textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes to the forecast overnight.

KEY MESSAGES

- Heavy rainfall arrives late Tuesday, continuing overnight and through the evening on Wednesday. This will lead to potential street and flash flooding throughout that period. The heaviest rain in eastern New England is expected on Wednesday, with street flooding possible but less favored than further inland where a Flood Watch is in effect.

- Periodic rain chances continue especially on Thursday, but may linger into Friday.

- An uncertain weekend forecast but trends point to a building surface high pressure and rising mid level heights which would lead to a drier period than earlier in the week.

DISCUSSION

KEY MESSAGE 1...Heavy rainfall arrives late Tuesday, continuing overnight and through the evening on Wednesday. This will lead to potential street and flash flooding throughout that period. The heaviest rain in eastern New England is expected on Wednesday, with street flooding possible but less favored than further inland where a Flood Watch is in effect.

The focus for this week's forecast continues to be on an anomalously deep mid-level low and the associated occluding surface low that together will bring impressive moisture and dynamics to southern New England over the next 48 hours. The mid level low digs into NY late today, becoming closed off and very sluggish to go anywhere thanks to a persistent blocking pattern. This will continue to plague us through the work week as this low languishes over the northeast, ever so slowly moving east and keeping unsettled weather around through Friday. That being said, by far the best moisture and dynamics line up later tonight through Wednesday evening. We'll have a very moist airmass ahead of the frontal boundary with PWATs up to 1.7". Expecting impressive dynamics as well with strong mid level divergence and upper diffluence along with low level convergence from a 35-45kt LLJ. This will produce deep layer ascent leading to widespread, at times heavy, rainfall.

Expected rainfall totals have not changed significantly since yesterday, generally 2 to 3 inches of rain with locally higher amounts. The most likely location of the higher totals will be in the orographically favored interior of western MA and CT where a Flood Watch remains in effect. Easterly flow into the Berkshires is likely to enhance rainfall totals, while potential for training storms (continually moving over the same location) leads to increasingly likely issues with flooding. Timing for the heaviest rain would begin as early as this afternoon for interior SNE, while eastern MA is likely to stay dry until late Tuesday night/Wednesday morning. Heavy rain will eventually move into these areas, in eastern MA and RI during the day on Wednesday. Some instability exists throughout with 500-1000 J/kg of CAPE which could lead to some embedded thunder and convectively enhanced localized downpours. Currently think flooding concerns are limited to street flooding (urban/poor drainage variety) as no rivers are currently progged to rise to flood stage.

Perpetual cloudcover and easterly flow off the cool ocean will keep temperatures below normal, only reaching the low 70s on Wednesday.

KEY MESSAGE 2...Periodic rain chances continue especially on Thursday, but may linger into Friday.

Previously mentioned vertically stacked low continues to slowly meander east and while the best dynamics are no longer present, PWATs remain elevated. This, together with the mid level PVA/mid level shortwaves rounding the trough will keep scattered showers around Thursday and even into Friday, though more scattered to isolated. Temperatures continue to be depressed by clouds and E/NE flow, in the low to mid 70s.

KEY MESSAGE 3...An uncertain weekend forecast but trends point to a building surface high pressure and rising mid level heights which would lead to a drier period than earlier in the week.

The synoptic pattern begins to finally show signs of breaking down as ridging downstream is displaced and high pressure tries to build over the northeast with some mid level height rises. Guidance is still unsure of how persistent this ridging will be, so confidence is low, but for now things at the very least look less unsettled than the work week. No anomolous warm ups on the horizon.

AVIATION /06Z TUESDAY THROUGH SATURDAY/

Forecaster Confidence Levels...

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

06z TAF Update:

Through 12Z..Moderate confidence.

VFR to start with deteriorating ceilings toward low end VFR/MVFR likely across the interior and eastern terminals with localized IFR/LIFR ceilings possible. Confidence regarding lower ceilings is still somewhat only moderate between 08z and 14z. Light S winds.

Tuesday...Moderate confidence.

IFR/MVFR early for eastern/interior terminals improving to low end VFR along with other terminals. Deteriorates to MVFR/IFR after 18Z at BDL, expanding east through 00z. Increasing chance for rainfall through the day. Low chance for embedded TSRA. SE winds 10-13 kt with gusts to 20 kt in the afternoon.

Tuesday Night...Moderate confidence.

LIFR/IFR for most in periods of heavy rain. SE winds 5-10kt with gusts to 20 kt possible.

Wednesday...Moderate confidence.

IFR/MVFR in continued periods of heavy rain. E winds 10-15 kts with gusts to 25 kts possible.

KBOS Terminal...Moderate confidence.

Only a moderate confidence in extent/thickness of MVFR cloud deck 11-14z.

KBDL Terminal...High confidence.

Outlook /Wednesday Night through Saturday/...

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

Thursday: Mainly VFR, with areas IFR possible. Breezy. SHRA, slight chance TSRA.

Thursday Night: Mainly VFR, with areas IFR possible. Chance SHRA, slight chance TSRA.

Friday: Mainly VFR, with local MVFR possible. Chance SHRA, slight chance TSRA.

Friday Night: Mainly VFR, with local MVFR possible.

Saturday: VFR.

MARINE

Forecaster Confidence Levels...

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

Through Wednesday: High confidence.

Winds and seas to remain below small craft advisory levels through the daytime hours on Tuesday. Southerly winds around 10 kt will be increasing to around 15-20 kt Tuesday, with seas building to 3-4 ft. A few showers are possible on southern waters during the Tuesday daytime hours.

Small craft advisories are in effect Tuesday evening into Wednesday as south to southeast winds increase to around 25 to 30 kt and seas in the 4 to 7 ft range, highest offshore. Showers and embedded thunderstorms can be expected through Wednesday night. Winds and seas diminish Wednesday night.

Outlook /Wednesday Night through Saturday/...

Thursday and Friday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers.

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...Flood Watch from 2 PM EDT this afternoon through Wednesday evening for CTZ002-003. MA...Flood Watch from 2 PM EDT this afternoon through Wednesday evening for MAZ002-003-008>011. RI...None. MARINE...Small Craft Advisory from 2 AM to 8 PM EDT Wednesday for ANZ230-231-250-251. Small Craft Advisory from 8 PM this evening to 8 PM EDT Wednesday for ANZ232>237-254>256.