Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jan 31 2023 20:24:18 FOUS30 KWBC 312024 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 323 PM EST Tue Jan 31 2023 Day 1 Valid 16Z Tue Jan 31 2023 - 12Z Wed Feb 01 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann Day 2 Valid 12Z Wed Feb 01 2023 - 12Z Thu Feb 02 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF EASTERN TX, NORTHWEST LA, & FAR SOUTHERN AR... ....2030 UTC Excessive Rainfall Outlook Update... The deterministic QPF from WPC and the QPF from various models were fairly consistent in terms of placement and amounts...so there was little need to make any changes to the on-going Excessive Rainfall Outlook area. The instability is still forecast to be the primary limiting factor. On the other hand, the backing/strengthening flow aloft riding up and over the low level frontal boundary and some weak elevated instability could still result in convective elements that produce locally rainfall rates that approach...if not exceed...1 inch per hour rainfall rate over an area that has experienced a recent period of wet weather. Bann ....0830 UTC Excessive Rainfall Outlook Discussion... Multiple rounds of rain will continue to impact portions of TX into the lower MS Valley over the next 3 days. The risk of some flooding impacts should increase Wednesday into Wednesday night as troughing approaching from the west and an upper jet to the north increases vertical ascent across the region. In the lower levels increasing 850mb moisture transport and convergence should help focus a shower and thunderstorm threat, and with the deep layer mean flow generally parallel to the convergence axis, some repeat activity is probable. Instability is likely to be a limiting factor for this event, with surface temperatures in the 30s and 40s not usually supportive of a notable flash flooding potential. However there will be a strong warm nose above the low level inversion, which will support some weak elevated instability. This combined with the increasing large scale forcing should support some locally heavier convective elements embedded within a broader region of showers...and there is some support for hourly rain locally getting towards 1" in an hour. The magnitude of the flash flood risk remains a bit uncertain given the aforementioned question regarding instability and thus rainfall rates. However a deciding factor for keeping this threat in the Slight risk level is antecedent conditions. Much of the Slight risk areas has seen rainfall the past couple days, with NASA SPoRT-LIS soil moisture percentiles above the climatological 90th percentile and USGS streamflows running above average. Some of this area will see some additional rainfall Tuesday as well, so soil conditions should remain pretty saturated heading into Wednesday. Thus the combination of sensitive soil conditions combined with the uptick in forcing/rainfall rate potential by Wednesday supports the continuation of the Slight risk across portions of east TX into western LA. This area has the best overlap of more saturated soil conditions and potential for rainfall rates getting close to or exceeding 1" in an hour. Locations most prone to flash flooding are atop the aforementioned overly saturated soils, in more heavily urbanized communities that contains a greater concentration of hydrophobic surfaces, and poor drainage areas. Chenard Day 3 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... Made only minor changes to the northern/eastern periphery of the Marginal and Slight Risk areas to encompass slightly higher QPF in parts of the Southeast U.S.. Now any 2+ inch amounts from the SREF and most of the GEFS members are covered by the Marginal and Slight risk areas. The axis of 1+ inch amounts extends farther eastward into a region that has had little rainfall lately and is less likely to be a flash flood concern. Bann ....0830 UTC Excessive Rainfall Outlook Discussion... A wet pattern will continue across portions of the lower MS valley into the Southeast over the next couple days. Most of the rainfall Tuesday into Wednesday will be on the lighter side, but we should see an uptick in intensity on Thursday. 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 instability surge northward out of the Gulf of Mexico with this system. However even along and north of the front we should be able to realize some weak elevated instability, which combined with the increasing forcing, still supports some heavier embedded convective elements. This, combined with PWs approaching the climatological 95th percentile, should be enough to support some hourly rainfall approaching or locally exceeding 1", and total rainfall amounts of 1-3". 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 the climatological 90th percentile and USGS streamflows running above average over most of the area. Some of this area will also see some additional rainfall Tuesday and Wednesday, so soil conditions should remain pretty saturated heading into Thursday. Absent these antecedent conditions this setup is probably more of a Marginal risk level event. However, the combination of sensitive soil conditions combined with the uptick in forcing/rainfall rate potential by Thursday supports the continuation of the Slight risk across portions of LA/MS/AL/GA. Locations most prone to flash flooding are atop the aforementioned overly saturated soils, in more heavily urbanized communities that contains a greater concentration of hydrophobic surfaces, and poor drainage areas. There remains some model spread with regards to the favored axis for heaviest rainfall. The 00z GFS remains a bit of an outlier with its quicker and more northeast progression of heavier rain, with the ECMWF farther southwest. While reality will more likely than not end up somewhere in between, ensemble data and the GEM/UKMET support something a bit closer to the ECMWF. Also the farther northeast you go the less sensitive soil conditions are and the weaker the signal for elevated instability. So we did cut back a bit on the northeast extent of both the Marginal and Slight risk with this update to account for the model preference and less susceptible antecedent conditions. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9pnqMTBRkQ2q_3vdcvqff82Y_wtxKm-4bVdle_TkkAL-= SScf_Dqjz2Ua437ug7CSxXv3kR4NhImZqksnkBIBxtnL7dk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9pnqMTBRkQ2q_3vdcvqff82Y_wtxKm-4bVdle_TkkAL-= SScf_Dqjz2Ua437ug7CSxXv3kR4NhImZqksnkBIBND1uD48$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9pnqMTBRkQ2q_3vdcvqff82Y_wtxKm-4bVdle_TkkAL-= SScf_Dqjz2Ua437ug7CSxXv3kR4NhImZqksnkBIBToTRZug$=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) .