Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Aug 12 2024 15:44:11 FOUS30 KWBC 121543 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1143 AM EDT Mon Aug 12 2024 Day 1 Valid 16Z Mon Aug 12 2024 - 12Z Tue Aug 13 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF EASTERN COLORADO AND WESTERN KANSAS, EASTERN OKLAHOMA AND FAR WESTERN ARKANSAS, AND THE COASTAL CAROLINAS... ....16Z Update Summary... The SLGT risk across Colorado and KS was expanded back to the west to include the foothills of the Rockies in coordination with WFO Boulder. The SLGT risk across Oklahoma was adjusted to remove areas where flash flooding potential is very low or no longer expected 16z. The threat across OK and AR will likely end after 18z with removal of the risk area anticipated at the 01z update. General continuity across the Carolinas with the current SLGT risk. Elsewhere, only minor adjustments were made to reflect recent radar and hi-res trends in the QPF field(s). There were no significant changes made to the MRGL risk across the west with the heaviest rainfall expected over Northern UT, but away from the more sensitive zones.=20 Kleebauer ....Northeast OK and Western AR... Complex from overnight is currently migrating south with a Flash Flood Warning in effect across a small portions of Southeastern OK. Environment is borderline in terms of relative instability, but upwind propagation vectors do indicate the chance at pockets of training/back-building convection within a small zone over Eastern OK into the OK/AR border. The threat has been handled okay by a few of the CAMs with a general consensus of the threat continuing until about 18z before dwindling as the energy associated with the threat shifts east and weakens through the afternoon. The previous SLGT risk was shrunk about 50% to only cover the current areas impacted at this hour. Expectation is for the threat to dissolve in a few hours with a removal of the risk area likely occurring the next update.=20 Kleebauer ....Eastern CO into KS... 16Z Update: The biggest change in the forecast was a westward expansion of the SLGT risk out towards the Foothills of the Rockies, including the Denver metro in coordination with WFO Boulder. Elevated PWATs located across Colorado with sufficient afternoon destabilization will likely spawn a scattered area of showers and storms capable of producing rainfall rates up to 1-1.5"/hr in the strongest cores. Burn scar locations within the foothills are at more sensitive state with regards to heavier precipitation, so the expansion was to help cover for the localized threat expected this afternoon and early evening before the threat shifts more into the Front Range and points east.=20 A general continuity in the HREF mean QPF and associated probability fields allowed for a maintenance of the SLGT risk area located over Eastern CO into portions of Western KS, mainly along and near I-70. A stationary front was analyzed across the above area with a focus of elevated theta-E's situated within the narrow corridor of the front. This aligns with the CAMs initiation point of convection across Eastern CO, shifting eastward with mean flow and expected cold pool propagation from any cells that develop. LLJ will materialize after 00z with the nose of the jet intersecting the region between 03-05z, a time frame where models do diverge on the intensity of any cells in the region. A solid neighborhood probability for >3" is positioned over East-Central CO (50-70%) with lower, but no insignificant probabilities located across Western KS (25-40%). Rainfall rates between 1-2"/hr will be most common within the stronger cells, but the convection could produce some intra-hour rates higher allowing an enhanced threat, especially across Eastern CO where diurnal destabilization pattern will be at its peak before shifting to a more nocturnal threat downstream. The SLGT risk was maintained across the region from=20 previous forecast given the signals above, but the threat is on the lower end of the SLGT risk threshold due to drier antecedent=20 conditions in place across the outlined area.=20 Kleebauer ...Previous Discussion.. Convection is expected to develop over portions of eastern CO by later this afternoon. Models indicate that a weak shortwave will eject eastward originating from the ongoing convective activity over UT...with an upper jet to the north supporting at least weak divergence aloft in its right entrance region. Meanwhile, easterly flow in the low levels will advect in higher moisture, with PWs forecast to increase towards 1.5", above the climatological 90th percentile for mid August. Given the forcing and moisture in place, the forecast ~2000 j/kg will allow for robust convective development. With deep layer mean flow from the west, and low level flow from the east, cells may initially end up slow moving, with cell mergers a possibility. Given the environment in place, tend to think some of more aggressive high res models may end up closer to reality for this event. This would support localized rainfall upwards of 3-5" over portions of eastern CO and far western KS. HREF QPF, environmental ingredients in place, and the CSU machine learning ERO are all supportive of an isolated to scattered flash flood threat. Thus we will go ahead and upgrade this region to a Slight risk. This activity is likely to grow upscale overnight and then move eastward across KS. Convection will likely be quicker moving by this time, although can not rule out a west to east training axis setting up. If this were to occur then Slight risk level impacts may continue overnight across KS. However given some lingering uncertainty on these convective details, and the fact that soil saturation and streamflow conditions across KS are running below average...will keep the risk level at Marginal for now. ....Carolinas... 16Z Update: Little to no change was made to the SLGT risk across the Eastern Carolinas as HREF blended mean QPF distribution of heaviest rainfall and relative probability fields remain fairly unchanged from previous forecast issuance. Expect the main threat to initiate between 18-21z with ongoing convection likely through 03z before dissipating overnight with the loss of diurnal heating. Kleebauer=20 ...Previous Discussion.. Upgraded portions of the eastern Carolinas to a Slight risk for=20 today. We should see another round of convection near the stalled=20 front this afternoon, with HREF 3" neighborhood probabilities in=20 the 40-70% range. Totals of this magnitude will likely stay=20 isolated to scattered in nature...however given the increased=20 sensitivity over the area after Debby and post Debby=20 rainfall...expect another day of isolated to scattered flash=20 flooding. Today is a bit trickier of a forecast with a weak low=20 along the front. While the higher probabilities of excessive=20 rainfall are along the coast this afternoon, we could see at least=20 a localized flash flood risk a bit further inland this morning into the early afternoon as well. Chenard ....Southwest... A broad Marginal risk remains across much of the=20 Southwest, with isolated flash flooding possible over this large=20 geographic region. The coverage/organization of intense rainfall=20 may be a bit less than what we saw on Sunday, but nonetheless an=20 isolated risk likely continues. Do see some more elevated=20 probabilities over southern AZ, so may end up with a bit better=20 concentration of convection there, but still seems to fall shy of=20 Slight risk levels. Portions of central/northern UT also stand out=20 as potentially seeing greater convective coverage...but this signal is generally north of the more sensitive basins in UT...and thus=20 not thinking a Slight risk is needed at this time. Chenard Day 2 Valid 12Z Tue Aug 13 2024 - 12Z Wed Aug 14 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF EASTERN KANSAS INTO MISSOURI AND NORTHERN ARKANSAS... ....Portions of KS/MO/AR... Likely to have some ongoing convection at 12z Tuesday across portions of southeast KS and southwest MO. This will be what remains of the Monday night organized convection that is expected to move across KS. Most indications are that this activity should be on a weakening trend after 12z, so while an isolated flash flood threat may persist, not currently anticipating a widespread threat. The greater threat is the likelihood of training convection Tuesday night across this corridor. This appears to be evolving into a potentially significant training convective setup. Robust instability will be advecting in from the southwest, likely resulting in a tight instability gradient somewhere in the vicinity of eastern KS into western to central MO. Meanwhile the low level jet will increase, resulting in strong and persistent moisture transport into the region. Corfidi vectors end up becoming quite weak, and what movement they do indicate is generally parallel to the expected moisture convergence axis and instability gradient. Thus we very well may end up with repetitive backbuilding upscale convective growth over this area Tuesday night. There remains some uncertainty on the exact details of convective evolution and placement of highest totals...but confidence is increasing in an excessive rainfall threat, with potentially significant localized impacts. The coverage of excessive rainfall will likely be quite a bit narrower than the Slight risk area...however it remains broader to account for uncertainty in the maximum rainfall axis. ....Southwest... Scattered convection is again likely from portions of AZ/NM northward into ID/WY. There will be a bit more in the way of westerly flow compared to earlier days, which may allow cells to move along at a faster pace. This should keep the flash flood risk isolated in nature...however heavy rainfall rates will again be probable in the more intense cells...which could result in a localized flash flood within the broad Marginal risk area. ....Southeast... Convection is again likely Tuesday across portions of GA/SC/NC. Overall it is an interesting setup, with model guidance suggesting a more well defined low over SC by this time, with stronger forcing and flow aloft moving in as well. This should seemingly allow for some strong convective development...with the stronger flow aloft supporting quicker cell motions off to the southeast. These cell motions would typically suggest a lowered flash flood risk...however we will need to keep an eye on how the convergence axis east of the low plays out. This boundary could act as a focus for some training convection, which would offset the quicker cell motions and still produce a flash flood risk. Overall think enough uncertainty on these details remains to keep the risk level at Marginal for now. However will continue to monitor trends and would not rule out an eventual Slight risk upgrade over portions of the area. Chenard Day 3 Valid 12Z Wed Aug 14 2024 - 12Z Thu Aug 15 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MIDDLE MISSISSIPPI VALLEY... The threat of excessive rainfall shifts into Plains and MS Valley on Wednesday. Likely to have some ongoing risk to start the day across portions of central MO...with the threat likely shifting to the north by late in the day into the overnight hours. It again looks like a favorable environment for training/backbuilding convection with an increasing low level jet into a warm front. Strong and persistent moisture transport into this boundary is typically supportive of backbuilding convection and a flash flood risk. There is some model QPF spread, with convection and locally heavy rainfall likely all the way from the Dakotas and MN into MO. However it looks like the best ingredients for excessive rainfall are over MO to start the day, and then across portions of IA and western IL later in the day/overnight. This is where the better instability and most persistent moisture transport is currently forecast. And while the location of greatest QPF differs from model to model and run to run...this axis is favored by the majority of solutions. Thus will place the Slight risk here for now, with the understanding that some adjustments are likely as the event nears. So while confidence on the exact location is only average...do think we are getting towards above average confidence in the impacts from this event...with at least isolated to scattered flash flooding expected, some of which could be locally significant. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!68cnQpoMqC-kjo7CBViV2SMi3_EXeWWwceVpkuJdBzXr= EvQbMYoNzWjmbSvqqYXJMzmAGhBHwg0jEzP_0CBhcEC6RSk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!68cnQpoMqC-kjo7CBViV2SMi3_EXeWWwceVpkuJdBzXr= EvQbMYoNzWjmbSvqqYXJMzmAGhBHwg0jEzP_0CBhYiVTCbU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!68cnQpoMqC-kjo7CBViV2SMi3_EXeWWwceVpkuJdBzXr= EvQbMYoNzWjmbSvqqYXJMzmAGhBHwg0jEzP_0CBh75curIM$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .