Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Apr 23 2023 20:30:09 FOUS30 KWBC 232030 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 429 PM EDT Sun Apr 23 2023 Day 1 Valid 1946Z Sun Apr 23 2023 - 12Z Mon Apr 24 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Day 2 Valid 12Z Mon Apr 24 2023 - 12Z Tue Apr 25 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....2030Z Update... The Marginal Risk for portions of the Central and Southern Plains has been removed. As noted in the prior forecast, widespread, organized convection is not anticipated and QPF amounts in the model guidance have continued to trend downward. The flash flood threat from any isolated stronger storms producing locally heavier rain will also be limited by the very dry antecedent conditions across the region. Any rainfall will more likely be beneficial. Putnam ....Previous Discussion... No significant changes were made to the Marginal Risk area encompassing much of KS and OK with this morning's forecast. A weak shortwave will move across the Marginal Risk area, which will support scattered showers developing along a front. The showers will be slow-moving, but will only support light rain. Behind this initial round, isolated to widely scattered stronger storms are expected to develop, particularly across the TX and OK Panhandles region. While these storms will be isolated, any stronger ones have the potential to cause isolated flash flooding if they move over sensitive areas. This is most likely overnight Monday night, as it will take until Monday night for any instability (which will still remain under 500 J/kg) to advect northward into this region. This remains a low-end Marginal Risk, as much of the area has well below normal soil moisture supporting high FFGs, and organized convection is not expected. A downgrade of the Marginal Risk area is possible with future updates. Wegman Day 3 Valid 12Z Tue Apr 25 2023 - 12Z Wed Apr 26 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE ARKLATEX REGION THROUGH EASTERN OKLAHOMA... ....2030Z Update... Minor adjustments were made to the current Slight and Marginal Risk areas centered on the ArkLaTex region based on updated model guidance. While there is a signal for rainfall totals in the 2-3" range, with locally higher amounts, the location for where convection will organize and propagate remains a bit nebulous and differs some amongst the guidance. Areal average rainfall totals over the Slight Risk area thus may be a bit too high in some locations, while underestimating the potential elsewhere, leading to a continued rather broad risk area. This covers a notable bi-modal distribution in the QPF with one maxima located over portions of northeastern Oklahoma/northwestern Arkansas to the north and a second maxima to the south over northeastern Texas. Regardless, given the noted environment conducive to heavy rainfall and the QPF totals, the potential for heavy rainfall has remained consistent while future hi-res guidance should help to narrow down any more specific adjustments to the location. The inherited Slight Risk over the Colorado Front Range was removed in coordination with the local Denver/Boulder and Pueblo forecast offices. An upper-level low will drop southeastward over the region as surface low pressure organizes over south-central Colorado, bringing moist southeasterly flow to eastern Colorado. While this will lead to a significant ramp up in low-level upslope flow during the day Tuesday along the Front Range, the precipitation for the event is expected to remain more stratiform in nature with forecast rainfall amounts likely distributed through the period. While some locally heavier short-term totals will be possible given the flow and previously noted increase in instability, any resultant impacts should be limited to minor street flooding. In addition, as the upper-level low approaches, snow levels will likely quickly lower starting Tuesday evening from around 8000 feet to 6000 feet, bringing a changeover to snow for locations in the foothills and Palmer Divide and limiting the areal coverage of any continued excessive rainfall threat. Putnam ....Previous Discussion... ....Arklatex Region... A shortwave moving through the flow across the Slight Risk area will help force convection as instability increases to around 1,000 J/kg Tuesday. A preexisting LLJ over central TX & OK will shift eastward through the day as a potent mid-level low ejects out of the Rockies across TX. Ahead of the low the flow will turn south to southeasterly, drawing the LLJ northward. The LLJ will also advect more unstable and moisture-laden Gulf air north into the Slight Risk area. While there is good agreement in the models that there will be swaths of precipitation exceeding 3 inches over the Slight Risk area, there's very poor agreement on where those swaths of higher rainfall totals will develop. Thus, the Slight Risk is largely covering the area of highest forecast rainfall totals, but it's expected that any flash flooding will occur with those higher and more localized swaths of higher rainfall, and not the entire Slight Risk area. Of course, as agreement (hopefully) improves with subsequent guidance, the Slight Risk area is likely to be refined to cover a smaller area. The inherited surrounding Marginal Risk area was trimmed towards the west quite a bit out of AL and eastern MS, as agreement that the focus for rain will be further west is also good. ....Colorado Front Range... The plume of moisture associated with the LLJ over the Arklatex Region will turn eastward into eastern CO on Tuesday. This easterly flow will also advect instability as high as 1,000 J/kg across eastern CO as well. The mid-level low will increase 850 mb surface flow to 15 kts along the Front Range. This orthogonal flow to the Front Range, the approach of a strong positively-tilted shortwave trough in the upper levels, and that instability will optimize precipitation rates in the region. There is much better agreement in the guidance that this will result in multiple inches of rain along the Front Range and adjacent foothills, as the upslope and instability work in tandem to increase total forecasted rainfall. The consensus rainfall totals have more than doubled from previous runs over a wide sampling of the guidance, which increases forecast confidence. Thus, a Slight Risk area was introduced with this forecast update. The elevation of the rain/snow line will need to be monitored, as it may approach some of the higher foothills, especially Tuesday night. Finally, this region has low FFGs, with hourly values between 1 and 1.5 inches over a large area of the northern Colorado Front Range. Localized areas around Denver are even lower. These rainfall rates are quite possible along the mountains as the upslope flow is supported by the atmospheric instability. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-I5TaFqWiOBB9p179XtHisIY3hbi7eWdVeOxTPTI1oQN= gr_-DJ3TxByNub3p3iEYvyjKvLVas0xppRBe3jpAkvrWEKk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-I5TaFqWiOBB9p179XtHisIY3hbi7eWdVeOxTPTI1oQN= gr_-DJ3TxByNub3p3iEYvyjKvLVas0xppRBe3jpAphN8PVk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-I5TaFqWiOBB9p179XtHisIY3hbi7eWdVeOxTPTI1oQN= gr_-DJ3TxByNub3p3iEYvyjKvLVas0xppRBe3jpALYdYjao$=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) .