Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Feb 01 2023 23:35:12 FOUS30 KWBC 012335 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 633 PM EST Wed Feb 01 2023 Day 1 Valid 01Z Thu Feb 02 2023 - 12Z Thu Feb 02 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN AND NEAR NORTHERN LOUISIANA... The wetness across portions of TX, LA, AR, and MS will continue through into Thursday morning. NASA SPoRT-LIS soil moisture percentiles and USGS streamflow anomalies continue to run above climatological averages. While virtually no instability is currently forecast by the RAP, SREF, or NAM guidance across this area, which would normally imply hourly totals capping around 0.25", overlapping low- to mid-level frontogenesis could allow for hourly totals to maximize in the 0.5-1" range. This rainfall could result in some excess runoff and some minor flooding concerns. The 18z HREF guidance is peaking hourly rainfall in the 0.25"-0.5" range, particularly early Thursday morning across northwest MS, with any 0.50-1" totals possible in perhaps very narrow, localized swaths early Thursday morning. Local totals around 2" appear to be the maximum from Thursday 00-12z. The expected rainfall would typically result in a flood risk below Marginal level. A one category bump to account for the saturated soil and above average streamflows seems reasonable. Some lower end flooding still possible given the overly saturated conditions. Roth Day 2 Valid 12Z Thu Feb 02 2023 - 12Z Fri Feb 03 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE LOWER MISSISSIPPI VALLEY INTO THE SOUTHEAST... 2030 UTC Excessive Rainfall Outlook Discussion... The axis of rainfall continues to waver slightly...but the impact on the placement of the Slight Risk and Marginal Risk areas was minimal. The bigger concern remains how much instability there is and what that implies for rainfall rates and any resulting run off problems. It is still perceived that the event is probably on the low end of the Slight Risk probability range...soil moisture still suggests a Slight Risk area is warranted given the latest deterministic and ensemble guidance. Bann 0830 UTC Excessive Rainfall Outlook Discussion... An uptick in rainfall intensity is expected on Thursday across portions of the Lower MS Valley into the Southeast. Troughing approaching from the west and a strong upper jet to the north will broadly increase vertical ascent across the region by Thursday. Meanwhile strengthening lower level moisture transport and convergence will help focus shower and embedded thunderstorm development. Instability is again expected to be a limiting factor. The positioning of the upper forcing and strongest moisture convergence suggest most of the organized rainfall will be along and north of the surface front within a cooler and generally more stable airmass. It currently does not look like we will see much in the way of an instability surge northward out of the Gulf of Mexico with this system. Compared to Wednesday (where we downgraded to a Marginal risk), rainfall rate potential looks a bit higher Thursday, as we see some better mid/upper forcing and tighter lower level convergence which may support more of a convective component to the rain and some localized swaths around 1" in an hour. Also, unlike the downward QPF trend in the guidance for Wednesday, trends are pretty steady for Thursday into Thursday night. The magnitude of the flash flood risk remains a bit uncertain given the aforementioned question regarding instability and rainfall rates. However a deciding factor for keeping this threat in the Slight risk level is antecedent conditions. Much of the Slight risk area has seen rainfall the past couple days, with NASA SPoRT-LIS soil moisture percentiles above climatological normals and USGS streamflows running above average over most of the area. Absent these antecedent conditions this setup is probably more of a Marginal risk level event. So while this event is probably on the lower end of the Slight risk probability range, a one category bump to account for the saturated soil conditions still seems warranted. Thus we will maintain the Slight risk across portions of LA/MS/AL/GA. Chenard Day 3 Valid 12Z Fri Feb 03 2023 - 12Z Sat Feb 04 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!48VRgBMpRCVO41ieYsynybMrj0CG_mKaa9MpwafXwzk3= F1rbkuaUt0oNeYH1LWxQivIIaOEYs04xTAcm2gaqxiAmKyg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!48VRgBMpRCVO41ieYsynybMrj0CG_mKaa9MpwafXwzk3= F1rbkuaUt0oNeYH1LWxQivIIaOEYs04xTAcm2gaqhqlPnQ0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!48VRgBMpRCVO41ieYsynybMrj0CG_mKaa9MpwafXwzk3= F1rbkuaUt0oNeYH1LWxQivIIaOEYs04xTAcm2gaqqF8sMgo$=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) .