textproduct: Boston / Norton

WHAT HAS CHANGED

Small Craft Advisory issued for the southern outer waters from 06z Thursday to 00z Friday.

KEY MESSAGES

- Scattered showers and isolated thunderstorms expected overnight.

- Heat Advisories advisories remain in effect for northern Connecticut and much of interior Massachusetts from noon Thursday to 8pm Friday.

- A few scattered strong to severe thunderstorms are possible Thursday and Friday.

- Above normal temperatures continue into next week. Decreasing humidity as well. Weekend looking mostly dry, while uncertainty remains high regarding potential for rain showers early next week.

KEY MESSAGE 1

Scattered showers and isolated thunderstorms expected overnight.

An approaching warm front pushes into the region overnight bringing a round of scattered showers and isolated elevated thunderstorms. Potential for severe will be held at bay as 0-6 km Bulk Shear values stay below 15kts. Still, some of the machine learning guidance shows a slight (5-15%) chance for some stronger storms. Notably, the HRRR Neural Network still highlights the potential for severe winds across far northern Massachusetts overnight. Higher confidence part of the forecast is areas of heavy rain as PWATs surge to 1.9"-2.0" across most of southern New England overnight into Thursday morning. The exact corridor of the most concentrated precipitation is still somewhat uncertain as forcing will be somewhat weak and nebulous. There may be two corridors of heavier rain, one near northwestern Mass, and the other across the Cape and Islands as a weak low pressure passes offshore.

KEY MESSAGE 2...Heat Advisories advisories remain in effect for northern Connecticut and much of interior Massachusetts from noon Thursday to 8pm Friday.

The warmest air mass of the season so far arrives Thursday and Friday as 925 mb temperatures soar to +25-29C. In addition to the heat, humidity will increase behind a warm front as it moves north of our CWA. Southwest flow brings oppressive 65-70 degree dew point temperatures especially across interior MA and northern CT. These high dew points combined with temperatures climbing into the low to mid 90s will lead to heat index values approaching the upper 90s to near 100F Thursday and likely above 100F Friday across the CT River Valley. As such, Heat Advisories remain in effect for these areas through 8pm Friday.

By Friday, a backdoor cold front will provide some relief from the heat as onshore flow develop. Lowest temperatures will be found mainly east of 495. Further inland, temperatures soar to the middle and perhaps upper 90s with heat index values as high as 100-101 degrees for the CT River Valley. Moderate heat-related impacts expand across the region, with major impacts expected in the CT River Valley, both Thursday and Friday, leading to an increased risk of heat-related illness for anyone spending extended time outdoors without adequate hydration or access to cooling.

There is still some uncertainty as to how far east heat index values >95F expand. The latest guidance shows apparent temperatures increasing to between 90 and 95 degrees in the Merrimack Valley. While just below heat advisory criteria, these temperatures are no less dangerous to those without adequate hydration or cooling.

KEY MESSAGE 3...A few scattered strong to severe thunderstorms are possible Thursday and Friday.

The increased heat and humidity set the stage for impressive levels of instability Thursday and Friday. In terms of aggregate severe weather probabilites, machine learning guidance continues to show a 15-20 percent chance for severe wind potential Thursday and Friday. In terms of CAPE, guidance continues to show moderate destabilization with values approaching 1500-2000 J/kg across the region. The limiting factor will be weak shear as values of 0-6km Bulk Shear struggle to get above 20kts Thursday afternoon. Forcing will also be weak and confined to western portions of the CWA. Guidance shows only modest height falls as a weak shortwave moves through the region Thursday evening. Still, any storms that do form will have the opportunity to tap into the richly buoyant and unstable airmass. Notably, forecast soundings show mid level lapse rates around 6-7.5C/km Thursday afternoon and evening. The primary threat with any stronger storm would be cold pool-driven damaging winds.

KEY MESSAGE 4...Above normal temperatures continue into next week. Decreasing humidity as well. Weekend looking mostly dry, while uncertainty remains high regarding potential for rain showers early next week.

A weak high pressure sandwiched between two fronts should dominate our weather for most of this weekend. The main impact of these fronts will be lowered humidity. It should still be plenty warm for most, if not outright hot. At least the peak heat index values should remain mainly in the 80s, with a few locations reaching the lower 90s.

Still quite a bit of uncertainty with the details of the forecast early next week. The latest guidance stalls yet another front towards the south coast of New England as a weak low pressure develops towards NJ sometime Monday night into Tuesday. The forcing looks rather weak, but cannot completely dismiss the idea of scattered showers in our area at times. A subtle shift of even just 20 miles to the south, and the forecast could be largely dry. This is one of those details yet to be ironed out. Changes with later forecasts are expected.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Forecaster Confidence Levels...

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

Through Tonight...High confidence to start, moderate confidence towards the nighttime hours.

VFR conditions continue for the rest of the afternoon. Some MVFR to localized IFR conditions may overspread parts of the region from southwest to northeast by early this evening along with a few showers. Showers become more likely tonight and coverage looks limited as the activity makes its way from west to east. Greater chance for -TSRA/-SHRA across western terminals.

Gusts to around 20 kt continue thru the early evening SW winds will shift more S in the wake of a passing warm front later today into tonight. Winds become light as an inversion moves overhead.

Thursday...Moderate confidence.

Generally MVFR/IFR conditions. Improvement from showers and fog overnight will likely be slow (not until 12z or later). Some areas could be on the cusp of LIFR conditions as any lingering showers push offshore. SW winds continue through the morning hours, generally around 10 kts once showers move out.

KBOS Terminal...High confidence in trends. Moderate confidence in timing. TEMPO for -SHRA continues from 01-05z.

KBDL Terminal...High confidence in trends. Moderate confidence in timing. Timing for SHRA and later TSRA still a bit uncertain. Included as TEMPOs for now.

Outlook /Thursday Night through Monday/...

Friday: VFR. Chance SHRA, slight chance TSRA.

Friday Night: Mainly VFR, with local IFR possible. Breezy. Chance SHRA, slight chance TSRA.

Saturday through 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 Thursday...High confidence.

Seas 2-4 feet for the majority of the waters except 3-5 feet across the southern outer waters overnight. SW winds increase to 15-25 kt with gusts as high as 30 kts. Areas of showers and perhaps a thunderstorm or two through Thursday morning. SCA conditions continue for the southern waters Thursday before gradually subsiding by the evening.

Outlook /Thursday Night through Monday/...

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

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

Saturday through Sunday: Winds less than 25 kt.

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

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...Heat Advisory from noon Thursday to 8 PM EDT Friday for CTZ002>004. MA...Heat Advisory from noon Thursday to 8 PM EDT Friday for MAZ002- 003-008>011. RI...None. MARINE...Small Craft Advisory from 2 AM to 8 PM EDT Thursday for ANZ254>256.