Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue May 02 2023 16:02:20 FOUS30 KWBC 021602 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Tue May 02 2023 Day 1 Valid 12Z Tue May 02 2023 - 12Z Wed May 03 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Campbell ....Southern High Plains... Convection is firing up along a frontal boundary across West Texas and eastern New Mexico this morning and is expected to lift north and east through the afternoon and evening hours. Return flow from the Gulf of Mexico has brought PW values up to 1/1.25 inches to the Southern/Central Rockies and adjacent High Plains, which is approximately an anomalously of +1.5 sigma (especially across the Texas/Oklahoma Panhandles). Several of the hi-res guidance depict isolated cells that pulse up to 1.5 inches/hour as they cross the Texas panhandle. 3-FFG over much of this region ranges 2-5 inches and the nature of these storms look to remain fairly progressive; which will be a limiting factor on the potential for excessive rainfall and isolated areas of flash flooding. There is a non-zero potential for some areas to see flash ponding or flooding but the exact location is uncertain due to the "popcorn" nature of convection expected. Day 2 Valid 12Z Wed May 03 2023 - 12Z Thu May 04 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley Day 3 Valid 12Z Thu May 04 2023 - 12Z Fri May 05 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE INTERMOUNTAIN WEST AND FROM PARTS OF THE LOWER MISSOURI AND MID MISSISSIPPI VALLEYS TO THE OZARKS AND ARKANSAS RIVER VALLEY... ....Central and Northern Intermountain West... Stubborn, slow-moving mid-upper low/trough will take on more of a negative tilt Thu-Thu night as shortwave energy pivots northward through the Southwest and Central Great Basin. The uptick in upper level forcing with the increased upper difluence/divergence/DPVA, coinciding with areas of decent instability for this time of year prior to monsoon season (MUCAPEs of 500-1500 J/Kg, owing in large part to steepening H7-H5 lapse rates of 7-8+ C/KM), will allow for more numerous, pulse-type convective clusters with the potential of isolated 0.50+"/hr rainfall rates. This as PWs climb between 0.50-1.00" per the guidance, highest (>0.75") across eastern WA-OR into northern ID, which would be 2-2.5 standard deviations above normal for early May. By themselves, the dynamical and thermodynamical parameters progged by the models would result in a minimal excessive rainfall threat (i.e. below the 5% threshold for a Marginal); however, when combined with the snowmelt in some locales and burn scar areas in others, do anticipate a more enhanced risk for localized runoff issues. As a result, the Marginal Risk area inherited from yesterday's experimental Day 4 ERO was expanded northward to the Canadian border with WA-ID. ....Eastern Kansas-Oklahoma into much of Missouri, Arkansas, and Western Illinois... Downstream of the aforementioned upper trough, flattening mid/upper ridge will remain across the central CONUS. Strong upper level confluence is expected to generate a 100-130 kt upper level jet streak across the Midwest to Lower OH Valley Thu-Thu night. The resultant uptick in low-level frontogenesis is noted from the guidance, while SW/SSW 850 mb flow increases to 40-50 kts and MUCAPEs between 1000-2000 J/Kg by Thursday evening and overnight across the outlook area. The dynamical and thermodynamical environment will foster the development and upscale growth of organized convection Thursday night, potentially leading to an MCS with an enhanced risk of cell training as the low-level jet (LLJ) ramps up and 850-300 mb thicknesses become more divergent downstream. Deep-layer moisture parameters are not overly impressive/anomalous at this point however (maximum PW values of 1.5-1.7" within the outlook area). As a matter of fact, the 00Z guidance did not indicate much of an overlap in terms of areas with the best forcing (upper divergence/strongest low-level frontogenesis across eastern KS-northeast OK into much of MO and northern AR) and best deep-layer moisture/instability profiles (south of the Red River and ArkLaTex). Given this, and the fact that the models currently depict more forward (downwind) propagation/Corfidi Vectors than otherwise, for now will maintain the Marginal Risk that was inherited from yesterday's experimental Day 4 ERO. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7nnrpBrQPGGwcTf_umHJxeR9WWb78OoEVhEwZ8ZDrAP5= 0a4a-CV5yzcKgqlCbja44DfxKrr1fBJ4QMYOnW5p6iULWT4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7nnrpBrQPGGwcTf_umHJxeR9WWb78OoEVhEwZ8ZDrAP5= 0a4a-CV5yzcKgqlCbja44DfxKrr1fBJ4QMYOnW5plqDXHQ8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7nnrpBrQPGGwcTf_umHJxeR9WWb78OoEVhEwZ8ZDrAP5= 0a4a-CV5yzcKgqlCbja44DfxKrr1fBJ4QMYOnW5pTzUOLKE$=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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