Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jul 24 2023 16:04:56 FOUS30 KWBC 241604 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1204 PM EDT Mon Jul 24 2023 Day 1 Valid 16Z Mon Jul 24 2023 - 12Z Tue Jul 25 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST/MID-ATLANTIC, MID-MISSISSIPPI VALLEY AND OHIO VALLEY, FLORIDA, AND THE SOUTHWEST/GREAT BASIN... ....Northeast/Mid-Atlantic... Anomalous trough centered over the Hudson Bay will maintain lowered heights across the eastern CONUS, and drive a cold front slowly eastward across the Great Lakes and towards Upstate NY today. A modest shortwave is progged to rotate through the base of the trough this aftn/eve causing subtle height falls and PVA into the Mid-Atlantic and Northeast, combining with at least modest upper diffluence in the tail of a jet streak to drive deep layer ascent across the region. This lift will act upon an environment favorable for heavy rain noted by PWs of 1.25-1.5 inches, around the 75th percentile according to the SPC sounding climatology, and MUCAPE of 1000-1500 J/kg to produce convection which the HREF and HRRR both indicate will have rain rates of 1-2"/hr due to weak mid-level lapse rates in deep warm cloud depths. The simulated reflectivity from the high-res today generally features scattered convection, but the presence of 30-40 kts of effective bull shear may help drive multi-cell organization in some areas, especially Upstate NY into western New England. In general these storms will move progressively to the E/NE on 0-6km mean winds of 20-25 kts, but some short term training is possible as Corfidi vectors align to this flow downstream of the primary trough axis. The HREF probabilities for 1" and 3" of rain are relatively modest, so despite compromised FFG due to 14-day rainfall that is 150-300% of normal, the FFG exceedance probabilities are low. For this reason the MRGL risk was just adjusted cosmetically, but should more organized convection develop and train, especially from eastern PA northward through the Hudson Valley and into western New England, which has at least a subtle signal in the HRRR and experimental RRFS, some scattered instances of flash flooding could be possible. ....MS VLY/Ohio Valley... An MCS moving through Missouri this morning is being driven by a weak shortwave dropping through NW flow on the upwind side of a large trough across the eastern CONUS. This MCS should weaken by the aftn in response to the veering and waning LLJ, but the accompanying shortwave impinging the weak low-level baroclinic zone along a weak warm front/stationary front will drive additional convection to the east this aftn/eve. Subtle height falls within the mid-level trough will additional aid ascent, which when acting upon an environment with PWs of 1.25-1.5 inches and a ribbon of MUCAPE exceeding 2000 J/kg will produce convection with heavy rain rates of 1-2"/hr. Although mean 0-6km winds will be progressive at 15-20 kts, these should remain aligned generally to the front, suggesting some short duration training, with effective bulk shear driving multi-cell clusters across the region. Where repeated rounds of thunderstorms can occur, rainfall could reach 2-3" as reflected by HREF exceedance probabilities. Where this rain falls atop more sensitive soils noted by FFG as low as 1.5"/3hrs, some isolated flash flooding is possible. ....Florida... Deep trough centered over the Hudson Bay will extend into the Gulf Coast today, leaving generally flat zonal flow across the Gulf of Mexico and Southeast. A weak impulse rotating through the flow will combine with the tail of a jet streak positioned offshore and an approaching cold front which will stall in the vicinity to produce scattered convection today. Nearly unidirectional flow over the Gulf of Mexico will drive convection west to east into the northern FL Peninsula today, driven by this ascent working over an airmass characterized by a ribbon of 2" PWs and MUCAPE above 1000 J/kg. Flow parallel to the approaching front will allow for training of cells into FL, while Corfidi vectors becoming increasingly aligned anti-parallel to the mean wind suggests backbuilding to lengthen rainfall duration. HREF probabilities for more than 3" peak along the Nature Coast and around the Big Bend of FL, with rainfall rates likely reaching 2"/hr at times. Although this area has generally been dry recently, training of these rates could produce isolated flash flood responses, especially in any urban areas. ....Southwest/Great Basin... Expansive mid-level ridge will become entrenched across the Southwest/Four Corners area, anchoring over New Mexico today. The placement of this ridge will allow for return flow to spread across the western portions of the Southwest and into the Great Basin, noted by PW anomalies on the NAEFS ensemble tables reaching +1 to +2 sigma from western Arizona northward through NV/UT and into ID/WY. While subsidence beneath the core of the ridge will suppress much in the way of convection due to a +18C 700mb cap, this PW plume around its periphery combined with weak shortwave impulses lifting around the ridge will likely result in scattered convection, primarily during peak heating as MUCAPE rises to 750-1000 J/kg. The inherited MRGL risk was adjusted via an expansion northward to account for the higher PW plume and at least modest signals in the HREF for 1"/hr rates. Storms are likely to be moving pretty quickly on 0-6km mean winds to the northeast at 15-20 kts, but could still produce 15-min rainfall totals of 0.25-0.5 inches as reflected by the HRRR sub-hourly. In general this will be less than FFG, but where storms can move a bit slower (primarily southern AZ) or where this rain can occur atop more sensitive terrain features like washes and slot canyons, isolated flash flood instances are possible. Weiss Day 2 Valid 12Z Tue Jul 25 2023 - 12Z Wed Jul 26 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST AND SOUTHWEST... ....Northeast... A mid-level trough axis and surface front will work together to trigger scattered convection across portions of northern New England into Tuesday. PWs of 1.0-1.4" look to be marginally above average, and instability (MU CAPE) is progged to increase to 1000-1500 J/kg. QPF from both the downscaled global and available CAM guidance depicts the potential for 1-2"+ amounts (which could fall over a relatively short period), but the overall spread and inconsistency suggests those totals will be fairly localized. That makes sense, given that the expectation is for convection to lack organizion given the modest mid/upper support and generally weak shear. Will continue to maintain a Marginal risk area as we wait for the full HREF suite to come into view (and because FFG values are lower due to above average soil saturation and streamflows over much of this region). ....Southwest... The core of the mid-level ridge remains centered over CO and NM, with enough subtle forcing on the periphery of the ridge to drive another day of isolated to scattered diurnally-driven convection. Any storms could drop a quick inch of rain, which could yield a localized flash flood risk within any more susceptible basins. The Marginal risk is quite large and pretty much covers most areas where there is at least some convective threat Tuesday. It seems unlikely that this area will technically meet the 5%+ coverage of a Marginal risk, but given the nature of convection in this region it's hard to pin down exactly where a localized flash flood could occur. Continued incorporation of the CAM guidance will likely refine this area for later updates. ....Elsewhere across the CONUS... The ECMWF has remained rather consistent in bringing a shortwave over the ridge and into the Dakotas Tuesday, which would result in an organized convective threat. However, it can be too strong with such features. Other models are not nearly as aggressive with this feature (with only the UKMET giving a modest signal for 1-2" QPF amounts), and the 95th percentile NBM QPF is still lower than the downscaled ECMWF. Will continue to re-evaluate with later guidance. There are also continued signs of subtle signs of a heavy rainfall potential across portions of IN/KY/TN and vicinity, but the model spread remains quite wide and with lesser amounts than on Day 1. There does appear to be an axis of enhanced instability in most of the guidance across this corridor, with some semblance of lower level moisture transport/convergence. The continued weak QPF signal amongst most of the deterministic and ensemble guidance suggests the threat is pretty low, and the 95th percentile NBM QPF has trended downward to near half an inch. Churchill/Fracasso/Chenard Day 3 Valid 12Z Wed Jul 26 2023 - 12Z Thu Jul 27 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST, NORTHEAST, AND SOUTH FLORIDA... ....Northern Plains/Midwest... The phasing of a southern stream system (rounding the southwest ridge) and northern stream system (digging south from Canada) and associated low-level fronts may result in enhanced low-level convergence with the potential for some vigorous convection from eastern SD into north-central MN and far northwest WI into Wednesday. Models still have quite the spread and disagreement on the resulting QPF, but odds are high enough that an inherited Marginal risk area was maintained (with the GFS indicating PWs upwards of 1.5", approaching the 90th percentile). ....Southwest... The same story across the Southwest as Days 1 and 2, with the potential for localized flash flooding driven by sub-hourly rates of 1"+. The coverage of the realization of these 1"+ totals will likely be very low, but becoming more emphasized on the northern the northeast periphery of the core of the ridge (as it is nudged a bit towards the southwest in response to the digging shortwave in the northern stream over Canada). ....South FL... Introduced a Marginal risk across portions of South FL (focused on the greater Miami metro area), as much greater low-level moisture is progged to arrive within strengthening easterly flow (with the GFS depicting PWs of >2.0" and reaching the 95th percentile). None of the downscaled global guidance is outputting impressive QPF signals at this point, but there is an expectation for much greater coverage of precipitation with the potential for localized totals causing problems in the poor drainage metro area of Miami (and this will be honed in on as CAM guidance starts coming into view). ....Elsewhere across the CONUS... Continued potential for the eventual introduction of Marginal areas, with the best potential across the southern Great Lakes region and into the northern OH Valley (where GEFS and ECENS depicts the best agreement for some 1" exceedance, though displaced rather far from one another). A weak quasi-stationary front is progged by WPC to lift northward as a warm front through the period, but confidence is too low at this juncture to even introduce a Marginal. Any excessive rainfall coverage will likely remain below 5% across other areas of diurnal convection. Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!65nghe1HmgHKX5uqxUro70Zq0jQdCNx2toy40vOJSIJ2= 8NlaDFj244PwT6SuqzgXr8ewfwASRGAefnpm34MIpZCbvNk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!65nghe1HmgHKX5uqxUro70Zq0jQdCNx2toy40vOJSIJ2= 8NlaDFj244PwT6SuqzgXr8ewfwASRGAefnpm34MIP3JpbE0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!65nghe1HmgHKX5uqxUro70Zq0jQdCNx2toy40vOJSIJ2= 8NlaDFj244PwT6SuqzgXr8ewfwASRGAefnpm34MI6U2Px4g$=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) .