Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Apr 29 2025 07:41:26 FOUS30 KWBC 290739 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 339 AM EDT Tue Apr 29 2025 Day 1 Valid 12Z Tue Apr 29 2025 - 12Z Wed Apr 30 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS... ....Southern Plains... A repetitive scheme of successive convective outputs across portions of West TX through Oklahoma will result in significant flash flooding for a large portion of the above areas. The=20 combination of several mid-level impulses ejecting northeast within a large scale diffluent pattern will interact with a quasi- stationary front aligned from the Western Rolling Plains of TX up towards the Red River Basin mainly west of I-35. Favorable mid-=20 level ascent will allow for waves of convective initiation beginning late-morning and afternoon Tuesday with a secondary pulse around 00z Wednesday with a final pulse forecast after 06z as the mean trough finally starts pivoting east out of the Southwestern CONUS. Instability within the confines of West TX up to the Red River will allow for rapid development of convective clusters with some supercellular modes plausible prior to nightfall, eventually merging into a more multi-cellular scheme with backbuilding favored within the confines of the front denoted pretty well within the theta_E gradient output in each deterministic. All CAMs are well supportive of the 3-wave convective evolution with heavy rainfall situated across the Southern High Plains near the eastern Permian Basin with a general QPF maxima aligned southwest to northeast across the northern extension of the Rolling Plains in TX, up through the Red River and into the southern portion of the OKC Metro. This area has been the benefactor of multiple convective episodes already, leading to flash flooding within the corridor extending from Lawton down to the Red River. This area will be the most primed location for not only widespread flash flooding, but significant life-threatening flash flood prospects due to the well above normal soil moisture anomalies likely curbing any potential for rainfall to soak into the top soil layer.=20 Latest 00z HREF probability fields are overwhelmingly indicative of the threat with one of the more robust indications occurring within the 6-hr FFG exceedance probability marker running between=20 70-90% across the area extending from Foard County in TX up to western Cleveland County in OK. Neighborhood probabilities of >5" total rainfall are running between 50-80% in the same location with >8" between 20-30% across Southwest OK between Lawton to the Red River. Neighborhood probs for >3" are significant across much of OK back into North TX with the southwest extension back into the southeast corner of the LUB CWA. Considering the NASA SPoRT soil moisture percentiles running between 70-99% for the 0-40cm layer across the entire area above, the propensity for flooding, even in rural zones will be higher than normal. Given expected rates between 1-2"/hr during peak storm cycles and the expectation for >3" of rainfall...the threat will be of the higher magnitude in terms of impact prospects. The previous MDT risk inherited was not only maintained, but expanded a bit further southwest to align with the higher probabilities with the neighborhood >5" potential and=20 the elevated HREF EAS >2" probabilities that have been depicted. A=20 high-end MDT risk is forecast across the Red River Basin west of=20 Wichita Falls up into the southern fringes of the OKC metro,=20 including the town of Norman. Lower end probabilities for flash=20 flooding will exist across areas of the eastern Permian Basin out=20 through the Concho Valley as well as across much of Oklahoma not=20 included in the MDT.=20 ....Mississippi and Ohio Valleys... Mid-level impulses that eject out of the Southern Plains will continue to press northeast towards the Mississippi and Ohio Valleys with a progressive cold front moving southeast out of the Midwest during the same time frame. Afternoon destabilization coupled with the favorable addition of mid-level forcing and surface frontogenesis will lead to scattered convective pulses across the region with some organized elements focused within the confines of the front and under the shortwave propagation. Soils remain moist across much of the region extending from MO through the Ohio River Basin with an eastern extension to the Central Appalachians and Western PA Highlands. PWAT anomalies on the order of +1 to as high as +2.5 deviations cements a favorable deep moist environment conducive for heavy rainfall prospects within any convective scheme. The greatest concentration of heavy rain will likely be back over Southern and Central MO towards the 3 river confluence zone near KY/MO/IN/IL. HREF probs for >1" are very high (70-90+%) across much of the Mississippi and Ohio Valley areas with >2" probs relatively high (50-70%) in-of the Ohio River Basin over Southern IL through much of Eastern KY. The combination of convection over areas that are still in recovery from previous rainfall episodes and complex terrain will create a threat for isolated to perhaps=20 scattered flash flood instances over a large area during the afternoon and evening hours Tuesday. A SLGT risk remains firmly in place across much of MO along and south of I-70 until the 3 river confluence zone referenced above. A MRGL risk encompasses all other areas east through Western PA and even the southwest corner of NY state.=20 Kleebauer Day 2 Valid 12Z Wed Apr 30 2025 - 12Z Thu May 01 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE ARKLATEX, EASTERN OKLAHOMA, AND WESTERN OZARKS... The pattern remains very active as we move into Wednesday as the mean trough slowly slips eastward with ample mid-level energy accompanying, creating an environment favorable for organized convective clusters through the entire D2 time frame. Rich Gulf air will be pulled poleward ahead of the advancing trough with several mid-level perturbations ejecting out of the ahead of the primary shortwave trough. This in combination with sufficient buoyancy across Central and East TX through the Mississippi Valley will maintain a widespread convective zone of impact with strong, heavy thunderstorm cores and clusters capable of producing prolific rainfall totals.=20 Two primary periods of interest have become fairly well documented within the recent deterministic cycles. The first is the=20 continuation of the convective complex that will develop overnight=20 Tuesday into Wednesday morning over the western Red River Basin and slowly trudge eastward as it maneuvers in tandem with the primary=20 shortwave energy, and across the remaining theta_E gradient running parallel to the river. 00z HREF on the back end of its run is=20 signaling some very impressive rainfall totals (2-5") within the=20 first 12 hr. period in the forecast cycle (12z Wed to 00z Thu)=20 across places along the Red River up through Central and Eastern OK as the multi-cell cluster advances. Prob fields for >3" within=20 that first 12 hrs. are between 50-90% across south-central OK,=20 along and east of the I-35 corridor with 25-40% probs for even >5". Considering this is only for the first 12 hrs., the probs are=20 pretty impressive and really highlight the threat on the initial=20 heavy QPF core.=20 The second period of interest will occur in the afternoon and evening hrs. as diurnally driven convection will flare up over Eastern TX within a broad moisture rich environment as NAEFS anomalies continue to point to +2 to +3 deviations for PWATs (1.5-2") situated over the area. LLJ introduction by early evening will only exacerbate the potential with increasing low-level shear capable of persisting updrafts and eventual cell clusters that will originate from cells moving northeast within the mean flow. Several mid-level perturbations will be in the vicinity across Northeast TX to truly enhance and maintain convective cores such that it becomes a fairly favorable prospect for heavy rainfall extending into the Lower Mississippi Valley. Totals between 2-4" within an areal expanse are forecast across the ArkLaTex up through Western AR to just before the MO Ozarks. Locally higher totals will be possible across that entire area given the favorable rates between 1-2.5"/hr forecast currently within global deterministic. Pending outflow=20 propagation within the setup, heavier totals could advance further=20 east towards Central AR, but at this juncture, the primary area of=20 focus aligns over East TX up through Eastern OK and Western AR.=20 This correlates well with the theta_E ridge extension up through=20 the area where convective threats will be highest during peak=20 diurnal destabilization and beyond.=20 The previous MDT risk was maintained with a westward extension back over portions of south-central OK down across the Red River into TX. The MDT risk is NOT within the DFW metro, however it does lie just north, so will have to monitor trends closely over the next 24-36 hrs. A broad SLGT risk extends south into East TX but well north of the TX Gulf Coast.=20 Kleebauer Day 3 Valid 12Z Thu May 01 2025 - 12Z Fri May 02 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF THE LOWER MISSISSIPPI VALLEY UP THROUGH THE CENTRAL AND EASTERN GREAT LAKES... Numerous areas of thunderstorms will develop across the Mississippi Valley through the east-central CONUS due to the progression of a surface low out of the Southern Plains and attendant cold front trailing the primary cyclone. Western Atlantic ridging will allow for a continued advection of rich Gulf air into the Eastern U.S. with a deep moist environment pretty much solidified from the Great Lakes down to the Gulf coast. Area PWATs will run between 1.5-2.5 deviations above normal for much of the CONUS east of the Mississippi, an environment very favorable for convective schemes on the eastern flank of the surface low, as well as along and ahead of the progressing cold front. Areal QPF within the ensemble means are generally between 0.5 and 1" with some locally higher forecast across the above area(s). Some of the deterministic output is of course higher with some 2-4" maxima littered over the Southeast and Ohio Valley where the convective scheme will be most prominent. There's little agreement within the suite on exactly where the heaviest precip will fall, but there's plenty of coverage where any one spot could be the beneficiary of some heavy precip. At this lead, would probably lean on the Tennessee Valley into the Southern Ohio Valley to have the best potential due to a combo of antecedent moist soils, as well as the greatest surface based ascent likely within proxy to the advancing cold front. A broad MRGl risk exists over parts of the Lower Mississippi up through the Central and Eastern Great Lakes with the furthest east expansion over Western NY state down to the Central Appalachians.=20=20 Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt= __;!!DZ3fjg!7l9JD6txOmlpxCv5F6V-tv3FaASvVKzfmhTW4N-RgMU3yeXtbtKcl2Gx1mQcHbc= AQM4hbLtMjcaqfBsc6DLund8k_Ok$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt= __;!!DZ3fjg!7l9JD6txOmlpxCv5F6V-tv3FaASvVKzfmhTW4N-RgMU3yeXtbtKcl2Gx1mQcHbc= AQM4hbLtMjcaqfBsc6DLuTYtTOtQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt= __;!!DZ3fjg!7l9JD6txOmlpxCv5F6V-tv3FaASvVKzfmhTW4N-RgMU3yeXtbtKcl2Gx1mQcHbc= AQM4hbLtMjcaqfBsc6DLuanmacb8$=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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