Plaincast

What has changed

No significant changes to the forecast overnight.

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 precipitable water 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 storm energy 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, precipitable water 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/northeast flow, in the low to mid 70s.