textproduct: Boston / Norton

WHAT HAS CHANGED

No changes to the forecast at this time.

KEY MESSAGES

- Periods of heavy rain possible later today into early Tuesday.

- Drier air with seasonable temperatures expected midweek. Chances for rain return for the end of this week.

DISCUSSION

KEY MESSAGE 1...Periods of heavy rain possible later today into early Tuesday.

Have high confidence in a widespread, soaking rain across our region later today into Tuesday morning. Even at this time range, there remain different ideas on where the bands of heaviest rainfall will be. Currently have the greatest confidence across northern CT and western and central MA, where terrain will assist the dynamic lifting processes.

Thinking total rainfall of 1.0-1.5 inches is the most likely outcome. The probability for 2+ inches of rainfall remains about 30 to 40 percent. Not expecting flooding, as our region remains abnormally dry. Flash flood guidance is 2-3 inches in an hour, which we should not achieve. Overall, this is looking like a beneficial rainfall for southern New England.

We should dry out from west to east as a low pressure along the south coast of New England moves offshore Tuesday.

KEY MESSAGE 2...Drier air with seasonable temperatures expected midweek. Chances for rain return for the end of this week.

Surface high pressure will keep the weather dry through the middle of this week, after the rain comes to an end on Tuesday.

A low pressure system is expected to move into the Great Lakes by Thursday, then track into southeastern Canada by the end of this week. In addition, a weak shortwave trough should pass across the northeast. This combination will bring the next rainfall chances, beginning some time Thursday night into Friday morning. Isolated to scattered thunderstorms are possible during this time as well, especially Friday afternoon, with the highest chances across western MA.

The GEFS has low, but non-zero, probability for severe storms Friday, as well as some of the other ensemble and machine learning guidance. However, this probability varies significantly amongst the guidance still. Given this spread, and how far out in the forecast it is, confidence on overall severe potential remains low.

AVIATION /12Z MONDAY THROUGH FRIDAY/

Forecaster Confidence Levels...

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

12Z Update

Today: High confidence overall, though trending moderate for this afternoon due to timing of rainfall.

VFR most of the day, though clouds thicken up and lower through the day. Sub-VFR ceilings and arrival of light -SHRA unlikely to be sooner than 20z across western New England, and not until 22-00z at earliest for central/eastern TAFs. Any -SHRA during this period should be light. W winds under 10 kt, then shift to S/SE then E late under 10 kt.

Tonight: Moderate confidence.

Conditions deteriorate to widespread MVFR-IFR. Steadier rainfall arrives. But the timing and location of the eventual moderate to heavy rainfall remains unclear. The earliest reasonable start time is 01-03z across western New England, and after 03-05z for the eastern TAFs. TSRA also possible.

Tuesday...High confidence in trends. Moderate confidence in timing.

IFR early, improving to VFR from W to E during the afternoon as rainfall ends. MVFR may linger across the Cape and islands through late afternoon.

KBOS Terminal...High confidence in TAF trends. Moderate confidence in timing. KBDL Terminal...High confidence in TAF trends. Moderate confidence in timing.

Outlook /Tuesday Night through Friday/...

Tuesday Night through Thursday: VFR.

Thursday Night: VFR. Chance SHRA, slight chance TSRA.

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

MARINE

Forecaster Confidence Levels...

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

Through Tuesday...High confidence.

A Small Craft Advisory continues for the outer coastal waters south of Block Island and east to Nantucket for this afternoon into Tuesday morning. Potential for gusts to 25 kt across some of the waters across the Cape and islands, as well as the eastern outer coastal waters. However, confidence is not high enough yet that it will persist long enough to warrant expanding the Advisory. This will need to be monitored with later forecasts.

Rain arrives across the waters tonight, before finally moving off to the east some time Tuesday afternoon into Tuesday evening. Moderate to heavy rainfall at times expected, resulting in poor visibility less than 1 mile at times.

Outlook /Tuesday Night through Friday/...

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

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

Wednesday Night through Thursday: Winds less than 25 kt.

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

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 4 PM this afternoon to 8 AM EDT Tuesday for ANZ255-256.