Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Apr 30 2023 00:53:28 FOUS30 KWBC 300053 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 852 PM EDT Sat Apr 29 2023 Day 1 Valid 01Z Sun Apr 30 2023 - 12Z Sun Apr 30 2023 ....THERE IS A MARGINAL RISK FOR PORTIONS OF THE GULF COAST/SOUTHEAST U.S. AND PENNSYLVANIA... ....Gulf Coast and Adjacent Southeast U.S.... Maintained the Marginal Risk area for areas along and near the Gulf Coast into portions of Georgia as a second line of convection capable of producing heavy rainfall moves in behind an area that brought locally heavy rainfall to some spots earlier in the day.=20 The second area of rainfall will be supported by a mid level circulation that was moving eastward and had reached central/southern Mississippi as of 30/00Z...which ramps up 850 mb winds into the 30 to 40 kt range after 30/06Z and induces cyclogenesis over Alabama. The 18Z HREF probabilities show an increase of 1 inch per hour over central Alabama by 06Z to 35 percent or so...with some low-end probabilities for 1 inch per hour rates developing farther north in Alabama and Georgia overnight before the instability erodes. Present indications are that this batch of rain should stay north of the areas that already got heavy rain earlier today...and that the progressive nature of the storms opted to maintain the Marginal Risk but remove the Slight Risk farther south where there were few signals for even 0.5 inch per hour rates. ....Pennsylvania to Portions of the Virginia Blue Ridge... Also maintained the Marginal Risk area over portions Pennsylvania and expanded it southward into northern Virginia along the Blue Ridge as surface low pressure deepens over the Great Lakes and Southeast. The outlook area will be sandwiched in a tightening pressure gradient between the two lows and a dome of high pressure near Nova Scotia. The atmosphere will quickly saturate between 00-12Z Sunday with LCL-LFC RH values up to 90% after midnight. Surface winds will be light out of the ENE while at the same time, southerly 850mb winds (associated with a surge in 850mb moisture flux) will accelerate from 15-20 knots 00Z Sunday to 35-40 knots by 09-12Z Sunday. A narrow convergence zone at 850mb is expected to set up over the heart of the Commonwealth at the same time the mean 850-300mb flow is uniformly out of the SW. This sets up a scenario for training convection in an environment where warm cloud layers are as deep as 7,000 feet, MUCAPE could reach up to 250 J/kg, and PWs will range between 1.0-1.25". Antecedent soil moisture conditions have grown more saturated from the Laurel Highlands and central PA to the Poconos and Catskills with 0-40cm soil moisture percentiles approaching 90%. Thunderstorms tonight will have the potential to generate 1.0-1.5"/hr rainfall rates, which is in line with current 1-hr FFGs in central PA. The signal from the 18Z HREF for 0.5+ inch per hour rates extending southward along the Blue Ridge had increased from earlier runs...so extended the Marginal southward. A Marginal Risk has been issued for these reasons, with the greatest chances for Excessive Rainfall coming after 05Z. Bann Day 2 Valid 12Z Sun Apr 30 2023 - 12Z Mon May 01 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF SOUTHERN MAINE, EASTERN NEW HAMPSHIRE, SOUTHERN NEW YORK AND CENTRAL/EASTERN PENNSYLVANIA... ....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 conveyor belt 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 Florida Straits and be directed north into the Northeast Sunday afternoon and into the overnight hours. NAEFS shows 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, 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 instability as of this forecast cycle. Max rainfall rates could approach 1"/hr, but this appears to be at the top end of the possible scenarios with most guidance coming in with hourly totals around 0.5"/hr. Should more guidance begin to show higher available instability and heavier rainfall rates, a Moderate Risk may be needed in future forecast cycles. Elsewhere, central PA and as far north as the Binghamton, NY area will have already been dealing with rounds of heavy showers and thunderstorms during the early morning hours that will have helped to further saturate soils. By midday, a cold front over western PA will be making its way east into central PA where just enough instability along with the strong mesoscale forcing from the front will help to generate thunderstorms capable of up to 1"/hr rainfall rates. Given the soils will have grown more sensitive after Saturday night's/early Sunday morning's rainfall, have maintained the Slight Risk for portions of the region. ....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. PWs out ahead of the front will hover round 2" with MUCAPE >2,000 J/kg and 850-300mb RH values between 80-90%. 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. Mullinax Day 3 Valid 12Z Mon May 01 2023 - 12Z Tue May 02 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Mullinax Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-ofHoexsmVAEGDB90QGSw38XMa2zH2uLG-mDWIedU8bX= q6scZIZ8Gt1E9qhNKUg8vW_2eE3mwhFhq_1es_KXtcX82eE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-ofHoexsmVAEGDB90QGSw38XMa2zH2uLG-mDWIedU8bX= q6scZIZ8Gt1E9qhNKUg8vW_2eE3mwhFhq_1es_KX8X_tuuw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-ofHoexsmVAEGDB90QGSw38XMa2zH2uLG-mDWIedU8bX= q6scZIZ8Gt1E9qhNKUg8vW_2eE3mwhFhq_1es_KXOHKjtr4$=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) .