textproduct: Boston / Norton

WHAT HAS CHANGED

Heat Advisories have been issued for interior MA and northern CT from noon Thursday until 8PM Friday. Increasing confidence in a few storms possibly becoming severe in nature Thursday evening.

KEY MESSAGES

- Chances for showers and thunderstorms return this afternoon into tonight.

- Heat Advisories have been issued across interior MA and CT Thursday into Friday. Diurnal thunderstorms remain a possibility during the afternoon and evening hours Thursday and Friday.

- Temperatures remain warm with decreasing humidity throughout the weekend, while uncertainty remains high regarding potential for rain showers.

DISCUSSION

KEY MESSAGE 1...Chances for showers and thunderstorms return this afternoon into tonight.

A warm front associated with low pressure system tracking through the Great Lakes and southern Ontario will sweep through southern New England will bring some showers and possibly thunderstorms to the region later this afternoon and into the overnight period. Potential for severe still seems low, with the CSU ML probabilities remaining at a 5-15 percent chance for severe wind across western MA and CT. Similar trends are being seen in the latest run of the HRRR Neural Network, with highest potential for severe winds across interior MA and CT tonight. Given winds turning southwesterly today, expecting to see dewpoints and PWATs surge... the latest model guidance is indicating PWATs 1.9"-2.0" across most of southern New England, with pockets of up to 2.2" possible across central and southern CT heading into the early Thursday morning hours. Unfortunately, confidence in where exactly storms could initiate remains on the lower side. Latest guidance continues to keep the heaviest rainfall in northern New England and offshore to the south... regardless, any rainfall that does form is likely to be heavy in nature.

KEY MESSAGE 2...Heat Advisories have been issued across interior MA and CT Thursday into Friday. Diurnal thunderstorms remain a possibility during the afternoon and evening hours Thursday and Friday.

Confidence continues to increase that heat and humidity will pose a risk Thursday and Friday. As the warm front from Wednesday lifts further north, 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 likely to top out in the upper 60s to low 70s, 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 Thursday and likely above 100F Friday across the CT River Valley, prompting Heat Advisories to be issued for northern CT and western MA from noon Thursday until 8PM Friday.

Temperatures across RI and eastern MA likely remain in the mid 80s to low 90s across both days. Dewpoints surging to near 70 will lead to heat indicies in the low 90s, especially away from the coastal plain. Heat Advisories may be expanded further east; however, a backdoor cold front is expected to drop into eastern MA sometime on Friday. Onshore flow will provide some relief from the heat Friday, but the question remains how far inland the front will drop. Regardless, widespread moderate heat impacts 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.

The increased heat and humidity combined with a series of weak upper- level disturbances will also set the stage for higher chances of showers and thunderstorms Thursday and Friday, with increasing potential that some storms Thursday may be severe in nature. CSU ML probabilities have increased to 15-30 percent for severe wind potential and 5-15 percent chance to see small hail across the interior. Latest guidance is showing CAPE values of 1300+ J/kg Thursday evening, with pockets of up to 2000 J/kg possible across far western CT... however, those higher values are likely to rapidly decay as they push east, which would lead to short-level cells. Regardless, confidence has increased enough to upgrade the western portion of southern New England from General Thunderstorms to a Slight Risk for severe weather Thursday.

KEY MESSAGE 3...Temperatures remain warm with decreasing humidity throughout the weekend, while uncertainty remains high regarding potential for rain showers.

Anticipating the passage of a couple fronts Saturday and Sunday. The main impact will be lower humidity, with Sunday anticipated the drier of the two. Lingering showers possible Saturday, but plenty of dry hours too. Some guidance members indicated a weak low pressure passing by towards the south coast of New England sometime Monday into Tuesday. Kept a mention of showers in the forecast, but confidence in these details is low.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

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 through much of today. 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, but coverage is still relatively uncertain, with western terminals most likely to see showers and a rumble of thunder.

Gusts to around 20 kt may make a return by the mid/late morning hours. SW winds will shift more S in the wake of a passing warm front later today into tonight.

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. Seabreeze possible today between 14-20z before winds flip back to SW. PROB30 for SHRA switched to a TEMPO.

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 Sunday/...

Thursday Night: VFR. Slight chance SHRA.

Friday: Mainly VFR, with local MVFR possible. Slight chance SHRA.

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

Saturday through Saturday Night: VFR.

Sunday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

Through Thursday Morning...High confidence.

Winds shift more SW then SSW Wednesday afternoon, sustained at around 15-20 kt with gusts remaining just below Small Craft criteria. Seas generally 2-4 ft through Wednesday night, though the southern outer waters may start to creep to 5 ft heading into Thursday morning.

Outlook /Thursday Night through Sunday/...

Thursday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers, slight chance of thunderstorms.

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

Friday Night through Saturday: Winds less than 25 kt. Slight chance of rain showers.

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

Sunday: Winds less than 25 kt.

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...None.