textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes.

KEY MESSAGES

- A cool and wet start to meteorological fall. Some rain could be heavy, leading to poor drainage flooding.

- Unsettled weather late Wed into Thursday, with several rounds of showers and thunderstorms. Low confidence on exact timing.

- Improving weather Friday and into the holiday weekend. Other than a spot diurnal shower each day, dry weather prevails with a mix of clouds and sun. Cooler than normal with highs in the 70s and lows in the 50s, along with onshore breezes.

DISCUSSION

KEY MESSAGE 1...A cool and wet start to meteorological fall. Some rain could be heavy, leading to poor drainage flooding.

The backdoor cold front that pushed southwest of New England this morning starts to shift back to the northeast today, with increasing cloud cover this afternoon. As advertised the past several days, there are multiple rounds of beneficial rainfall this week, with the first round coming late tonight through Tuesday. With the frontal boundary shifting northeast, PWATs increase to between 1.8-2.0 inches, interacting with a weak wave of low pressure riding along the boundary.

Timing-wise, rain aims to arrive around midnight west of the CT River Valley and around the time of the Tuesday morning commute for the I-95 corridor. Looking at BUFKIT, do see warm rain processes with the warm cloud layer around 10k feet. Locally heavy downpours, especially during the daytime Tuesday, are possible, which may lead to localized street and poor drainage flooding. This coincides with the Day 1-2 ERO Marginal Risk (level 1 out of 4) for excessive rainfall.

Do think there will be a thin axis of heaviest rainfall across the southwest section of our CWA. Rainfall amounts are in the neighborhood of half an inch to an inch, with lesser amounts east of I-95, where totals are in the range of a quarter to half an inch. Since these are showers versus stratiform rain, there could be areas of higher and lower amounts within a relatively short distance. However, a subtle shift northeast could put some of the heaviest rainfall across parts of central Massachusetts and eastern Connecticut. Additionally, there's a lower chance for thunder, mainly Tuesday afternoon, with a few hundred J/kg of CAPE across mainly western Massachusetts and Connecticut, although it's not a significant risk.

Rain tapers off late Tuesday, with drier conditions overspreading the region Tuesday night into early Thursday morning. Given the clouds, expect a much cooler start to Tuesday, with highs mainly in the upper 60s to lower 70s. A true nod to the first day of meteorological fall.

KEY MESSAGE 2...Unsettled weather late Wed into Thursday, with several rounds of showers and thunderstorms. Low confidence on exact timing.

1025 mb high pressure over the Maritimes should provide dry weather much of Wed, especially across eastern MA, while a frontal boundary lurks to the west and southwest of SNE. This front will eventually move northeast as a warm front later Wed and especially Wed night, resulting in an increased risk for showers and thunderstorms across SNE late Wed/Wed night. Cooler temps Wed, with 1025 Maritime high and ESE onshore flow, limiting max temps in the upper 60s and lower 70s.

Trailing/secondary short wave and associated cold front impact the region around Thu. Potential for thunderstorms to become strong to severe during this time, as warm sector airmass overspreads SNE. However, better instability/higher theta-e airmass appears to remain southwest of SNE. Thus, potential for severe weather and flash flooding here looks limited at this time range. Heat and humidity return Thu with warm sector airmass, highs in the lower 80s and dew pts in the upper 60s and lower 70s.

KEY MESSAGE 3...Improving weather Friday and into the holiday weekend. Other than a spot diurnal shower each day, dry weather prevails with a mix of clouds and sun. Cooler than normal with highs in the 70s and lows in the 50s, along with onshore breezes.

Definitely a sign of fall approaching, with robust northern stream Friday yielding a baroclinic 998 mb maritime low late Fri/Sat. This will yield a cool, brisk NE flow into the SNE over the weekend. Highs likely remaining in the 70s and overnight lows in the 50s. It will feel even cooler over southeast MA, including Cape Cod and the Islands, with northeast winds 15-25 mph. Cool cyclonic flow aloft will yield diurnal clouds each day and possibly a spot shower. Otherwise, dry weather prevails. Possibly warmer and less wind for Memorial Day Monday, as high amplitude trough moves offshore and 1025 mb high settles over SNE.

AVIATION /18Z MONDAY THROUGH SATURDAY/

Forecaster Confidence Levels...

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

18Z TAF Update:

Through 00Z...High confidence.

VFR and dry. However, mid-level clouds are increasing from west to east a slow-moving front lifts east-northeast. Coastal sea breezes continue today at around 10 kt, while inland winds remain southerly at less than 6 kt.

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

Ceilings lower to low-end MVFR, if not IFR, as SHRA arrives from west to east between 06Z and 12Z. Conditions eventually deteriorate to LIFR across interior terminals by 08-10Z, with visibilities lowering to 2-5 SM. There is a low chance of lightning, with the greatest probability across western terminals in southern New England, particularly KBDL. Winds shift from south to southeast overnight, remaining light across interior southern New England and increasing to 8-10 kt at coastal terminals.

Tuesday...Moderate confidence.

Pockets of LIFR to start; otherwise, a mixture of IFR and MVFR ceilings persists, with periodic showers and isolated TSRA possible. East-southeast winds of 10-15 kt are expected at coastal terminals and 6-10 kt at interior terminals.

Tuesday Night...Moderate confidence.

MVFR ceilings generally prevail along and east of I-95, while areas farther inland remain IFR to LIFR. Onshore easterly winds and chances for SHRA continue.

KBOS Terminal...High confidence in trends, moderate confidence in timing.

KBDL Terminal...High confidence in trends, moderate confidence in timing.

Outlook /Tuesday Night through Saturday/...

Wednesday: Mainly VFR, with local IFR possible. Chance SHRA, slight chance TSRA.

Wednesday Night: MVFR/IFR conditions possible. SHRA, chance TSRA.

Thursday: Mainly VFR, with local IFR possible. Chance SHRA, chance TSRA.

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

Friday: VFR. Slight chance SHRA.

Friday Night through Saturday: VFR. Breezy. Slight chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Through Tuesday Night...High confidence.

High pressure situated over the Gulf of Maine shifts northeast toward the Canadian Maritimes tonight as a semi-stationary front lifts northeast. A series of low pressure systems will ride along this boundary, beginning tonight and bringing unsettled weather. Rain develops late tonight into Tuesday, potentially leading to poor visibility. Southerly winds overnight become east-southeast Tuesday and easterly Tuesday night, with speeds of 10 to 15 kt and gusts around 20 kt. Seas remain below 2 ft tonight, then slowly build to 2 to 3 ft Tuesday and possibly 3 to 5 ft by late Tuesday night or early Wednesday morning. Building seas may warrant a Small Craft Advisory for the outer waters, but no headlines have been issued at this time.

Outlook /Tuesday Night through Saturday/...

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

Wednesday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Rain showers likely, chance of thunderstorms.

Thursday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers, chance of thunderstorms.

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

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

Friday Night: Winds less than 25 kt.

Saturday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...None.