Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 11 2023 00:33:35 FOUS30 KWBC 110033 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 832 PM EDT Thu Aug 10 2023 Day 1 Valid 01Z Fri Aug 11 2023 - 12Z Fri Aug 11 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN NEW ENGLAND, MISSISSIPPI AND ALABAMA, AND THE SOUTHWEST... ....Northeast... A maturing low pressure system will track across portions of southern New England overnight, where an inherited Slight risk was maintained (for portions of eastern CT, RI, and southeastern MA). These areas have the best potential of realizing localized short-term totals of 2-3" (per hourly HRRR runs since 18z), where instability is maximized with SB CAPE of 1000-1500 J/kg (per 23z SPC RAP analysis). Farther west and north (across the remainder of New England), localized streaks of 1-2" may lead to isolated instances of flash flooding (where a Marginal risk remains in effect). Wet antecedent conditions are largely driving the flash flood threat across interior portions of New England, as NASA SPoRT-LIS 0-1 meter soil moisture anomalies are at or above the 90th percentile for the majority of the area. The risk of flash flooding is expected to end for the bulk of the region by 06z, but will likely last beyond 12z for portions of ME. ....Interior Southeast... The inherited Slight risk area was maintained for this update for portions of east-central MS and west-central AL, as convection is beginning to proliferate in association with a weak shortwave impulse embedded within northwest flow aloft. High instability (SB CAPE of 2500-5000 J/kg) and precipitable water (1.9-2.2 inches, above the 90th percentile of JAN/BMX sounding climatology). Should convection grow into an MCS overnight (which seems likely, given SPC RAP analysis indicating MCS maintenance values of 50-70%), some training along the southward flank of the MCS is possible (given deep flow being nearly parallel to the likely outflow boundary orientation). This could result in short-term localized totals of 2-4", and the Slight risk remains generally confined to areas with wetter antecedent conditions and lower FFG (and thus greater potential vulnerability to impacts from heavy rainfall). ....Southern Nevada, Southern Utah, Northern Arizona... 01z Update: The only changes for this update were to shrink the western extent of the inherited Slight and Marginal risk areas (and expand the eastern extent of the Marginal risk), based on recent observational trends. The bulk of the rainfall should end by 06z as instability wanes with rapidly decreasing solar insolation. Previous discussion: The outlook was upgraded to a Slight Risk for portions of the Southwest, largely centered on the UT-AZ border region. Although there will be a broader area of typical isolated to scattered convection over a greater portion of the Southwest region, we did have concern about a concentrated area with greater potential for high rainfall rates and even some backbuilding of convective elements. This was being facilitated by a mid-level circulation from the remnants of Eugene in the East Pacific, now working into Southern California. A corridor of enhanced 700mb flow, around 20-25 knots was expected to develop up the Lower Colorado River basin and pointed into NW AZ. This low-level inflow should be roughly equivalent (or even slightly stronger) than the overall mean flow, a configuration that can favor some backbuilding. Furthermore, RAP forecasts do show a pool of 1000-2000 j/kg SBCAPE centered in NW/NC AZ this afternoon, and this upstream pool of instability would also favor backbuilding. The 12Z FGZ sounding was quite moist and unstable, with over 1000 j/kg of CAPE (above the 90th percentile) and nearly 1" PW at high elevation (near the 90th percentile), which provides some observational evidence for these concerns. Hi-res models do show a concentrated area of convection near the UT-AZ border with some backbuilding evident in the simulated reflectivity. Given this, a Slight Risk was coordinated with the WFOs in the region. The Slight Risk does overlap some vulnerable terrain with numerous slot canyons, raising concern with any convective elements that can sustain heavy rain in a particular basin for an hour or more. Churchill/Lamers/Bann Day 2 Valid 12Z Fri Aug 11 2023 - 12Z Sat Aug 12 2023 ....THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL IN THE SOUTHEAST, OZARKS, CENTRAL GREAT LAKES, DESERT SOUTHWEST, AND CENTRAL SIERRA NEVADA... ....Ozarks and Mid-Mississippi Valley into the Southeast... The broad Marginal Risk was maintained over the Southeast region, and was expanded back to the northwest to include a large portion of the Ozarks and Mid Mississippi Valley. The Marginal Risk area was placed near a slow-moving front that will persist across the region, with the front embedded in the northern portions of a plume of strong instability and deep moisture. Ensemble probabilities indicate that CAPE in excess of 2000 j/kg and PWs in excess of 1.8 inches are likely (over 60 percent chance) during the afternoon and early evening hours over most of the area outlined in the Marginal Risk. Hi-res models show a scattering of more intense rainfall with associated scattered non-zero chances of FFG exceedance. However, there is not a consistent signal on placement or convective evolution, so the risk level was kept at the Marginal level for now. ....Southern Michigan, Northern Indiana, Northwest Ohio... Models continue to show convection developing ahead of an advancing cold front in the Great Lakes region on Friday. For the most part, the convection depicted in hi-res model simulated reflectivity appears to be progressive and unlikely to linger in any one location for very long. Nevertheless, the Marginal Risk was maintained due to an increasingly focused signal for heavy rainfall on southeast Michigan, and to a lesser extent in some of the adjacent portions of the region. The HREF does show 40km neighborhood probabilities of 3 inches of rainfall (total) in the Day 2 period as high as 30 percent in southeast Michigan, which would correspond to a 5-10 year average annual recurrence interval. However, this rainfall is also likely to fall in much shorter bursts, so there does seem to be some risk of excessive rainfall and flash flooding. We did consider the possible need for a small Slight Risk, but trends will be monitored in future guidance. ....Eastern Arizona and Western New Mexico... A northward surge of moisture, increasing precipitable water values to slightly above average, and the entrance region of an anticyclonically curved, strengthening, upper level jet streak may combine to lead to an increase in convective coverage near the Arizona/New Mexico border region on Friday. Instability tends to be a factor more highly correlated to flash flood activity in the Southwest region, so overall the threat looks to be fairly isolated at this time. The instability should be limited by a fairly deep, dry sub-cloud layer. ....Central Sierra Nevada... A localized excessive rainfall and flash flooding threat continues to be forecast for the Central Sierra Nevada region, where there is expected to be a surge of deeper moisture and a lingering mid-level vorticity center from the now-decayed tropical cyclone (Eugene) in the East Pacific. A Marginal Risk was maintained, and expanded slightly into central Nevada, where hi-res models do indicate the potential for some localized heavy rainfall in the lee of the Sierra Nevada. The main question continues to be whether or not denser cloud cover associated with the deeper moisture limits heating and instability more than models are currently anticipating. Lamers Day 3 Valid 12Z Sat Aug 12 2023 - 12Z Sun Aug 13 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A BROAD AREA FROM THE NORTHEAST, TO THE OHIO VALLEY, THE CENTRAL PLAINS, AND INTO THE FOUR CORNERS... The main change to the Day 3 ERO was to connect two large Marginal Risk areas along an extensive front, and trim out the portion of the risk area on the Gulf Coast where model QPF signals had become much weaker. The broad Marginal Risk area was supported by CSU machine learning guidance which also showed a large area of 5 to 15 percent excessive rainfall probabilities, and this was used to fine tune the placement of the Marginal Risk. Although the convective evolution and forcing mechanisms may be slightly different in various regions of the country, the fundamentals are basically the same. Ensembles indicate that above average PWs (greater than 1.5 inches in the central and eastern U.S.; greater than 1.0 inches in the Southwest) and ample instability are likely along most of the front, particularly in the afternoon and evening hours. Therefore, the threat of 1-2 inch per hour rain rates and localized excessive rainfall issues are expected in the entire Marginal Risk area, but confidence on placement and timing is low. Lamers Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9niNJXmDeovrKGbfgPkLXqyWSDFfkyzNQTRfktLuLQH2= eH833IhfDpUu__E_vYhw2hht_MJnF7mJ2RPjtCAOsByYc-U$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9niNJXmDeovrKGbfgPkLXqyWSDFfkyzNQTRfktLuLQH2= eH833IhfDpUu__E_vYhw2hht_MJnF7mJ2RPjtCAODbGWEoU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9niNJXmDeovrKGbfgPkLXqyWSDFfkyzNQTRfktLuLQH2= eH833IhfDpUu__E_vYhw2hht_MJnF7mJ2RPjtCAO5B9-eEw$=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) .