Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Oct 04 2023 08:36:36 FOUS30 KWBC 040836 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 435 AM EDT Wed Oct 04 2023 Day 1 Valid 12Z Wed Oct 04 2023 - 12Z Thu Oct 05 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS... ....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 TEXAS, THE ARKLATEX, AND THE MIDWEST.. ....2000Z Update... The previous SLGT risk was dropped for the D3 time frame as the QPF forecast has continued to slowly trend down with the heaviest precip now being aligned over the southwest Rio Grande Valley (RGV) between Laredo and Zapata and more localized heavy rainfall to the northeast over east-central TX. There's still an opportunity for an upgrade to a targeted SLGT over the RGV if guidance trends better for convergence over south TX and heavier precip signals re-appear within the population center along the river valley. Until then, the current MRGL should suffice, and was in agreement with some of the local WFO's within the risk area. An additional Marginal Risk was added to the D3 time frame across part of the Midwest. More on this potential below.... ....Midwest... A digging upper trough over the northern plains will create a favorable jet streak pattern across the Midwest with the central Midwest area over IL/IN/southern MI in the best ascent focused under the influence of the right-entrance region of a departing speed max. Shortwave energy ejecting to the northeast out of the Mid-Mississippi Valley will also traverse overhead creating a better dynamical structure for the affected area with widespread showers and storms propagating northeast within the main steering pattern ahead of the mean trough. Local QPF maximum between 1.5-2" is now forecast with even the ensemble bias-corrected QPF indicating the potential for 2+" over northern IN where the best ascent and low-level instability axis is located, meaning more convective potential. Rich Gulf moisture advecting ahead of the mean trough and attendant cold front will lead to PWATs approaching 1.5-2 deviations above normal, more than sufficient for heavy rainfall potential for a 3-6 hr period when precip moves through the area. Locally heavy rainfall will affect urban corridors the most, especially places like northwest IN over into the Chicago south side where FFG's are low due to urban sprawl on tip of Lake Michigan. A Marginal Risk was introduced given the threat. Kleebauer ....Southern Texas... There's a decreasing signal for heavy rain over parts of southern Texas along the southern periphery of a cold front on Thursday. Upper-level energy associated with the deep parent low in Canada will swing through Texas and provide forcing for showers and thunderstorms to develop across southern portions of the state. The guidance has trended farther north with the favorable jet dynamics, however this does not mean that the excessive threat does not exist as there are still some areas which have received heavy rain recently and are susceptible to flash flooding within the current risk area. The latest guidance shifting the axis of heaviest rainfall into northern Mexico just south of the Rio Grande among other factors has lead to a decrease in risk from a SLGT to a Marginal Risk across the state, extending north into the Arklatex, mainly from residual impact from the morning MCS expected on Thursday. Kebede/Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4cIPXvKCMY6yu0ngIexNhftNEVf0z8gTcBVOOpXmtzqx= vl9f3wVXS51-gEPwlOvX5E0MAHwYrJpkgTruwdW4rE67UW4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4cIPXvKCMY6yu0ngIexNhftNEVf0z8gTcBVOOpXmtzqx= vl9f3wVXS51-gEPwlOvX5E0MAHwYrJpkgTruwdW4S5IinZs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4cIPXvKCMY6yu0ngIexNhftNEVf0z8gTcBVOOpXmtzqx= vl9f3wVXS51-gEPwlOvX5E0MAHwYrJpkgTruwdW4V3rIyIc$=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) .