textproduct: Boston / Norton

WHAT HAS CHANGED

A Flood Watch has been issued for Rhode Island and southeast Massachusetts for Sunday.

KEY MESSAGES

- Showers, and possibly a few thunderstorms, with pockets of torrential rainfall possible early Sun am into the afternoon. A Flood Watch has been issued for RI and southeast MA.

- Any lingering scattered showers depart early Mon...otherwise dry/seasonable Mon-Tue but a brief bout of summer warmth and humidity possible Wed and/or Thu ahead of a cold front.

DISCUSSION

KEY MESSAGE 1...Showers, and possibly a few thunderstorms, with pockets of torrential rainfall possible early Sun am into the afternoon. A Flood Watch has been issued for RI and southeast MA.

One more beautiful day is on tap with plenty of sunshine and high temps this afternoon in the 70s.

Otherwise...the main forecast concern will be an approaching shortwave and associated warm from very late tonight into Sunday. As this shortwave approaches...it will result in a very strong instability burst. Showalter indices drop near 20 units to near or below zero inside 12 hours. That rapid change in stability is often a good sign for a complex of widespread showers and the potential for a few embedded thunderstorms. In addition...Pwats climb to around 2.25 inches across RI/SE MA which is quite high for this part of the country and supportive of pockets of torrential rainfall. The GEFS situational awareness tables indicate Pwat values outside the models climatology near the south coast Sunday morning.

So in a nutshell...the ingredients above are concerning for a narrow band of torrential rainfall and the potential for localized flash flooding. The HREF has probabilities for 6-24 HR precipitation values to exceed 10 year ARI across parts of eastern CT/RI and southeast MA. There are even very low probabilities to exceed 100 year ARI values near the south coast. That being said...whether or not this actually materializes remains uncertain and will depend on the mesoscale. And on top of that...even if it occurs trying to determine the specific location is still beyond the state of the science. But we do know this risk will hinge on the potential for where a mesoscale low tracks...which would setup an area of enhanced low level convergence for the southerly LLJ to impinge upon.

Given the above...we have opted to issue a Flood Watch very late tonight into Sunday for RI and southeast MA. Whether or not this narrow band of torrential rain falls across this region or remains to our south is uncertain...but given the potential felt it is just a Watch and worth highlighting the potential. Widespread rainfall of 0.50" to 1.50" expected across much of our region...But localized narrow bands of 2"-4" of rain possible inside 3 hours. If this occurs...the potential for localized flash flooding would increase especially if it occurs over a vulnerable urban center.

KEY MESSAGE 2...Any lingering scattered showers depart early Mon...otherwise dry/seasonable Mon-Tue but a brief bout of summer warmth and humidity possible Wed and/or Thu ahead of a cold front.

The actual cold front crosses the region later Sunday night into early Monday morning. This may be accompanied by a few brief showers. Otherwise...high pressure builds back into the region Mon and Tue bringing dry and seasonable weather. High temps will mainly be in the upper 60s to the middle 70s with lows in the 40s and 50s both days. This high pressure system moves east of the region by the middle of next week. This may allow for a brief return of summerlike warmth/humidity sometime Wed and/or Thu.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

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

12z TAF Update...

Today...High confidence.

VFR. Winds becoming S at 6-12 knots except for sea breezes developing along much of the immediate coast.

Tonight and Sunday...High confidence tonight with moderate confidence Sunday.

Mainly VFR conditions tonight. A large cluster of showers and potentially a few embedded thunderstorms will overspread the region from west to east roughly between 9z-15z Sunday. This will result in low end MVFR-IFR conditions developing with even brief LIFR conditions in narrow bands of torrential rainfall. The bulk of the heaviest showers should be exiting the southeast New England coast by 20z/21z. Winds will be light from a S-SE direction. There also may be a period of LLWS.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Sunday Night through Wednesday/...

Sunday Night: Mainly VFR, with local IFR possible. Breezy. Slight chance SHRA, patchy BR.

Monday through Monday Night: VFR. Breezy.

Tuesday through Wednesday: VFR.

MARINE

Forecaster Confidence Levels...

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

Today through Sunday...High confidence.

High pressure over the waters today shifts to the east tonight. A warm front than approaches and crosses the waters on Sunday. Surface gradient should remain weak enough to keep winds/seas below small craft advisory thresholds through Sunday. However...the main concern for mariners will be heavy showers and potentially a few embedded thunderstorms on Sunday with reduced visibility.

Outlook /Sunday Night through Wednesday/...

Sunday Night: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers, patchy fog. Local visibility 1 to 3 nm.

Monday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers.

Monday Night through Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft.

Tuesday Night through Wednesday: Winds less than 25 kt. Seas locally approaching 5 ft.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...Flood Watch from late tonight through Sunday afternoon for MAZ017>024. RI...Flood Watch from late tonight through Sunday afternoon for RIZ001>008. MARINE...None.