Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Oct 04 2023 16:00:21 FOUS30 KWBC 041600 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Wed Oct 04 2023 Day 1 Valid 16Z Wed Oct 04 2023 - 12Z Thu Oct 05 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS... ....16Z Update... The main components of the D1 synoptic pattern remain on track for the rest of today and tonight. Currently have an initial round of convection centered over the Arklatex with the main corridor extending from NE TX into the southwest corner of AR. The convergence area is along the frontal boundary positioned along and just north of the Red River into the Lower Mississippi Valley. 12z sounding from KSHV this morning indicated well-above normal PWATs around 2.0-2.1" with guidance showing a slightly higher PWAT location bisecting the area where precip is ongoing. This area will remain the focal point of convection through this afternoon before the attention turns upstream from an expected MCS developing over western OK down into north TX. Hi-res deterministic is very consistent on the core of the heaviest rainfall centered over northeast TX and southwest AR due to the multi-round convective impact anticipated. In fact, the 12z HREF mean is now up over 5" for the intersection point in the Arklatex, positioned right over Texarkana and Bowie county in TX. Associated probabilities from the HREF for 5"/12-hrs (18-06z) and neighborhood probabilities of 5"/24-hrs are both very high running 70-80% and 90+% respectively lending high confidence in a higher impact threshold exceedance necessary for a higher end risk category. Further to the west over the DFW metroplex, the signal is not as pronounced due to the main wave of convective impact being confined to the evening and overnight time frame with the MCS propagation. 5"/24-hrs probability is around 20-25% with the 3"/24-hrs a very high 60-70% which indicates the potential for a higher impact possibility within the urban corridor. Local FFG guidance is low given the urban sprawl factors in place and impervious surfaces within much of the metroplex. Considering the forecast for 2-4" with local 5+" potential for the DFW area, maintained continuity on the MOD risk in place and will monitor trends in short range guidance for the evolution of the nocturnal MCS. Widespread localized flash flooding concerns will be felt over areas outside the MOD as well with 12z HREF probabilities for rates over 2" and 3"/hr running over 50% within the OK/TX/AR area north of the Red River. Training echoes across the MOD area will be a concern due to the positioning and attenuation of the front as it interacts with cold pool environments stemming from nearby convective clusters. This well documented when looking at the theta-E indices overlayed with the sfc wind field as adjustments on the northern extent of the best instability axis wavers before being pushed south with the MCS overnight when the larger cold pool source finally propels the frontal boundary south. Overall, the highest impact zone will be within that Arklatex region, followed by areas along the Red River from Wichita Falls and points east. This correlates well with the MOD which encompasses the higher signals from both the 12z HREF blended mean QPF and higher end probabilistic means for at least 3" of QPF. Kleebauer ....Southern Plains... Training bands of convection are expected to impact a north-south axis from the Texas Gulf Coast up through the ArkLaTex beginning this morning before a squall line, emanating over northern Texas and Oklahoma pushes through the region this evening. Rain rates at around 0.5"/hr aren't expected to be too high from this early round of storms, but the training aspect should still be able to cause some flash flooding concerns this morning and afternoon. A complex of thunderstorms, initiated ahead of a cold front, which is supported by a deep upper level trough centered over southern Canada, are expected to congeal into a squall line with 1-2" (locally higher) rain rates. There will be ample upper level forcing with strong 250mb jet and substantial divergence streaking across Oklahoma and northern Texas this afternoon and evening. Several of the CAMs including the HRRR suggest outflow boundaries propagating from the squall line, surface boundary draped across the Red River and Lower Mississippi Valley, and other preceding convective cells could support another axis of training convection around the eastern portions of the Red River this afternoon and evening. These higher rain rates combined with training could overcome some of the dry soils present in much of the Slight Risk area. The Moderate Risk area was extended back into the Dallas/Fort Worth metro area to account for the vulnerability of the urban environment to the high rain rates associated with this afternoon and evening's storms. ....Texas coast... Moist southerly flow from the Gulf will interact with a shallow coastal front and generate rounds of training thunderstorms beginning this morning. Substantial instability (~1000J/Kg) and high PWATs (2-2.5") will be present as well. The 00z HREF 40km neighborhood probabilities of 24hr QPF exceeding 5 inches is between 20-60% for much of the central Texas coast from Houston to Corpus Christi. Recent heavy rainfall over this area made extending the slight risk into this area an easy call especially given the potential for training over Houston's metro area. Kebede Day 2 Valid 12Z Thu Oct 05 2023 - 12Z Fri Oct 06 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS... ....Texas... A slow moving cold front will support some moderate to locally heavy rainfall across southern Texas. The squall line propagating across the eastern part of the state at the end of the day 1 period is expected to make its way into the Gulf Coast by the beginning of the day 2 period (Thursday morning). This will lead to pockets of moderate to potentially heavy rainfall between Houston and Brownsville. Guidance has kept the heaviest QPF in northern Mexico just south of the Rio Grande for the past several runs. The lack of dynamic support for the squall line might be partly to blame for keeping QPF amounts low over south Texas. Kebede Day 3 Valid 12Z Fri Oct 06 2023 - 12Z Sat Oct 07 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST AND FAR SOUTH TEXAS.. ....Northeast... A strong trough for early October (below the 10th percentile) will be pushing across the Great Lakes and OH valley through Friday night. This trough will begin to take on a negative tilt, increasing mid and upper level forcing, and likely driving an uptick in shower and thunderstorm activity over portions of PA and NY by later Friday into early Saturday. Instability is weak, but given the height falls and overall increasing synoptic forcing, there will likely be enough instability to drive a shallow convective threat. Activity will progress eastward, but the tilting of the trough and generally unidirectional southerly flow should be enough to slow this eastward progression enough for there to be local areas of heavy rainfall. Areal averaged rainfall will probably not get much over 1", however swaths of 2-3" are possible...which could drive a localized flood risk. A second area to watch is further east across portions of Long Island into CT and western MA. The ECMWF has been consistent for a few runs now depicting a narrow axis of convergence and potential training convection Friday into Friday night. There is some signs of this in other models as well, but overall the signal is weaker and more inconsistent in the non-ECMWF guidance. Thus this is a low confidence forecast...but there does appear to be at least a low threat of excessive rainfall with this setup. There may be some moisture connection to Philippe by this time as well...so the moisture will be in place...just a matter of whether we can focus enough convergence and instability to drive the heavy rainfall threat. Certainly worth watching over the next few days and hopefully confidence increases with time. ....South Texas... A small Marginal risk was maintained across far south TX. Post frontal coastal convergence may be enough to trigger additional convective development on Friday, but this threat really comes down the timing of and speed of the cold front. Some guidance clears the front quicker and dries out south TX by Friday, so this is more of a conditional Marginal risk. The ECMWF remains a bit slower bringing in the drier air, and would support a localized flash flood risk. Thus opted to maintain the Marginal for now, but it may be removed on future updates if the quicker non-ECMWF solutions pan out. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_7O4NqTklnYoIVkKvW2hjLyMQmP-yzAmPMssxmce3Te9= NYlXfghXnUoeHcLHJZO3KKoDC7icJBTJO7-r9pQ4znELjUE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_7O4NqTklnYoIVkKvW2hjLyMQmP-yzAmPMssxmce3Te9= NYlXfghXnUoeHcLHJZO3KKoDC7icJBTJO7-r9pQ4fgkz5Ro$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_7O4NqTklnYoIVkKvW2hjLyMQmP-yzAmPMssxmce3Te9= NYlXfghXnUoeHcLHJZO3KKoDC7icJBTJO7-r9pQ4U_q10vE$=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) .