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.
Temperatures this weekend should make it to above 95 both Saturday and Sunday, with either of the days likely to surpass the 96 degree "gold standard" of heat that was established earlier this month and equaled on Monday.
Obviously, the heat alone won't be the biggest story of the upcoming weekend. The combination of dew point values in the upper 60's and lower 70's will add a few degrees to the heat index, which means that most days will result in a "feels like" temperature that pushes 100. There will likely be some level of heat watch issued for Saturday and Sunday, perhaps extending into Monday. The big caveat on Monday is the timing of a cool front through the region and how much cloud cover and thunderstorm activity it tosses at us. Guidance has waffled a fair bit on timing the front through but odds favor the passage on Monday, which should bring the end of this round of heat to our region with it.
It's been a while since the region hit 100 - we last did so on July 18th, 2012. It's been three years since we hit 98 (7/18/2013), although we've been paraded eight 95 degree highs in 2015 and 2016. While we probably don't break 100 this weekend, we will probably get darn close to it at least once, perhaps twice.