textproduct: Boston / Norton

WHAT HAS CHANGED

Slightly increased rainfall totals for today with the Connecticut River Valley likely seeing the highest totals. Otherwise, cloudy and dry weather Wednesday and an increasing chance for showers and perhaps isolated severe weather Thursday.

KEY MESSAGES

- Widespread rain today, possibly heavy at times. Watching for the potential for localized urban and poor-drainage flooding.

- Cloudy but drier on Wednesday with decreasing shower chances. Precipitation chances return Thursday with an increasing chance for showers and thunderstorms throughout the day.

- 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...Widespread rain today, possibly heavy at times. Watching for the potential for localized urban and poor-drainage flooding.

Rain continues to overspread the region this morning and likely reaches the I-95 corridor by sunrise. Radar shows areas of heavier rainfall along and ahead of a warm front that will bring in a more moisture-rich higher theta-e airmass. Will see enough low level convergence for banded precipitation and areas of locally heavy rainfall through much of the day today. CAMs show that the greatest risk for heavy rains will be found across portions of central Massachusetts and Connecticut. Not entirely surprising given BUFKIT soundings depicting warm cloud depths increasing to 10-12k feet to the SW of the warm front. Notably, the HREF shows 10-30% probs of >1" of QPF in 3 hours in these same areas. Thus, this corridor will have the greatest risk for isolated urban/ poor drainage flooding. Further NE, cooler stratiform rain processes will be more dominant resulting in lighter rainfall rates.

In terms of general rainfall totals, expecting a half an inch to an inch, with locally higher amounts of 1.5 to 3 inches SW of I-95. Further northeast, 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. 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 from W to E this afternoon, with drier conditions overspreading the region overnight. Guidance shows lingering areas of drizzle and light showers but this will be the exception and not the rule. Given extensive cloud cover, expect a much cooler day today, with highs mainly in the upper 60s to lower 70s. A true nod to the first day of meteorological fall.

KEY MESSAGE 2...Cloudy but drier on Wednesday with decreasing shower chances. Precipitation chances return Thursday with an increasing chance for showers and thunderstorms throughout the day.

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.

Greater risk for more widespread shower and thunderstorm activity Thursday as a short wave and surface cold front arrive from the west. Guidance is coming into better agreement with respect to increasing instability Thursday afternoon. The latest NBM shows widespread 60 to 80% probs of CAPE exceeding 1000 J/kg mainly along and S of the Mass Pike Thursday afternoon. These values are higher than previous runs so it is likely that guidance has trended north with the extent of the warm sector over our CWA. Latest NSSL and CSU machine learning probabilistic guidance support these trends and highlight southern New England in a 15 percent chance for severe weather Thursday. 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 /12Z TUESDAY THROUGH SATURDAY/

Forecaster Confidence Levels...

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

12z TAF Update...

Today...Moderate confidence.

Widespread showers with locally heavy rainfall were moving into the region from the west early this morning. This will result in conditions lowering to MVFR levels from northwest to southeast through the morning. Pockets of heavy rain will result in periods of IFR conditions as well. A few embedded thunderstorms are possible too...especially across interior MA and CT. Winds becoming E at 5-13 knots.

Tonight and Wednesday...Moderate confidence.

Widespread MVFR with pockets of IFR conditions will dominate tonight. The bulk of the steady showers will have come to an end by early this evening...but spotty light showers and pockets of drizzle may persist. We do expect conditions to improve Wednesday morning with mainly VFR dominating by Wednesday afternoon. Most of the daylight hours on Wednesday will end up dry. Winds will continue with an E component...generally at 5-13 knots.

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

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

Outlook /Wednesday Night through Saturday/...

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

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

Thursday Night through Friday Night: VFR.

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 Tonight...High confidence.

Rain and areas of poor visibility overspread the waters this morning ahead of an approaching warm front. Expecting SW winds to shift to the SE as the front passes through around sunrise this morning 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 today with marginal Small Craft conditions developing for the outer waters overnight into Wednesday. Later forecasts may warrant a Small Craft Advisory if trends continue.

Outlook /Wednesday Night through Saturday/...

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

Thursday: Winds less than 25 kt. Seas up to 5 ft. Rain showers likely, slight chance of thunderstorms.

Thursday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers, slight chance of thunderstorms.

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

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

BOX WATCHES/WARNINGS/ADVISORIES

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