Subj : Heavy Rain/Flooding NY/NJ To : All From : Mike Powell Date : Sat Oct 07 2023 09:10:00 AWUS01 KWNH 071128 FFGMPD VTZ000-NYZ000-NJZ000-PAZ000-071700- Mesoscale Precipitation Discussion 1137 NWS Weather Prediction Center College Park MD 728 AM EDT Sat Oct 07 2023 Areas affected...The Poconos and northern NJ through eastern Upstate NY Concerning...Heavy rainfall...Flash flooding likely Valid 071127Z - 071700Z Summary...Showers and thunderstorms ahead of a cold front will expand in coverage and train northward through the morning. Rainfall rates at times may reach 3"/hr, resulting in bands of total rainfall of 2-4" with locally higher amounts. Flash flooding is possible. Discussion...The regional radar mosaic this morning shows widespread showers with isolated thunderstorms stretched from the St. Lawrence Valley southward through the Catskills, with more scattered activity noted along the I-95 corridor from New York City to Baltimore, MD. This convection is blossoming immediately ahead of a cold front analyzed by WPC draped from near Lake Ontario through eastern PA, and this front is expected to advect eastward into New England later today. Downstream of this boundary, moisture advection is increasing into the front noted by regionally backed 850mb winds of 15-25 kts driving PWs of 1.2 to 1.5 inches northwestward, above the 90th percentile for the date. Additionally, a ribbon of MUCAPE above 250 J/kg is beginning to surge northward from NJ, further enhancing the thermodynamics to support heavy rain. Rainfall rates have been estimated via local radars above 2"/hr already this morning, with the low reflectivity centroids of these cells indicating efficient warm rain processes. The most recent runs of the HRRR are initializing pretty well this morning, but in general all of the high-res is in pretty good agreement with radar evolution the next several hours. As the front continues to push eastward, it will combine with the tail of an intensifying jet streak pivoting poleward over Canada and convergence along the nose of the 850mb inflow to drive more intense ascent. This will act upon a thermodynamic environment that will remain ripe for heavy rainfall, characterized by PWs nearing 1.5" and modest 250-500 J/kg SBCAPE, to produce rainfall rates for which the HREF indicates has a 10-20% chance of reaching 2"/hr, with 15-min rainfall from the HRRR exceeding 0.75" at times (brief 3"+/hr rates). Although Corfidi vectors indicate the entire line ahead of the front will advect progressively eastward, increasing convective coverage and 0-6km mean winds aligned to the front will still permit enhanced training from south to north. This could produce 2-4" of rainfall, and the HREF suggests a 10-15% chance of more than 5" in isolated areas. Antecedent conditions are extremely favorable for rapid runoff from the Capital District southward through the Hudson Valley and into the Poconos where 14-day rainfall has been 150-300% of normal resulting in 40cm soil moisture around the 95th percentile. This has compromised FFG to 1.5"/1hr and 2-2.5"/3hrs, and the HREF FFG exceedance probabilities peak around 40% late this morning into the early afternoon. This region has the greatest risk for flash flood impacts today. Farther north into the Adirondacks, isolated runoff leading to flash flooding is also possible where training can move atop sensitive terrain features. Weiss ATTN...WFO...ALY...BGM...BTV...OKX...PHI... ATTN...RFC...MARFC...NERFC...NWC... LAT...LON 45057390 45047343 44787332 43857339 42387350 41747379 41497421 41177459 40677487 40837531 41337551 41917549 42527531 43877482 44797456 = = = --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .