Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Feb 27 2023 20:06:59 FOUS30 KWBC 272006 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 302 PM EST Mon Feb 27 2023 Day 1 Valid 16Z Mon Feb 27 2023 - 12Z Tue Feb 28 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN GREAT LAKES AND UPPER OHIO VALLEY...=20 1600Z Update... Only minor adjustments were made to the Marginal Risk area, with some trimming of the outlook area across portions of WI/IL/IN where the heaviest rains have begun to advance away off to the east. Generally no changes to the larger scale thinking for rainfall amounts with the 12Z HREF still advertising some pockets of 1 to 2 inch rainfall amounts. Some localized runoff/flooding problems remain possible. Orrison Previous discussion... =20 A strong low pressure system moving through Iowa will begin to weaken as the upper-level shortwave shears out while rounding the top of an anomalous Southeast ridge. Ongoing convection associated with a previous MCS ejecting out of the central Plains should be entering Illinois Monday morning. Potent moisture flux along a strong low-level jet into the Great Lakes and Ohio Valley will support a marginal heavy rainfall threat. Precipitable water values should nose upward to at or above 1" -- 2-4 sigmas above the mean for late February per the NAM guidance -- while the better moisture slides eastward into the Upper Ohio Valley by the early evening. MUCAPE values are expected to reach up to 500 J/kg. Maximum rainfall amounts across the Marginal Risk area could locally reach up to 2" if redeveloping convection overlaps previous rainfall throughout the Ohio Valley by early evening.=20 Soils have been saturated by 300-600% of average rainfall during the past week across portions of northern IL, southeast WI, and the southern Mitt of MI. However, this remains a low chance due to the overall quick system progression/limited residence times of the better ingredients, thus the Marginal Risk designation. These totals suggest only minor and localized flooding is possible as opposed to more scattered flooding concerns, with ponding water and rapid runoff most likely over areas around the Great Lakes where recent snowmelt also comes into play.=20 =20 Roth/Snell Day 2 Valid 12Z Tue Feb 28 2023 - 12Z Wed Mar 01 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent.=20 Orrison Day 3 Valid 12Z Wed Mar 01 2023 - 12Z Thu Mar 02 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF TENNESSEE... ....Mid-South... Low-amplitude southern stream shortwave energy embedded within strong, and increasingly diffluent mid to upper-level west-southwest flow aloft downstream of the longwave trough over the Southwest will be interacting with a quasi-stationary front draped across the Mid-South for multiple rounds of convection late Wednesday afternoon through Wednesday night. The convective threat will be facilitated by an increasingly moist and unstable boundary layer pooling along and south of the front with aid of a 30 to 50 kt low-level jet. PWs are forecast to rise to locally over 1.5 inches over the Mid-South with these values reaching as much as 2 to 3+ standard deviations above the mean. A dual moisture feed regime is forecast with a combination of low-level Gulf of Mexico moisture working in tandem with the northeast transport of mid to upper-level eastern tropical Pacific moisture which is already seen pooling in the latest CIRA-ALPW imagery. MUCAPE values are forecast to rise in the vicinity of the front to between 1000 to 1500 J/kg and this coupled with deep layer forcing associated with the aforementioned shortwave energy, increasing vertical shear, and larger scale placement of right-entrance region jet dynamics/forcing should result in regional development of organized convection. Multiple convective clusters are expected that will tend to be aligned west-southwest to east-northeast with the deeper layer mean flow and this will be suggestive of a threat of repeating/training cells. Rainfall rates are likely to reach up into the 1 to 2 inch/hour range with the stronger/organized convective cells and with some storm totals that may reach up into the 2 to 4+ inch range given concerns for repeating/training cells. The latest NASA SPoRT soil moisture data indicates relatively moist antecedent conditions across portions of the Mid-South and this is suggested as well with USGS data showing locally above normal streamflows. Given this and the environmental support for heavy rainfall, a sizable portion of the Marginal Risk area was upgraded to a Slight Risk with an emphasis on Tennessee where the best overlap of moist soil conditions and heavy rainfall potential overlap. Isolated to scattered instances of flash flooding will be possible across this region. Orrison Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9F3FlazOKfHe-YknHRDJ0xO5sZjoEHl93LIECrF1ndbN= imXQuEC7gATEvReqZwMzE66ubYSruxSnLCLpRiD4nVH0oJs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9F3FlazOKfHe-YknHRDJ0xO5sZjoEHl93LIECrF1ndbN= imXQuEC7gATEvReqZwMzE66ubYSruxSnLCLpRiD432NvOF8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9F3FlazOKfHe-YknHRDJ0xO5sZjoEHl93LIECrF1ndbN= imXQuEC7gATEvReqZwMzE66ubYSruxSnLCLpRiD4qB852RM$=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) .