Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Apr 02 2023 08:36:57 FOUS30 KWBC 020836 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 436 AM EDT Sun Apr 02 2023 Day 1 Valid 12Z 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... 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. ....Potential for Risk Category Upgrades... Consideration was given to a targeted Moderate Risk upgrade along the LA/AR border, but with unfavorable antecedent conditions noted by both SHV and JAN, all agreed to keep the risk at Slight for this issuance. Despite the high likelihood of training, the fast moving nature of the individual cells should work to keep flash flooding from becoming more numerous, and at this point a future upgrade is not anticipated. 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 FLORIDA PANHANDLE... The same mesoscale convective complex (MCC) from Day 1/Sunday will continue into Day 2/Monday across portions of the Southeast. By Monday morning, the main forcing associated with the rapidly moving upper level shortwave will have largely departed east of the MCC. While the LLJ will still be gusting at 30-40 kt, its orientation will have turned more southwesterly by Monday morning across the Marginal Risk area. Instability will remain plentiful south and west of the boundary the MCC will track along, as values increase to between 1,500 and 2,000 J/kg. The newly formed convection on the southern edge of the MCC will propagate southeastward with time, as the convection develops into the instability, however PWATs are expected to come down a bit closer to 1.25 inches. As such, much of the guidance shows the convection will separate from the previously congealed area of rain further to the north, with just a single round of strong thunderstorms likely to impact much of the Marginal Risk area. Further, this portion of southeast AL, southwest GA, and the FL Panhandle has about average soil moisture based on NASA Sport imagery. Prior coordination with the impacted forecast offices suggests this area is very difficult to flash flood. While rainfall rates will remain elevated, and the storms will be moving, albeit individually slower than in Day 1, the inherited Marginal Risk for this area was largely kept unchanged for this forecast issuance. The rain will likely be over by or even an hour or 2 before sunset, so the main threat for excessive rainfall will be during the first half of the day Monday. Peak rainfall totals are most likely in far southeastern AL and the southwest corner of GA, where amounts may locally exceed 2 inches. This may result in an isolated flash flooding risk. ....Potential for Risk Category Upgrades... Very low at this time. There are several reasons for this: the steady, if somewhat slower storm motion towards the southeast, and lack of training and/or additional rainfall from prior convection moving over an area with high FFGs, historically quite difficult terrain to flash flood, and generally rural area all point towards the threat being closer to risk-free than to a Slight at this point. Wegman 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 UPPER AND MIDDLE MISSISSIPPI VALLEYS... ....Southern Wisconsin and Adjacent Areas... 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. ....Potential for Risk Category Upgrades... At this point, the potential for a risk upgrade is low. The cold front is likely to need a much more robust response in the consensus rainfall such that 2 inches or more of rainfall is much more likely than the current guidance suggests. This is more likely in the southern-more of the two Marginal Risk areas, given the higher total moisture and greater likelihood of the cold front stalling out, favoring training convection. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_5WpRSgOg5hgi7I92tkb9vCOISIhdx1heNdhWEiUqwHm= GFnIiWwykwF3nTGnR7ylx9iq-aTaIadu9L-SAmWX6zuVOy4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_5WpRSgOg5hgi7I92tkb9vCOISIhdx1heNdhWEiUqwHm= GFnIiWwykwF3nTGnR7ylx9iq-aTaIadu9L-SAmWXHG9x0AU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_5WpRSgOg5hgi7I92tkb9vCOISIhdx1heNdhWEiUqwHm= GFnIiWwykwF3nTGnR7ylx9iq-aTaIadu9L-SAmWXT_aB7wQ$=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. 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