textproduct: Boston / Norton

WHAT HAS CHANGED

The risk for severe weather has diminished somewhat with best chance near the south coast.

KEY MESSAGES

- Scattered showers and thunderstorms developing through early evening with main focus across CT, RI and SE MA, especially near the south coast. A few strong to severe storms possible.

- Another round of scattered showers later tonight into Thu morning for CT, RI and SE MA. Much cooler Thu then dry and seasonably mild Fri.

- 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...Scattered showers and thunderstorms developing through early evening with main focus across CT, RI and SE MA, especially near the south coast. A few strong to severe storms possible.

Cold front has pushed southward into northern MA with wind shift to W-NW. Winds have veered to the W ahead of the front which will limit low level convergence somewhat. As front moves south and approaches south coast, expect scattered showers and t-storms to develop. This is where best instability, up to 1000 J/kg of MLCAPE, is located along with best convergence as winds are more SW near the coast. Effective shear is marginal, up to 25 kt which is a bit lower than you want to see for storm organization. However, steep low level lapse rates and inverted V soundings support localized strong to damaging wind gusts with any stronger storms that develop. The best chance for a few strong to severe storms are near the south coast from 5-8 pm, with HREF max updraft field targeting southern RI. However, severe threat is greatest to the SW.

KEY MESSAGE 2...Another round of scattered showers late tonight into Thu morning for CT, RI and SE MA. Much cooler Thu then dry and seasonably mild Fri.

The cold front moves south of New Eng tonight. A weak wave is forecast to develop along the boundary as shortwave energy approaches from the west toward Thu morning. Northern edge of higher PWATs and deep moisture plume lingers near the south coast where scattered showers may redevelop late tonight and linger into Thu morning, with best chance along and south of HFDL-PVD-PYM. Otherwise, drier air will be pushing south from northern New Eng leading to partly sunny skies developing across northern MA during the afternoon. Much cooler day Thu as 925 mb temps drop to 7-9C leading to highs in the 60s.

The shortwave moves east of New Eng by late Thu followed by subsidence then upper ridge builds over the Gt Lakes with dry NW flow across New Eng through Fri. The drier air will reach the south coast Thu night with clearing skies. Good radiational cooling will result in a rather chilly night with lows ranging from the upper 30s to mid 40s. Sunshine and seasonably mild temps Fri as high pres builds into the region. Highs will reach the upper 60s and lower 70s but cooler along the coast where sea-breezes are expected to develop.

KEY MESSAGE 3...Cooler temperatures through Memorial Day Weekend. Unsettled weather returns for the weekend with periods of rain possible, but confidence is low.

High pressure and mid-level ridging overhead for Friday bringing dry conditions and seasonal temperatures. Expect high temperatures in the upper 60s/low 70s for most inland areas while easterly flow will keep the east-facing coastline cooler in the 60s.

For the holiday weekend, we continue to see a signal for a potentially unsettled weather pattern with a period or two of showers. However, there continues to be a notable spread among deterministic and ensemble guidance on the track and timing of disturbances associated with a larger system moving through the Ohio Valley. While the ECMWF and some runs of the GFS show rain chances later Saturday into Sunday, their AI-renditions have been dry for Saturday. Beyond that point, differences even emerge between the AI guidance on the timing of rain sometime Sun or Mon. This spread in solutions has lead to a bullish view when assessing the ensemble guidance for rain probabilities over the weekend. Ensembles have blanketed southern New England with moderate probabilities for at least 0.01" of rain later Sat into Sun and Monday. Given the notable spread in guidance, rain chances are still low confidence for the holiday weekend. Leaning toward a wetter weekend based on range of ensemble guidance leaning in that direction. On the positive side, if Saturday ends up wetter, there is some agreement with the showers coming in later Saturday which at least keeps part of that day potentially dry! Given that guidance is leaning more unsettled, temperatures this weekend will be cooler in the 50s and low 60s. There is a signal for temperatures to trend warmer into early next week.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Forecaster Confidence Levels...

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

18Z TAF Update...

Through 02z...Moderate confidence.

Mainly VFR. A few showers possible along and north of the MA Pike through 20z. Scattered SHRA/TSRA developing across CT/RI/SE MA after 20z. Any t-storms will be accompanied by downpours and brief gusty winds. Localized stratus may impact ACK this evening. W gusts to 25 kt diminishing this evening.

Tonight and Thursday...High confidence.

Mainly VFR cigs. Scattered showers redeveloping near the south coast overnight into Thu morning with brief MVFR possible. NW wind 5-15 kt this evening becoming light N overnight. Sea- breezes developing Thu.

Thursday night...High confidence.

VFR. Light wind.

KBOS...High confidence in TAF.

KBDL...High confidence in TAF.

Outlook /Friday through Monday/...

Friday through Friday Night: VFR.

Saturday: VFR. Chance SHRA.

Saturday Night through Sunday: VFR. Breezy. SHRA likely.

Sunday Night: VFR. Chance SHRA.

Memorial Day: Mainly VFR, with local MVFR possible. Chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Through Wednesday Night: High confidence.

SW gusts to 25 kt over nearshore waters will diminish this evening with seas gradually subsiding tonight. Winds will shift to NW this evening then N overnight. NE wind Thu morning with sea-breezes developing over nearshore waters in the afternoon. Light winds Thu night becoming N-NE overnight.

Outlook /Friday through Monday/...

Friday through Friday Night: Winds less than 25 kt.

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

Saturday Night through Sunday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Rain showers likely.

Sunday Night: Winds less than 25 kt. Seas up to 5 ft. Rain showers likely.

Memorial Day: Winds less than 25 kt. Seas up to 5 ft. 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 MAZ005>007- 010>019. RI...Heat Advisory until 8 PM EDT this evening for RIZ001>004. MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ230- 232>235-237. Small Craft Advisory until 2 AM EDT Thursday for ANZ254>256.