Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Apr 20 2025 21:23:02 FOUS30 KWBC 202121 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 521 PM EDT Sun Apr 20 2025 Day 1 Valid 2120Z Sun Apr 20 2025 - 12Z Mon Apr 21 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE LOWER MISSOURI AND UPPER MISSISSIPPI VALLEYS... ....2100 UTC Update... Based on the latest observational and guidance trends, updated the Day 1 ERO to trim the back edges of both the Slight and Marginal Risk areas across eastern parts of the Central-Southern Plains. Hurley ....1600 UTC Update... Closed circulation over the TX Panhandle is beginning to lift northeast with a steadier frontal progression to the east as noted via the theta_E advection panels via recent mesoanalysis. Large scale ascent is still plentiful in-of the front and just out ahead leading to scattered convective elements from south-central TX up through the Mid-Mississippi Valley. The previous forecast outline remains consistent with what is occurring and general expectations moving forward. Two main areas of concern this afternoon and evening will be separated by two distinct forcing mechanisms. The first will be the northern precip extent over eastern OK extending up through MO/IA through much of the forecast period. Much of this convective scheme will be driven by significant height-falls and primed mid and upper ascent driven by the shortwave trough/closed ULL pattern propagating to the northeast along with primed RER jet dynamics from a broad anti- cyclonically oriented jet streak positioned over the Upper Midwest. The maturation of the surface cyclone across the Plains and points northeast will benefit to some training potential within the developing triple-point likely located over northern MO into eastern IA by the evening, correlating well with a zone of enhanced neighborhood probs of >3" (50-80%) located in the aforementioned region. Heavy rain signature is forecast within those zones in particular with rates reaching 1-2"/hr at peak intensity within organized convective schema. Areas of MO are already well saturated due to previous periods of rainfall the past 24-48 hrs, so inherited FFG's are fairly low upfront for the setup. This allowed for a continuation of the SLGT risk across the above area with little deviation in the risk area outline. For more information on the initial stages of afternoon flash flood concerns over OK/MO, please refer to MPD #0149. Secondary focus will align with the flow oriented out of the Western Gulf up into east TX providing reasonable theta_E advection within the confines of a slowing cold front, eventual quasi- stationary boundary forecast by late-afternoon through the evening. Radar/Sat composite already indicates convective streaks of cumulus moving north-northeast just north of the TX Gulf coast. This outlines a sufficiently buoyant environment capable of convective development once initiated via either thermodynamic processes, surface convergence, or a combination of the two. At this juncture, expectation is for an area of thunderstorms to develop east of the the terrain focused region of the I-35 corridor with the target being closer to the Piney Woods area of east TX. Convective initiation will likely settle along the frontal boundary with some training potential due to mesovorticies becoming more anchored to the theta_E gradient positioned in proxy to the front. HREF blended mean output is very aggressive in forecast totals of 2-3" and locally higher in those areas that see the slow-moving convective pattern that initiates. The only good news is the area that will see the heaviest precip is relatively dry with FFG's running near and above climo for all relevant indices (1, 3, and 6-hr FFG). Considering the low probability (10-30%) of even seeing 2"/hr rates and 3 or 6-hr FFG exceedance probs <20% at any point, the prospects for significant flash flooding are low. This led to maintaining the MRGL risk across the area with only some minor eastward adjustments to reflect the current hi-res trends in heaviest precip placement. Kleebauer 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!8L0HgLMzrq5axjT9pJBHMWP0Hz7hjeSqs3Pi1K2j9Hg_= akT-W0uN9xNX1w4gn9Ho97Z1lSA_KgNDark0zjJWUUR0IiQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8L0HgLMzrq5axjT9pJBHMWP0Hz7hjeSqs3Pi1K2j9Hg_= akT-W0uN9xNX1w4gn9Ho97Z1lSA_KgNDark0zjJWIcKl9Z4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8L0HgLMzrq5axjT9pJBHMWP0Hz7hjeSqs3Pi1K2j9Hg_= akT-W0uN9xNX1w4gn9Ho97Z1lSA_KgNDark0zjJWRp-lJQk$=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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