Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Oct 29 2023 00:52:11 FOUS30 KWBC 290052 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 851 PM EDT Sat Oct 28 2023 Day 1 Valid 01Z Sun Oct 29 2023 - 12Z Sun Oct 29 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF NORTHEAST TEXAS THROUGH THE LOWER OHIO VALLEY...=20 Only minor changes were needed to the Slight risk area, with an isolated flash flood threat continuing into the overnight hours. One area of focus overnight will be across central AR, where continued rounds of convection near a stalled front will persist. Model guidance and water vapor imagery suggests weak mid level shortwave energy should help increase synoptic ascent over this area tonight, with convergence in the lower levels near the front also forecast to increase. Cell motions should remain generally parallel to the front, so repeated rounds of moderate to heavy rain appear likely. The main limiting factor will continue to be instability. Generally low CAPE values should keep convection from getting too robust overnight, and keep rainfall rates from getting too high. Recent HRRR runs suggest hourly rainfall totals up to 1.5" on a localized basis...and that seems reasonable. For the most part rainfall of this intensity should not exceed FFG. However portions of this area have already received 1-3" of rainfall, with an additional 1-3" (locally up to 4") likely. Thus with time ground conditions may saturate enough that the aforementioned 1.5" of rain within an hour could very well be enough to result in some pockets of flash flooding. The other area of focus will be across portions of east central TX near the stalled front. We should see a gradual uptick in convective activity near the front tonight as low level convergence increases along the boundary. Initially these cells should stay pretty tied to the boundary, and thus training will be a possibility. By 12z the front will finally begin to accelerate southward, which should bring to an end any flood risk. Activity tonight is not expected to be as intense or organized as last night...with generally lower instability and weaker overall forcing. The expectation is that the heaviest rainfall tonight will also stay just southeast of where the biggest impacts were last night. For these reasons a Slight risk should still suffice. With that said, would still expect to see some localized flash flooding where cells develop and stay tied to the front, with localized amounts of 2-4" expected. Chenard Day 2 Valid 12Z Sun Oct 29 2023 - 12Z Mon Oct 30 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM NORTHEAST TEXAS INTO THE MID-SOUTH... ....20Z Update... No significant changes were made to the ERO risk areas with this update. There remains some uncertainty as to the flood threat from Arkansas into the mid-Mississippi Valley, as the Day 2/Sunday flood threat in these areas will be highly dependent on how much rain falls during the Day 1/Saturday period. The area has been very dry lately, and there remains very little to no instability for the rainfall to feed off of. The area has been really dry prior to today/Saturday, so it will take quite a bit of rain between today and Sunday to cause widely scattered impacts. The greatest chance of those impacts is across central to northeastern AR. The broader Slight covers the hard hit areas of north-central and northeast TX, and in similar fashion, the mid-Mississippi and lower Ohio Valleys, though the flood threat is low here as well. More scattered convection with a bit better instability will be present across the TX Hill Country on down to the Big Bend area, but that would result in widely scattered footprints of heavier rain, not a pattern that is overly favorable to flash flooding. Wegman ....Previous Discussion... =20 ....Southern Plains into the Ohio Valley...=20 Precipitable water values will be in slow decline this period, maximizing above 1.75" early on before falling into the 1.25-1.5"range by Monday morning. A positively-tilted upper level trough will be approaching the region from the west and north causing some forward progression to the front across Texas and the Lower Mississippi Valley with time. The degree of low-level inflow and effective bulk shear available could organize convection into training bands during the day Sunday, assuming sufficient instability, as the flow is fairly uniformly out of the west-southwest with height, but should be fading somewhat Sunday night into Monday morning. MU CAPE of 250-500 J/kg should be sufficient to achieve hourly rain totals of 1-2" at times early on, which would be a problem across area terrain and places that rained heavily on preceding days, but the 00z NAM indicates the instability fades with time on Sunday and Sunday night. Local amounts in the 2-4" range are indicated by the available guidance overall. The biggest change to continuity was to extend the Slight Risk area farther southwest, which has as much to do with current soil saturation and ongoing heavy rainfall as it does with the possibility of additional heavy rainfall. There was some temptation to raising the threat level to a Moderate Risk, particularly early in the period when the parameters for heavy rainfall look best which would account equally for heavy rainfall continuing into Sunday morning and how much rain fell the preceding day in and near AR, but the minimal instability forecast was the biggest drawback. =20 Roth Day 3 Valid 12Z Mon Oct 30 2023 - 12Z Tue Oct 31 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Some consideration was given to a targeted Marginal south and east of Dallas, TX where lingering rainfall from the overnight will continue on Day 3/Monday, but amounts should almost universally remain under a half inch. Despite the low FFGs, those amounts should not be enough to cause flash flooding. It's likely the FFGs may slowly come up with more time under no or even light rain. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-06sLcYQnhw_U3NKi4OqR9LnGDpdNY2dexl8QzbFg_3q= 6mdz-ONICfXiE8Wlrk6uYK49txq3m0kAofTKirNGniPoIr8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-06sLcYQnhw_U3NKi4OqR9LnGDpdNY2dexl8QzbFg_3q= 6mdz-ONICfXiE8Wlrk6uYK49txq3m0kAofTKirNGjL1gGv8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-06sLcYQnhw_U3NKi4OqR9LnGDpdNY2dexl8QzbFg_3q= 6mdz-ONICfXiE8Wlrk6uYK49txq3m0kAofTKirNGghOjcsk$=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) .