours. At this point, it's unlikely the fog and/or stratus will erode this afternoon/evening with the ongoing onshore flow. Therefore, this will likely continue into tomorrow morning/mid- day. However, we will continue to monitor observations and trends through the period and make adjustments as needed.

Additionally, showers should persist across the southern coastline with isolated thunderstorms continue to develop across central and southeastern MA through the evening.

KBOS Terminal...Low confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Monday through Thursday/...

Monday through Monday Night: VFR.

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

Tuesday Night: Mainly VFR, with local MVFR possible. Breezy. Chance SHRA, slight chance TSRA.

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

Wednesday Night: VFR. Breezy.

Thursday: VFR.

MARINE

Forecaster Confidence Levels...

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

Through Sunday...Moderate confidence.

A weak pressure gradient will yield sub-SCA thresholds through the period. Light showers should continue across the southern waters through the afternoon/evening. Fog and/or stratus continues to impact the waters off the eastern MA. With the ongoing onshore flow, this will likely stick around into tomorrow mid-day, before eroding on Sunday afternoon. Seas will range from 2-3ft. Fog should greatly diminish on Sunday afternoon.

Outlook /Monday through Thursday/...

Monday through Tuesday: Winds less than 25 kt.

Tuesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Chance of rain showers, slight chance of thunderstorms.

Wednesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers.

Wednesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Slight chance of rain showers.

Thursday: Winds less than 25 kt. Seas up to 5 ft.

BOX WATCHES/WARNINGS/ADVISORIES

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