textproduct: Boston / Norton

WHAT HAS CHANGED

No changes to the forecast at this time.

KEY MESSAGES

- Showers expected to continue today with some embedded thunder and heavier downpours possible.

- Drier through midweek. Turning mostly cloudy, humid and unsettled by late this week into the weekend. Though it won't be raining all the time, chances for showers/storms will exist on most days Fri- Sat, but severe weather potential looks very limited.

DISCUSSION

KEY MESSAGE 1...Showers expected to continue today with some embedded thunder and heavier downpours possible.

Showers will continue to impact southern New England today as another wave tracks over the region. Latest mesoanalysis has an area of increased moisture transport at 925 mb from the S/SW, so showers moving into this area over RI and SE MA may intensify and produce higher rainfall rates, especially as any pockets of MUCAPE are encountered. Based on the latest guidance, a more south track for the next wave is favored with heavier, more widespread rain remaining mostly to the south of southern New England. The exception would be along the south coast and into the Cape and Islands where heavier showers remain possible. PWATs over this area heading into the afternoon hours are also forecast around 2.0-2.10". High-res guidance also has some MUCAPE over much of SE MA into the Cape and Islands through the afternoon, which would help produce periods of heavier downpours and some embedded thunder within any showers that develop. The RAP also has a burst of some elevated MUCAPE values over Cape Cod early this morning that could favor thunder there. With the showers we've seen overnight producing occasional lightning and heavier downpours in CT and now heading into RI, this is not out of the question. With regards to totals, thinking has not changed much as totals in the 1-2" range remain the main expectation with this system. Flash flooding is not a widespread concern considering the drought conditions we've been experiencing, but flashier areas including urban and poor drainage areas may see some flooding under any heavier rainfall rates. With persistent cloud cover today, highs will likely not climb higher than the 70s.

Rain should clear out of the region heading into tonight as the wave moves offshore, though showers may linger early tonight over the Cape and Islands. Winds shift NW and drier air makes its way into the region in the wake of the passing cold front, which will also aid in clearing remaining cloud cover heading into Wednesday. Lows tonight are expected to be in the upper 50s and low 60s.

KEY MESSAGE 2...KEY MESSAGE 2...Drier through midweek. Turning mostly cloudy, humid and unsettled by late this week into the weekend. Though it won't be raining all the time, chances for showers/storms will exist on most days Fri-Sat, but severe weather potential looks very limited.

After the cold front pulls away later tonight, we get into a drier weather pattern Wed onward thru the daytime hrs Thurs.

Our weather pattern then turns more unsettled for late in the week into at least the early part of the weekend. Broadly-cyclonic, quasi- zonal pattern with enhanced westerly 500 mb flow stretches from the Gt Lakes region to the Northeast states in this period. Salient large-scale features in this period by early Thurs night include one mid-level shortwave trough over the Gt Lakes region, another weaker 500 mb disturbance near the central Plains, with an elongated, quasi- stationary frontal boundary draping itself from the OH/mid-MS Valley eastward to the mid-Atlantic/southern New England vicinity. Zonal regimes like this one are typically prone to timing errors when it comes to precip/thunder chances and are also sensitive to daily convective details which are unknown at this time horizon. Adding to that uncertainty is there are differences in the strength of the initial/lead Gt Lakes shortwave, which influence whether or not better rain chances move in by Thurs night. GFS guidance is weaker/broader and thus more progressive with rain chances into New England by Thursday night, while the international guidance feature a stronger trough and is slower to move PoP in until Fri.

I would rate confidence in the timing of rain/thunder chances later this week into the weekend as lower than average, given the above sources of uncertainty. Forecast carries a rather non-descript Chance PoP for most of this timeframe; it won't be raining/thundering the whole time, but ID'ing those periods is subject to the timing details become more in-focus. That said, an early estimate for rain/thunder chances is more centered into Fri aftn/Fri evening with the lead shortwave, then increasing again into at least the south coast to areas south of the Mass Pike Sat/Sat night as the central Plains shortwave moves through or just south of Southern New England. Although mid-level flow is seasonably strong, much of this period is mostly cloudy and rather humid, but also one with limited diurnal heating leading to weak instability. Indeed, NSSL machine-learning severe weather progs offer tempered severe weather probabilities on Fri. Wouldn't rule out a strong storm or two on Fri, but early lean is that the prospects of storms becoming severe seem limited.

High pressure and dry weather return for the weekend.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

Forecaster Confidence Levels...

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

12Z Update

Moderate confidence at best.

Widespread IFR conditions currently, with scattered showers and an isolated TSRA currently pushing east across the Cape. With a weak frontal boundary currently extending from NE Mass to SW Connecticut drifting slowly south, this will keep the threat for scattered showers and general IFR conditions going most of the day. Various model guidance although they show that general scenario for the day, differ on timing and location of the showers. So went a little broadbrush with prevailing -SHRA at many TAF sites with TEMPOs for when there might be a potential for a little heavier showers. Any risk for TSRA from late morning into early evening will be generally south of the region, but can't rule it out across ACK.

Expect any remaining -SHRA to have moved away from the region by 02z. Given the slow moving boundary and no real push of drier air into the region, IFR/MVFR conditions will likely persist for most of the night, but confidence is low on timing of seeing improvement to VFR. However, once VFR sets in, it will remain in place for all of Wednesday.

KBOS Terminal...Moderate Confidence. Expect a wind shift to NW in the 14-15z timeframe, and then veering to NE by mid-afternoon although winds will stay less than 10kt the entire day. Scattered -SHRA expected at times through the day, but timing is difficult to pinpoint.

KBDL Terminal...Moderate confidence. Scattered -SHRA expected at times through the day, but timing is difficult to pinpoint.

Outlook /Wednesday Night through Saturday/...

Wednesday Night through Thursday: VFR.

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

Friday: Mainly VFR, with areas MVFR possible. Breezy. SHRA likely, slight chance TSRA.

Friday Night: Mainly VFR, with local IFR possible. Chance SHRA.

Saturday: VFR. Chance SHRA, slight chance TSRA.

MARINE

Forecaster Confidence Levels...

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

Through Wednesday...High confidence.

Rain and a few embedded storms continue before finally moving off to the east this evening. Moderate to heavy rainfall at times expected, resulting in poor visibility less than 1 mile at times. Seas 2-4 ft with SW winds at 10-15 kt shifting NW in the wake of a passing cold front this evening into tonight. Winds then shift more SW for Wednesday, remaining around 10 kt.

Outlook /Wednesday Night through Saturday/...

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

Thursday through Thursday Night: Winds less than 25 kt.

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

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

BOX WATCHES/WARNINGS/ADVISORIES

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