textproduct: Boston / Norton

WHAT HAS CHANGED

The Flood Watch has been expanded across the rest of southeast Massachusetts into Tuesday.

KEY MESSAGES

- Periods of heavy rain & possibly a few embedded t-storms today into Tue across parts of CT/RI/SE MA. Localized flash flooding possible dependent on if the heaviest bands setup in our region. Otherwise...rain overspreads the rest of the region through tonight but amounts will be lighter especially near the NH border.

- Summer warmth & humidity return for the latter half of the week. Mainly dry weather returns Wed into Thu...But the risk for a few showers & t-storms may return by Fri perhaps lingering into Sat.

DISCUSSION

KEY MESSAGE 1...Periods of heavy rain & possibly a few embedded t-storms today into Tue across parts of CT/RI/SE MA. Localized flash flooding possible dependent on if the heaviest bands setup in our region. Otherwise...rain overspreads the rest of the region through tonight but amounts will be lighter especially near the NH border.

The main forecast concern is the potential for localized flash flooding across parts of CT/RI/SE MA into Tue. This remains a very difficult forecast as the narrow bands of potentially extremely heavy rain are quite difficult to forecast in terms of timing and locations. However...the environment in place does favor the potential for them to form. We will discuss this in more detail below.

The environment in place is favorable for the potential of localized flash flooding. Weak high pressure across eastern Canada was enforcing a shallow boundary in the vicinity of CT/NY/NJ. This boundary was being impinged upon by a modest southeast LLJ coupled with a Pwat plume of 1.5 to 2.0+ inches. The idea though is that you are combining winter-like synoptic features with summertime moisture parameters. That combination can lead to localized extreme rainfall events. This potential is indicated by various model suites printing out narrow bands of very high QPF. The issue is that these narrow bands of potentially extreme QPF are depicted in a lot of different locations depending on what model you are looking at. This is quite common in these cases that the models are good at indicating the potential for a localized torrential rain events...But struggle to pin the location.

Given the above...we have expanded the Flood Watch from CT/RI across the rest of southeast MA. We are looking at widespread rainfall amounts of 1-3"...but localized 4-6" amounts would be possible within 6 hours if any narrow bands setup. Not all the models show these narrow bands developing...but many of the CAMs do and it is reasonable given the parameters in place. If they do develop...some CAMs focus it to our south while other bring it into our region. Regardless...the Flood Watch is certainly warranted give the potential if these bands are realized.

Across the rest of the region...rain amounts will be lighter especially towards the NH border. In fact...it may take until later tonight for the steady rain to finally reach northeast MA. Periods of rain will persist into Tuesday especially across eastern MA/RI as closed 850 mb low may setup a mid level deformation zone.

KEY MESSAGE 2...Summer warmth & humidity return for the latter half of the week. Mainly dry weather returns Wed into Thu...But the risk for a few showers & t-storms may return by Fri perhaps lingering into Sat.

As the shortwave departs the region...a mid level ridge axis builds over the region with mainly dry weather returning Wed into at least a good part of Thu. Rising height fields should allow summer-like warmth and humidity to return. Highs may take a run at 90 in some spots by Thu. The next shortwave trough may bring the risk for a few showers and thunderstorms returning by Fri which may linger into Sat. This will depend on the timing of the shortwave/cold front.

AVIATION /12Z MONDAY THROUGH FRIDAY/

Forecaster Confidence Levels...

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

12Z TAF Update...

Today through Tuesday...Moderate confidence.

VFR conditions early this morning will gradually deteriorate to MVFR levels today into tonight from southwest to northeast with brief bouts of IFR conditions possible too. This will be associated with periods of rain...which may fall heavy at times across parts of CT/RI/SE MA where there may even be an embedded t-storm or two. The process of lower cigs/vsbys and steady rain will be slowest to arrive across northeast MA...where it may take until this evening or even later for MVFR conditions to develop. Periods of rain will continue into Tue especially across eastern MA/RI. MVFR with localized IFR conditions will also continue into Tue. E winds 6-12 knots today gradually becoming NE at 8-16 knots by Tue.

KBOS Terminal...Moderate confidence in TAF.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Tuesday Night through Friday/...

Tuesday Night: Mainly VFR, with areas IFR possible. Breezy. Slight chance SHRA.

Wednesday: VFR. Breezy.

Wednesday Night: VFR.

Thursday through Thursday Night: VFR. Breezy.

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 through Tuesday...High confidence.

Weak high pressure over eastern Canada will combined with low pressure tracking south of the waters. The result will be E wind gusts of 20 to 30 knots developing from south to north today into tonight and continuing into Tue shifting to more of a NE direction. Therefore...have small craft headlines for the open waters and seas building to between 3 and 5 feet.

Outlook /Tuesday Night through Friday/...

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

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

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

Thursday: Winds less than 25 kt. Seas locally approaching 5 ft.

Thursday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Slight chance of rain showers.

Friday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers, slight chance of thunderstorms.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Flood Watch through Tuesday afternoon for CTZ002>004. MA...Flood Watch through Tuesday afternoon for MAZ017>022. RI...Flood Watch through Tuesday afternoon for RIZ001>008. MARINE...Small Craft Advisory from 2 AM to 8 PM EDT Tuesday for ANZ231>234-250-251-254. Small Craft Advisory from 11 AM this morning to 8 PM EDT Tuesday for ANZ235-237-255-256.