Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Wed May 03 2023 15:42:00 FOUS30 KWBC 031541 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1140 AM EDT Wed May 03 2023 Day 1 Valid 16Z Wed May 03 2023 - 12Z Thu May 04 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Campbell Day 2 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 THE LOWER MISSOURI VALLEY AND EASTERN OKLAHOMA TO PARTS OF THE MID-SOUTH... ....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 0.50-1.00"/hr rainfall rates per. 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, western Tennessee, and northwest Mississippi... 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 robust MUCAPEs between 1500-3000 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). Meanwhile, there continues to be a multi-modal distribution with the axes of heavier rainfall, owing to the lack of overlap with the strongest dynamical forcing (northern portion of the outlook area) and strongest deep-layer instability and low-level southwesterly inflow/moisture flux (southern portion). As a result of the continued model spread with the QPF, and the fact that the models currently depict more forward (downwind) propagation/Corfidi Vectors than otherwise, will maintain the Marginal Risk that was inherited from yesterday's Day 3 ERO. Hurley Day 3 Valid 12Z Fri May 05 2023 - 12Z Sat May 06 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE MID-SOUTH AND TENNESSEE VALLEY... ....Mid-south and Tennessee Valley into the Southern Appalachians and Parts of the Southeast... Organized elevated convection north of the surface warm front at the start of Day 3 (12Z Friday), again possibly an MCS, will track east along with WNW-ENE oriented vort lobe, while individual cell motions will be southeasterly per the 850-300 mb thickness orientation and layer Corfidi vectors. While not as robust compared to west of the Mississippi, per the guidance the veering LLJ Friday night (850 mb flow becoming more westerly) will align more parallel to the mean 850-300 mb flow. This will allow the downwind Corfidi vectors to decrease significantly, thereby increasing the risk for cell training. Once again, the models show latitudinal differences with respect to the stripe of heaviest rainfall. However, given the strong pool of deep-layer elevated instability (MUCAPEs 1000-2000 J/Kg) nearby into the S-SW portion of the TN Valley, this time closer to/more overlap with the region with most favorable divergence aloft, WPC continued to side with a QPF closer to the operational ECMWF, which is notably south of the other operational guidance. Given the enhanced potential for cell training with 1.5-2.0+ inch/hr rainfall rates, we opted to continue the Slight Risk inherited from yesterday's experimental Day 4 ERO, only shift it south across northeast AR and the MO Bootheel east through much of TN, southern KY, and far northern MS-AL-GA. ....Central and Northern Intermountain West... Upper level pattern changes very little across these areas Fri-Fri night, with broadly difluent upper level flow east of the H5-H3 trough axis (though the favorable upper level forcing does shift a bit northeastward into western-central MT). As was noted in the Day 2 ERD, the favorable forcing and weak deep-layer instability (relatively steep mid-level lapse rates) along a ribbon of above-average PW will favor isolated hourly rainfall rates of 0.50-1.00". These rates, along with the recent snowmelt and possibility of occurring over burn scar areas, could foster localized short-term runoff issues. As a result, the Marginal Risk that was inherited from yesterday's experimental Day 4 ERO continues in today's Day 3 ERO. Hurley $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .