textproduct: Boston / Norton

WHAT HAS CHANGED

Signs of potentially salvaging at least one dry weekend day, but uncertainty remains high.

KEY MESSAGES

- Final day of record warmth today with highs in the 90s away from the immediate coast. Chance for strong to severe thunderstorms by the afternoon with a risk for damaging winds and some hail.

- Cooler temperatures through Memorial Day Weekend. Unsettled weather returns for the weekend with periods of rain possible, but confidence is low.

DISCUSSION

KEY MESSAGE 1...Final day of record warmth today with highs in the 90s away from the immediate coast. Chance for strong to severe thunderstorms by the afternoon with a risk for damaging winds and some hail.

One more day with near record temperatures on tap before a drastic return to your regularly scheduled May weather. The Heat Advisory remains in effect as we see an anomolously warm airmass remaining overhead, even as the upper level ridge begins to break down. Expecting a mix of sun and diurnal clouds, with westerly downslope flow once again contributing to temp maxima in the CT and Merrimack valleys. Again, will have to go to the immediate south coast to escape the heat but a slight shift in wind direction may bring those hot temps closer to the shore than on Tuesday. Relief then arrives behind a cold front which is dragged through the region from west to east between late morning and this evening.

This warm and moist airmass (dewpoints climb into the mid 60s) produces a decent amount of instability, albeit a bit less than Tuesday, around 1,000 J/kg while model soundings show very steep low level lapse rates (8.5 C/km). Convergence ahead of the cold front will act on this airmass to produce a broken line of relatively shallow thunderstorms, limited in depth and organization by poor ML lapse rates and marginal 0-6 km bulk shear respectively. Unlike Tuesday's more northward severe risk, the best instability and resultant t-storm chance looks to be south of the MA pike where some damage wind gusts would be the main risk.

KEY MESSAGE 2...Cooler temperatures through Memorial Day Weekend. Unsettled weather returns for the weekend with periods of rain possible, but confidence is low.

High pressure slowly builds in on Thursday into Friday suppressing any rainfall to the south; some guidance wants to keep rain close enough to the southern periphery to brush the south coast with rain showers the first half of Thursday. Ensemble guidance is a bit more bullish on measurable rainfall Thursday morning than the current forecast, so that may end up needing to be bumped up in the future.

Looking to the holiday weekend, as discussed last night the AI guidance continues to be dry, with a stronger high suppressing unsettled weather just to the south over the mid Atlantic while the traditional models are wetter and colder. That being said, the GFS continues to also suppress the rain south until Monday while the ECMWF has at least slowed the arrival, to perhaps salvage Saturday. That being said, ensemble guidance doesn't seem to have caught on to any drying ideas, still bullish on rain both days (both the GEFS and EPS). The main idea is, there remains a great deal of uncertainty with regard to the holiday weekend, but still leaning the forecast on a wetter and cooler (50s and low 60s) solution. Temperature-wise, after a seasonable Thu/Fri in the upper 60s/low 70s the weekend is looking like it may be colder than normal for late May.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

Forecaster Confidence Levels...

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

12Z TAF Update...

Wednesday...Moderate confidence (TSRA chances).

VFR. Scattered TSRA along and ahead of a cold front in the afternoon and evening. Given widely scattered nature and uncertainty in location of TSRA, have included PROB30 groups for terminals PVD south, and again for ACK with possible visby drop overnight under -SHRA. WSW winds 8-12 kts with gusts 20-25 kts turning NW behind the cold front between 16-00Z. Brief strong gusts possible in any thunderstorms.

Wednesday night...Moderate confidence.

VFR. Some SHRA may linger/expand over southern terminals overnight into Thursday morning. Light NW winds becoming N/NE.

Thursday...High confidence.

VFR. Light winds.

KBOS...High confidence in TAF.

KBDL...High confidence in TAF.

Outlook /Thursday Night through Sunday/...

Thursday Night through Friday: VFR.

Friday Night: VFR. Slight chance SHRA.

Saturday: Mainly VFR, with local MVFR possible. Chance SHRA.

Saturday Night: Mainly VFR, with local MVFR possible. Breezy. SHRA likely.

Sunday: Mainly VFR, with local IFR possible. Breezy. Chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Through Wednesday: High confidence.

Small craft advisories remain in effect through 06z Thurs. SW winds of 15-25 kt continues this afternoon and overnight Seas will also be building to around 4-6 ft, with some 7 footers possible tonight over the southern waters.

Mainly dry weather prevails through tonight with a few showers or thunderstorms possible for the northern waters., Some fog may persist and expand across the southern waters tonight on increasing southwest winds.

Outlook /Thursday Night through Sunday/...

Thursday Night through Friday: Winds less than 25 kt.

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

Saturday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers.

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

Sunday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Chance of rain showers.

CLIMATE

Record Highs for Wed May 20...

BOS 91/1996 BDL 99/1996 PVD 95/1996 ORH 91/1903

BOX WATCHES/WARNINGS/ADVISORIES

CT...Heat Advisory until 8 PM EDT this evening for CTZ002>004. MA...Heat Advisory until 8 PM EDT this evening for MAZ003>007- 010>019-026. RI...Heat Advisory until 8 PM EDT this evening for RIZ001>004. MARINE...Small Craft Advisory until 2 AM EDT Thursday for ANZ254>256.