Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Feb 11 2023 20:02:10 FOUS30 KWBC 112002 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 301 PM EST Sat Feb 11 2023 Day 1 Valid 16Z Sat Feb 11 2023 - 12Z Sun Feb 12 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM THE EASTERN FLORIDA PANHANDLE THROUGH COASTAL SOUTH CAROLINA... The potential for heavy to possibly excessive rainfall will be focused along and near a quasi-stationary boundary draped over portions of the Southeast U.S. as the flow aloft backs in response to a closed mid- and upper-level low approaching from the southwest. Precipitation grows in coverage and intensity today as low pressure forms along the front, with an additional 1-3 inches of rainfall possible from the Florida Panhandle to southeastern North Carolina, including much of central and southeast Georgia. Instability remains at a premium, but the forcing aloft from a coupled upper-level jet structure and the focus provided by a surface cold front that sweeps across the area later today into this evening will likely lead to high enough rainfall rates to lead to mostly isolated to scattered flash flooding. Precipitable water values this morning range from 1.4 to 1.7 inches across northern Florida and southern Georgia (along the frontal boundary), which is close to the daily max for TLH according to SPC's sounding climatology website. This should also support locally efficient rainfall rates and could overlap with areas that received locally heavy rainfall on Friday. Even if rainfall amounts are not as great as they were on Friday, the antecedent conditions warranted maintaining the Marginal and Slight Risk areas without too many changes. The fact that the front should keep the convection progressive will help to mitigate the flash flooding risk somewhat. More specifically, the greatest threat for 1-2"+/hr rainfall rates is most likely near the coastal Georgia-South Carolina border region this evening over the course of a few hours. As the warm front lifts northward to the Southeast coast, a nose of MUCAPE around 500-1000 J/kg could inch close to places such as Savannah, GA and coastal areas to the east. Training convection is possible along the front, with a second round approaching from the west along the cold front by around 03z. The 12z HREF 40 km neighborhood probabilities for 6 hr QPF exceeding 3" are between 20-40% for the region, but do also highlight the higher probabilities just offshore. Elsewhere, underneath the closed upper-level low, steady and slow-moving rainfall over the Alabama-Mississippi border could add up to around 2 inches by the end of the outlook period, which could lead to some isolated runoff concerns and prompted the small expansion of the Marginal Risk. Snell Day 2 Valid 12Z Sun Feb 12 2023 - 12Z Mon Feb 13 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS A SMALL PORTION OF FAR SOUTHEASTERN VIRGINIA INTO EASTERN NORTH CAROLINA... Rain will continue to spread northward and eastward at the start of the Day 2 period on Sunday morning as a mid- and upper-level low tracks across portions of the Southeast. Present indications are that the higher rainfall rates should be confined close to the North Carolina coast where the upper level support from a coupled jet structure is initially in place followed by increasingly difluent flow to the north and east of the approaching upper low. Precipitable water values near 1.5", which ranks just above the 90th climatological percentile (per the GEFS) along with 850 mb easterly winds of 40 knots should result in decent moisture transport into the region. So this limited area has the best chance for rainfall rates to be high enough for flooding concerns. 12Z HREF probabilities confine low-end neighborhood probabilities of 1 inch per hour rates to this area, which includes the Outer Banks, through roughly 18Z before shifting out to sea. In reality, any heavy rainfall will be falling into coastal swamp and sandy soils, which are unlikely to respond with much flash flooding, although urbanized areas are more likely to see isolated flash flooding. This also includes parts of southeast Virginia, where rainfall rates will not be particularly impressive, but rainfall totals up to 2" could pose an urban runoff or flood risk. Snell Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4VEFZ5AgChWBDNOa04XFPzGckQ6hddkrAXwII7VhxbBY= pWfp3-Dp3GfY2QEaBVauNV81UAscvsD_a9VYoagypoTwG_Q$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4VEFZ5AgChWBDNOa04XFPzGckQ6hddkrAXwII7VhxbBY= pWfp3-Dp3GfY2QEaBVauNV81UAscvsD_a9VYoagyCuI2hrs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4VEFZ5AgChWBDNOa04XFPzGckQ6hddkrAXwII7VhxbBY= pWfp3-Dp3GfY2QEaBVauNV81UAscvsD_a9VYoagyghc57L0$=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) .