Ida Remnants Prompts Flash Flood Potential
Thanks to heavy thunderstorms in recent days and now a slow moving tropical system in Ida, Flash Flood Watches have been issued for much of the Lower Ohio River Valley, Appalachians, Mid-Atlantic and into parts of New England.
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
410 PM EDT Tue Aug 31 2021
Valid 12Z Wed Sep 01 2021 - 12Z Thu Sep 02 2021
...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL THROUGH PORTIONS OF THE NORTHEAST...
Heavy signals and strong model agreement for portions of southern PA northeastward into northern NJ and southern NY has led to the issuance of a High Risk for flooding. The latest Hi-Res guidance has really narrowed in on this corridor with 4 to 6 inches possible and locally heavier amounts near 6 to 8 inches not out of the question. HREF probs for ARI exceedance of 100 years was well over the 95th percentile for this region increasing confidence even more. Models over the last 12 hours have shifted things a bit further north, and the surrounding Moderate, Slight, and Marginal Risk areas were adjusted to account for this shift. Much of this area is still recovering from Henri and already has fairly low FFG.
...Gulf Coast states into northern Florida...
Latest guidance has shifted much of the precipitation earlier expected to stay offshore, now onshore leading to the expansion of the Marginal Risk area to encompass portions of southern LA into central MS/AL down into northern FL. Areas in LA and MS/AL have extremely low thresholds right now due to Ida impacts and any precipitation at this time could lead to further flooding concerns.
The Slight Risk area for the Southwest was left in play as remnants from Nora continue to impact the region. The Marginal Risk area surrounding this region was also left in play as well as the Marginal Risk for the Northern Plains.
...Central Appalachians/Mid-Atlantic northeast into southeast New York and southern New England... Very heavy rainfall will continue to spread northeastward on Wednesday across portions of the central Appalachians, central/northern Mid-Atlantic region, and up across southeast NY and southern New England as the extratropical low center of Ida makes its way across northern VA, central/eastern MD and southern NJ before then heading offshore to the south of Long Island. Moisture and instability will continue to surge northward around its eastern flank and over a rather strong frontal zone draped across the region. Enhanced moisture convergence, strong frontogenetical and isentropic ascent just north of the front, and focused right-entrance region upper jet dynamics over the region will foster locally several inches of rain, including the potential for some areas to see 4 to 6+ inches totals from the WV/MD Panhandles east/northeast across south-central PA. A fairly broad Moderate Risk area is again highlighted to account for the latest forecast guidance and storm track of Ida. While a High Risk area may ultimately be needed, but too much uncertainty lingers to Ida's exact path at this point to warrant an upgrade now.
I'm particularly concerned about parts of PA where locally 5 to 7 inches of very heavy thunderstorm rains are expected (Euro Forecast shown above) through early Thursday mornning. Most of this will fall during a relatively short window during the day on Wednesday creating the potential for excessive runoffs into local streams and tributaries. Flash Flood Warnings will likely be issued.
Watch for ponding if you have to drive and remember to turn around, down drown!
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