Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Tue Apr 30 2024 08:09:00 FOUS30 KWBC 300820 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 420 AM EDT Tue Apr 30 2024 Day 1 Valid 12Z Tue Apr 30 2024 - 12Z Wed May 01 2024 ....A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS SOUTHEAST KANSAS AND NORTHERN OKLAHOMA... ....Kansas and Oklahoma... A surface cyclone will develop over the northern plains with a cold front extending along the southern periphery of the surface reflection. A strong southerly component downstream of the surface low will allow for a pool of elevated theta-E's to advect poleward into eastern KS and Oklahoma during the late afternoon time frame with an associated increase in regional instability within the axis of higher theta-E's. There is a growing consensus for convective initiation across east-central KS towards the 23-01z period where the LLJ kicks and the aforementioned cold front converges on the region with rapid CI likely somewhere north of Wichita. A moisture rich environment thanks to low-level moisture advection coupled with primed instability in-of the area will lead to locally heavy rain within any convective development, especially under more prominent supercellular modes thanks to increasing shear within the area of positive buoyancy. 00z HREF blended mean QPF has really become bullish with totals approaching 2.5-3" now across that area in and north of Wichita with some guidance expanding further as cold pool propagation to the south would expand some pretty hefty totals to areas that were just recently impacted. Rates are likely to approaching 2-3"/hr based on the latest HREF probability fields (30-40%) which would easily eclipse the lower FFG indices in place after the recent deluge. The area near the KS/OK border is conditional to the cold pool propagation and the cold front expecting to struggle to gain latitude leading to an elongated front where convection can situate through the end of the period. Some models are aggressive with totals exceeding 3" if that were to occur, so the prospects for higher end impact lead to the addition of a SLGT risk over the area where the threat is maximized. ....Northeast... A stalled frontal boundary is analyzed across northern NJ, extending west through northern PA, southern NY, into Ontario. A cold front is currently moving through the Great Lakes in response to surface low moving through the northern Midwest. The stationary front, cold front, and attendant mid-level energy will all aid in convective development this afternoon with initiation over western NY and northern PA, moving eastward through peak diurnal instability. Consensus for scattered thunderstorms has grown across all guidance leading into the period with the latest CAMs persistent on an axis of locally heavy rainfall impacting areas from the Finger Lakes, eastward into the Hudson Valley. 00z HREF blended mean QPF footprint depicts a large swath of 1-1.5" precip totals in-of NY state with the northern fringes of PA out near Bradford and Susquehanna also within the mean QPF swath formed via the convective regime expected. Hourly rates will be the primary driver for localized flooding across the area in question with HREF probabilities for 1"/hr now topping out at 50-60% from a line extending between Utica down to northeastern PA with the time frame between 18-00z as the period of interest for convective initiation and impact. 2"/hr rates are non-zero, but limited in spatial coverage with probabilities peaking closer to 10-15% over portions of southern NY state. The area of interest will lie along the stalled front where the best low-level convergence axis will reside, as well as where the favored mid-level vorticity maxima advects overhead. Where these two align will likely lead to totals approaching 2" within a course of 1.5-2 hrs before convection steadily moves eastward. Areas with more urbanization will be subject to the best flash flood potential due to runoff, as well as areas that can pick up a quick 1.5-2" when impacted. The MRGL risk was maintained from the previous forecast with some minor adjustments on the fringes. ....Midwest to Upper Mississippi Valley... A quick-moving shortwave will eject out of the southern periphery of a negatively tilted longwave trough over the northern Plains and southern Canada leading to convective genesis over the northern and central plains. The latest HREF and associated CAMs highlight the potential within the mean QPF forecast with a swath of 1-1.5" possible over portions of MN/IA/Northwest WI with a signal for local 2-3" amounts given the potpourri of deterministic. There is less consensus on exactly where the stronger cells will reside, but the initiation point across southeastern SD and eastern NE is fairly consistent on guidance. The threat is low to mid-tier for the MRGL risk, but probabilistic signals for at least 1"/hr and up to a quick 2-3" are enough for some local impacts to areas who have substantially lower FFG indices thanks to the previous series of storms. The MRGL risk was maintained with little deviation from the previous forecast. ....Southern Plains... A dryline will focus over west TX into OK during the afternoon today with scattered convection likely to fire after 20z within the Stockton Plateau and portions of north TX towards the Red River. Precip rates will be capped between 1-1.5"/hr which is enough for some localized flood concerns in small towns with urbanization factors that could enhance runoff potential. The threat is on the lower end of MRGL, but resides within the scope of the threshold of 5%. Kleebauer Day 2 Valid 12Z Wed May 01 2024 - 12Z Thu May 02 2024 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS FORECAST OVER PORTIONS OF THE MIDWEST & CENTRAL/SOUTHERN PLAINS... ....Midwest/Central Plains... A broad, closed upper level cyclone will persist over eastern MT into the adjacent northern plains and southern Canadian Provinces by Wednesday with a potent, mid-level shortwave rounding the base of the mean trough and exiting into the northern high plains by the second half of the period. A vigorous upper diffluent pattern will transpire by Wednesday afternoon and evening with a corridor of convection initiating towards nightfall. A stout LLJ will enhance areal shear in-of the central and northern plains within the Quad state area between NE/KS/IA/MO. Any convective development will congeal and grow upscale into a large cluster of heavy thunderstorms within the above area leading to widespread heavy rains capable of flash flooding as rates likely reach towards that 1-2"/hr marker with locally higher possible. The one factor that may limit the higher end potential will be the progressive nature of the convection as pattern yields faster storm motions limiting the prospects for training. This does not mean there will not be some areas that can overachieve and maximize the potential based on the prime mid-level energy involved, along with a strong meridional push of some very moist air approaching 2-2.5 deviations above normal for PWATs based on the latest GEFS and NAEFS output. The threat remains solidly within the SLGT with a chance at any upgrade if future CAMs indicate a better potential for totals to exceed 3" in a larger footprint than what is currently depicted. ....Southern Plains... The location on a secondary rainfall maximum across TX is becoming more clear as a quick moving shortwave embedded within the subtropical jet ejects out of MX and moves to the northeast by Wednesday evening. Guidance suggests that somewhere across eastern TX would be the primary target for convective development with the heaviest rainfall located within the tongue of higher theta-E's positioned east of the line from ABI to DRT. A cluster of thunderstorms will develop over west TX with a steady propagation to the east during the afternoon and early evening hours. By nightfall, a prominent LLJ aligned across central and east TX will provide necessary maintenance to any convective clusters that will migrate over the state after peak diurnal instability. Surface dew points running into the upper-60s and low-70s relay the moisture rich environment that will provide the foundation for significant rainfall potential as the mid-level vorticity maxima continues to move over the region providing the additional large scale forcing that will keep the convection rolling through the evening. Current deterministic output is consistent with local totals breaching 4" across portions of central and north TX with the latest ensemble bias corrected QPF sufficiently in the range of 2-3" over an area extending from southwest OK down through the DFW metroplex and adjacent surrounds. This is quickly gaining steam for a higher impact event, especially when you compound the idea that the northern portion of the I-35 corridor in TX becomes a focal point. There is some spread on the exact positioning of the heaviest precip expected, but ML outputs have been persistent in the area over north TX for the past succession of runs. Should deterministic guidance converge further with similar amounts, or if ensemble support becomes more aggressive with totals and probabilities for exceeding higher end QPF markers, a Moderate Risk cannot be ruled out in later issuance's. Moderate Risk impacts are certainly possible wherever the heavy rainfall materializes with the highest prospects within the major urban centers along I-35. Kleebauer = = = --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .