Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Apr 02 2023 19:44:08 FOUS30 KWBC 021944 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 343 PM EDT Sun Apr 02 2023 Day 1 Valid 16Z Sun Apr 02 2023 - 12Z Mon Apr 03 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHEAST TEXAS INTO THE LOWER MISSISSIPPI VALLEY... 16z Update: Previous outlook still looks in good shape, thus just made some minor adjustments (mainly tweaking the northern edge of the risk areas a bit further south). Still looks good for organized convective development and some west to east training by this evening into the overnight hours. The heaviest convection will move west to east, and eventually take on a bit of a southerly component as it follows the instability gradient and propagates into the strong low level inflow. 12z HREF neighborhood probabilities for 3"+ are 50-80% from northeast TX into northern LA and far southern AR, with HREF EAS probabilities of 3"+ showing 10-20% and 2" 40-60%. The neighborhood probabilities indicate the high end potential, while the EAS probabilities are more indicative of coverage. Thus this data would support good coverage of 2" totals over the area, with high confidence in low coverage of 3"+ totals. Where these localized 3"+ totals do occur an isolated to scattered flash flood risk could evolve, especially if it overlaps with any more flood prone areas. The probabilities of exceeding 5" remain very low, likely due to the overall progressive nature of the convective complex. Tend to think it would take some 5"+ amounts to start getting more numerous and/or higher end flood impacts, and with totals this high appearing unlikely, at this time do not think anything more than a Slight risk is needed. Chenard ....Previous Discussion... A small but potent shortwave disturbance starting out the day over southern NM will race eastward across TX, reaching southeastern OK by the evening. Meanwhile, a leeside low will develop over southeastern CO and move southeast into OK. This low will help draw substantial Gulf moisture northward on a 30-40 kt LLJ. PWATs feeding the convection could get as high as 1.75 inches, which would be 2.5 sigma above normal for this time of year. For early April that is quite notable. The final piece of the puzzle will be a stalled out trough set up in southern AR, that will further help (along with the Ozarks) with uplift of that moisture into the developing convection. As far as the timetable goes, convection is likely to break out starting during the mid-afternoon across north central TX, then quickly race E to ESE along the aforementioned boundary across northeast TX and into southern AR/northern LA through the evening. Once the initial round of storms moves through, potentially as many as 3 more additional lines of storms is likely to quickly develop behind the leading line, which all will also quickly race eastward, but also congeal into a more solid area of moderate to heavy rain as they move. Thus, the possibility of many distinct rounds of storms following one another in quick succession is being portrayed in much of the guidance with storm total rainfall as high as 4 inches, though locally amounts could certainly be higher. This approaches the values for FFGs in the area. In coordination with SHV/Shreveport, LA and JAN/Jackson, MS forecast offices, largely kept the Slight as is, generally aligned east-west covering an area roughly 50-100 miles on either side of the LA/AR border. Multiple CAMs are also suggesting the heavy rain associated with the storms gets going further west than previous guidance indicated. While the storms won't have time to congeal like further east, they still have the potential of increasing in intensity to include rain rates as high as 2 inches per hour, even if that intensity of rainfall only lasts for an hour or so. Thus, in coordination with FWD/Dallas-Fort Worth, TX forecast office, expanded the Slight risk westward to include the city of Dallas, as 2 inches in an hour, should that occur over the city, would result in localized flash flooding concerns. Wegman Day 2 Valid 12Z Mon Apr 03 2023 - 12Z Tue Apr 04 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL IN AND AROUND SOUTHERN AL, SOUTHWEST GA AND THE FL PANHANDLE... Only minimal changes were made to the inherited Marginal risk. Organized convection should be ongoing at 12z Monday across the region. This activity will likely still be producing heavy rainfall Monday morning, however it should be progressing off to the southeast at a decent clip keeping any flood risk localized in nature. The bigger question concerning the flood risk is what happens for the remainder of the day. The better forcing from the shortwave departs the area, but we still have some dynamics in the left exit of the southern stream jet, and lingering 850mb moisture convergence. There continues to be a signal in some of the guidance for backbuilding convective cells behind the early day main convective complex. If this were to occur then an increased flash flood risk could result. The 12z HREF neighborhood probabilities of exceeding 3" are over 50% and 5" probabilities are even 15-30%. However HREF EAS probabilities (a better indicator of potential coverage) are much lower. This would indicate that the potential is certainly there for some higher end 3-5"+ totals, however at the moment the coverage of these higher amounts (if they occur) is expected to be pretty small. Thus while some flash flooding could certainly evolve from this setup, the coverage is currently expected to stay below Slight risk levels. So while a Slight risk upgrade was contemplated, we opted to stick with the Marginal for now and we will continue to monitor trends. Chenard Day 3 Valid 12Z Tue Apr 04 2023 - 12Z Wed Apr 05 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE MISSISSIPPI VALLEY AND GREAT LAKES REGION... 1930z Update: Mainly just some minor adjustments to the Marginal risk areas. The northern area from IA/IL into MI will have good dynamics but cells should be quick moving. The northern edge of this Marginal from central WI into northern lower MI does have more substantial snow water equivalent (SWE) that could come into play if they get into the true warm sector of the system...so something to keep an eye on. While we did not expand the Marginal further northwest into more of WI and eastern MN, we will need to keep an eye on that area. Higher rainfall rates and melting snow may drive some flood risk here, but some question remains on the northern extent of higher dewpoint air and on QPF totals. The southern risk area from northeast TX into AR/MO was shifted a bit west with this update given trends in the latest guidance. Prefer a scenario more similar to the 12z ECMWF and Gem Reg for this warm sector convection. Still looks more like a severe mode as opposed to flash flood, nonetheless some pre frontal convection could briefly train ahead of what should then be a more progressive line of storms along the cold front. This multiple rounds and brief training potential should be enough to pose an isolated flash flood risk. Chenard ....Previous Discussion... ....IA/IL INTO WI AND MI... A rapidly strengthening low moving northeast across the northern Plains is expected to be most impactful on the cold/blizzard side of the low. However, on the warm side of the storm, expect two separate areas where an isolated flash flooding threat will be possible. On the northern end, the warm frontal precipitation is likely to be most intense immediately north and east of the surface low across southern Wisconsin and the adjacent areas, including the Lake Michigan shoreline of the L.P. A large fetch of Gulf moisture will race northward ahead of the cold front, with the LLJ strengthening to as strong as 60 kts. While this will help the warm front move along, it will still have to dislodge the much colder air in place out ahead of it, which is a common problem for northward advancing warm fronts trying to dislodge an abnormally cold air mass in place over the upper Great Lakes. With the surface low tracking towards the Marginal Risk area, expect the warm front to most likely get hung up in this area, as the surface low will only be playing a minor role at helping advance the warm air northward out ahead of it. Thus, expect a local maximum of heavy rainfall on the warm side of the boundary. While rainfall amounts are only expected to get into the 1 to 2 inch range for storm total, this amount of rain over urbanized areas, particularly Madison, Milwaukee, Green Bay, or Rockford, IL may result in localized isolated flash flooding concerns. ....Middle Mississippi Valley from Northeast Texas to Central Indiana... Guidance is also in agreement that there will be a local maximum in precipitation along a corridor from northeast Texas through central Indiana on Tuesday. Unlike further north, this area will be entirely caused by the cold frontal portion of the strong low moving into the upper Midwest. As the low center moves into Minnesota and Wisconsin Tuesday night, the trailing cold front is expected to stall out over the Marginal Risk area. While this alone wouldn't normally be cause for too much concern, the area out ahead of the cold front will feature a strengthening 50 kt LLJ of Gulf moisture, which will raise PWATs up to 1.75 inches. This will result in a local increase in both convective coverage and intensity Tuesday night. Since the parent low will be moving parallel to the axis of the front, the cold front will be slowing down, resulting in an increasing likelihood of training storms across the Marginal Risk area. Secondly, this area was slammed with severe weather yesterday, and as such the soils have been recently saturated by that convection, and may not be fully recovered before this next round of storms moves through. Thus, these areas may be more sensitive than usual to flash flooding. The inherited Marginal Risk area was expanded a little to the southwest into northeast Texas, and quite a bit to the northeast into southern Indiana to account for these more favorable antecedent conditions. The general consensus is that the convection will not be too extensive, so the rainfall amounts are generally only expected to be in the 1 to 2 inch range. Thus, widespread flash flooding is certainly not expected. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6dQv8flxHskKHmvQSS_cMmLcgCSE30fM8yeNcWyhY9IG= CldViAr9QoTzJW-ihq78ymgjxzOxnMptdMUBpZZVHedQT3k$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6dQv8flxHskKHmvQSS_cMmLcgCSE30fM8yeNcWyhY9IG= CldViAr9QoTzJW-ihq78ymgjxzOxnMptdMUBpZZVvqdJmeY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6dQv8flxHskKHmvQSS_cMmLcgCSE30fM8yeNcWyhY9IG= CldViAr9QoTzJW-ihq78ymgjxzOxnMptdMUBpZZVMPFrM4g$=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) .