Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Apr 04 2025 20:15:13 FOUS30 KWBC 042014 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 414 PM EDT Fri Apr 4 2025 Day 1 Valid 16Z Fri Apr 04 2025 - 12Z Sat Apr 05 2025 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OZARKS AND MID-SOUTH... ....LIFE-THREATENING FLASH FLOODING EXPECTED FRIDAY AFTERNOON THROUGH EARLY SATURDAY... ....16Z Update... The forecast remains on track as model guidance from the CAMs and global models show good agreement on placement of the highest rainfall totals today through tonight. Overall, fairly minimal changes were needed for the Day 1 ERO outside of some small adjustments. Based on latest high-res model and radar trends, adjustments were made to expand the Slight Risk area in Kentucky and West Virginia where precipitation has lingered through the morning hours and in western and northern Ohio where precipitation is expected later this afternoon and tonight. Adjustments were also made to the High and Moderate Risk areas, extending them a bit north and northeast based on latest high-res deterministic and ensemble guidance. The largest expansion was to bring the Moderate area further into Kentucky where heavy rain fell this morning and additional rainfall is expected tonight. The HRRR was trending more south than other guidance with the higher rainfall totals in Arkansas, but it seems to be an outlier at this point and doesn't seem to have a good handle on the current conditions either as compared to the other CAMs. However, if precipitation trends south this afternoon, further adjustments may be needed to the High Risk area. Dolan ....Previous Discussion... Both global models and CAMs have come into better agreement over the past 24 hours and have largely maintained that agreement in the 04/00Z model production cycle. The development of at least one north- northeast to south- southwest oriented band of convection located generally from the ArkLaTex into southeastern Missouri was still depicted...with convection being oriented generally parallel to fast, meridionally oriented flow aloft, with abundant moisture, low- level shear, and instability profiles all favoring abundant rainfall (potentially exceeding 10 inches on a localized basis). These totals will fall on areas that will have experienced 2-6 inches of rainfall leading up to the forecast period, with wet soils/terrain variations all suggestive of a potentially widespread and significant flash flood event. The overall outlook was on track with an eventual broad axis of heavy precipitation expected from north/central Texas northeastward to Ohio/Pennsylvania. The previous synoptic scale forecast was relatively unchanged. What should be the final surface wave had already started to enhance convection across over southwestern Texas in the predawn hours...and it should continue to make its way across the Southern Plains to Mississippi Valley by later this morning through the remainder of the period. This wave will likely be the strongest of the 3 allowing a much more amplified surface evolution that will shift the focus of heavier precip further northwest while also providing some vigor in the overall QPF distribution. The trend the past 48 hrs is for a zone of significant (3-6+") totals to be recognized across portions of the Southern Plains up through the Ozarks and Mid-Mississippi Valley of Missouri, spanning as far north as south- central Illinois.The heaviest precipitation axis based off the latest NBM probabilities still has a significant footprint of at least 3" located near the Dallas metro and points just east (40-60%) to as high as 60-80% in a large area spanning the Red River of northeast TX up through much of western and northern AR, through the Ozarks of MO, finally reaching the Mississippi River south of St.Louis. Ensemble mean QPF output signals a massive area of at least 2" with urban centers like Dallas, Little Rock, St. Louis, and Springfield, MO all within a core of the heavy precip footprint. Considering the neighborhood probabilities from the 04/00Z HREF...maintained the Moderate Risk area with only minor adjustments based on the model and WPC QPF. Also made a slight eastward expansion of the Marginal and Slight Risk areas in the Ohio Valley and the Mid-South where Flash Flood Guidance was largely reduced to less than half an inch per 3 hours. Bann Day 2 Valid 12Z Sat Apr 05 2025 - 12Z Sun Apr 06 2025 ....THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF ARKANSAS, THE MID-SOUTH, AND THE LOWER OHIO VALLEY... ....LIFE-THREATENING FLASH FLOODING IS EXPECTED ON SATURDAY=20 AFTERNOON THROUGH SUNDAY MORNING... Saturday is forecast to be the final day in the multi-day high- impact flood event that is unfolding across the Mid-South, Lower and Mid-Mississippi Valleys, and Lower Ohio Valley. Deep slow- moving convection is anticipated to be ongoing across the Mid-South at the beginning of the Day 2 period, and another wave of=20 widespread convection should develop by late morning/early=20 afternoon. Saturday night, veering flow aloft and increasing=20 eastward momentum of the frontal boundary draped from Texas to the=20 Ohio Valley will result in faster forward storm motion as=20 convection pushes into the South. The ingredients will be in place and the synoptic setup will=20 support widespread, deep convection with efficient/high rain rates across the Lower and Mid-Mississippi Valleys. Deep southerly flow=20 across the region will continue to push warm, moist, unstable air=20 into the region. Anomalous precipitable water values 2-3 standard=20 deviations above normal, and high MUCAPE values of 2000+ J/Kg will=20 support rainfall rates in excess of 1-2 inches per hour. Additionally, areas from Arkansas to the Lower Ohio Valley will be in the favorable right entrance region of the upper level jet with a slow-moving frontal boundary across the region. The combination=20 of slow storm motion and efficient/high rainfall rates is expected=20 to result in widespread significant additional rainfall totals of=20 3-6 inches, with locally higher amounts.=20 The High Risk of excessive rainfall encapsulates areas from=20 Arkansas through southeastern Missouri to western Tennessee and=20 Kentucky. All major deterministic models depict an expansive=20 footprint of 3-6 inches of rainfall, with locally up to 7-9 inches. These areas have already received significant rainfall since=20 Wednesday, and any additional rainfall could result in a very=20 dangerous flash flooding scenario with considerable to potentially=20 catastrophic impacts.=20 For the afternoon forecast, the High Risk area remained in place with small adjustments based on latest high-res model guidance and WPC QPF. A broad Moderate Risk extends around the High, stretching from the Ark-La-Tex to southern Ohio, which encapsulates areas that could also be prone to significant flash flooding. This remains a=20 life-threatening situation for areas of the Lower and Mid-=20 Mississippi Valley regions. Dolan/Bann Day 3 Valid 12Z Sun Apr 06 2025 - 12Z Mon Apr 07 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST ON SUNDAY... On Sunday, upper level southern stream energy that had been=20 supporting the heavy rainfall event over the past 4 days will begin to interact with a northern stream trough and accelerate northeastward. This will push the surface cold front across the=20 southeast, with an axis of high moisture ahead of and along the=20 front. There should be enough instability and shear to support thunderstorms with locally heavy rainfall from the Central Gulf Coast through Central Alabama and Northern Georgia. Rainfall totals are expected to reach 2-3+ inches in these areas, and rainfall=20 rates could challenge FFGs of 2-2.5 inches per hour (although the=20 3+ inches per 3 hour FFG may be be less likely to be met/exceeded). Dolan/Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9n3GJxPLXI0s1qn0OORONupLgYjtrxxFLnOjl6_Rzj4A= UnwW_RgKln_0OuF4g0IBfB8DXgGyrMB8rqn4Qz3Wewk59XA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9n3GJxPLXI0s1qn0OORONupLgYjtrxxFLnOjl6_Rzj4A= UnwW_RgKln_0OuF4g0IBfB8DXgGyrMB8rqn4Qz3WAtRop7g$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9n3GJxPLXI0s1qn0OORONupLgYjtrxxFLnOjl6_Rzj4A= UnwW_RgKln_0OuF4g0IBfB8DXgGyrMB8rqn4Qz3WDcAhAd4$=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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