Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Sep 28 2023 08:21:58 FOUS30 KWBC 280821 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 421 AM EDT Thu Sep 28 2023 Day 1 Valid 12Z Thu Sep 28 2023 - 12Z Fri Sep 29 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MID-ATLANTIC, NORTHEAST, FLORIDA AND THE OHIO VALLEY... ....Mid-Atlantic into Northeast... We added a Marginal risk across portions of eastern PA into NJ and southeast NY with this update. Lower confidence than normal for a day 1 forecast as an area of low pressure develops offshore and attempts to push an inverted trough/coastal front inland. The extent of the flash flood risk really comes down to the positioning of this front/convergence axis later tonight into Friday morning. If this axis stays offshore then any instability will also remain offshore, resulting in just some stratiform rainfall moving inland. However if the axis is along the coast or inland, then some weak instability should get ashore as well, and set the stage for the potential of shallow efficient convection. Most of the HREF members do bring some heavier rain onshore, with the highest probabilities focused over NJ. However each HREF member has a slightly different location, and most of the 00z global models keep the bulk of the heaviest rain offshore through 12z Friday. Given this spread and uncertainty opted to stick with a Marginal risk for now. The main threat for day 1 is the last 6 hours (06z-12z Fri), so will let the day shift take a look at the 12z HREF and make a call on whether to upgrade to a Slight or not at that time. ....Ohio Valley... A Marginal risk was maintained across portions of TN/KY into southern OH. Convection will be ongoing at 12z this morning, with some brief training/backbuilding remaining possible. We might end up with a downward trend in convective activity by mid to late morning...but with an elongated area of vorticity continuing to traverse the area through the day good synoptic ascent will remain in place. Thus once instability recovers from the morning activity we could see some additional convective development across the region this afternoon/evening. The magnitude/organization of this additional activity is conditional on the amount of recovery we get....but if we area able to ramp instability back up then some additional brief training could occur given the persistent low level inflow and convergence in place. Isolated flash flooding will be possible, both this morning and potentially again this afternoon/evening. ....Florida... Only minimal changes to the inherited Marginal risk area over FL. Anomalous moisture will remain in place, with a stationary front and stronger upper level flow helping trigger above average convective coverage again today. High rainfall rates will drive a localized flood risk over any more susceptible urban locations. Chenard Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4P8yBaEr0FHh4X1xM3ZQ36fUbVuKwoDd33r3QPYliK3e= cBw3PCFZB2dB5ep2TrGZabQ10C6hBKYZPxEzBhWq8_WZs1k$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4P8yBaEr0FHh4X1xM3ZQ36fUbVuKwoDd33r3QPYliK3e= cBw3PCFZB2dB5ep2TrGZabQ10C6hBKYZPxEzBhWqnwJ4m5A$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4P8yBaEr0FHh4X1xM3ZQ36fUbVuKwoDd33r3QPYliK3e= cBw3PCFZB2dB5ep2TrGZabQ10C6hBKYZPxEzBhWqS3Hj4YA$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .