Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Sep 12 2024 16:02:12 FOUS30 KWBC 121601 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1201 PM EDT Thu Sep 12 2024 Day 1 Valid 16Z Thu Sep 12 2024 - 12Z Fri Sep 13 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF CENTRAL AND NORTHERN ALABAMA... 16Z Update... The 12Z HREF guidance did not suggest any large-scale changes were required, therefore made only minor adjustments to the previous issuance based on recent observational trends. Pereira=20 Previous discussion... ....Mid-South into the Southeast... As of 08z Tropical Storm Francine continues to move northward out of Louisiana and into Mississippi. The consolidated core of heavy rainfall to the north of Francine will lift north across MS today, and eventually into portions of far eastern AR and western TN by this afternoon. This is the corridor that will likely see the most widespread axis of 3-6" of rainfall. However rainfall rates along this axis will generally be on the decline through the day. At 12z we still may seeing some 1-2" per hour rainfall just north of the center, but by 18z we are probably below 1" per hour for the most part, and by 00z we may be struggling to even get 0.5" per hour. Thus even though this axis has the highest HREF EAS probabilities of 1,2 and 3", the flash flood risk is likely mitigated by the lower rainfall rates. In fact despite the high QPF in the HREF, the probability of exceeding 3 or 6 hr FFG in the HREF is negligible over this area. Now 3-6" is still a lot of rain, and will probably see some urban and perhaps small stream flooding, however currently not expecting to see more widespread higher end impacts. Thus the Slight risk should cover the threat. The higher risk of flash flooding may actually end up well east of Francine over portions of central and northern AL. Areal averaged rainfall may be lower here than areas along the track of Francine, but rainfall rates and upper bound rainfall potential is likely higher. A narrow axis of enhanced convergence appears likely, with south southwesterly flow east of Francine, and easterly flow from the building high over the western Atlantic. This axis may stay rather persistent though the period, with around 1000 j/kg of instability advecting northward behind Francine as well. This environment does appear conducive for possible training convective bands and several high res model solutions suggest we do see periodic training convective bands across this area by this afternoon and evening. HREF neighborhood probabilities of exceeding 5" are 50-70% and exceeding 8" are as high as 30-40%. Not necessarily expecting widespread coverage of these amounts, however localized swaths of 6-10" of rain are a possibility today/tonight. This could lead to locally considerable flash and urban flood impacts. For this reason we have upgraded to a small MDT risk where the conditional threat of high end impacts appears greatest. Slow moving/backbuilding convection also appears possible today over portions of the FL Panhandle. Model agreement on the potential is not quite as strong as over AL, but certainly some potential for higher end impacts here as outer banding east and southeast of Francine could train/backbuild at times near the coast. Given a bit more uncertainty and higher FFG here, we will not upgrade at this time, but do consider this area as a higher end Slight risk. ....Montana... We did trim the Marginal and Slight risk areas some with this update. Overall not convinced we will see much in the way of true flash flooding today over MT. Instability is lacking over much of the state which will keep rainfall rates generally below flash flood thresholds. However with strong and persistent forcing the latest WPC deterministic rainfall forecast is 1-3" near and just south of Great Falls. If this occurs it would be an event that falls in the 5-25 year ARI range. So despite weak rates rainfall of this magnitude could certainly cause some areal flood concerns. Over northeast MT there should be more instability and the potential for rainfall totals over 1"/hr. Current indications suggest this stronger convection would be progressive in nature likely limiting the flood risk...however given the high PWs and instability forecast could certainly see pretty impressive sub hourly rates which could cause some flood concerns in any more sensitive areas. Thus even though the Slight risk appears lower end...we opted to maintain it over areas that may either see a higher ARI longer duration rain or the potential for at least short duration high rates. Chenard Day 2 Valid 12Z Fri Sep 13 2024 - 12Z Sat Sep 14 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE TENNESSEE VALLEY INTO THE SOUTHEAST... By Friday the remnant surface circulation of Francine will likely be somewhere near the AR/TN border. By this time the surface feature will not be a main driver of heavy rainfall. However the pattern will remain favorable for periodic training convective bands from portions of TN into AL/GA and the FL Panhandle. While the surface circulation of Francine will be dissipating, we will still have plenty of mid level energy left over. Meanwhile we will also have a trough of low pressure off the Southeast coast and a high pressure to the north over the Western Atlantic. The combination of these features will likely result in a corridor of persistent low level convergence over TN/AL/GA. We should have plenty of moisture and instability in place as well, and thus a notable flash flood risk could continue. We will carry a higher end Slight risk across portions of TN/AL/GA with this update. Depending on what happens on day 1 (impacting soil and streamflow conditions) can not rule out needing a MDT risk over a portion of this area. However there is a bit more uncertainty on convective details by Friday compared to Thursday, and so would like to continue to evaluate future high res model runs, as well as see how convection evolves today, before making any decision on an upgrade. Either way flash flooding appears possible, and a locally considerable threat could evolve. Uncertainty increases with northeastward extent. There is some flash flood risk over the western Carolinas, but unclear how far northeast the risk will get. Quite possible the better convergence and convective threat stays further west and southwest on Friday as that appears to be the trend at the moment. Thus will only carry a Marginal risk here. There is also some risk over the coastal Carolinas where a low pressure may try to form offshore. Again at the moment the most likely scenario sees the heaviest rainfall stay offshore...however we could see some heavier convective cells along coastal areas, warranting a Marginal risk. Chenard Day 3 Valid 12Z Sat Sep 14 2024 - 12Z Sun Sep 15 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE TENNESSEE VALLEY INTO THE SOUTHEAST... ....Tennessee Valley into the Southeast... A similar setup to day 2 continues into the day 3 saturday time period. The environmental setup remains favorable for a narrow axis of significant rainfall. Broadly convergent flow will exist across the region...with easterly flow off the Atlantic and south to southwesterly flow east of the remnants of Francine. Within the broader flow there will probably be a narrower axis of stronger low level convergence. And with the upper flow broadly divergent and plenty of moisture and sufficient instability forecast...the stage remains set for localized heavy rainfall. The trickiest part of this forecast will likely be pinning down the axis of heaviest rain, as it will probably be quite narrow in nature. At the moment portions of central TN, eastern AL and central/northern GA appear most favored. Like day 1 and 2, there is high impact potential on Saturday. Locally considerable flash and urban flooding is certainly possible in this setup. Hopefully the axis will shift some each day and not repeat...but there is some chance similar areas could see training convection on successive days...which would only exacerbate the flood potential. Given the inherent uncertainty in a day 3 forecast and the expected narrow nature of heaviest rainfall, can not really go any higher than a higher end Slight risk at this time. However will need to closely monitor model trends over time. ....Southwest... We will maintain a small Marginal risk over southern AZ. Moisture looks to stream northward out of the Eastern Pacific ahead of a tropical disturbance. This increasing moisture and some weak forcing may result in convective development and a localized flash flood risk. The 00z model consensus is for a slightly different tropical evolution over the eastern pacific, which would result in less of a convective threat over AZ on day 3. However the predictability of this pattern appears low, and run to run model consistency has been poor. Thus while the 00z guidance verbatim would support removing the Marginal risk, the preference was to maintain it for now and continue to monitor trends. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5mBkXiHFYWQ_XBRXdZGMLPvtI_vzKQhl-TbiYl8R0eE9= 2xuCEOQwxUGSJz7tol_U32U-JWs4Jugm9ElY3mxfMZb6YmY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5mBkXiHFYWQ_XBRXdZGMLPvtI_vzKQhl-TbiYl8R0eE9= 2xuCEOQwxUGSJz7tol_U32U-JWs4Jugm9ElY3mxfsz_nYLE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5mBkXiHFYWQ_XBRXdZGMLPvtI_vzKQhl-TbiYl8R0eE9= 2xuCEOQwxUGSJz7tol_U32U-JWs4Jugm9ElY3mxfYxD4QI8$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .