textproduct: Boston / Norton

WHAT HAS CHANGED

Expanded the Flood Watch to include Windham, Southern Worcester, Suffolk, and Suffolk Counties. Monitoring a very low chance for waterspouts and a weak tornado across the south coast. The risk for any severe thunderstorms is dwarfed by the hydrologic risk, however.

KEY MESSAGES

- Confidence continues for a period of torrential and a risk of flooding across much of Rhode Island and southeast Massachusetts.

- High pressure in control next week brings generally quiet weather. Brief bout of summer warmth and humidity expected Wed-Fri.

DISCUSSION

KEY MESSAGE 1...Confidence continues for a period of torrential and a risk of flooding across much of Rhode Island and southeast Massachusetts.

Main forecast focus for the next 48 hours continues to be on a shortwave and warm front later tonight through much of the day on Sunday. Aloft, the area will find itself in the right entrance region of a 80-90 kt jet by 12z Sunday morning. So, synoptically, there are plenty of jet and frontal dynamics at play to allow for a heavy rain event. The thermodynamic environment certainly isn't lacking either, with model soundings showing PWATs climbing to >2" which is near the 99th percentile of climatology for September 13th. These highly anomalous PWATs pair with a nearly saturated atmospheric column and warm cloud heights of 12-14kft and allow for efficient rainfall processes and areas of torrential rain. Taking these conditions into account it makes sense that the 12z HREF shows a 40-55% chance for 3 hour rainfall amounts to exceed flash flood guidance covering an area from eastern Connecticut to Rhode Island and SE Massachusetts. Additionally, there is still a strong (55-70%) chance for 6-24 HR precipitation values to exceed the 5 year ARI and a moderate (25-40%) chance of exceeding the 10 year ARI. The greatest concern for areas of flash flooding is across the urban corridor from Providence to interior SE MA. Still some uncertainty with regards to the exact placement of the heaviest rain it will be contingent upon the placement of a mesoscale low. Subtle differences in deterministic guidance have this band as far south as the south coast and as far north as northern Massachusetts. Think the HREF and RRFS handle these differences best by centering the highest rainfall totals along the I-95 corridor.

Given the uncertainty and the N jog in guidance, opted to expand the Flood Watch to include Windham county in eastern Connecticut, southern Worcester County, Norfolk county, and Suffolk County including Boston. Elsewhere, a widespread 0.50" to 1.50" expected across much of our region Within the watch there will likely be localized narrow bands of 2"-4" of rain possible inside 3 hours. The narrow but intense area of rainfall likely comes to an end from west to east between 2 and 5 pm.

On a final note, while the main threat is areas of flash flooding, there is a low albeit non-zero chance for isolated severe weather across coastal RI and MA. CAMs show a narrow area of MLCAPE approaching 300-500 J/kg across coastal areas. Paired with 0-1km shear of ~20kts and curved hodographs there is a very low chance for a spin up or a waterspout as the mesolow passes late tomorrow morning into the afternoon. What's lacking is adequate instability or lapse rates for deeper convection but worth keeping an eye on.

KEY MESSAGE 1...High pressure in control next week brings generally quiet weather. Brief bout of summer warmth and humidity expected Wed- Fri.

The start of the new work week starts with some lingering showers as the cold front makes its way offshore. Rain comes to an end after sunrise for western MA/CT while showers linger into the early-mid afternoon across eastern MA/RI. Eventually dry NW post frontal flow advects 40-50F dewpoints overhead which combined with highs in the low to mid 70s Mon and Tue will make for a comfortable and seasonable first half of the week. This, as high pressure is in control at the surface through much of the week keeping us dry. As the high shifts offshore Wednesday flow turns southerly bringing the return of more summer-like temperatures and dewpoints in the upper 70s and mid 60s respectively. Uncertainty increases with regard to the forecast for late week next weekend. Zonal mid level flow continues into the weekend with potential for a low to cross somewhere in the vicinity of southern New England; whether that stays to our north or goes overhead is uncertain, but generally looking at a better shot of unsettled weather late Thursday onward.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Forecaster Confidence Levels...

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

18Z TAF Updates: There are no significant changes to the near term forecast, but a rather impactful system brings a period of moderate and heavy rain, low ceilings, and poor visibility for Sunday.

This afternoon and Tonight...High confidence.

VFR. South winds between 6 and 12 kt, however, sea breezes for much of the immediate coast. The sea breeze will give way to southerly wind overnight. Ceilings remain VFR most of the night, though area gradually lowering to low-end VFR between 08-12z.

Sunday...High confidence in trends, moderate confidence in timing.

Widespread showers and potentially a few embedded thunderstorms will overspread the region from west to east roughly 09z-14z Sunday morning. This will result in low-end MVFR to 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 22z/23z. Winds will be light from a S-SE direction. With eastern coastal MA going E. There also may be a period of LLWS.

Sunday Night...High confidence.

Low-end VFR to MVFR ceilings. Wind shift to the NW behind the front. KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Monday through Thursday/...

Monday through Monday Night: VFR. Breezy.

Tuesday through Thursday: 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.

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. There is a very low chance for a water spout or two mainly across the near-shore waters along the S coast.

Outlook /Monday through Thursday/...

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

Monday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft.

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

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

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...Flood Watch from late tonight through Sunday afternoon for CTZ004. MA...Flood Watch from late tonight through Sunday afternoon for MAZ012-013-015>024. RI...Flood Watch from late tonight through Sunday afternoon for RIZ001>008. MARINE...None.