textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes.

KEY MESSAGES

- Coastal clouds, drizzle, and fog give way to widespread beneficial rainfall tonight into Thursday.

- Cooler and mainly dry this weekend then milder next week with a few showers.

DISCUSSION

KEY MESSAGE 1...Coastal clouds, drizzle, and fog give way to widespread beneficial rainfall tonight into Thursday.

Low cloud deck finally eroded a bit thanks to daytime heating and was confined to areas near the immediate coast, although we're seeing some substantial breaks in cloud cover especially near Cape Cod and Islands. Cloud deck should begin to expand westward again this evening as we lose daytime heating and boundary layer cools, and deepening marine layer should promote patchy fog and drizzle along coast. Farther inland, mostly clear skies will persist with increasing mid and high clouds early tonight.

We're still looking at two areas of showers tonight into Thursday. First is later this afternoon and evening across eastern MA in association with surface trough from offshore low. Showers will then become more widespread later tonight into Thursday morning as we come under broad larger scale lift ahead of upper trough (as seen in model Q-vector fields) and primary low tracking through the St. Lawrence Valley. Secondary low tracking from mid Atlantic has trended weaker since yesterday so between that and the speed of the system overall rainfall will be less, on the order of around 0.5" for most, but still much needed given recent rainfall departures.

Guidance does show some marginal instability on the edge of the dry slot that lifts through southern New England Thursday morning, focused near Cape Cod and Islands, but moreso offshore. It's possible we see some additional showers or even a thunderstorm pop up in those areas but that is conditional on seeing better instability.

KEY MESSAGE 2...Cooler and mainly dry this weekend then turning milder early next week with a few showers.

Amplified pattern over the weekend should feature a building ridge of high pressure over central Canada which traps a closed upper low over southern Ontario. As mentioned before, southern New England should be far enough removed from coldest air aloft to keep us mostly dry, especially given W/NW flow, but pattern certainly favors diurnal clouds and perhaps a few afternoon sprinkles over weekend.

We're also watching a weak low also passing offshore that could come close enough to bring a few showers to parts of eastern MA and RI later Saturday into Sunday, but right now any rainfall looks to be fairly light and may end up being more focused near Cape Cod and the Islands or even offshore.

Pattern relaxes a bit early next week as closed low lifts farther north, but we remain under broad cyclonic flow aloft. Eventually longer range guidance shows closed low setting up near upper Great Lakes, putting us in SW flow aloft. This could end up bringing more moisture into region by Tuesday/Wednesday as weak front approaches resulting in scattered showers. Temperatures should return to more typical levels for early May however.

AVIATION /00Z THURSDAY THROUGH MONDAY/

Forecaster Confidence Levels...

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

00z TAF Update: Moderate confidence due to timing of flight categories.

MVFR/IFR closer to coast will become IFR this evening, roughly as far west as KORH/KIJD with VFR conditions prevailing farther to the west. Scattered -SHRA possible later this afternoon and evening in eastern MA before showers become more widespread overnight with IFR ceilings/visibilities. Showers come to an end around midday Thu with gradual improvement to MVFR ceilings, and possibly VFR later in day with the exception of Cape Cod and Islands.

KBOS Terminal...Moderate confidence. Return of IFR/LIFR could be a few hours too fast.

KBDL Terminal...High confidence in TAF.

Outlook /Thursday Night through Monday/...

Friday: VFR. Breezy.

Friday Night: VFR.

Saturday: VFR. Slight chance SHRA.

Saturday Night: VFR. Chance SHRA.

Sunday: VFR. Breezy. Slight chance SHRA.

Sunday Night: VFR.

Monday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

High confidence through the end of the week.

Pressure gradient between slow-moving low pressure SE of the 40/70 benchmark and high pressure over the Canadian Maritimes keeps E/NE winds in play through the end of the week.

Main issue will be the long fetch of NE winds resulting in rough seas of 5 to 8 feet on outer waters through Thu where SCA remains posted. Areas of fog will persist through Thu along with showers, especially later tonight through early Thu afternoon.

Outlook /Thursday Night through Monday/...

Friday through Friday Night: Winds less than 25 kt. Seas up to 5 ft.

Saturday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers.

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

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

Sunday Night: Winds less than 25 kt. Areas of seas approaching 5 ft.

Monday: Winds less than 25 kt. Seas up to 5 ft.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 AM EDT Friday for ANZ250-254>256.