Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jan 28 2023 00:16:31 FOUS30 KWBC 280016 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 715 PM EST Fri Jan 27 2023 Day 1 Valid 01Z Sat Jan 28 2023 - 12Z Sat Jan 28 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann Day 2 Valid 12Z Sat Jan 28 2023 - 12Z Sun Jan 29 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHEAST TEXAS & SOUTHWEST LOUISIANA... ....2030 UTC Update... Only minor modifications were made to the current Marginal Risk area to adjust for the latest 12Z global and hi-res guidance. There remains some concern with the timing in development of storms as the southerly low-level flow ramps up and a coastal warm front pushes northward, with storms potentially not initiating until the end of the day 2 forecast period early Sunday morning. However, several pieces of guidance from both the global and hi-res models agree on the chance for heavy rainfall prior to 12Z, specifically the potential for a few swaths of more intense rain rates in the hi-res guidance. This is supported by a gradual increase in instability near the coast as well as the previously noted influx of anomalously high Gulf moisture. Putnam ....Previous Discussion... A digging longwave positively-tilted trough will direct a weakening shortwave across the Red River Valley and into the Ozarks Saturday night. Meanwhile, deep Gulf moisture will be directed northward from the Bay of Campeche into eastern TX and southwest LA, as a front pushes south and east out of the Plains and into northeast TX through AR. As is typical of most January nights, instability will be the primary limiting factor. MUCAPE across AR will struggle to even get to 100 J/kg. Meanwhile we'll be doing a bit better in that arena further south, especially across southeast TX. Thus, the main change from inherited was to remove the Marginal Risk across much of AR in favor of areas closer to the Gulf Coast. In deference specifically towards the GEM Regional, which is typically a reasonably accurate representation of convective potential around the Gulf Coast, much of the Marginal Risk area was retained across southeast TX and southwest LA. Besides greater instability, the GEM-REG indicates the potential for multiple inches of rain from training convection in the Marginal risk area. Antecedent conditions remain favorable in the Marginal Risk area with nearly saturated soils, though that may be mitigated somewhat by the dry conditions expected between now and Saturday night. PWATs will increase Saturday to 1.5" along the TX Gulf Coast, which is over 2 sigma above normal for this time of year. The LLJ will increase to 50 kt across northern LA/southern AR, while further south in the Marginal Risk area, expect the LLJ to be closer to 30 kt. Nonetheless this will advect plentiful Gulf moisture into the area which will support training convection. While the forcing will be greater further north, again the lack of instability should disallow short-term heavy rainfall rates, even if there are training cells into southern AR. Wegman Day 3 Valid 12Z Sun Jan 29 2023 - 12Z Mon Jan 30 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST... ....2030 UTC Update... A few small adjustments were made to the current Slight and Marginal Risk areas to account for the latest 12Z model guidance. These adjustments followed the updated orientation of the heavier QPF as well as some trimming of the northward extent of the Marginal Risk given concern over realizing sufficient instability values for more intense, short-duration rainfall, despite the potential for some 1-2" totals in the Tennessee Valley. There is some uncertainty in how far more buoyant air will push northward during the day following an advancing coastal warm front. Higher values of instability, contributing to the potential for more intense rainfall rates, are most likely for southern/western portions of the outlook area ahead of the advancing cold front to the north, specifically the western/central Gulf Coast and adjacent inland areas over the South. However, despite these concerns, there remains a strong signal in the guidance for a continuous band of widespread heavy rainfall totals between 2-3 inches from southwest Louisiana east-northeastward to southern Georgia, with locally higher totals of 4-5 inches possible, as noted with the overnight update. This is supported by the earliest runs of the hi-res guidance, specifically the NAM NEST, with a long west-east band of storms moving roughly parallel to the front centered within the Slight Risk area. The progression of the cold front should also slow later into the day Sunday, further enhancing the chance for a longer period of heavier rainfall as storms continue to train over the same region.=20 One more notable adjustment was to trim back the Slight Risk northwestward across portions of coastal Georgia and South Carolina. Current drought conditions and less confidence in the chance for more significant rainfall will keep the risk of flash flooding lower than areas to the west. The risk remains higher further inland where wetter antecedent conditions along with high streamflows will be a concern despite some uncertainty in the northeastward extent of higher QPF and sufficient instability for heavier rainfall rates.=20 Putnam ....Previous Discussion... A nearly stationary positively-tilted longwave trough will remain anchored over the northern tier of states and the Southwest Sunday. A weak shortwave trough and associated vorticity advection will move across the Southeast through the day, which will help force the convection across the Slight Risk area. Instability is likely to remain confined to close to the immediate Gulf and later Atlantic coasts, and struggles to move very far inland. Even so, values will remain under 1,000 J/kg. Despite that, moisture will be plentiful as PWATs around 1.75 inches are expected from coastal LA early in the day that moves over northern FL by late in the day into Sunday night. These PWAT values are over 2.5 sigma above normal for this time of year. The inland front will also help force the convection as it slowly sags south into the Southeast through the day. All of these combined factors will result in a long swath of 2-3 inch rainfall totals from southwest LA early Sunday morning to coastal SC late Sunday night. These will occur due to multiple rounds of convection, which will become more likely to train later in the day as Corfidi vectors turn west-southwesterly, or parallel to the front, which will work to offset declining instability further east. Changes from the inherited risk areas include expanding the Slight Risk further west into southeast TX, to include metro Houston. This is largely a continuation of the Marginal risk from the tail end of Day 2. While much of the rain in the Houston metro region is likely to fall in Day 2, most of the impacts and the end of the rain will continue into Day 3. As mentioned in the Day 2 discussion, antecedent conditions should favor flash flooding as soils remain nearly saturated, and streamflows currently are well above average. This may be mitigated somewhat by the dry conditions expected up until Saturday night, with heavy rain likely continuing into the early morning hours of Sunday. The northern edge of the Marginal Risk area across northern MS/AL/GA was trimmed a row of counties or 2 south from inherited as these areas are unlikely to realize any instability, and therefore are unlikely to see rainfall rates heavy enough to produce flash flooding. In fact, additional trimming towards the south is possible with future updates with model agreement and continued consistency as to how far north any atmospheric instability can reach. Finally, a small expansion of both the Slight and Marginal risk up the Carolina coast was included as instability is likely to be realized there due to the warm water temperatures. Further, more training potential is likely in this region and antecedent soils are close to saturated in this region. Regardless, any rainfall along the Carolina coast is expected well after sunset on Sunday night. ....Peninsular Ranges of Southern California... A small Marginal Risk area was considered in this region. However, the upper level low forcing the rainfall into these Ranges will be diving south down the CA coast, and will therefore not have any subtropical connection. Thus, despite continued high flow levels in the rivers draining the Peninsular Ranges, rainfall totals of 1 inch or less are unlikely to cause flash flooding. Continued monitoring of this region is likely with future updates. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4A7vieAzVm5Rzn-pTfXT4MRG7KljdrTWrlGuTCEjiBrt= QcLGbguHdqhrD1l14H3kJ-PKqm3IXoU0zCqgeWYGvliKFVU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4A7vieAzVm5Rzn-pTfXT4MRG7KljdrTWrlGuTCEjiBrt= QcLGbguHdqhrD1l14H3kJ-PKqm3IXoU0zCqgeWYGVlLsjes$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4A7vieAzVm5Rzn-pTfXT4MRG7KljdrTWrlGuTCEjiBrt= QcLGbguHdqhrD1l14H3kJ-PKqm3IXoU0zCqgeWYGefQ-1Jk$=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) .