textproduct: Boston / Norton

WHAT HAS CHANGED

Forecast remains on track at this time.

KEY MESSAGES

- Heat Advisories advisories remain in effect for northern CT and much of interior MA from noon today to 8pm Friday.

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

- Above normal temperatures continue into early next week, but humidity will be on the decrease. Weekend looking mostly dry, while uncertainty remains high regarding potential for rain showers early next week.

DISCUSSION

KEY MESSAGE 1...Heat Advisories advisories remain in effect for northern CT and much of interior MA from noon today to 8pm Friday.

Increasing heat and humidity pose a risk today and Friday with the arrival of the warmest airmass of the season. Prolonged southwesterly flow will bring a surge of very hot and humid air, especially as 925mb temperatures increase to +27C Thursday and up to +30C Friday. Surface dewpoints are already in the low 60s just after 2AM and will only continue to climb throughout the day, likely topping out at or near 70, especially across interior MA and northern CT. These high dewpoints combined with temperatures climbing into the low to mid 90s will lead to heat index values approaching 100F today and likely above 100F Friday across the CT River Valley. As such, Heat Advisories remain in effect for these areas through 8pm Friday.

Temperatures across RI and eastern MA likely remain in the upper 80s to low 90s across both days. Dewpoints surging to at or near 70 across most of southern New England both today and Friday will lead to heat indicies in the low 90s, especially away from the coastal plain. A backdoor cold front is also expected to drop into eastern MA sometime on Friday. Onshore flow will provide some relief from the heat Friday... likely leading to temperatures along the immediate coast topping out in the low 80 with heat indicies in the mid to upper 80s. Some uncertainty still remains how far inland the front will push, but currently thinking it'll fall short of reaching ORH. Regardless, widespread moderate heat impacts are expected across the region, with pockets of major heat impacts possible in the CT River Valley, both today and Friday, leading to an increased risk of heat-related illness for anyone spending extended time outdoors without adequate hydration or access to cooling.

KEY MESSAGE 2...A few scattered strong to severe thunderstorms are possible Thursday and Friday.

The increased heat and humidity combined with a series of weak upper- level disturbances continue to be the catalyst for showers and thunderstorms later today and again Friday. While confidence has waned somewhat, the potential for some severe storms across western CT this evening remain. CSU ML probabilities continue to show 15-30 percent for severe wind potential, and the latest guidance shows CAPE values upwards of 1500 J/kg with pockets of up to 2200 J/kg possible across far western CT... however, those higher values are likely to quickly decay as they push east, which would lead to short- level cells. Latest CAMs continue to show 0-6km Bulk Shear values struggling to reach 20 kts, with highest values concentrated across western and southern CT. Regardless, any storms that do form will quickly tap into the unstable airmass, with the primary threat continuing to be damaging winds.

Friday looks to be very similar, with the exception that CSU ML probabilities have a narrow corridor in western CT favoring a 30 to 45 percent chance for severe winds. Some model sounding are indicating significantly higher DCAPE values Friday evening compared to this evening, which would support a chance for higher thunderstorm driven winds. Uncertainty remains Friday regarding how widespread storm coverage could be.

KEY MESSAGE 3...Above normal temperatures continue into early next week, but humidity will be on the decrease. 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, bringing lowering dewpoints and decreased humidity. While it will still be plenty warm for most of southern New England, 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. Changes with later forecasts are expected.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

This afternoon...Moderate confidence.

VFR except for ACK where IFR cigs/vsbs persist. There is a risk for thunderstorms between 19Z and 02z, but the risk is highest at BDL. There is a chance for -SHRA/-TSRA at the other terminals, but confidence is lower.

Tonight...Moderate confidence.

-SHRA/-TSRA tapers off by 03-04Z. Mainly VFR, with some MVFR/IFR vsbys developing in patchy fog mainly over Cape/islands. Light and variable winds.

Friday...Moderate confidence.

VFR. Light onshore winds developing. 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.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...Moderate Confidence in TAF. Risk for -TSRA this afternoon/evening between 23Z and 02Z but may depend on if the seabreeze makes it into the terminal which is becoming more uncertain the later into the day we get.

Friday night...High confidence in TAF.

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

Outlook /Saturday 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...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. 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 late evening.

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. RI...Heat Advisory until 8 PM EDT Friday for RIZ001>004. Air Quality Alert until 11 PM EDT this evening for RIZ001>008. MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ254>256.