Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 30 2023 19:45:42 FOUS30 KWBC 301945 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 345 PM EDT Thu Mar 30 2023 Day 1 Valid 16Z Thu Mar 30 2023 - 12Z Fri Mar 31 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF THE UPPER MIDWEST... ....16Z Update... The main change to the previous ERO issuance was to trim and narrow the Marginal Risk particularly on the northeastern side given the likelihood for wintry precipitation especially for northern Wisconsin into Michigan. Additionally, the west-east oriented warm front where sufficiently high dewpoints will be and rain will fall to its south looks to also stay south of areas with notable snow cover, so snowmelt is not too large of a threat. More issues could arise if the warm front ends up being farther north but that appears unlikely at this point. See the previous discussion for more details on the overall pattern and meteorological ingredients. Farther south, there may be a nonzero threat for isolated flash flooding mainly this evening in the general vicinity of the Oklahoma/Arkansas border and eastward where precipitable water anomalies rise above the 90th percentile. Storms that form generally appear to move quickly eastward in the hi-res guidance though, which would limit any flash flooding threat. Interestingly the probabilities for exceeding an inch of QPF for example from the EC ensemble is much higher than the HREF and GEFS probabilities--perhaps indicative of QPF likely right around an inch. And QPF in those amounts should not cause too many issues, hence no Marginal Risk looks needed at this point. Tate ....Previous Discussion... A vigorous deep-layer trough traversing the Great Basin and Southwest today will lift across the central-southern Rockies and High Plains tonight and early Friday. Increasingly difluent upper-levels across the Upper Midwest will make for a more favorable environment for deep-layer ascent, particularly later tonight as the the models depict an enhanced right-entrance region of a departing northern stream jet streak (of 130+ kts). A rapid increase in low- to mid-layer moisture transport from the western Gulf of Mexico is expected, as 850-700 mb moisture flux anomalies climb quickly to between 4 and 5 standard deviations above normal tonight into early Friday (on the heels of robust SSW 850 mb flow of 50-60 kts). On the nose of that strong flow, enough instability (500-1000 J/kg of MUCAPE) and convergence will materialize to support deep convection that may become surface-based along the warm front across northeastern Nebraska and vicinity this evening before migrating east-northeastward into a slightly more stable airmass. The orientation of convection could allow for spotty areas of convective training, with FFG thresholds generally around 1"/hr supporting isolated potential for excessive runoff. The main limitation to a higher excessive rainfall threat is the transient upper flow pattern, with the progressive closed 500 mb low and associated surface frontal features (along with relatively dry antecedent conditions, per NASA SPoRT-LIS 0-40 cm soil moisture anomalies at or below the 10th percentile for much of the region). Such a pattern, one without the benefit of more prolonged low-mid level frontogenesis, would limit the risk of cell training (though that starts to become more of a factor going into Day 2). Churchill/Hurley/Cook Day 2 Valid 12Z Fri Mar 31 2023 - 12Z Sat Apr 01 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MID-SOUTH AND LOWER MISSISSIPPI/OHIO/TENNESSEE VALLEYS... ....2030Z Update... The overall meteorological pattern described below still looks on track for this update cycle, with the potential for quick-hitting storms but with high rain rates, as initially possibly more discrete convection is likely to organize by evening/night into a cold frontal squall line. This squall line should be mainly progressive farther north but with the most potential for the front to "lay down" a bit, becoming more southwest to northeast or even west-east oriented rather than south to north, across the Mid-South (southern Tennessee into northern Mississippi and Alabama). This could lead to more training of convection, and indeed the highest probabilities for elements like QPF to exceed thresholds of an inch, QPF to exceed FFG, etc. in the first run of the HREF to go through this full period are also maximized there. Thus the Slight Risk was maintained across these regions, though it was trimmed back on the northwestern side per the newer model guidance and WPC QPF. Additionally, the northwestern side of the Marginal Risk was shaved off due to QPF being predominately snow/ice. See the previous discussion for more details on the pattern. Tate ....Previous Discussion... The guidance remains in fairly good agreement with the timing of the compact mid-upper level trough traversing the Central Plains into the Upper Midwest from Friday afternoon/evening into early Saturday. The relatively swift progression of this feature will limit the excessive rainfall potential over the northern OH Valley and western Great Lakes; however, farther south, under a more confluent, zonal mid-upper flow, there will be a better chance of cell training along outflow-generated effective fronts oriented quasi-parallel to the deep-layer westerly flow. Where instability is plentiful and the dynamics are most impressive, deep convection is expected to rapidly develop with quasi-discrete supercells initially expected to the preferred storm mode. Some of the strongest activity is anticipated to be across the newly introduced Slight Risk area, where hourly rates of 1-2"+/hr are possible (with these hourly rates largely driving the flash flood threat, as the bulk of the forecast precipitation is expected over a period of 3-6 hours). Low-level moisture transport is expected to be quite impressive, as a large low-level jet (50-70 kts at 850 mb) ushers in precipitable water values of 1.2-1.8 inches (above the 90th percentile for the bulk of the MS/OH/TN Valleys). Farther to the north in the Mid-West and Great Lakes region, rainfall rates will be less impressive (perhaps as high as 1.5"/hr at times), but the prolonged nature of the rainfall may lead to some higher areal average totals (closer to ~2", though localized totals will likely be higher to the south). The Slight may need to be expanded northward in future cycles, which may be partially dependent on how rainfall totals evolve late on Day 1. Farther south of the new Slight Risk (into portions of the Deep South), flash flood guidance is generally higher and the dynamics of the system are less impressive (so an expansion of the Slight Risk southward is less likely). Churchill/Hurley Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_WbArdyhIPYc1vOCrCgfK4dgMTTcT9-94Sq5k8IOeHq5= hysk0OX0Pjzxrq5giDtKRVt19sKctQlAmAkE0eOhUZuoQxI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_WbArdyhIPYc1vOCrCgfK4dgMTTcT9-94Sq5k8IOeHq5= hysk0OX0Pjzxrq5giDtKRVt19sKctQlAmAkE0eOhNFavsK4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_WbArdyhIPYc1vOCrCgfK4dgMTTcT9-94Sq5k8IOeHq5= hysk0OX0Pjzxrq5giDtKRVt19sKctQlAmAkE0eOhLqBWZX4$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .