textproduct: Boston / Norton

WHAT HAS CHANGED

A Flood Watch has been expanded into Rhode Island and parts of southeast Mass.

KEY MESSAGES

- Confidence still increasing in the potential for bands of heavy rainfall and even some flash flooding starting late tonight but more likely Monday into Tuesday. The greatest risk area right now remains in northern CT where recent heavy rain has fallen and a Flood Watch remains in effect and has been expanded.

- Summer warmth & humidity return for the latter half of the week. A few showers and thunderstorms possible by Friday to Saturday.

DISCUSSION

KEY MESSAGE 1...Confidence still increasing in the potential for bands of heavy rainfall and even some flash flooding starting late tonight but more likely Monday into Tuesday. The greatest risk area right now remains in northern CT where recent heavy rain has fallen and a Flood Watch remains in effect and has been expanded.

To start with the highest confidence, we look at antecedent conditions. Heavy rainfall yesterday impacted mainly northern CT, especially southern Hartford County. Areas where soil moisture is approaching 50 percent is roughly northern CT into the southern half of RI and into the EWB area. Similarly 6 hr FFG is lowest across southern Hartford, Tolland, and Windham Counties, with values generally at or below 3 inches. Farther north and east that increases to around 4 to 5 inches.

Also higher confidence is the timing of precip. There is a chance that heavy rain could begin this evening, but as of this writing, CAM guidance wants to focus that more towards Long Island Sound than northern CT. Mon into Tue looks more favorable for rounds of heavy rain/showers to push into northern CT and parts of RI.

More moderate confidence will be the location and amounts of heavy rain. One thing I can say for sure is that all available guidance, from global to convection allowing mesoscale weather models have some impressive QPF numbers. Many include max QPF values of 7 inches or more. One such piece of guidance is the 05.12z HREF. 24 hr max QPF is in the 7-10 inch range. From past experience and local research, the HREF max QPF usually does occur even if the blend struggles to pinpoint the location. So I think it is reasonable to prepare for rainfall amounts on that order. Best guess at this range would be in the BDL to PVD corridor somewhere, but anywhere south of the Mass Pike would be wise to keep an eye on trends. With that in mind I feel more comfortable expanded the Flood Watch east into RI and the flash flooding hotspot of Fall River to cover those higher soil moisture areas, and where synoptic forcing do not look meaningfully different from CT.

KEY MESSAGE 2...Summer warmth & humidity return for the latter half of the week. A few showers and thunderstorms possible by Friday to Saturday.

There isn't a whole lot more to say about this period. Temps will continue to climb until by Fri a few locations may be flirting with 90 degrees again. A cold front will approach from the northwest and provide a focus for showers and thunderstorms. At this time the best chances for rain will be northwest of the forecast area Thu, but by Fri chances will be much better to see at least scattered convection across southern New England.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

Forecaster Confidence Levels...

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

18Z TAF Update...

Today through Monday...Moderate confidence.

Mainly VFR this afternoon but some lower MVFR conditions possible at times today in a few showers across western MA/CT and near the south coast. MVFR conditions become more widespread across western MA/CT later tonight and Mon along with periods of showers with locally heavy rainfall.

Light ESE winds today and tonight becoming E at 6 to 13 knots on Mon.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Monday Night through Friday/...

Tuesday: MVFR/IFR conditions possible. Breezy. SHRA likely, slight chance TSRA, patchy BR.

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

Wednesday through Wednesday Night: VFR.

Thursday: VFR. Breezy.

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

Friday: VFR. Breezy. Chance SHRA, chance TSRA.

MARINE

Forecaster Confidence Levels...

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

Today and tonight...High confidence.

High pressure over eastern Canada combined with a wave of low pressure passing to our south will result in ESE winds this afternoon and tonight...But the winds/seas should remain below small craft advisory thresholds.

Monday...Moderate confidence.

Wave of low pressure passing to our south should result in E wind gusts increasing to around 25+ knots across our southern waters. Later shifts will likely need to issue small craft advisories for our southern waters and we may need to expand them further north Monday night.

Outlook /Monday Night through Friday/...

Tuesday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Seas up to 5 ft. Rain showers likely, slight chance of thunderstorms, patchy fog. Local visibility 1 to 3 nm.

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

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

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

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

Friday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers, slight chance of thunderstorms.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Flood Watch from 2 AM EDT Monday through Tuesday afternoon for CTZ002>004. MA...Flood Watch from 2 AM EDT Monday through Tuesday afternoon for MAZ020. RI...Flood Watch from 2 AM EDT Monday through Tuesday afternoon for RIZ001>008. MARINE...None.