Sept 10: Rain and Storms Possible Tomorrow

Jonathan Carr
By Jonathan Carr September 10, 2014 12:51

Sept 10: Rain and Storms Possible Tomorrow

A low pressure system will be moving from the Great Lakes region into E. Canada between now and tomorrow evening. Attached to the southern side of this system will be a frontal passage with showers and thunderstorms ahead of it. This is impacting areas further to our west today and will impact the east coast tomorrow afternoon-evening with gusty winds and possible flash flooding from heavy rainfall. Here’s a radar image showing the current location of the system with the center of circulation about to enter Lake Michigan:

weather nj radar image of low pressure system over the great lakes heading into eastern canada bringing a cold frontal passage to new jersey

Let’s talk about instability first. Most storms this summer have featured high levels of instability, both at the surface and higher levels. This typically results in towering cumulonimbus clouds with frequent lightning. Tomorrow will be different. CAPE (Convective Available Potential Energy) levels will be hovering around 1200 J/Kg (Joules per Kilogram). That’s considered marginal at best for thunderstorm development. With that being said, I expect thunderstorm cloud tops to be lower with less lightning. Here’s the high-res NAM showing surface-based CAPE values right before rain/storm time in the late-evening:

high-res NAM showing low cape levels tomorrow in new jersey

Now lets talk wind. Bulk effective wind shear from the surface up to the 500mb level (18k feet) will be varying in the 30-40kt range. If those winds are enhanced by convective downdrafting and brought to the surface, it’s enough to trigger severe thunderstorm warnings. 58mph is the criteria for that and I have to allow for isolated instances of such. This is probably why the National Weather Service Storm Prediction Center is going with an elevated risk for severe weather (as seen below):

national weather service storm prediction center issues slight SLGT risk for new jersey on september 11 2014

In English: First, watch out for tidal flooding near the ocean and back bays today. The coastal storm is pulling away but onshore flow timed with high tide can still push water through the drains and into the streets. Expect it to be warmer and muggier tomorrow with light southwest flow. At some point tomorrow afternoon-evening (will narrow as we approach), showers and storms should move through the state from northwest to southeast over the period of a few hours. Expect gusty winds, heavy downpours (flash flooding) and infrequent lightning (rolling thunder). Isolated cases of severe winds are possible (greater then 58mph) so batten down the hatches. I’ll be tracking. Be safe! JC

 

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Jonathan Carr
By Jonathan Carr September 10, 2014 12:51

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