textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes.

KEY MESSAGES

- Cooler temperatures but generally dry weather and onshore breezes through Saturday. Less cloud cover Friday, but turning overcast with even cooler temps (60s) for Saturday, with rain developing later Saturday evening.

- Cool with widespread rain Sunday.

- Warming trend next week. Scattered showers lingering Monday with another chance of showers Wed.

DISCUSSION

KEY MESSAGE 1...Cooler temperatures but generally dry weather and onshore breezes through Saturday. Less cloud cover Friday, but turning overcast with even cooler temps (60s) for Saturday, with rain developing later Saturday evening.

A slowly-sagging frontal boundary along the southern coast will continue to sag southward into the coastal portion of the northern Mid-Atlantic region. Meanwhile a 1028+ mb high pressure cell over northeast Ontario, associated with low-level temperatures running several degrees cooler than late-May normals, will be building into Southern New England tonight and remain across eastern MA/adjacent eastern waters Fri and Sat. After the first taste of summer-like warmth the last couple days, we'll flip the calendar back to temperatures more reminiscent of late April. We should also see relatively dry weather, although the risk for approaching rain develops later Sat and particularly Sat night.

Plenty of mid-level cloud cover ongoing on the cold side of the frontal boundary, although expectation is for that to decrease from north to south as we move into tonight. As this cloud cover has tempered daytime warming and with otherwise good radiational cooling, it sets stage for a cool and dry night. Lows tonight should bottom to the upper 30s to low 40s, with the coolest readings in northwest MA. There could be patches of frost in the more sheltered areas in northwest MA, but the anticipated coverage of frost seems low enough to not rise to the level of warranting frost headlines. Pretty stark change in airmass and sensible weather compared to the past few days!

Fri and Sat feature more-or-less similar conditions with onshore breezes, with the only differences being the extent of cloud cover being quite a bit more on Sat. With less cloud cover Fri, expect the mildest temperatures to reach into the 60s with a few spot 70s out in the CT Valley away from the onshore flow, and lows in the 40s. Then temps may struggle to reach the mid 60s Sat with increasing cloudiness; the better chance for light rains into western New England then begins to arrive late Sat evening, spreading eastward through the nighttime hours. The exact timing of rains late Sat/Sat night is still subject to adjustment given rain falling into a cool and dry airmass. But the bulk of the rain arrives into Sunday.

KEY MESSAGE 2...Cool with widespread rain Sunday.

Surface wave will be lifting north from the mid Atlc coast on Sunday. Ahead of the low pres a 40-50 kt SE low level jet will transport deep moisture plume into SNE with PWATs exceeding 1.5". Favorable forcing for ascent acting on this moisture will result in widespread soaking rain across SNE which will essentially be a washout. EPS ensembles show high probs (70-100%) for QPF over 0.50" and moderate probs over an inch. Current thinking is rainfall averaging 0.50" to 1.0" with locally higher amounts. There is also some elevated instability on the northern gradient of higher theta-e air moving up from the south so can't rule out an isolated t-storm, mainly near the south coast. It will be a cool day with temps mainly in the 50s, and it's possible temps struggle to get to 50F over the higher elevations in interior MA. Gusty E winds will impact coastal locations Sun afternoon and evening associated with the low level jet. Steadier rain will begin to move out Sun night as the moisture plume moves to the east, but risk of showers will continue as the low pres lifts NE across SNE.

KEY MESSAGE 3...Warming trend next week. Scattered showers lingering Monday with another chance of showers Wed.

Conditions should improve somewhat Mon as the low pres moves to the NE, but scattered showers may linger as a shortwave approaches from the west. GFS and its ensembles are most aggressive with another period of rain moving in, especially south of the MA Pike so will have to watch this. Otherwise, warming trend next week with warmest days Tue and Wed when temps should get back above 80F away from the coast. GEFS and EPS ensembles both show moderate probs of 80+. Looks dry Tue then chance of showers return Wed as a northern stream shortwave rotates southward into the region. Somewhat cooler airmass follows for next Thu.

AVIATION /00Z FRIDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

00z TAF Update...

Through Tonight...High confidence.

VFR with high clouds. Calm to light N winds this evening, then N-NE 5-10 kt overnight.

Friday...High confidence.

VFR. NE winds 8-15 kt, with gusts to 20 kt Cape/Islands diminishing in the afternoon. Sea-breezes developing on the eastern MA coast.

Friday Night...High confidence.

VFR. Increasing mid and high-level clouds. NE winds less than 5 kt.

KBOS...High confidence in TAF.

KBDL...High confidence in TAF.

Outlook /Friday Night through Tuesday/...

Saturday: VFR. Chance SHRA.

Saturday Night: VFR. Breezy. SHRA likely.

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

Sunday Night: MVFR/IFR conditions possible. Breezy. Chance SHRA.

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

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

Tuesday: VFR.

MARINE

Forecaster Confidence Levels...

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

Winds and seas to remain below small craft advisory criteria through at least midday Sat. NE to E winds tonight through Fri night around 10-15 kt, with E/ESE winds at similar speeds for the southern waters, and seas 4 ft or less all waters.

Into Sat of the holiday weekend, ESE winds around 15-20 kt with fringe small craft advisory winds/seas over the southern waters especially later in the day. Rain will also develop around late Sat/Sat night over the southern waters. Increasing, more solid-SCA E/SE winds 25-30 kt and seas 5-9 ft (higher southern waters over the open-ocean) then develop for Sunday for your holiday boating plans, to go along with steady rain which could reduce visibility.

Outlook /Friday Night through Tuesday/...

Saturday: Winds less than 25 kt.

Saturday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Seas locally approaching 5 ft. Chance of rain showers.

Sunday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Rain showers.

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

Memorial Day: Winds less than 25 kt. Areas of rough seas. Chance of rain showers.

Monday Night: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers.

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...None.