textproduct: Boston / Norton

WHAT HAS CHANGED

Areas of locally dense fog overnight, otherwise no significant changes to the forecast.

KEY MESSAGES

- Rain ending this afternoon, areas of fog develop overnight, warm and pleasant conditions Tuesday afternoon.

- Warm conditions continue Wednesday and slightly cooler Thursday, with only a low risk for isolated afternoon showers and thunderstorms.

- Temperatures trend near or below normal heading into the weekend as rain chances return Saturday.

DISCUSSION

KEY MESSAGE 1...Rain ending this afternoon, areas of fog develop overnight, warm and pleasant conditions Tuesday afternoon.

A frontal boundary shifts offshore today, bringing an end to the rain along with gradual clearing this afternoon. Behind the front, somewhat drier air moves in and clouds begin to disperse. A surface 1020 mb high pressure system builds across the Northeast tonight, with continued clearing, especially north and west of I-95. A combination of clear skies and light to calm winds should support radiational cooling; however, given the high surface moisture and elevated dew points, lows will only fall into the 50s. That said, lingering moisture from the weekend rainfall will support the potential for areas of locally dense fog overnight into early Tuesday morning. Expect fog to linger through at least the first half of the Tuesday morning commute. As temperatures warm after sunrise, boundary layer mixing will increase and allow the fog to dissipate. Tuesday afternoon looks fairly pleasant as the atmospheric column continues to dry out, with perhaps some passing high clouds filtering the sunshine. However, Cape Cod and the Islands may hold onto scattered to broken low-level cloud cover, as less dry air remains over this portion of the region. As for temperatures Tuesday afternoon, mixing appears to reach roughly 850mb, where temperatures are forecast to be +12C to +14C, supporting highs in the lower to middle 80s inland. It will not be as warm along the south coast due to the development of a sea breeze, where coastal areas can expect highs in the lower to middle 70s. Some locations, such as Block Island and Nantucket, may struggle to get out of the 60s.

KEY MESSAGE 2...Warm conditions continue Wednesday and slightly cooler Thursday, with only a low risk for isolated afternoon showers and thunderstorms.

Wednesday and Thursday will feature the chance for pop-up showers and thunderstorms, though a washout is not expected. In fact, most locations will remain dry for a significant portion of both days. There will be a relatively small window each afternoon for the development of isolated convection. On Wednesday, the better mid- level forcing remains south of southern New England, although the 12z NAM depicts a subtle 850mb trough and weak shortwave that could provide enough ascent for a few storms to develop. However, moisture will be somewhat limited, especially aloft, which could hinder updraft strength. Instability also appears limited, which should further inhibit storm development. Wind fields are not particularly impressive, though 30-35 kt of deep-layer shear could help organize any storms that do develop. At this point, the worst-case scenario appears to be a couple of rogue showers or rumbles of thunder, along with localized sub-severe wind gusts. On Thursday, a modest mid- level trough and shortwave pivot through New England, although PWATs decrease through the afternoon. As the previous forecaster mentioned, cooling aloft may support some diurnally driven pop-up showers or thunderstorms. However, mid-level dry air remains a limiting factor and should inhibit stronger updraft development. Machine learning guidance has not highlighted a severe weather threat at this time, which supports the current thinking of isolated storms with general thunder at most. Temperature-wise, Wednesday looks to be the warmest day of the week, with west to west-northwest flow promoting warmer conditions and some downsloping/compressional warming off the higher terrain. Highs should climb well into the 80s and 70s at the coast. Thursday becomes more complicated, particularly across eastern Massachusetts, where temperatures will depend heavily on wind direction. A northeast wind would advect cooler marine air inland from the relatively cold ocean waters, supporting highs only in the upper 60s to lower 70s, with the warmest readings likely south and west of Boston. A land breeze influence toward the south coast of Massachusetts and Rhode Island could allow temperatures there to approach the mid-70s. Farther inland, winds should trend more northerly to north-northwesterly, promoting some compressional warming off the higher terrain of southern Vermont into the Connecticut River Valley. Combined with greater sunshine compared to eastern Massachusetts and Rhode Island, this would support highs in the middle to upper 70s.

KEY MESSAGE 3...Temperatures trend near or below normal heading into the weekend as rain chances return Saturday.

Overall model guidance has continued to trend on the cooler side for temperatures in the wake of a cold front heading into the weekend, but there remains uncertainty in just how cool we could get.

Ensemble guidance continues to be in good agreement that a surface low pressure will spin up over the James Bay sometime Friday and move south along the fringes of an upper-level trough into New England, bringing another round of potentially beneficial rains for Saturday. The latest runs of deterministic guidance have also honed in on this development; however, there remains significant uncertainty in when and where the low could travel...therefore, confidence in how much rain SNE could see remains low at this time. Canadian guidance indicates more of an eastward progression, keeping the low (and the rainfall) further north into the Eastern Townships and northern New England, but ECMWF guidance brings that low further south along through VT, NH, and further into MA. GFS remains more of a centrally located solution and indicates more of a graze along the northeastern MA/NH coast. Regardless, portions of SNE are likely to see at least some rain this coming weekend.

Given the high amount of uncertainty in the guidance, anticipating that both PoPs and temperatures for this weekend remain in flux for the next few forecast updates.

AVIATION /18Z MONDAY THROUGH SATURDAY/

Forecaster Confidence Levels...

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

18z TAF Update:

This Afternoon and Tonight...Moderate confidence in trends and timing.

While showers have ended, ceilings will be slow to lift and scatter out. Conditions trend toward VFR from west to east after 21z, although IFR to MVFR ceilings will likely persist across the Cape and Islands. Areas north and west of I-95 will be most favorable for the development of radiational fog, with localized dense fog possible; however, confidence in exact placement remains low. Otherwise, winds become light and variable to calm overnight.

Tuesday...High confidence.

Becoming VFR and dry across southern New England by 12-15z. Winds become SW 8 to 12 kt.

Tuesday Night...Moderate confidence.

VFR. Marine stratus may develop south of the Islands overnight and drift toward the south coast. Winds remain SW less than 10 kt.

Wednesday and Thursday...Moderate confidence.

Generally VFR, though there will be a daily chance for afternoon SHRA/TSRA. Winds on Wednesday will be WNW at 5 to 10 kt inland and 10 to 15 kt along the coast. Winds shift northerly on Thursday, although eastern Massachusetts may see an ENE to NE wind. Winds across the interior will be 10 to 15 kt, while coastal areas see 8 to 12 kt.

KBOS...Moderate confidence in TAF.

Low confidence regarding fog development tonight, though current thinking favors less restrictive conditions than suggested by LAMP/GLAMP guidance.

KBDL...Moderate confidence in TAF.

Localized fog remains possible overnight, though confidence in occurrence and exact timing remains moderate.

Outlook /Thursday Night through Saturday/...

Wednesday: VFR. Breezy. Isolated SHRA, isolated TSRA.

Wednesday Night: VFR. Isolated SHRA.

Thursday: VFR. Breezy. Isolated SHRA, isolated TSRA.

Thursday Night: VFR.

Friday: VFR. Slight chance SHRA.

Friday Night: Mainly VFR, with areas MVFR possible. Chance SHRA.

Saturday: Mainly VFR, with local IFR possible. Chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Monday Night through Thursday...High confidence.

Marginal Small Craft Advisory conditions continue tonight across the outer ocean waters, with seas of 4 to 6 ft. These seas gradually subside by daybreak Tuesday, followed by generally tranquil boating conditions through Thursday, with seas mainly 2 to 4 ft. Winds remain light and somewhat variable on Tuesday as high pressure moves overhead. By Wednesday, winds become more southwesterly around 15 kt, with occasional gusts up to 20 kt possible over the nearshore waters south of the coast. While conditions appear marginal, there is a low chance of localized stronger gusts that could warrant a brief Small Craft Advisory. Given the start of the boating season, this potential bears monitoring despite the marginal setup. Heading into Thursday, winds shift more ENE to NE. Both Wednesday and Thursday afternoons feature the chance for isolated pop-up showers or thunderstorms, though a washout is not expected.

Outlook /Thursday Night through Saturday/...

Wednesday: Winds less than 25 kt. Seas locally approaching 5 ft. Isolated rain showers, isolated thunderstorms.

Wednesday Night through Thursday: Winds less than 25 kt. Isolated rain showers, isolated thunderstorms.

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

Friday Night: Winds less than 25 kt. Chance of rain showers.

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

BOX WATCHES/WARNINGS/ADVISORIES

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