Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Apr 21 2025 00:20:23 FOUS30 KWBC 210020 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 820 PM EDT Sun Apr 20 2025 Day 1 Valid 01Z Mon Apr 21 2025 - 12Z Mon Apr 21 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE MID TO UPPER MISSISSIPPI VALLEY... ....0100 UTC Updates... Further curtailed the back (western) edges of both the Slight and Marginal Risk areas based on the latest observational and guidance trends, along with the more current (18Z) HREF exceedance probabilities. HREF continues to show the 1-2+ inch/hr rates will be supportive of the Slight Risk through about ~0300Z.=20 Hurley Day 2 Valid 12Z Mon Apr 21 2025 - 12Z Tue Apr 22 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE LOWER MISSISSIPPI VALLEY... 20Z Update: Sufficient buoyancy across the Southeast will enable periods of scattered convection through much of Monday into early Tuesday morning across the Lower Mississippi Valley with the greatest coverage over LA/MS/AL. A slow-moving front will bisect the central portion of the region with prevailing southeast flow along and ahead of the front thanks to persistence within the western flank of a broad surface ridge extending from the Western Atlantic. Modest PWATs entering the 75-90th percentile will pool over the above area creating a better environment for pulse convective potential, as well as some organized convective elements within the confines of the front. Storm motions are forecast to be pretty weak for the first half of the forecast leading to a risk of training thunderstorm activity upon initiation. 12z HREF probs for >3" has increased closer to 45-70% within the corridor from southern MS up through portions of western AL. Despite drier antecedent conditions compared to points north and west, the environment is still favorable for locally heavy rainfall in portions of southeast LA up through the southern half of MS into western AL, a region susceptible to flash flooding prospects due to a plethora of urbanized areas. In coordination with the local Jackson, MS WFO...a MRGL risk has been added across the aforementioned area(s). Across coastal TX, the front mentioned in the previous discussion above will be co-located over southeast TX, slowly migrating to the southeast which will eventually press off the TX coast. A deeper pool of moisture will be situated along the coastal plain within the confines of the front as convergent flow in proxy to the front will generate a period of scattered to widespread convective clusters that will take time to move away from impacted areas. The good news in this setup is the rates anticipated are generally well-below FFG indices, especially for the 1 and 3 hr markers (~4 and 5" respectively) that would limit the flash flood concerns. However, the slow nature of the precip could still yield some local totals reaching 3-5" over a span of 2-4 hrs prior to the full convective pattern moving away from a given area. The best threat is likely within the immediate Houston metro given the lower FFG's locally due to urbanization factors. Outside the metro, sandier soils will limit flash flood threats below the threshold necessary for a risk, but still non-zero overall. After some coordination with the local Houston WFO, will opt to maintain a nil ERO signature over the area, but will mention the threat is non- zero and could spur a highly localized flash flood threat. Kleebauer Day 3 Valid 12Z Tue Apr 22 2025 - 12Z Wed Apr 23 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF TEXAS AND/OR THE ADJACENT SOUTHERN PLAINS... 20Z Update: Relatively unchanged thoughts for the period as guidance remains spread on the exact evolution of the convective pattern across the Southern Plains. The greatest threat will likely be within the overlap of the lower FFG indices due to previous rainfall the prior 72 hrs and the area where the approaching shortwave ascent pattern will yield a greater convective coverage. As of this time, the best chance will lie over north-central TX towards the Red River Basin down into the northern Hill Country. Ensemble bias corrected QPF output remains very bullish over those areas with >1.5" of precip forecast in that zone. That would be sufficient for flash flood potential given saturated grounds from this past series of convective impacts. Until we get a clearer picture across guidance, it will be difficult to pin-point any targeted upgrades. Until then, maintained general continuity of the MRGL risk with some trimming on the northern and southern flank of the risk area given a combination of ensemble probabilities favoring less of a threat for flash flooding to the north, and very high FFG's co-located within lower probs for any precip totals >2". Kleebauer ...Previous Discussion.. Largely maintained a Marginal Risk area over portions of the Southern Plains from Tuesday into early Wednesday morning due to the suite of 00Z numerical guidance continuing to show the potential for repeat/training of cells during the day and with overnight activity within an emerging moist and unstable return flow into and over several frontal boundaries. With the models struggling to latch onto a coherent signal...as shown by some guidance focusing heaviest QPF near the Gulf coast in proximity to a weakening cold front and other guidance developing the threat closer to the Red River in response to the approach of a mid-level wave...have opted to highlight concern for excessive rainfall in broad and rather unfocused Marginal Risk area close to a consensus placement of the precipitable water axis. With PW values generally being 1.25 inches or less...would not be expected numerous torrential downpours. The cumulative effect of what has already fallen from this weekends event and what falls on Tuesday/early Wednesday needs to be watched. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4EZuVde3vvfxjldjr-zSEhZMe1PrsQO28FozABR9oiYu= QCEw_Exo2nu7AbFYsy_uDzAhgT3rjZ-D_XtHpNJEsYVFNyo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4EZuVde3vvfxjldjr-zSEhZMe1PrsQO28FozABR9oiYu= QCEw_Exo2nu7AbFYsy_uDzAhgT3rjZ-D_XtHpNJEO8J6Uno$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4EZuVde3vvfxjldjr-zSEhZMe1PrsQO28FozABR9oiYu= QCEw_Exo2nu7AbFYsy_uDzAhgT3rjZ-D_XtHpNJEZsw9ldQ$=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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