Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 24 2024 19:47:03 FOUS30 KWBC 241946 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 346 PM EDT Sat Aug 24 2024 Day 1 Valid 16Z Sat Aug 24 2024 - 12Z Sun Aug 25 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST...MIDWEST...AND SOUTH FLORIDA... ....Southwest... 16Z update... Localized heavy downpours will remain possible given the plume of moisture still being drawn northward out of Mexico and northward around the western side of a sprawling upper level ridge. No changes were needed at this time. Campbell Another round of daytime convection is expected in the Southwest again on Saturday, although the primary threat area should be relatively narrow, focused in a plume of deeper moisture along the periphery of an advancing deep trough. The Marginal Risk was generally maintained in the same area as the previous outlook issuance with only minor adjustments around the southern periphery. Localized heavy downpours will remain possible given the plume of moisture still being drawn northward out of Mexico and northward around the western side of a sprawling upper level ridge. However, the mid-level flow will be much stronger than usual for this time of year helping to mitigate the flash flooding concern by providing a strong and persistent steering flow. Nevertheless, around an inch of rain falling in a short period of time may still lead to flash flooding, and the Marginal Risk was maintained. ....Midwest... 16Z update... A minor northeast expansion of the Marginal further into Illinois was made to account for the trends of the latest hi- res guidance. There was an uptick in coverage and amounts of the round of convection expected after 00Z from southeast Iowa to northwest Illinois. Campbell A complicated forecast exists with several conditional threat areas that still need to be sorted out. Models are generally in good agreement in showing several rounds of elevated convection along the periphery of a pronounced elevated mixed layer, with 700mb temperatures approaching record highs in the sounding archive and ERA reanalysis dataset. One of the rounds is focused early in the Day 1 period in or near Missouri. Satellite and radar imagery from the overnight hours was showing thunderstorm development along the Kansas/Nebraska border moving westward around the 1000-700 mb thickness gradient and being fed by south to southwesterly winds of 35 to 50 kts per area VAD Wind profilers upstream over Kansas and Oklahoma. The 06Z run of the HRRR continued to generate a narrow stripe of moderate to heavy rainfall in western Kansas...its areal coverage apparently being in check from the warm mid level temperatures noted above but providing support to maintain the Marginal risk area without much need to adjust the placement. The other round looks to be focused after 00Z farther to the north from southeast Iowa into northwestern Illinois. Ingredients favoring heavy to excessive rainfall extend to the north and northwest in so those solutions extending as far north and northwest into Minnesota or southeast South Dakota cannot be ruled out yet. Multiple hi- res models also show much of the northern sections of the region remaining almost entirely dry. However, the risk should an inch of rain falling in a short period of time may still lead to flash flooding, and the Marginal Risk was maintained.be assumed to be non- zero in the areas described above, and above 5 percent in the Marginal Risk areas. Further updates and expansion of the Marginal Risk is possible. ....South Florida... 16Z update... No changes were needed at this time. Campbell Incoming deep moisture with roots from the Caribbean will make headway into South Florida today with area precipitable water around the 90th percentile, which is well above 2 inches. The increasing moisture will be in proximity to a quasi-stationary front to the north...with a sharp delineation point of where the moisture pooling will reside. A Marginal risk area was maintained across the Southern FL Peninsula. Bann Day 2 Valid 12Z Sun Aug 25 2024 - 12Z Mon Aug 26 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NEW MEXICO AND COLORADO...AND THE MIDWEST... ....New Mexico and Colorado... 21Z update... The Marginal Risk area continues to highlight the region where there is an elevated threat for excessive rainfall with local flash flood concerns. No Changes were made with this forecast cycle. Campbell As the western periphery of the Southern Plains upper level high begins to erode, the heavy rain threat area should expand eastward on Sunday into more of New Mexico and Colorado. Scattered daytime convection should develop in a plume of deeper moisture and moderate instability, and the thunderstorms will be capable of producing flash flooding within complex terrain and especially around any burn scars. Hourly rain rates may locally exceed 1 inch. The Marginal Risk was mostly maintained in the same areas. ....Midwest... 21Z update... The Marginal Risk area continues to highlight the region where there is an elevated threat for excessive rainfall with local flash flood concerns. No Changes were made with this forecast cycle. Campbell The latest guidance shows potential for for organized convection and an associated heavy rainfall threat to persist into the Day 2 period over portions of southeast Iowa into western Illinois and northeast Missouri. The latest CSU machine learning Excessive an inch of rain falling in a short period of time may still lead to flash flooding, and the Marginal Risk was maintained. Rainfall Outlook still appeared to be displaced north and west of where the ensembles favored heaviest rainfall...and the 00Z runs did little to resolve the considerable degree of mesoscale uncertainty both from the overall setup across the region. The model consensus was best on the idea that heavy rainfall was possible but they varied widely on placement. Therefore, the Marginal Risk should still be viewed as the most likely placement for heavy rain on Sunday morning, but future updates are possible. Lamers/Bann Day 3 Valid 12Z Mon Aug 26 2024 - 12Z Tue Aug 27 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST US AND THE NORTHERN PLAINS/UPPER MIDWEST ON MONDAY... ....Southwest US... 21Z update... The Marginal Risk area continues to highlight the region where there is an elevated threat for excessive rainfall with local flash flood concerns. No Changes were made with this forecast cycle. Campbell The threat of excessive rainfall will linger into Monday across portions of the Southwest US as precipitable water values of 0.75 to 1.00 inches remain in place from extreme southeast Arizona into New Mexico. Scattered daytime storms remain possible within the plume if deeper moisture...but strengthening steering flow should help limit residence time to mitigate some of the flash flooding concern. As mentioned earlier...though...even brief heavy rainfall rates in a short period of time may still lead to flash flooding, and a Marginal Risk was maintained in the region. ....Northern Plains... 21Z update... There has been some spread in the guidance over the=20 last few runs that support a slightly less progressive=20 trough/front to pass through the northern tier states. If this=20 comes to fruition then modest to heavier rain amounts/rates will=20 materialize over eastern Montana and the western Dakotas rather than focused over the eastern Dakotas and Minnesota. To account for this possibility, the Marginal Risk area was expanded to the west=20 to now include eastern Montana and western North and South Dakota. Campbell An upper trough over the Intermountain region will begin lifting northeastward across the High Plains helping to sharpen a front across parts of the Upper Midwest/Northern Plains. Precipitable=20 water values are forecast to approach 1.75 inches over portions of=20 the eastern Dakotas prior to the arrival of the surface front and=20 increasing divergence/difluence associated with the upper level=20 system. The expectation is that moisture flux convergence along the front will focus convection moving into an unstable environment=20 driven by diurnal heating that leads to locally heavy downpours.=20 Farther west where instability is more driven by cold mid level=20 temperatures...rainfall should be less focused and less intense. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6HulbpFQk4EfEjw-TsaRM1qD7DVFagtvY7qDIjK5OUHI= 9Ssf0sGz7WgKXagiZtXeT90HYE9tSeW-_UJtStZypXwSmlo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6HulbpFQk4EfEjw-TsaRM1qD7DVFagtvY7qDIjK5OUHI= 9Ssf0sGz7WgKXagiZtXeT90HYE9tSeW-_UJtStZyuWzMFC4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6HulbpFQk4EfEjw-TsaRM1qD7DVFagtvY7qDIjK5OUHI= 9Ssf0sGz7WgKXagiZtXeT90HYE9tSeW-_UJtStZytQSrj1I$=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) .