Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri May 23 2025 19:41:42 FOUS30 KWBC 231941 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 341 PM EDT Fri May 23 2025 Day 1 Valid 16Z Fri May 23 2025 - 12Z Sat May 24 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEASTERN KANSAS AND NORTHEASTERN OKLAHOMA INTO THE OZARKS... ....Kansas, Oklahoma, Missouri, Arkansas... There remains a good model signal for the development of heavy rainfall later today, in what is expected to be the first of several rounds impacting the region over the next few days. The synoptic pattern will evolve today into one that will support heavy rain within periods of organized convection/MCS beginning tonight. This will be in response to the central part of the CONUS being sandwiched between an elongated trough to the northeast and a building ridge over the Four Corners, which leaves broad W/NW flow across the area. Within this flow, ascent will occur through periodic impulses moving through the flow combined with the RRQ of a jet streak aloft. The resultant frontogenesis from the RRQ of the jet streak will efficiently overlap a warm front wavering in the area, providing additional lift, which will then be enhanced by isentropic ascent of the LLJ atop this front. Forcing will be pronounced, and will act upon favorable thermodynamics, which will become increasingly robust the latter half of today and tonight. This will occur as the impressively broad 850mb LLJ emerges from the Gulf and surges to 40+ kts, angling orthogonally into the warm front. This will produce strong moisture convergence as reflected by the moisture transport vectors, and resupply elevated CAPE of 3000 J/kg northward into the Plains. As convection blossoms this evening, generally after around 04Z, it will expand rapidly and organize thanks to 40-50 kts of bulk shear. This will result in clusters of storms, potentially growing upscale into an MCS, and training of echoes is likely due to aligned flow to the warm front. There continues to be some uncertainty into how quickly the convection will grow into an MCS allowing it to push faster to the southeast towards daybreak Saturday, but until that happens, some upstream growth to the NW is expected which will prolong training of echoes. With a high probability for rain rates exceeding 2"/hr, any training could lead to rainfall of 2-5", with locally higher amounts possible (10-20% chance), especially near the MO/KS/AR/OK intersection. Another fly-in-the-ointment for the risk area is how elevated convection north of the warm front, especially early tonight, will impact the progression SW. There are a few high-res CAMs, and several global models, that suggest an outflow boundary will be pushed south more into Oklahoma tonight, which will be aided by residual boundaries from any dry line convection moving east, and the front itself. This could result in a bi-modal distribution to rainfall, and a secondary maxima of flash flood risk across eastern OK. While confidence is higher in the isentropic/elevated heavy rain risk farther north, the SLGT risk was tailored a bit SW from inherited to account for this potential. Otherwise, the inherited risk areas were modified only cosmetically for recent guidance, primarily being pulled NW (the MRGL risk area) more into KS for backbuilding convection during the development stage. ....South Florida... A weakening front across South Florida will serve as a focus for slow moving thunderstorms once again today, drifting east to the Gold Coast through the aftn/eve. This front and a lingering tail of an upper jet streak will provide synoptic lift into favorable thermodynamics characterized by PWs of 1.75 to 2.00 inches, and SBCAPE above 2000 J/kg. This overlap will support rainfall rates within thunderstorms for which the HREF indicates has a 40% chance of exceeding 2"/hr, and the 15-min HRRR suggests brief rainfall intensity of 4"/hr possible. Where these storms slow/stall, primarily along the southeast coast where they interact with the sea breeze, this could result in spots of 3-5" of rainfall. Should this rain occur atop urban areas or over locations that have received heavy rainfall the past 48-hrs, isolated flash flood instances could result, so the MRGL risk was maintained. Weiss Day 2 Valid 12Z Sat May 24 2025 - 12Z Sun May 25 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEASTERN KANSAS and NORTHEASTERN OKLAHOMA INTO THE OZARKS... ....Central/Southern Plains to Ozarks... The period will begin with decaying convection from what is expected to be a pronounced MCS moving across the region overnight tonight into Saturday morning. As this decays and drifts southward, it will leave lingering boundaries across the region. Exactly where these boundaries track is uncertain, but each one will be critical to redeveloping convection later Saturday. Otherwise, the synoptic setup continues to look favorable for another round of heavy rainfall within organized convection/MCS Saturday night, especially after 03Z or so. The placement of this remains uncertain as the warm front will be meandering, and likely influenced by residual convective boundaries pushing it subtly south. At the same time, the mid-level ridge over the Four Corners will at least drift eastward, causing some bulging of mid-level thicknesses across the High Plains, and somewhat offsetting the southward advance of the front itself. It is not possible to decipher exactly where the boundary will be when convection initiates with the onset of the robust LLJ Saturday night (SW 850mb winds above 40 kts), but it will again be a focus for renewed=20 thunderstorms as PWs and MUCAPE surge to 1.75 inches and 3000 J/kg, respectively. The accompanying impressive moisture flux will help fuel convective development, and as bulk shear remains above 40 kts, thunderstorms will again quickly intensify and grow upscale into a new MCS Saturday night. Training and backbuilding of echoes with rainfall rates 1-2+"/hr are expected, leading to total rainfall of 3-5" in some areas. Considered a targeted MDT risk due to potential overlap of this impressive rainfall atop soils that will be primed from anticipated heavy rain on D1. However, the high-res CAMs and globals have shifted just far enough south (suggesting the OFB will win out over the slow ridge build) to preclude a perfect overlap of rainfall footprints. After coordination with the affected WFOs, opted to instead shift the SLGT risk southward a bit and leave room for a potential upgrade with later updates should confidence in MCS placement become more certain. ....Central High Plains... Mid-level trough amplifying across the Great Basin will yield downstream divergence into the High Plains on Saturday. This will occur into an environment that will become increasingly moist and unstable as 850mb flow emerging from the Gulf carries moisture and instability northward into NE/SD/WY, and upslopes into the region. The guidance has trended a bit more robust with rainfall across=20 this area on Saturday, as convection potentially becomes widespread within PWs that are progged to exceed the 90th percentile=20 according to NAEFS ensemble tables. Rainfall rate probabilities=20 from the HREF and REFS both feature a low end risk (10-20%) for=20 brief 1"/hr intensity, and cells may repeat from west to east through the evening, leading to rainfall that may exceed 2" in a few locations. The signal is modest, and after coordination with NWC there appears to be limited NWM signal as well, so no risk area was introduced at this time. However, a MRGL risk may be needed with future updates, especially if the focus translates a bit farther north into wetter antecedent soils. Weiss Day 3 Valid 12Z Sun May 25 2025 - 12Z Mon May 26 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM SOUTHEASTERN KANSAS and NORTHEASTERN OKLAHOMA INTO THE OZARKS... Rinse and repeat for D3 as the pattern remains stagnant and supportive of yet another round of heavy rain due to repeating thunderstorms and an MCS, especially the latter half of D3. This will produce heavy rainfall on top of soils that will likely be quite saturated (NASA SPoRT 40cm soil moisture forecast above the 98th percentile) from heavy rain anticipated on D1 and D2. For Sunday, despite still some latitudinal differences in the placement of the heavy rainfall due to uncertainty in the warm frontal location, influence of any residual convective boundaries from prior thunderstorms, and some eastward translation of the mid-level ridge, the risk for flash flooding remains elevated. This is because renewed convection beneath a shortwave moving across the Southern Plains Sunday evening/night will again help generate and focus thunderstorms. The environment will remain extremely=20 favorable for heavy rain, with PWs progged by NAEFS to exceed the=20 97th percentile late D3 as the renewed 850mb LLJ surges moisture=20 out of the Gulf and into the boundary.=20 The biggest difference between D3 and D1/D2 is that the guidance suggests convection will blossom a bit farther SW than previous days, but with uncertainty continuing, the MDT risk was adjusted only slightly near the MO/KS/AR/OK borders where heavy rain is possible all of the next 3 days. Farther SW into OK and even beyond the Red River Valley into TX, some southward advancement of forcing could produce heavy rainfall, so the SLGT risk was expanded into this region as well where GEFS and ECENS probabilities are at least modest for 3"/24 hrs, although the highest probabilities remain farther to the NE within the MDT risk contour. While the latest CSU first guess field driven by the UFVS does not suggest a MDT risk, it is likely suffering from not recognizing the D1 and D2 antecedent rainfall, so after coordination with the WFOs the MDT risk was tailored only slightly and maintained for potentially considerable flash flooding, despite some future adjustments to the position likely. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7lRg0Jn2Ilkl4IZmta_WTrZ0ZO5feW0g8KhY6GqzyRWR= E5U81FISagDCngwG1I_y5KL7W-ksIizadsclNvSq09T5C4M$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7lRg0Jn2Ilkl4IZmta_WTrZ0ZO5feW0g8KhY6GqzyRWR= E5U81FISagDCngwG1I_y5KL7W-ksIizadsclNvSqEgq7L7k$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7lRg0Jn2Ilkl4IZmta_WTrZ0ZO5feW0g8KhY6GqzyRWR= E5U81FISagDCngwG1I_y5KL7W-ksIizadsclNvSqk5RA62c$=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. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .