The summer of flash flooding continues

Watching 93L for development in the northern Gulf as well

I posted this on BlueSky last night in response to Jeff Berardelli’s post about serious heavy rain and flash flooding in Plant City, FL yesterday afternoon. Flash flooding has been the weather story the last several weeks, and yesterday may have been the peak as far as number of locations seeing major flash flooding. My colleague Michael Lowry pointed out in his post this morning that yesterday saw the NWS issue 96 flash flood warnings, the most of any July calendar day on record. The resulting impacts were significant and spanned areas from the Northeast to Florida to Texas as seen by these news articles from all over the country:

At Least two killed after heavy rain in New York region floods subways and strands vehicles

Flash Flooding Hits the DMV

Massive flooding forces multiple water rescues in Lancaster County (Pennsylvania)

Plant City sees heavy flooding amid storms in Tampa Bay

Heavy rains return to Texas 10 days after catastrophic flash flood

Again, none of this flooding is resulting from large scale weather systems producing large organized areas of heavy rainfall. Rather, it is the result of smaller scale atmospheric processes focusing thunderstorms for a 2-6 hour time period over a rather small area, with anomalously high levels of moisture supporting intense rainfall rates. I have highlighted on the 24 hour MRMS multi-sensor rainfall map areas from the media articles I listed above. Even on a more national scale graphic, they stand out fairly well as pockets of 3-8” of rain, rainfall that often occurred on a matter of just a few hours. As an example of the type of rainfall rates we are seeing, NYC Central Park had 2.07” of rain in an hour last night (7-8 pm), the second highest hourly rainfall on record (behind only 3.47” during the remnants of Ida). When this sort of rainfall intersects with areas vulnerable to flash flooding such as urban areas or a flashy river basin like the south fork of the Guadalupe River, major societal impacts can occur as we have tragically seen several times over the last few months.

Today will be another active day for thunderstorms with high rainfall rates, with several areas across the country in slight (level 2 of 4) risks of flash flooding as seen with the red areas in the map above. The Mid-Atlantic area continues to be a particular area of concern given that many parts of this region have seen 200-300% of normal rainfall over the last month and flash flooding will result more quickly from heavy rainfall.

Severe thunderstorms will also be a concern today, primarily from the central Plains into the upper Midwest where an increasingly unstable atmosphere along a slow moving cold front will fuel more intense thunderstorm activity. Conditions appear favorable for a number of severe storms over central Nebraska to produce particularly intense damaging wind gusts, and an enhanced (level 3 of 5) risk of severe storms is in place here.

We continue to watch Atlantic tropical disturbance 93L off the east coast of Florida this morning. Morning visible satellite imagery suggests the strong northeasterly wind shear we discussed in yesterday’s newsletters is relaxing. While that is allowing the thunderstorm activity to develop closer to the weak center of circulation, it has also allowed the center to form back a bit farther to the north, now looking to be a bit northeast of Cape Canaveral.

As I discussed in last night’s weekly tropical newsletter, a farther north trend is good in that it means the system will be dealing with more land influence, making development harder and the few model scenarios that showed a stronger system even less likely, which in fact is a trend that can be seen in the most recent model runs this morning. For now NHC continues to show a medium (40%) chance for development once the system crosses Florida and starts to move west near the northern Gulf Coast.

Again, regardless of any development, heavy rainfall is expected to be the primary threat with 93L. Additional heavy rainfall is possible over the Florida Peninsula today, where a slight (level 2 of 4) risk of flash flooding is in place across central Florida. The risk for heavy rainfall will spread west along the Gulf Coast the next couple of days, and eventually increase over the central Gulf Coast by Friday as the system turns more northwest.

A moderate (level 3 of 3) risk of flash flooding is already in place in southern Louisiana for Friday.

Heat continues to be an issue in parts of the West, and several heat advisories are now also in effect for parts of the eastern half of the country where extreme humidity in in combination with somewhat above normal temperatures will yield significant heat stress. Fire danger will be a serious concern for parts of the Great Basin and Intermountain West again today with red flag warnings in place. The increasingly active wildfire season continues to be a concern, with the National Interagency Fire Center now reporting more than 100 uncontained large fires across the western US and Alaska.