Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Apr 20 2025 15:42:50 FOUS30 KWBC 201542 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1142 AM EDT Sun Apr 20 2025 Day 1 Valid 16Z Sun Apr 20 2025 - 12Z Mon Apr 21 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE CENTRAL US TODAY INTO TONIGHT... 16Z 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=20 eastern IA by the evening, correlating well with a zone of enhanced neighborhood probs of >3" (50-80%) located in the aforementioned=20 region. Heavy rain signature is forecast within those zones in=20 particular with rates reaching 1-2"/hr at peak intensity within=20 organized convective schema. Areas of MO are already well saturated due to previous periods of rainfall the past 24-48 hrs, so=20 inherited FFG's are fairly low upfront for the setup. This allowed=20 for a continuation of the SLGT risk across the above area with=20 little deviation in the risk area outline. For more information on=20 the initial stages of afternoon flash flood concerns over OK/MO,=20 please refer to MPD #0149.=20 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=20 initiation will likely settle along the frontal boundary with some=20 training potential due to mesovorticies becoming more anchored to=20 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=20 see the heaviest precip is relatively dry with FFG's running near=20 and above climo for all relevant indices (1, 3, and 6-hr FFG).=20 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=20 prospects for significant flash flooding are low. This led to=20 maintaining the MRGL risk across the area with only some minor=20 eastward adjustments to reflect the current hi-res trends in=20 heaviest precip placement.=20 =20 Kleebauer Day 2 Valid 12Z Mon Apr 21 2025 - 12Z Tue Apr 22 2025 The probability of rainfall exceeding Flash Flood Guidance is less than 5 percent. Bann 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... 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!9t2hxyFpu04x1m_aaKwVjvDj8YFaNyG7aUO4BDYACrLs= LuCmJ7XVdSrFnZVfJziGgs5InpWadlrOFVpjEs3mJ80eR3I$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9t2hxyFpu04x1m_aaKwVjvDj8YFaNyG7aUO4BDYACrLs= LuCmJ7XVdSrFnZVfJziGgs5InpWadlrOFVpjEs3mYzdDIok$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9t2hxyFpu04x1m_aaKwVjvDj8YFaNyG7aUO4BDYACrLs= LuCmJ7XVdSrFnZVfJziGgs5InpWadlrOFVpjEs3m163OM1I$=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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