Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Apr 08 2024 08:31:00 FOUS30 KWBC 080830 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 AM EDT Mon Apr 8 2024 Day 1 Valid 12Z Mon Apr 08 2024 - 12Z Tue Apr 09 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS AND ADJACENT SOUTHERN MISSISSIPPI VALLEY... The potential for heavy to excessive rainfall looks to develop=20 over portions of the southern U.S. as shortwave energy diving=20 southward over the Intermountain region helps carve out a deep=20 layer southern stream trough. Flow aloft over the Slight Risk area=20 backs and begins to accelerate Monday evening and Monday=20 night, with increasingly divergent flow aloft and increasing low=20 level moisture transport helping to set the stage for convection=20 capable of producing downpours. Toward the latter half of the day one period (Monday night),=20 coupled upper jet dynamics will lead to more favorable deep-layer=20 lift, with an increase in low-level theta-e/moisture transport from the western GOMEX into a quasi-stationary surface boundary (which=20 is maintained by the persistent FGEN within the right-entrance=20 region of the northern jet streak across the Lower MO Valley). This is when the moisture and kinematic anomalies begin to increase more appreciably per the GEFS and SREF, with 850 mb southerly flow and moisture flux anomalies nearing +3 standard deviations above=20 normal by 12Z Tue. Meanwhile, current NAEFS forecast IVT (integrated waver vapor transport) increases above 750 kg/ms late Monday night as well. WPC has maintained the Slight Risk area that was inherited from yesterday's Day 2 ERO. Inhibiting a more enhanced risk is the relatively lack of deep-layer instability=20 later in the period when the dynamical forcing would be increasing, as implied with the latest HREF probabilities (40-60% chance of exceeding 1"/hr rates, but only 20-30% probs of exceeding 2"/hr). The other factor is the relatively dry (at least for now) antecedent soils, with the top meter soil depth moisture percentiles generally 30% or less per the latest NASA SPoRT analysis. This will undoubtedly change going into the day 2-3 period (more below).=20 Hurley Day 2 Valid 12Z Tue Apr 09 2024 - 12Z Wed Apr 10 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE SOUTHERN MISSISSIPPI VALLEY... Heavy to potentially excessive rainfall should be on-going at the=20 beginning of the outlook period on Tuesday morning that persists=20 into the daytime hours. Flow aloft should be increasingly diffluent east of an approaching deep-layer trough and closed low while=20 moisture from the Gulf continues to be drawn northward. Meanwhile, the surface front to the north of the ArkLaTex to ArkLaMiss corridor will drift slowly southward, which given the aforementioned FGEN support from the northern stream jet streak, will maintain widespread rainfall with repetitive convection and heavier rainfall rates across many of the same locations. Helping to boost the rainfall rates will be the uptick in deep-layer instability as well, with MUCAPEs climbing between 500-1000 J/Kg across the Moderate Risk area (and closer to 1500+ J/Kg along the Gulf Coast). Low-mid level moisture and kinematic anomalies remain impressive (around +3 standard deviations) ahead of the upper trough, so given the boost in instability, along with the priming of soils with the rainfall during day 1, WPC has maintained the=20 Moderate Risk area inherited from yesterday's Day 3 ERO.=20 Hurley Day 3 Valid 12Z Wed Apr 10 2024 - 12Z Thu Apr 11 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE LOWER MISSISSIPPI VALLEY AND MID-SOUTH TO THE GULF COAST... Per collaboration with the WFOs, have upgraded to a Moderate Risk=20 across the ArkLaMiss region through much of MS-AL, southwest TN, and the far western portion of the FL Panhandle. Still quite a bit of spread among the 00Z models, however most of them continue to=20 show multiple (2 to 3) footprints of heavy rain, to the tune of=20 3-6+ inches. Over northern portions of the Moderate (including the=20 western TN Valley), the multi-day rainfall in Days 1-2 (and=20 Slight- Moderate ERO risk areas) factored in terms of maintaining a Moderate Risk on Day 3, even though WPC leaned toward depicting=20 the heaviest QPF farther south toward the Gulf Coast. Farther=20 south, the FFG going into the event will be notably higher given=20 the drier antecedent soils, however the more favorable=20 thermodynamic profiles (especially greater deep-layer instability)=20 will allow for more intense short-term rainfall rates, as noted per the swath of 3-6+ inches of rain per the global models.=20 Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6ObyfJ8j9G5BPZKj7BpeDj2mogXmzQZbCLUZRUcr5fzW= vCaC3K9JhWc7aOcTEPEL-GqBgJ1e9-sAlo7UmHByF3pduJQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6ObyfJ8j9G5BPZKj7BpeDj2mogXmzQZbCLUZRUcr5fzW= vCaC3K9JhWc7aOcTEPEL-GqBgJ1e9-sAlo7UmHByBmISaZg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6ObyfJ8j9G5BPZKj7BpeDj2mogXmzQZbCLUZRUcr5fzW= vCaC3K9JhWc7aOcTEPEL-GqBgJ1e9-sAlo7UmHByukm0vzY$=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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