Daily Forecast Discussion 6/19/17- Update 150 PM

The Flash Flood Watch Continues.  A line of severe thunderstorms is marching across Berkshire County.  These areas have already had one round of thunderstorms.  Rainfall totals of up 4″ are possible in Western MA.  Elsewhere, localized amounts of 2-4″ are possible in Northern Worcester County.  In Southern Worcester County and the Blackstone Valley between 730-800 PM.  The storms will be weakening as they enter the more stable air of Eastern MA/RI.

The Storm Prediction Center in Norman, OK has issued a Severe Thunderstorm Watch until 8 PM.  This includes all of Central and Western Massachusetts and all of Connecticut.

SPC Severe Thunderstorm Watch 354

Another severe thunderstorm watch has been posted for most of Northern New England.

SPC Severe Thunderstorm Watch 355

These storms are firing well ahead of the cold front.  This first batch in Western Massachusetts is expected to track Northeast into S VT and S NH this afternoon.  Damaging wind and an isolated tornado are possible with these storms.

NWS Northeast Regional Radar

These storms are slow moving with torrential rain and lots of lightning.  More to come.

Original Post

A slow moving cold front will bring heavy rain and thunderstorms to Western New England this afternoon and this evening.  Flash flooding. hail. and strong winds remain the greatest concern.  Most of the afternoon will be dry in Central/Eastern MA and Northern RI before precipitation arrives around 7-8 PM.  Some clearing skies will be possible across Northeast/Central MA this afternoon before clouds fill back in.  Max temperatures will be in the mid to upper 80’s away from the South Coast where onshore flow will result in temperatures in the mid 70’s.  Dewpoints will be in the low 70’s so temperatures will feel like they are in the low 90’s in the Merrimack Valley back into NW Connecticut.

NWS Max Temperatures Monday (image Weatherbell)

General Overview

Here is the Surface map as of 5 AM this morning.  Low pressure is tracking through Ontario while a cold front that trails through the Ohio Valley all the way into the Southern Mississippi Valley and back into Texas.

WPC Surface Analysis Monday 5 AM

Out ahead of this cold front there is a deep connection to the Tropical Atlantic/Gulf of Mexico.  Plenty of moisture is available for showers and thunderstorms this afternoon.  That blob in the Gulf of Mexico is a potential Tropical Storm (its name would be Bret/Cindy depending on timing).   For more on this please check out the Tropical Discussion page.

10z HRRR Precipitable Water Monday 7 AM (image Weatherbell)

With all of this potential moisture, the Weather Prediction Center (WPC) has placed interior New England in a marginal to a slight risk of exceeding flash flooding levels this afternoon and evening. For more on this, please see the Golf and Landscape report page.

WPC Excessive Rainfall Outlook Monday

In addition to the flash flood threat, there is a damaging wind threat with any thunderstorms that develop today.  Parts of New England are under an enhanced risk of severe weather this afternoon and evening.  The risk is less in Southern Worcester County and Eastern MA/RI due to the cold front passing later in the evening.

SPC Day 1 Convective Outlook

Forecast

Ongoing convection in Western NY/PA will continue moving NE into Eastern NY and Vermont.

NWS Northeast Regional Radar

Severe thunderstorms, even a few supercells, will break out in Vermont, New Hampshire, and Maine early this afternoon.  Keep an eye to the ski up in the Green/White Mountains and the Lakes Region between 12-2 PM.  Some strong thunderstorms are possible during this time.   Strong thunderstorms will reach Hartford and Springfield between 3-4 PM.

10z HRRR Simulated Radar through 11 PM (image Weatherbell)

Notice how the heaviest of the rain/thunderstorms lifts NNE on an axis from Springfield to Worcester North and West.  There will be a sharp divide between those who get heavy rain and those who don’t.

NWS Precipitation Forecast through Wednesday 2 PM (image Weatherbell)

Zoomed in closer to the Boston metro, it is clear the rain stops around the Pike/395 in Webster up to Worcester up to Groton and Tysngsborugh.  That leaves less than 0.5″ of precipitation in Southeast Worcester County

NWS Forecast Precipitation Boston Area (image Weatherbell)

Because of this, there is a flash flood watch for Worcester County and points north and west.

NWS Headlines Monday AM

I will update the front page as needed this afternoon to address this threat.

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