Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Oct 05 2023 00:54:55 FOUS30 KWBC 050054 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 853 PM EDT Wed Oct 04 2023 Day 1 Valid 01Z Thu Oct 05 2023 - 12Z Thu Oct 05 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN AND NEAR THE ARKLATEX... ....Southern Plains... The degree of instability and effective bulk shear present late this afternoon was an environment conducive to mesocyclones and forward propagating lines of convection. Across much of northern TX and OK, the forward propagation has begun. However, warm advection showers and thunderstorms are moving back into the ArkLaTex, which received 5-9" of rainfall today. Over the next few hours, the highest risk of flash flooding will be there, an area that has been saturated. Hourly rain totals to 3" are possible in this environment before the entire convective pattern can progress as a forward propagating arc that drives south to southeast later tonight into Thursday morning. Given the combination of factors, and that the 18z HREF has a 15% chance of 8" of rain in the area hit hardest, was strongly tempted to upgrade to a High Risk -- High Risk impacts are possible. Flash flood emergencies cannot be ruled out in the ArkLaTex over the next few hours and it's possible that local rainfall amounts, including the earlier activity, exceed 12" before the rain comes to an end. Mesoscale precipitation discussions should be consulted for more up to date information overnight. ....Texas coast... The guidance suggests an uptick in heavy rainfall again for portions of the Middle and Upper Texas coasts as a squall line approaches from the northwest. The incoming convective line could broaden as it reaches the richer moisture in this region (precipitable water values towards 2.25") which would increase rainfall efficiency, and cell mergers between any north-south banding coming in off the Gulf of Mexico and the incoming forward propagating convection would lead to hourly rain totals of 3"+.=20 As such, left the Slight Risk for areas from Houston/Galveston southwest intact, which was supported by the 18z HREF guidance.=20 Localized Moderate Risk impacts cannot be ruled out. Roth 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... ....2000Z Update... Main area of interest continues to reside across TX where a cold front will be propagating south through the south-central portion of the state. Current forecast consensus is actually fairly solid in the overall evolution of the convective pattern through Hill Country and points south. A little more deviation on the convective coverage over the RGV and Gulf coast of TX thanks to some mesoscale features that will enter the fray by Thursday afternoon. The frontal convergence signature over the Gulf coast coincides with an expected sea breeze propagation inland during and ahead of the time of fropa. Ensemble bias-corrected PMM QPF and 75th percentile NBM are both indicating totals between 2-3" locally from around KCRP down into South Padre. 12z HREF probabilities for 3"/3-hrs are running between 20-30% with a few local 40% maxes being indicated within the zone of best convergence. This will be the focal point of highest impact for the period with perhaps the area southwest of Houston being a secondary focus early on as the MCS overnight pushes into the area and finally fizzles out overhead. Overall the probabilistic overview maintains a more MRGL flash flood risk given the evolution and higher FFG's within the coastal plain of TX.=20 Kleebauer ....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... ....2000Z Update... Very little change to the D3 ERO as guidance remains steadfast on the synoptic scale evolution and progression of the upper trough over the Great Lakes. Cold front across south TX has trended somewhat faster with global deterministic which is reflected in a slight southward shift of the QPF max within the ensemble means. Other guidance like the NAM and some hi-res beyond 12z Fri have the cold front closing in on the RGV by the end of the run. This setup and expected evolution is typical with early season cold fronts, but the bias has been for globals to be a bit on the slower end of observation and hi-res to be a bit too aggressive in the earlier runs before correcting. Taking bias into consideration, did cut the northern extent a bit further south, but maintained the MRGL risk at least into Deep South Texas where a convective scheme is still possible on the first half of Friday given the forecast SBCAPE along and ahead of the cold front. Main target area will likely be the urban corridors along the RGV. The Northeast U.S is still looking solid for some potential convective induced flash flooding concerns, especially over the interior where terrain elements and location with the best mid and upper-level forcing will be located. General instability axis is meager with MUCAPE forecasts running under 500 J/kg, but when looking at the forecast bufr soundings across the Northeast, you can see the steepening mid-level profile associated within the corridor of best ascent ahead of the mean trough. Area moisture field will be improving as well as advective pattern on southwesterly flow aloft leads to areal PWAT anomalies approaching 1-1.5 deviations above normal. Latest SREF member plots show a decent opportunity for 1" totals across the Northeast with even a few ARW members showing the possibility for 2" totals within a neighborhood probability scheme. The threat itself is on the lower end of the MRGL definition, but with more hi-res depictions coming into play, we could easily see some examples of potential convective impacts allowing flash flooding concerns. Kleebauer=20 ....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!8eJxq7RJCeg7e6JEGteewWb9yyQjOAj6Lo1qHatI-sYt= j9CjYgEDB0zy8kQvwjjifb4W7dTbraOVm5IVPLX6ZwHpQ7s$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8eJxq7RJCeg7e6JEGteewWb9yyQjOAj6Lo1qHatI-sYt= j9CjYgEDB0zy8kQvwjjifb4W7dTbraOVm5IVPLX6v4NTF6o$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8eJxq7RJCeg7e6JEGteewWb9yyQjOAj6Lo1qHatI-sYt= j9CjYgEDB0zy8kQvwjjifb4W7dTbraOVm5IVPLX6qS9T-0Y$=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) .