Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Feb 22 2023 19:59:57 FOUS30 KWBC 221959 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 258 PM EST Wed Feb 22 2023 Day 1 Valid 16Z Wed Feb 22 2023 - 12Z Thu Feb 23 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM NORTHEASTERN MISSOURI TO SOUTHWESTERN MICHIGAN... A distinct mid-level wave over Texas this morning is ejecting northeastward and forecast to reach the Mid-Mississippi Valley by tonight. As this occurs, a surface low will continue to organize over Kansas and northern Missouri along a well-defined frontal boundary. Extending to the east of the low through the Ohio Valley and into the Mid-Atlantic, a slowly lifting warm front will provide a focus for sufficient lift as a 55+ kt 850 mb jet surges anomalous moisture into the boundary. This front is also forecast to stall as it lifts into central Illinois, Indiana, and Ohio later today, leading to the potential for rainfall to train over the same region. Meanwhile, a Pacific front will migrate eastward across the Ozarks and could produce brief periods of heavy rain. The primary focus for heavy rainfall (and potential for flash flooding) will reside across central/northern Illinois and northern Indiana during the forecast period (particularly through 03Z). During that time frame, persistent convergence along the warm front and lift/ascent from the approaching central Plains mid-level wave will result in scattered to widespread convection along that axis. That axis should also be favorably oriented to mid-level flow aloft for training at times. Areas of 1-2 inches of rainfall are probable, with locally higher totals possible where training or repeat convection can occur. Ensemble maximums show the ceiling for this event is around the 3 to 4 inch mark, specifically from central Illinois to northern Indiana. The convection will benefit from very anomalous moisture for this time of year (1.3+ inches just south of the front, which is about 4 standard deviations above the climatological mean according to the 06z GEFS) and speed convergence on the nose of 50-70 knots of low-level flow. Furthermore, FFG thresholds are fairly low (only 1-2 inches/3 hours) and are likely to be exceeded in spots where heavier rainfall can develop. Latest indications from models is that heavy rain ongoing over Missouri and developing cells along the warm front over central Illinois will migrate eastward and merge near central Illinois while spreading into northern Indiana as well. The risk on the northern extent of the outlook is mitigated by the extent of colder air and frozen precipitation, which should limit runoff where temperatures are below freezing. Changes to the outlook area for the 16z update include a small southward shift of the Slight Risk in Illinois and Missouri based on latest radar trends and 12z HREF probabilities. Most notably, the 12z HREF 40 km neighborhood probabilities for 6 hr QPF exceeding 2 inches has a stripe of 25-50% throughout the center of the highlighted Slight Risk. Additionally, the western portion of the Marginal Risk was shaved as the cold front progresses eastward. Snell Day 2 Valid 12Z Thu Feb 23 2023 - 12Z Fri Feb 24 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Snell Day 3 Valid 12Z Fri Feb 24 2023 - 12Z Sat Feb 25 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE GREATER LOS ANGELES METRO AREA OF SOUTHERN CALIFORNIA... A very anomalous and deep upper-level trough over the western U.S. this week is expected to spawn a cut-off upper-level low by early Friday as it slides down the California coast. The sub 540 dam closed low will then slide eastward towards southern California by Saturday. The result of this storm track will allow for a ribbon of subtropical moisture to round the trough and eventually reorient so that it impacts almost perpendicular to the Transverse Ranges and LA metro region of southern California. Impressive upper-level diffluence is also forecast over much of southern California due to the depth and location of the system. The axis of heaviest rainfall and the potential for rainfall rates between 0.5-1.0" per hour is expected to gradually slide eastward by the latter half of Friday and peak on Friday night near the San Gabriel and San Bernardino mountains and down to the coastal areas of southern California. PWATs are forecast to rise to around 0.75", which is around the 75th-90th percentile compared to climatology. IVT values up to 700 kg/m*s from the ECMWF highlight the increasing atmospheric moisture content as well. Instability will be limited, but MUCAPE of 250 J/kg may reach just inland per the 12z NAM, supporting the potential for excessive rainfall rates. As far as actual rainfall amounts, most guidance depicts the potential for 2 to 4 inches of rainfall throughout the elevations that remain below the freezing level during the entirety of the event. Snow levels will be quite low compared to normal, starting below 3000 feet but rising up to as high as 4000-5000 feet during the period with the most intense precipitation rates. Locally higher rainfall amounts are possible. Most of the recent burn scars should be protected and within the heavy snowfall, but urban flooding and rapid runoff will be likely in the lower elevations of Ventura and Los Angeles counties. After coordinating with the local forecast office (LOX) a Moderate Risk was issued for this region. A few limiting factors for excessive rainfall include the low snow levels and the relatively dry antecedent conditions. Snow in the elevated terrain will limit runoff to the lower elevations and coastal regions, bottling up the 5-7" QPF amounts within the mountain ranges. NASA SPoRT-LIS also depicts normal to below normal moisture percentiles in southern California, so soils may be able to soak in the initial bursts of rainfall (this is somewhat different from the previous atmospheric river to impact the region in early January). A broader Slight Risk incorporates more of the Transverse Ranges and coastal regions of southern California. Snell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8odMqwHTrp-NQkJy0wsTfMDdxgwEDEkXusrH7OxqTcmJ= WurxIfRxPAKmEn3of0Pp9oMQ07YHao-3reaUdxp6kOVn8gg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8odMqwHTrp-NQkJy0wsTfMDdxgwEDEkXusrH7OxqTcmJ= WurxIfRxPAKmEn3of0Pp9oMQ07YHao-3reaUdxp6FIuhRVY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8odMqwHTrp-NQkJy0wsTfMDdxgwEDEkXusrH7OxqTcmJ= WurxIfRxPAKmEn3of0Pp9oMQ07YHao-3reaUdxp659DiwW0$=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) .