Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jan 21 2023 15:45:30 FOUS30 KWBC 211545 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1044 AM EST Sat Jan 21 2023 Day 1 Valid 16Z Sat Jan 21 2023 - 12Z Sun Jan 22 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE GULF COAST AND SOUTHEAST... ....16Z Update... No changes to the inherited Marginal Risk area. Sporadic flareups of convection formed overnight across central and southern LA, resulting in narrow areas of rainfall between 1-2 inches as the storms moved through. One of those areas happened to move over Baton Rouge, LA resulting in a Flash Flood Warning. Since then the convection has weakened and shifted north towards the LA/MS border. Two areas within the Marginal Risk will be monitored more closely today: First is the potential development of convection across the Gulf near Houston that tracks northeastward across western LA and into portions of southern MS north of eastern LA. There is considerable agreement in both the CAMs and even the global models of a stripe of heavier precipitation over this area. Despite low instability, the storms that have developed so far have still produced local rainfall rates that have approached 1.5 inches/hour. Fortunately so far they have been short-lived, suggesting any existing instability is not enough to maintain convection currently. The concern is with daytime heating and advection of Gulf moisture that instability may increase in the region, adding fuel to the storms tracking northeast along the boundary in this area. For now once again the area will be monitored and no changes to the risk areas are imminent for this region. The other area of concern will be late this afternoon through the overnight across southern AL and the FL Panhandle. By this point full daytime heating (such as it is in January) will increase instability over the area such that MUCAPE will approach 500 J/kg especially during the overnight as an LLJ develops. While certainly far from impressive, this will support more upright convection as a front develops over the western Gulf. The developing front will be slow-moving, allowing the possibility of training thunderstorms. This exact scenario is depicted in a few of the CAMs (though certainly not all). The FV3 shows the strongest storms, with the 12Z HRRR in decent agreement. Radar trends and newer guidance will be monitored for the possibility of an upgrade to a Slight in this area later today. Wegman ....Previous Discussion... A southern stream trough will amplify as it moves through the Great Plains today. A surface wave will glide across the Gulf Coast and direct moisture up the eastern seaboard through Sunday. Modest southerly mid-level flow will support PWATs in the 1-2" range with hourly rates of around 0.25-0.5in/hr over portions of the central Gulf Coast and Southeast. Increased frontogenesis is expected in the late afternoon hours of Saturday as the Gulf system moves ashore. Fortunately, soils in the Southeast are relatively dry, so the 1-2" of forecasted rainfall won't cause widespread flash flooding concerns. There's still a chance for isolated flash flooding within convective cells, therefore a broad marginal risk is in place. Kebede Day 2 Valid 12Z Sun Jan 22 2023 - 12Z Mon Jan 23 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST... A southern stream system will phase with a northern stream trough over the Ohio/Tennessee Valley on Sunday. Mid-level flow out of the South will continue to supply moisture advection into the Southeast and eventually up the East Coast later into Monday morning. There's a decent signal for a line of convection to develop along a strengthening cold front and deepening low pressure system moving from the central Gulf Coast to Southeast coast late Sunday afternoon into the evening. PWATs between 1.5-2" and rainfall rates between 0.25-0.5in/hr may form along the axis of enhanced frontogenesis linked to convection/instability Sunday evening. The latest deterministic guidance is in reasonable agreement with the orientation and magnitude of the maximum qpf stripe extending from the Florida Panhandle to the Carolina coast. The GFS and EC have had great run-to-run consistency with respect to their 24 hour totals along the East Coast. The 00z NAM and 00z UKMET appear to be on the northern and southern extent, respectively, of the 00z suite of guidance with respect to their axis of greatest qpf. Kebede Day 3 Valid 12Z Mon Jan 23 2023 - 12Z Tue Jan 24 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF Texas... An upper-level low will swing from the four corners region into the Southern Plains on Monday. The primary rainfall threat won't begin until late Monday evening when the mid-level low arrives and starts pumping moisture into the Southern Plains. Mid-low-level southerly flow of around 50kts and shallow instability could support strong thunderstorms and enhanced rain rates, but a marginal severe risk will suffice for now according to the Storm Prediction Center. PWATs between 0.5-1in and rain rates between 0.1-0.25 over portions of north-central Texas could cause isolated flash flooding. Some of this early evening rainfall over northwestern Texas will likely turn cold as the surface wave shifts southward and cold air advection ensues overtop the system. The latest deterministic qpf guidance diverged a bit from continuity given a signal for the upstream ridge to flatten a bit over the Pacific Northwest. The resulting 00z GFS and ECMWF guidance have trended drier and farther south with their respective to their 24hr qpf footprints. Ensemble mean 2 inch exceedence probabilities remain low while 1 inch exceedence probabilities are modest at around 5-20%. High FFG and dry soils support this marginal risk area on day 3. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-7gjtncxoZd3RJNNoC5TuYck_aa352yCXFNtJSNyJZW8= zwqTeYPj5LbVSOOGU15zjsfk6Wt11uC4yTfB0kRVJXjVeL4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-7gjtncxoZd3RJNNoC5TuYck_aa352yCXFNtJSNyJZW8= zwqTeYPj5LbVSOOGU15zjsfk6Wt11uC4yTfB0kRVUMHD36A$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-7gjtncxoZd3RJNNoC5TuYck_aa352yCXFNtJSNyJZW8= zwqTeYPj5LbVSOOGU15zjsfk6Wt11uC4yTfB0kRVaIyVZsI$=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) .