Rain, Thunderstorms for Tuesday

After nearly perfect weather Friday-Monday (my opinion) unsettled weather will move into Southern New England for Tuesday.  A widespread severe weather outbreak like last week doesn't appear likely.  However the rainfall will be widespread and embedded within the rain will be thunderstorms with isolated severe weather.  The Storm Prediction Center currently anticipates a marginal severe weather threat
SPC Day 2 Severe Weather Outlook
Here is the SPC outlook chart
SPC severe thunderstorm risk
Forecasting severe weather is hard.  The SPC does a great job overall but they sometimes struggle in New England as our severe weather outbreaks tend to be atypical.  That threat could jump to slight in the next update.    The system responsible for the active weather is currently in the Great Lakes
WPC 11 am surface analysis
In the upper levels the trough is rather shallow, with just a few shortwaves (at the surface the two low pressure systems to the North and South of Michigan) moving under a stronger shortwave in Ontario. 
12z NAM 500 mb vort valid 8 pm (image NCEP)
Temperatures are mild tonight, staying in the mid 60s as dew points and cloud cover increase.  By 6-8 am rain should be overspreading much of Southern New England from SW to NE.  I am anticipating two rounds of heavy rain, one for the early rush and one for mid afternoon.  Here is the 8 am projected surface chart
WPC 8 am projected surface chart
Southerly flow will allow for very moist, humid air to enter the region.  Upper level winds will also be potent for August so any thunderstorms will have the potential for damaging winds.  Still not expecting a lot of thunderstorms with the early rainfall.
12z hires NAM Precipitable Water forecast 2 pm (image weatherbell)
Pwats (precipitable water) are approaching and exceeding 2' in Southern New England.  The mid afternoon will be a period to watch as low pressure may form along the cold front as it pushes through.  That will enhance the thunderstorm threat for the 12-6 PM timeframe. 
WPC 8 PM Tuesday surface forecast
This projection would bring heavy rain/thunderstorms to the Cape and Islands tomorrow afternoon so heads up vacationers (let's see if my brother reads this, he is on the Cape this week).    The WPC is calling for 1.25-1.5' of rain regionwide and in general this will be correct.

WPC 1-2 day rainfall forecast through 8 PM Wednesday
Anyone who gets caught in one of the heavier showers/storms will be looking at over 2' of rain in a short period of time.  Areas prone to urban flooding will be difficult to navigate if this happens.  Guidance has been hinting at a more widespread thunder threat so I will monitor that threat and report an update tomorrow AM.

-Zack Green


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