Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Mar 06 2023 19:59:46 FOUS30 KWBC 061959 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 259 PM EST Mon Mar 06 2023 Day 1 Valid 16Z Mon Mar 06 2023 - 12Z Tue Mar 07 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Taylor Day 2 Valid 12Z Tue Mar 07 2023 - 12Z Wed Mar 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF EASTERN OKLAHOMA AND NORTHWEST ARKANSAS... Afternoon Update: Minimal changes to the risk areas were made based on the latest guidance with the afternoon update. The synoptic setup and ingredients still remain in place for heavy rainfall to develop late in the period (late Tuesday into early Wednesday) across portions of Oklahoma with very favorable ingredients in place characterized by a persistent inflow of anomalous moisture (PWs +3 sigma) and mean flow oriented nearly parallel to the storm motions favoring backbuilding convection on the southwest flank of the convective clusters. 12Z HREF neighborhood probabilities line up well with the Slight Risk area with a slight signal (15-25 percent) for 24-hr QPF ending 12Z Wednesday to exceed 3" with a small signal embedded over eastern Oklahoma of QPF exceeding 5". Hourly totals between 1-2" will be common and given some of this rainfall will fall on areas more sensitive due to recent heavy rainfall, scattered instances of flash flooding are likely. Taylor ---previous discussion--- ....Southern Plains...Ozark Plateau...Lower MS Valley...=20 The upper-level pattern over the CONUS will feature a persistent longwave trough over the West going through Tuesday into early Wednesday. Shortwave energy rotating through the base of this trough will amplify the downstream mid-level flow with a ridge axis focusing over the OH Valley and Mid-South. The backing flow aloft over the southern Plains and lower MS Valley will encourage the return of low-level southerly flow with moisture becoming increasingly focused north along a well-defined front draped across the Red River Valley of the South and stretching east across the lower MS Valley. Initially, the forcing along the front will be rather modest, but by Tuesday evening and into Tuesday night, the arrival of low-amplitude shortwave energy in the deep layer southwest flow aloft along with some uptick in the low-level jet should encourage the development of showers and thunderstorms along and north of the front for areas of central and eastern OK and gradually into adjacent areas of northwest AR and southwest MO in vicinity of the Ozark Plateau. Aside from increasing isentropic ascent, there will also be the nose of at least a modest axis of elevated instability with MUCAPE values of 500+ J/Kg advancing up across central/eastern OK that will facilitate convective development. The PW anomalies increase with time and reach upwards of 3 standard deviations above normal in the 00Z-12Z/Wednesday time frame. This will be reflective of not only the low-level Gulf of Mexico moisture transport, but also the infusion of mid and high-level subtropical moisture from the eastern Pacific Ocean which is showing up in a well-defined manner in the global models. The convection that does develop will likely be capable of producing rainfall rates as high as 1 to 1.5 inches/hour given the relatively efficient moisture transport regime and available instability, and the guidance supports the potential for multiple rounds of convection that may repeat/train over the same area. There are some wetter trends showing up in the guidance, and the potential will exist for some localized 2 to 4 inch rainfall totals Tuesday evening through early Wednesday morning where any repeating clusters of convection occur. The highest confidence in the placement of these heavier rains is over eastern OK where there are also some relatively sensitive soil conditions as a result of the most recent rainfall event. The latest NASA SPoRT soil moisture anomalies in the 0-40 cm layer are running high (locally in the 80 to 95 percentile range). USGS data shows streamflows running high along the OK/AR border and over into the Ozark Plateau. Thus, given the sensitive antecedent conditions, and heavier rainfall potential for the latter part of this period, we have opted to upgrade a portion of the Marginal Risk area to a Slight Risk with concerns for at least isolated to scattered instances of flash flooding. Orrison Day 3 Valid 12Z Wed Mar 08 2023 - 12Z Thu Mar 09 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM EASTERN OKLAHOMA THROUGH CENTRAL ARKANSAS... Afternoon Update: A Moderate Risk was introduced this forecast update given the increasing confidence for another round of heavy rainfall to develop late Wednesday evening through Thursday across portions of eastern Oklahoma into western/central Arkansas. Another surge of highly anomalous moisture (PW anomalies +2 to +3) combined with sufficient instability (MUCAPE upwards of 1000 J/kg) developing on the surging low-level jet Wednesday night (NAM 850 mb inflow around 40 kts 06-12Z) provide the necessary ingredients to clusters of showers and thunderstorms producing heavy rainfall. 12Z guidance has begun to cluster spatially on eastern OK into western/central AR with fairly strong signal for another round of 1-3" (locally higher amounts) for the 24-hr period ending 12Z Thursday. This rainfall, combined with the previous day convection, could yield areal averages of 3"+ in the 48-hr period. Given the antecedent conditions (AHPS 14 day precip departures are 200-300 percent of normal and NASA SPoRT soil moisture percentiles in the 10-40 cm layer are above 80 percent in areas) and likely overlap of heavy rainfall areas, a Moderate Risk was collaborated with the affected WFOs for the potential for more numerous instances of flash flooding, some locally significant, late Wednesday night through Thursday. Taylor ....Southern Plains and Lower MS Valley... Some relatively organized areas of heavy showers and thunderstorms may be impacting portions of southeast OK and western to central AR early in the period as the shortwave energy driving convection Tuesday night over eastern OK advances off to the east across the lower and middle MS Valley. This batch of convection should encounter a lack of instability in time as it advances downstream and should gradually weaken which will foster at least a solid 6 to 12 hour break in any organized threat of heavy rainfall across the southern Plains and lower MS Valley. However, by Wednesday evening, the low-level jet should begin to reload/strengthen along a quasi-stationary front still situated near the Red River Valley and extending east-southeast across the lower MS Valley which will refocus favorable moisture and instability transport. This boundary will likely lose some latitude during the day as a result of strong high pressure attempting to build south across the Midwest and into the eastern U.S., but there will be a stronger shortwave trough crossing the Four Corners region that should begin to edge out into the High Plains by early Thursday and this will favor increasing ascent over the southern Plains and should facilitate new convective development along and north of the front. The guidance suggests multiple rounds of nocturnally enhanced convection focusing over areas of central and eastern OK Wednesday night, with convection then attempting to advance downstream across a large area of western and central AR through Thursday morning. Additional rainfall amounts of 2 to 4 inches may be possible locally, although the guidance does show some spread latitudinally with the placement of where the more organized convection will tend to focus. Given the expectation of wet antecedent conditions for areas of especially eastern OK and western/central AR, this area will see the greatest threat of flash flooding and the Slight Risk area inherited for this period will be focused predominantly over these locations. Meanwhile, a broader Marginal Risk will continue to encompass adjacent areas of the southern Plains and lower MS Valley region where there will certainly be a threat of locally heavy showers and thunderstorms given close proximity of the aforementioned front. Orrison Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6RbvS8K_DFBhTnxeJOt9gLuYDmLTFCQwmhkWwxkuU7oY= v_r8JRvrkKS4Bq97VInWYdwLSh1oxI20UuTFLOnmwjS0xsg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6RbvS8K_DFBhTnxeJOt9gLuYDmLTFCQwmhkWwxkuU7oY= v_r8JRvrkKS4Bq97VInWYdwLSh1oxI20UuTFLOnmmV__Lo8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6RbvS8K_DFBhTnxeJOt9gLuYDmLTFCQwmhkWwxkuU7oY= v_r8JRvrkKS4Bq97VInWYdwLSh1oxI20UuTFLOnmYNwQ5xA$=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) .