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Between four and six inches of rain fell in suburban Baltimore last night, yielding flood scenes like this...
...and like this...
Closer to home, heavy rain in Central New Jersey caused some pretty epic flooding in the Princeton area.
The culprit was the combination of tropical moisture over our region, being squeezed out by a "warm" front that lifted northeast through the region on Saturday (causing the New Jersey flooding) while a second "cool" front moved in from the west and crossed through Maryland and Pennsylvania Saturday evening. These fronts really don't separate warm and cold air like a front outside of summer often does - the air in both scenarios is warm and humid but the fronts are really boundaries that separate tropical air from not-as-tropical airmasses.
These boundaries move over geographic feature or atmospheric boundaries (outflow from other storms, for instance) and help enhance rainfall rates in some instances. In the case of the Maryland storms last night, the flooding is similar to what would occur if a storm dropped that level of rain in Conshohocken or along the Fall Line in Southeast PA...and we've seen several instances over the years where I-76 floods out because of heavy rain (and it did early this morning from a soaking thunderstorm that parked over the western burbs before many of us got up). The Jersey storms likely got some enhancement from the Fall Line as well given it runs relatively close to the Route 1 and Amtrak corridors in Central New Jersey.
Any thunderstorm over the next couple of days has the potential to yield more flooding due to the combination of slow moving pace and the muggy airmass in place. More thunderstorms will be around at times later this afternoon, tonight, and again tomorrow afternoon as weak disturbances track through the region. We should get into a less humid airmass on Tuesday.
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