textproduct: Boston / Norton

WHAT HAS CHANGED

The risk for fire weather concerns Thursday now looks limited, as cloud cover and potential for showers could develop during the daytime hours.

KEY MESSAGES

- Mostly sunny and dry today gives way to overcast later tonight, with scattered rain showers Wed as low pressure passes to our south and west.

- Slow-moving upper-level low over Atlantic Canada maintains cooler unsettled conditions Thursday through the weekend. Better chances for showers on Thursday before trending drier for Friday into the weekend.

DISCUSSION

KEY MESSAGE 1...Mostly sunny and dry today gives way to overcast later tonight, with scattered rain showers Wed as low pressure passes to our south and west.

Midlevel flow pattern begins to (briefly) deamplify thru Wed. Some pesky cloud cover across the Cape and Islands moves offshore early this morning; expecting a mostly sunny rest of the day today with good mixing allowing for dewpoints to plummet into the single digits to lower teens in the interior and RHs falling to around 20 percent. Seabreezes develop near the coasts which will keep temps cooler in the 40s, but should reach into the low to mid 50s away from the coast, coming out of a chilly overnight.

Cloud cover then increases tonight from west to east as a low- amplitude shortwave in a fast WNW flow brings an associated weak low pressure and warm frontal boundary into Southern New England. Looks to be a decent period of isentropic ascent which could allow for warm frontal precip to break out as soon as the Wednesday pre-dawn hours in interior western New England; an earlier arrival of precip would open up the door for light rain showers to mix with wet snowflakes through dynamic/wet-bulbing effects with perhaps some coatings on non-paved surfaces. It leads to a rather gray, dreary Wed with periods of rain showers and modest S winds. Despite modest warm advection, temps on Wed could struggle to get out of the 40s with reduced insolation and some of the precip lost to wet-bulbing, leading to cooler temps. QPF amts are on the light side - more of a nuisance than anything significant or impactful. Cloud cover should begin to decrease somewhat by Wed night but considerable cloudiness could linger for eastern MA/RI.

KEY MESSAGE 2...Slow-moving upper-level low over Atlantic Canada maintains cooler unsettled conditions Thursday through the weekend. Better chances for showers on Thursday before trending drier for Friday into the weekend.

An increasingly blocky mid-level pattern downstream of us in the North Atlantic then allows for the trough currently moving offshore to develop into a broad closed circulation near the Canadian Maritimes. The western extent of this anticipated closed low seems poised to drive our weather pattern for late in the week and into the weekend, with a cooler maritime-polar flow governing our weather. This will also mean a period of temps running near to slightly cooler than late-April normals.

What was previously looking like a potential dry and gusty day Thursday now could feature variable amts of cloudiness with more east and less west and at least isolated to scattered diurnal- instability showers. Progged wet- bulb-zero heights are extremely low at around 3500 ft, which suggests showers would be accompanied by graupel. Guidance sources more bullish on shallow instability (e.g. NAM) even suggest potential for low- topped storms with lightning and small hail in strongest of cores, best chance over the eastern half of Southern New England, with a pretty potent shortwave digging southward. Something to consider in later forecasts as we get into the mesoscale NWP forecast horizon. Areas in western New England probably have the best chance to mix down lower dewpoints but was hesistant to bring dewpoints too low in case the upper trough continues to course- correct westward.

Otherwise, Fri into the weekend are unsettled with a cool pocket of air aloft, but generally drier. Expect a similar sky- cover picture in this period, with diurnally driven cumulus/fair weather clouds with less of them during the evening. As sfc ridging looks to build in from the west on the weekend, this could bring a modest onshore flow to eastern coastal areas. The upshot is that the retrograding Maritimes low should suppress storminess coming out of the Northern Plains/Gt Lakes to our south and west.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

Forecaster Confidence Levels...

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

12z TAF Update:

Today: High confidence.

VFR. Light NNE/NE winds to shift to southerly by late afternoon at 5-10 kts. SE seabreezes along the eastern MA and RI coasts starting around 14-17z around 10 kts.

Tonight: High confidence.

VFR with initial high clouds lowering and thickening thru first half of the night. Showers move into western southern New England ~07-10z Wed. While mainly rain predominates, it could mix with wet snowflakes for higher terrain locations especially if timing is on the earlier end of that window. Showers spread east with ceilings dropping to MVFR with the arrival of showers. Light S/SSW winds.

Wednesday: High confidence overall, though moderate on timing.

Showers spread east thru 10-15z as rain showers. Arrival of precip brings ceilings dropping to mainly MVFR (with intervals IFR). Sub-VFR ceilings beginning to slowly improve for western terminals after 22z but probably takes until Wed night before more significant improvement develops. Southerly winds 6-12 kts.

KBOS Terminal...High Confidence in TAF. SE seabreezes could develop as soon as 14z but is more likely around 15-16z today. Winds slowly shift S/SSW by ~03Z tonight.

KBDL Terminal... High Confidence in TAF.

Outlook /Wednesday Night through Saturday/...

Wednesday Night: Mainly VFR, with local IFR possible. Slight chance RA.

Thursday: VFR. Breezy. Slight chance SHRA.

Thursday Night through Friday Night: VFR.

Saturday: VFR. Slight chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Winds and seas remain below SCA criterion today and tonight. Light northerly winds turn SE around 5-10 kt today, and become southerly around 10-15 kt tonight. Outside chance at SCA southerly gusts around 20 kt for Wed on southern waters, but seems marginal for headlines given scattered showers limiting degree of mixing. Seas 4 ft or less all waters. Some brief/local reduction in visby possible in scattered showers Wed but probably not persistent.

Outlook /Wednesday Night through Saturday/...

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

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

Thursday Night through Friday Night: Winds less than 25 kt.

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

BOX WATCHES/WARNINGS/ADVISORIES

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