textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes, the forecast remains on track.

KEY MESSAGES

- Scattered late day and evening showers and thunderstorms with a low risk of severe weather.

- Summerlike warmth continues Wednesday away from the coast, but backdoor front will bring cooler temps to portions of eastern MA. Another round of scattered afternoon and evening showers and thunderstorms with a low risk for severe weather.

- Temperatures trend milder western MA/CT and cooler in eastern MA/RI Thurs-Sat, with periods of showers/t-storms mainly focused on Fri.

- Strong cold frontal passage Sunday ushers in cooler temperatures for early next week.

DISCUSSION

KEY MESSAGE 1...Scattered late day and evening showers and thunderstorms with a low risk of severe weather.

Latest hi-res guidance has delayed onset of scattered convection moving into SNE to late in the day and this evening. We still have height rises into early afternoon then weak height falls developing late today and this evening as a weak shortwave passes to the north. There are favorable mid level lapse rates approaching 7 C/km and there is sufficient 0-6km shear for storm organization. However, instability is marginal with MLCAPES generally 500-1000 J/kg and with potential convection delayed into the evening instability will be diminishing. Also forcing for ascent is marginal as well. So while there are some conflicting signals for severe weather, if robust convection can get going to the north and west later this afternoon favorable deep layer shear should be enough to support a few strong to severe storms with the timing mainly from 5-10 pm across northern and western MA where HREF is showing some updraft helicity tracks. Strong to damaging wind gusts will be the primary threat but can't rule out some hail given steep mid level lapse rates and favorable deep layer shear.

KEY MESSAGE 2...Summerlike warmth likely to continue Wednesday away from the coast, but backdoor front will bring cooler temps to portions of eastern MA. Another round of scattered afternoon and evening showers and thunderstorms with a low risk for severe weather.

Backdoor front expected to slip southward into NE MA Wed morning and eventually move through rest of eastern MA by late afternoon but timing is uncertain. 925 mb temps 20-21C south and west of the boundary which will result in temps reaching low-mid 80s, especially across CT and portions of central/W MA which will remain on the warm side of the boundary. Meanwhile, temps will likely fall into the 50s in NE MA with temp forecast uncertainty highest across interior E and SE MA which will be close to the boundary. It is possible temps here warm well into the 70s and possibly lower 80s before the front moves through then falling sharply by late afternoon but this will depend on the timing of the front.

Another round of scattered showers and thunderstorms expected Wed afternoon into the evening as another low amplitude shortwave approaches. Environment is fairly similar to today with MLCAPES 500- 1000 J/kg although mid level lapse rates are less favorable. 0-6 km shear is up to 40 kt which is quite favorable for storm organization. Hi-res CAMs do show varying areal coverage of convection developing into the afternoon in the interior then eventually becoming focused near the south coast during Wed night. CSU machine learning probs and Nadocast showing low risk for severe across the interior so can't rule out a few strong to severe storms again with damaging wind the primary threat. HREF severe wind probs are a bit lower tomorrow and focused across CT.

KEY MESSAGE 3...Temperatures trend milder western MA/CT and cooler in eastern MA/RI Thurs-Sat, with periods of showers/t-storms mainly focused on Fri.

Challenging temperature and rain chances forecast Thurs and Fri as unseasonably warm temperature profiles aloft are met with quasi- zonal mid-level flow with several hard-to-time impulses (some convectively-influenced) moving through the WSW flow. There is also the potential for a backdoor frontal boundary to bring significantly cooler temperatures to at least eastern and northeast MA, but potentially as far westward as central MA and RI.

Ended up keeping a rather generic/broad-brush to PoPs, with lesser chances on Thurs and most of the day could be dry. Better chances could exist on Fri with NWP showing a stronger/more-coherent shortwave disturbance moving in.

For temps, modified NBM highs Fri and Sat to bring cooler temps (mid 60s to low 70s) along the eastern MA coast to RI/central MA, but highs should end up well into the 70s to lower 80s in western MA/CT under SW winds. Highs are somewhat cooler on Fri with more cloud cover and periods of showers/t-storms, in the 60s to low 70s. By Sat, we become more entrenched in cooler onshore flow with highs in the 50s to mid 60s. The potential exists for large temperature busts given placement of the backdoor frontal boundary.

KEY MESSAGE 4...Strong cold frontal passage Sunday ushers in cooler temperatures for early next week.

Strong cold front moves in for the latter half of the weekend bringing a risk for showers and thunderstorms for Sunday. Passage of this front will usher in an unseasonably cool airmass (850 mb temps as low as -8C Mon). Expect cool temperatures more typical of early/mid March vs mid-April, with highs in the mid 40s to lower 50s, but blustery conditions will make it feel quite a bit cooler.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

Forecaster Confidence Levels...

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

Tonight...Moderate Confidence

VFR. Steady west winds shifting to northwest after 06Z. A line of showers and thunderstorms moves over interior terminals between 00Z and 02Z. -TSRA most likely west of BOS/BED but can't completely rule out thunder chances at those terminals. West wind gusts to 30+ knots possible when the line moves through BDL/ORH.

Tomorrow...High Confidence in trends.

VFR through about 18Z. Then MVFR ceilings settling in. Sea- breeze likely at BOS again between 16-17Z. Additional rounds of -SHRA/-TSRA possible tomorrow afternoon and evening.

Tomorrow Night...High Confidence

MVFR/IFR ceilings with -SHRA and light and variable winds.

KBOS Terminal... Moderate confidence in TAF.

This afternoon's sea-breeze/stable marine air should weaken the line of thunderstorms to a line of showers by the time it reaches the terminal. High confidence in a sea-breeze again tomorrow.

KBDL Terminal... Moderate confidence in TAF.

The line of showers and storms should reach BDL by 0030-0100Z.

Outlook /Thursday through Sunday/...

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

Thursday Night: VFR. Chance SHRA.

Friday: VFR. Chance SHRA, slight chance TSRA.

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

Saturday: Mainly VFR, with areas MVFR possible.

Saturday Night: Mainly VFR, with areas IFR possible. Slight chance SHRA.

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

MARINE

Forecaster Confidence Levels...

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

SCA for outer southern waters through tonight for marginal seas. S- SW winds will gust to 20+ kt at times, shifting to E over NE MA waters during Wed with SW flow persisting over southern waters. Variable winds Wed night less than 20 kt, mostly N-NE over NE MA waters and SW over southern waters. Late night and early morning fog possible.

Outlook /Thursday through Sunday/...

Thursday: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers.

Thursday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers.

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

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

Saturday through Saturday Night: Winds less than 25 kt.

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

CLIMATE

Upcoming record highs:

Tue 4/14 Boston 83 in 2023 Hartford 96 in 2023 Providence 87 in 2023 Worcester 90 in 2023

Wed 4/15 Boston 82 in 1896 Hartford 88 in 1941 Providence 82 in 1938 Worcester 85 in 1941

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 AM EDT Wednesday for ANZ255-256.