Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jan 21 2023 20:19:04 FOUS30 KWBC 212018 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 318 PM 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... ....2030Z Update... Some notable changes were made to the ERO for the Sunday time period. The axis of heaviest rainfall associated with this event has been shifting south, and note that there is an ongoing moderate drought across nearly all of the coastal Carolinas into the Virginia Beach area. Due primarily to these factors, the inherited Marginal Risk was trimmed significantly towards the south, in coordination with the AKQ/Wakefield, VA and RAH/Raleigh, NC forecast offices. The current Marginal Risk area follows the heaviest QPF in the latest WPC forecast as well. Another primary limiting factor across NC and VA is a lack of instability, which will struggle even to get to 500 J/kg of MUCAPE. There was even some pause given to the severe drought ongoing across portions of the eastern FL Panhandle, but more abundant moisture and some preconditioning of the soils from tonight's convection may mitigate that. Overall, the threat appears to be even on the low side of the Marginal Risk (closer to 5%), and continued lowering of QPF in this area in the guidance should result in the consideration of removal of the Marginal Risk altogether. PWATs in this area have come down a bit closer to the 1.5", and local and brief rainfall rates around 1/2 inch/hour remain possible, but these rates are unlikely to overcome high FFGs, dry soils, and aforementioned drought conditions in any except the most vulnerable risk areas. Wegman ....Previous Discussion... 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 ....2030Z Update... The probability of rainfall exceeding flash flood guidance is less than 5 percent. The inherited Marginal Risk across a swath of Texas was removed with this update. There has been a notable decrease in nearly all of the guidance of rainfall expected to occur in this area during this period. As such, the chance of excessive rains has diminished below criteria. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7MCkOyJmli-u3eiAg3lHCNRB8jYhUJLdBlWOMOwvUWCV= gOuyjLc3J1s9ZzwZKk0zdEUbEVPZ9_Sy_JlwzTZMoOApKLc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7MCkOyJmli-u3eiAg3lHCNRB8jYhUJLdBlWOMOwvUWCV= gOuyjLc3J1s9ZzwZKk0zdEUbEVPZ9_Sy_JlwzTZM5zCUpG8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7MCkOyJmli-u3eiAg3lHCNRB8jYhUJLdBlWOMOwvUWCV= gOuyjLc3J1s9ZzwZKk0zdEUbEVPZ9_Sy_JlwzTZMGv33Ke8$=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) .