Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Apr 05 2023 08:16:18 FOUS30 KWBC 050816 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 415 AM EDT Wed Apr 05 2023 Day 1 Valid 12Z Wed Apr 05 2023 - 12Z Thu Apr 06 2023 ....THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL FROM SOUTHEAST TEXAS TO THE OHIO RIVER VALLEY & IN PORTIONS OF NORTHERN NEW YORK... ....Southeast Texas to the Ohio River Valley... The upper trough over the North-Central U.S. and strong ridging along the Southeast coast will work in tandem to funnel rich Gulf of Mexico moisture north and east from southeast TX into the Great Lakes. The region from east TX to the Mid-South remains favorably positioned beneath the diffluent right-entrance region of a 150 knot jet streak. PWs up to 1.5" are expected as far north as the Ohio Valley with PWs above 1.75" likely in the Lower MS Valley and along the Upper Coast of TX. The 00Z HREF still shows up to 1,000 J/kg of MUCAPE as far north as the central KY, with upwards of 2,500 J/kg in central LA. One of the biggest supporting factors for higher chances for flash flooding lies with a much more saturated profile. The NAEFS showed mean specific humidity values at the 925-850-700-500mb height levels that were all above the 90th climatological percentile through Wednesday afternoon and into Wednesday night, indicating the atmospheric column through much of the lower-mid levels of the atmosphere is forecast to contain an exceptional amount of moisture. One supportive proponent for flash flooding is that as the intense cyclone over the Midwest crosses into Canada Wednesday evening, the trailing cold front over the Mid-South and the Upper TX coast slows down to becoming stationary. Mean 850-300mb winds and surface-500mb wind shear will become oriented quasi-parallel to the front while southerly 850mb winds continue to pump in Gulf of Mexico moisture into the frontal boundary. This setup supports training convection, and with warm cloud layers as deep as 10k feet in some cases, efficient warm rain processes within these training segments of convection have better odds to cause flash flooding. FFGs are lowest in portions of the OH and TN Valleys where 1-hr FFGs are as low as 1.5". Given the anomalous moisture and sufficient instability, hourly rainfall rates topping 1.5"/hr are doable Wednesday afternoon and into Wednesday night. 1-hr FFGs are higher from southeast TX to the Lower MS Valley (lowest values are 2-3"), but greater instability and PWs do give a chance for hourly rainfall rates to reach 2"/hr. Then add in the slowing progression of the front with possible training convection, and 1-3hr FFGs could be surpassed in these locations late Wednesday night and into early Thursday morning. Some portions of southern AR, northern LA, and central MS are more vulnerable due to 0-40cm soil moisture percentiles as high as 70-80% according to NASA SPoRT-LIS. These areas with more sensitive soils, as well as in poor drainage areas and urbanized communities, are most prone to potential flash flooding. The Slight Risk remains in place with just minor tweaks to account for the latest QPF forecast. Farther north, the Marginal Risk still appears to be in good shape for portions of the eastern Great Lakes and interior Northeast. Instability is a little harder to come by, so with hourly rates on the low side compared to those in the South (topping out around 1"/hr), potential flooding will be more confined to poor drainage areas and nearby creeks/streams. There is one exception to this premise that is listed below. ....Northern New York... The warm front lifting across the Great Lakes Wednesday morning will progressively push north through the Northeast during the daytime hours. The atmospheric moisture content is exceptional for the time of year across the Great Lakes and Northeast. NAEFS are showing PWs (1.25-1.5" PWs) that are greater than all 18Z, 00Z, and 06Z in the CFSR climatology for the time of year. The integrated vapor transport (IVT) are also an impressive 750 kg/m/s, which is close to values that would also be above the CFSR climatology for the region. With the warm front also so far north, there will be some instability to be had with the 00Z HREF suggesting 250-500 J/kg of MUCAPE may be present. The other factor prompting a Slight Risk upgrade here is snow melt. The Tug Hill Plateau still contains anywhere from 1 to as much as 8 inches of liquid equivalent snowpack. Most guidance suggests dew points could climb to 50 degrees and breezy southerly winds, which are snow eaters in their own right, let along periods of rain. The concern is for the combination of hourly rainfall rates up to 1"/hr in an area with a combination of thawing soils and lingering snow pack. The 00Z HREF does show 30-40% probabilities for >3" of rainfall in 24-hours between Pulaski and Watertown, suggesting there could even be a higher ceiling for possible rainfall totals. Given the near record PWs for the date, sufficient instability, and expected snowmelt, opted to upgrade a Slight Risk just east of Lake Ontario and into the Tug Hill Plateau. The heavy rainfall rates and increasing snow melt may lead to rapid runoff and resulting flash flooding, particularly along complex terrain and in poor drainage areas. .... Mullinax Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6ocSgnTaqUAXUDVK2UVxwghNv1-SdgdJUEI23arlIALB= ANVuU5qk3gFjjXKhokgUWJRff2y2JbUfWq77A8KiqSC5-Ko$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6ocSgnTaqUAXUDVK2UVxwghNv1-SdgdJUEI23arlIALB= ANVuU5qk3gFjjXKhokgUWJRff2y2JbUfWq77A8KiavziSxc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6ocSgnTaqUAXUDVK2UVxwghNv1-SdgdJUEI23arlIALB= ANVuU5qk3gFjjXKhokgUWJRff2y2JbUfWq77A8KiO56vnio$=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) .