Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Feb 27 2023 07:45:59 FOUS30 KWBC 270745 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 243 AM EST Mon Feb 27 2023 Day 1 Valid 12Z 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 =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 Roth Day 3 Valid 12Z Wed Mar 01 2023 - 12Z Thu Mar 02 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN AND NEAR TENNESSEE... A boundary at 850 hPa wavers near southern TN this period as one cyclone ejects eastward from the Great Lakes and another moves into the Southern Plains. Precipitable water values are expected to rise into the 1.25-1.75" range, which is 1-2.5 sigmas above the mean for early March. Inflow from the Gulf is expected to range from 25-50 kts at 850 hPa into the region -- with effective bulk shear to match -- which should organize convection when combined with the 500-1500 J/kg of MU CAPE arriving late in the period/Wednesday night into Thursday morning. Temperatures at 700 hPa are generally 3-6C, implying little to no mid-level capping inversion exists. There is a signal in the current guidance for local amounts in the 2-4" range. Flash Flood Guidance values are particularly low in the southern Appalachians and modest across the remainder of the area -- roughly 2"/3 hours. However, the region has been relatively dry this past week with much of the region experiencing below average rainfall...and the rainfall forecast for Monday and Tuesday is minimal currently. There's still some placement issues in the guidance across TN, with the 00z GFS/00z UKMET splitting the difference between the southern 12z ECMWF and northern 00z Canadian Regional guidance. The 00z NAM shows unusually low QPF volume when compared to the rest of the guidance as well. Considering the above, a Marginal Risk still appears the best bet for the time being, until the guidance can show greater convergence on volume and placement. An upgrade to a Slight can't be ruled out at some point in the future as the forecasted ingredients available appear conducive to widely scattered flash flood issues. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7BN2IU5N-OkbhWIO6GHUS5Zy4brMV5-wjVKKmPxfHV1e= 6-ek2k5IW62_P3wapxUEf-OG5ccXKCRLReyZxv4Qd9bXC4E$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7BN2IU5N-OkbhWIO6GHUS5Zy4brMV5-wjVKKmPxfHV1e= 6-ek2k5IW62_P3wapxUEf-OG5ccXKCRLReyZxv4Ql1wzqZo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7BN2IU5N-OkbhWIO6GHUS5Zy4brMV5-wjVKKmPxfHV1e= 6-ek2k5IW62_P3wapxUEf-OG5ccXKCRLReyZxv4Q0IXvhsI$=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) .