Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu May 11 2023 08:28:33 FOUS30 KWBC 110828 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 427 AM EDT Thu May 11 2023 Day 1 Valid 12Z Thu May 11 2023 - 12Z Fri May 12 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL PLAINS AND THE LOWER MS VALLEY... ....Lower MS Valley into the OH/TN Valley and Southeast... A broad mid level trough with embedded vorticity centers is located over the Southern Plains and Lower MS Valley this morning. Convection is likely to be ongoing at 12z across portions of AR/LA and far northeast TX. There is some concern that a locally significant flash flood risk could exist into this morning across portions of far northeast TX into northern LA and possibly portions of AR. The south/southwest flanks of these shortwaves can often be a favored region for training/backbuilding convection. Persistent convergence on this flank, along with plentiful upstream instability can allow for continued repeat/training convection. The environment is certainly conducive for heavy rainfall with a deep saturated layer and high freezing levels supporting efficient warm rain processes. Thus 2-3" in an hour rainfall is possible...which combined with any backbuilding/training can quickly result in excessive rainfall totals. Many of the 00z HREF members do show an evolution like this...which has resulted in storm total 3"+ neighborhood probabilities over 70% and 5"+ over 40%. The environmental setup and 00z HREF signal does raise a concern for a potential organized, and locally significant, flash flood event into the morning hours. There remains some uncertainty with regards to coverage and organization, so we will need to closely monitor observational trends. However do think the threat is enough to go ahead with a Moderate risk in the new ERO. This is mainly for the morning hours, with the expectation that convection should begin to forward propagate eastward a bit more by later this morning into the afternoon...reducing the flash flood risk. Elsewhere a broad Marginal risk extends from OK east into the OH/TN valley and southeast into FL. An isolated flash flood risk likely exists anywhere within this region, although at this time not seeing a strong signal for a more organized threat. .....Central Plains... A broad area of rain will likely be ongoing at 12z across eastern CO. A strong deep layer low will provide ample forcing for heavy rainfall, and PWs will remain well above average...so the main driving factor for where flash flooding may become an issue will be instability. The 00z HREF suggests activity will become increasingly convective in nature through the morning, with 1"+ in an hour probs quickly increasing. The better chance of these higher rates is expected to be from northeast CO into northwest KS and southwest NE...with rates probably gradually dropping off as you head west in CO. Thus think a Slight risk should suffice for most of north central CO, where prolonged rainfall with embedded higher rates should produce an isolated to scattered flash flood risk...but the lack of more significant rates preventing anything more widespread. However we are a bit more concerned with portions of southwest NE, northwest KS and immediate adjacent areas of CO. Much of this area has already received locally heavy rainfall over the past 24 hours, likely making the region a bit more susceptible to additional flooding. The 00z HREF has neighborhood probs of 40-60% chance of exceeding 3" in 24hr here, and as mentioned above, 1"+ an hour probabilities ramp up between 16z-00z. There is some chance the heavier convection today ends up moving through pretty quickly...but given the persistent slow moving forcing, still worried there could be some repeat/backbuilding activity. Given this additional rainfall potential on top of what has already occurred...do think more numerous flash flooding could occur, some of which could be locally significant. Thus we will go ahead and upgrade to a small MDT risk over this area. ....Northern Plains... Plentiful forcing and moisture will result in widespread moderate to heavy rainfall across portions of eastern MT into the western Dakotas. Persistent low level convergence will help focus areas of convection...with instability likely the main differentiator between areas that see a prolonged steady rainfall vs higher rates and a potential flash flood risk. Some higher rates are possible over eastern MT early in the period, but then the focus is expected to shift eastward into ND/SD. There has been a notable eastward shift in the 00z guidance...putting more of a focus into central/western ND and western SD instead of eastern MT. This isn't to say the threat is gone in MT, as scattered to widespread 1" rainfall is excepted with localized amounts of 2-3". So still enough to result in an isolated to scattered flash flood threat. However the greater risk for more prolonged inch plus an hour rates, and thus higher total rainfall amounts, now appears to be a bit further east into ND and SD, where forecast instability is higher. The 00z HREF is highlighting a maximum near or just west of Bismarck, along what appears to be a corridor of lower level convergence maximum on the nose of the better instability. Chenard Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6k-RrT1bp4LDlwTm4iCD1i6rLkYFJRPd3ceWVtBS0ffj= vEMJP9bO9kWF4ON4q_cKoGQ6QtZmCNGnTEdX7YI-udmjTj8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6k-RrT1bp4LDlwTm4iCD1i6rLkYFJRPd3ceWVtBS0ffj= vEMJP9bO9kWF4ON4q_cKoGQ6QtZmCNGnTEdX7YI-O8-61mE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6k-RrT1bp4LDlwTm4iCD1i6rLkYFJRPd3ceWVtBS0ffj= vEMJP9bO9kWF4ON4q_cKoGQ6QtZmCNGnTEdX7YI-9QBOIpw$=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) .