Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Apr 29 2023 20:04:56 FOUS30 KWBC 292004 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 404 PM EDT Sat Apr 29 2023 Day 1 Valid 1938Z Sat Apr 29 2023 - 12Z Sun Apr 30 2023 ....THERE IS A SLIGHT RISK FOR PORTIONS OF THE FLORIDA PANHANDLE AND SOUTHERN GEORGIA... 1930Z Update... The Marginal Risk was expanded farther west to include more of the Lower Mississippi Valley as convection begins to flare up beneath the upper low. For more information, please see MPD #215 recently issued for the area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/metwatch/metwatch= _mpd_multi.php?md=3D02__;!!DZ3fjg!-dC3SoSPIvtC4LPsDhBIMKOnMMz3L_--4Op8a31-r= SJMkGP3fCVlTl1stlrTU424FkNYuLUA_rnrDegmK9MPpa6dVDs$=20 15&yr=3D2023. 16Z Update... ....Southeastern U.S... Latest 12Z HREF has continued to come in incrementally drier over northern FL, southern AL, and southern GA. One area that still shows potential for flash flooding is in the FL Panhandle where some members of both hi-res CAMS and global deterministic guidance show a scenario where some of the robust convection in the Gulf of Mexico makes its way northeast (thanks to southwesterly mean 850-300mb winds at 40-50 knots) around midday and into the afternoon hours. The good news is the bulk of the convection should remain progressive due to being influenced by such strong steering flow. That said, thunderstorms will still be capable of producing >2"/hr rainfall rates as a result of PWs surging close to 1.75", MUCAPE rising above 1,000 J/kg, RH values within the LCL-LFC that approach 90%, and warm cloud layers that are 11,000 feet in depth. NASA SPoRT-LIS continues to show an area north and west of Tallahassee where 0-40cm soil moisture percentiles are up to 90% due to 7-day rainfall totals that are 400-600% of normal over the past 7 days. The forward propagation of the storms and the bulk of the most intense convection weakening as it approaches the FL Peninsula will help to reduce the Slight Risk area this forecast cycle, but opted to maintain a Slight Risk for portions of the FL Panhandle and southern GA given their more sensitive soils. Portions of central MS and AL were considered for a possible Slight Risk upgrade as the upper low moves overhead, but more marginal instability levels and soils that are not as sensitive were the primary reasons a Marginal Risk was kept in place. The organizing MCS in the Gulf will head for the northern Florida Peninsula, but based on its progressive storm motions, opted to maintain the Marginal Risk. ....Central PA... The other change to the Day 1 ERO was to introduce a Marginal Risk area in central PA. As low pressure deepens over the Great Lakes and Southeast, the pressure gradient will tighten as central PA becomes sandwiched between these two areas of low pressure to the west and south, 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 will be present 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. A Marginal Risk has been issued for these reasons, with the greatest chances for Excessive Rainfall coming after 00Z this evening. ....Southern New England, NYC Metro Area, Long Island, & Lower Hudson Valley... Not much change from the overnight shift as the region at-large will continue to deal with periods of moderate-to-heavy rainfall through this evening. Latest RAP guidance suggests the atmosphere will continue to further saturate to the point where RH values in the LCL-LFC layer will climb above 90%. That said, the biggest limiting factor in rainfall rates and flooding potential is the lack is instability. Given the long duration nature of this setup, opted to maintain the Marginal Risk as of this update with any instances of flash flooding being highly localized, and likely confined to poor drainage areas and the most urbanized of communities. Mullinax ---Previous Discussion--- ....Southeastern US... No significant changes were made to the Day 1 ERO area. It's very worth noting the latest runs have backed off significantly on the total expected rainfall in the Slight Risk area, favoring most of the convection remaining south of the area into the Gulf. However, there remains several models that at least depict strong storms locally producing multiple inches of rain in a short time as the main round of convection moves through around mid-afternoon and a second separate and weaker round of mostly showers moves through around midnight. There is still some potential for training convection, but that chance is fairly low. Due to recent rainfall, especially over the Florida Panhandle, the soils remain nearly saturated over most of the Slight Risk area, and thus any rain that falls today is likely to quickly run off into the local watersheds. The Slight Risk area therefore remains, but it's certainly a lower confidence and therefore lower end Slight Risk, that with model trends, could be downgraded later today as the convective forecast by then will be a now-cast for many areas. Elsewhere across east central MS and west central AL, a strong and developing upper level wave will race across the area today. Given some modest instability to 500 J/kg and PWAT around 1.25 inches, think the added forcing will be more effective at adding to the strength of the storms expected to traverse that area this evening. Soil moisture is a little bit drier in that area compared to the surrounding area, and the modest instability may be enough to prevent anything more than isolated flash flooding. Thus, that area in particular is in a higher-end Marginal. ....Southern New England, New York City Metro Area, Long Island, and the Hudson Valley... Convective rainfall has already begun across the Marginal Risk area for CT and points south and west. Rainfall rates as of the time of this writing are approaching 1 inch per hour. In more sensitive areas of urbanized northern NJ and NYC, these rates may be enough to cause localized flooding in poor drainage areas, so southwestern portions of the Marginal Risk are already seeing the rain that may cause flash flooding, and the focus for the Day 1 ERO is for the morning hours. Meanwhile further northeast across MA and RI the threat will be more for later this morning into tonight. Some backbuilding along the back edge of the precipitation is likely across southern New England, so the potential for higher rainfall totals will be there. Wegman 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!-dC3SoSPIvtC4LPsDhBIMKOnMMz3L_--4Op8a31-rSJM= kGP3fCVlTl1stlrTU424FkNYuLUA_rnrDegmK9MPUH4rsbU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-dC3SoSPIvtC4LPsDhBIMKOnMMz3L_--4Op8a31-rSJM= kGP3fCVlTl1stlrTU424FkNYuLUA_rnrDegmK9MPkZ7xRaM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-dC3SoSPIvtC4LPsDhBIMKOnMMz3L_--4Op8a31-rSJM= kGP3fCVlTl1stlrTU424FkNYuLUA_rnrDegmK9MPf9MyHoM$=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) .