Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Feb 01 2023 20:27:12 FOUS30 KWBC 012027 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 326 PM EST Wed Feb 01 2023 Day 1 Valid 16Z Wed Feb 01 2023 - 12Z Thu Feb 02 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF EASTERN TEXAS INTO THE LOWER MISSISSIPPI VALLEY... 16Z Update...Tightened the areal coverage of the Marginal Risk area but kept it effect. The risk of excessive rainfall persists given the potential for modest rainfall amounts on a region that experienced a wet period over the past week or so.=20 Instability...even elevated instability...should limit rainfall rates/instability and the big question is whether any of the heavier rainfall falls on the soils that are most wet/primed to result in problems with runoff or ponding. Bann The wet last couple days will continue into today/tonight across east TX into LA and southern AR. NASA SPoRT-LIS soil moisture percentiles and USGS streamflow anomalies continue to run above climatological normals. Thus even moderate rainfall amounts will likely result in some excess runoff and some minor flooding concerns today into tonight across this region. With that said, it is becoming harder to justify a Slight risk across this area. Model QPF trends have generally been down, and just not seeing much in the way of even elevated instability. The increasing large scale forcing (troughing approaching from the west and an upper jet to the north), combined with pretty strong lower level moisture transport/convergence, should compensate some for the lack of instability and produce some embedded heavier convective elements. However most guidance is peaking hourly rainfall in the 0.25"-0.5" range, and the lack of even a couple hundred j/kg of MUCAPE will make it hard to exceed 0.5" in an hour outside of perhaps very localized swaths. The expected rainfall rates and totals across the area 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, but hard to justify a two category bump at this time. Thus we will remove the Slight risk with this update and carry a Marginal instead, with some lower end flooding still possible given the overly saturated conditions. Chenard 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 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-ulDg4yp4pwkXoMnvA8r4V1JvuDfO8d5U1jLScxsnyQx= mkcnkL0v5naBbaGvs9suPPJFk__njL86dejlAs8DKXpwsvA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-ulDg4yp4pwkXoMnvA8r4V1JvuDfO8d5U1jLScxsnyQx= mkcnkL0v5naBbaGvs9suPPJFk__njL86dejlAs8DWVLPGek$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-ulDg4yp4pwkXoMnvA8r4V1JvuDfO8d5U1jLScxsnyQx= mkcnkL0v5naBbaGvs9suPPJFk__njL86dejlAs8DRUBvDAg$=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) .