Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Sun Apr 30 2023 09:05:00 FOUS30 KWBC 300734 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 333 AM EDT Sun Apr 30 2023 Day 1 Valid 12Z Sun Apr 30 2023 - 12Z Mon May 01 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF EASTERN NEW ENGLAND AND FROM SOUTHERN NEW YORK TO THE MID ATLANTIC REGION... ....Northeast... A powerful upper low over the Great Lakes working in tandem with a vigorous 500mb vort max racing north along the East Coast will lay the ground work for a robust Warm Conveyor Belt (WCB) of highly anomalous moisture being directed into the Northeast. NAEFS 500-850mb heights by 18Z Sunday are lower than all 850-500mb heights in the CFSR climatology for late April-early May over the Lower Great Lakes. This remains the case throughout the eastern Great Lakes and even into parts of Upstate New York by Sunday night, eventually leading to a large footprint of MSLP values falling below the CFSR climatology from the eastern Great Lakes to central New England. With the strong high pressure over the eastern Canadian Maritime, the 850mb jet will steadily strengthen throughout the day over the Northeast, as well as off the East Coast. This massive storm system will tap into moisture as far south as the northern Caribbean and Florida Straits, and will be directed north into the Northeast Sunday afternoon and into the overnight hours. The latest NAEFS continues to show a remarkable integrated vapor transport (IVT) that will be the catalyst for the threat of Excessive Rainfall. By 00Z Monday, much of central New England will see IVT values (up to 750 kg/m/s) that are in the 97.5-99th climatological percentile range. Then by 06Z Monday, IVT values are even more anomalous over northern NH and across southern ME with values approaching the highest observed at 06Z in the CFSR climatology. PWs at their peak will be somewhere between 1.0-1.25" from Northern VA to northern New England, which is between 2-2.5 standard deviation above normal per the SREF and GEFS, although along the New England coast, PWs could come awfully close to 1.5". The strong ESE 850mb moisture transport will also support strong upslope enhancement in the higher elevations of central and northern New England Sunday afternoon and into the early morning hours on Monday. Latest WPC precipitation forecasts call for as much as 3-4" in southern ME with localized amounts >5" possible in the White Mountains. What has led to maintaining a Slight Risk and not an upgrade to Moderate Risk is the lack of deep-layer instability as of this forecast cycle, as the guidance consensus is for rather meager MUCAPEs (100-200 J/Kg). Max rainfall rates could approach 1"/hr based on the 00Z hourly QPF from the CAMs, but this appears to be at the top end of the possible scenarios (i.e. localized) as most areas should see peak hourly rates between 0.50-0.75" Sunday night. Should more guidance begin to show higher available instability and heavier rainfall rates, a Moderate Risk may be needed in future forecast cycles. Elsewhere, have expanded the Slight Risk across southern NY into the mid Atlantic (including the urban corridor from DC/Balt to NYC), due in large part to the heavy rainfall Fri-Fri night and the resultant lowering of FFG and rapid increase in the top 40cm soil moisture percentiles (aoa 90 percent for much of the area). Pre-frontal low-topped convection will help to generate thunderstorms capable of up to 1"/hr rainfall rates per the CAMs. ....Florida... A cold front tracking through the Sunshine State's peninsula will spark additional strong-to-severe thunderstorms that may generate hourly rainfall rates >2"/hr, as pre-frontal PWs average between 1.75-2.00" with MUCAPEs of 1000-2000 J/Kg. The front will race through rather quickly throughout morning and track south and east of the Florida Peninsula and Florida Keys by mid-afternoon. Areas most at-risk for potential flooding are in areas where soils remain overly saturated (NASA SPoRT-LIS 0-40cm moisture percentiles up to 90% from Cape Canaveral on south to West Palm Beach), or within the more urbanized corridor of eastern and southern Florida where a greater concentration of hydrophobic surfaces exist. Hurley/Mullinax Day 2 Valid 12Z Mon May 01 2023 - 12Z Tue May 02 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley Day 3 Valid 12Z Tue May 02 2023 - 12Z Wed May 03 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .