Subj : MESO: Heavy rain - floodi To : All From : Mike Powell Date : Mon Jun 12 2023 17:06:00 AWUS01 KWNH 121954 FFGMPD NYZ000-NJZ000-PAZ000-DEZ000-130200- Mesoscale Precipitation Discussion 0472 NWS Weather Prediction Center College Park MD 353 PM EDT Mon Jun 12 2023 Areas affected...Eastern Pennsylvania through central Upstate New York Concerning...Heavy rainfall...Flash flooding possible Valid 122000Z - 130200Z Summary...Showers and thunderstorms intensifying along a warm front will train to the north this afternoon before shifting eastward later this evening. Training of rainfall rates which may eclipse 2"/hr at times could produce 3+ inches of rain with locally higher amounts. Isolated instances of flash flooding are possible. Discussion...The regional radar mosaic this afternoon shows a broad band of moderate rain extending along a warm front from Lake Ontario southward through the Chesapeake Bay. The southern end has shown rapid convective development in the past 1-2 hours in response to a bubble of SBCAPE reaching 1500-2000 J/kg according to the SPC RAP, overlapping PWs that are around 1.8 inches on GPS measurements, above the 90th percentile according to the SPC sounding climatology. In this favorable thermodynamic environment, ascent is being provided through convergence along the warm front and a lobe of vorticity rotating northward around a closed low centered near the Great Lakes. The steadily improving environment has supported rain rates that are estimated above 0.5"/hr according to the MRMS operational viewer and above 1"/hr on KLWX WSR-88D. As the evening progresses, persistent S/SE flow around the closed low and within the warm sector ahead of an approaching cold front will drive 850mb winds to as high as 25-35 kts, drawing even better moisture and instability northward. The RAP indicates that PWs could reach above 1.5" as far north as Lake Ontario, with 500 J/kg or more of SBCAPE rising to the PA/NY state line. Although there will be some eastward translation of this entire band of rain, and individual cells will race northward at 20-25 kts of 0-6km mean flow, there should be an increasing training potential as flow becomes increasingly parallel to the advancing front and Corfidi vectors collapse to around 5-10 kts. This impressive setup should drive training of rain rates which the HREF indicates could reach 2"/hr, and the HRRR sub-hourly fields suggest some locations could receive up to 0.75"/15 min. Where these train, likely in a narrow corridor from far eastern PA into central NY as shown by focused HREF EAS probabilities, as much as 3-5" of rain could occur in a few areas. This region has generally been dry recently noted by AHPS 14-day rainfall below the 25th percentile and USGS streamflows near all-time lows. This has allowed FFG to recover to 2-3"/1hr and 3-4"/3hrs. The very dry antecedent conditions will generally limit the flash flood risk, and much of this rain could instead be beneficial. However, where the most intense rain rates can train, especially if they occur over more urban areas or within sensitive terrain, isolated instances of flash flooding could occur. Weiss ATTN...WFO...BGM...BUF...CTP...PHI... ATTN...RFC...MARFC...NERFC...NWC... LAT...LON 43527653 43477637 43187603 42577559 41767519 40957500 40117472 39707491 39527534 39807593 40177651 40917696 41757727 42427738 42897739 43207717 43347689 = = = --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .