Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Aug 21 2023 01:00:24 FOUS30 KWBC 210100 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 859 PM EDT Sun Aug 20 2023 Day 1 Valid 01Z Mon Aug 21 2023 - 12Z Mon Aug 21 2023 ....A HIGH RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF SOUTHERN CALIFORNIA AND SOUTHERN NEVADA... ....Portions of the West... Heavy rainfall and flooding associated with Hilary will continue through tonight over portions of CA and NV. The worst of the impacts are likely now into Monday morning. The historic amount of rainfall is expected to result in life-threatening to locally catastrophic flash, urban, and arroyo flooding including landslides, mudslides and debris flows. Flooded, washed out and impassable roads are likely...making travel difficult and unsafe tonight across the region. The worst of the impacts are expected to focus across the Transverse and Peninsular ranges of southern CA, including the San Bernardino Mountains, spreading northeast along the eastern slopes of the southern Sierra, into the Mojave Desert and adjacent areas of southwest NV. The broad area of stratiform rainfall will continue lifting north northeast across this corridor tonight. Embedded heavier convective elements are likely as well...especially along the eastern edge of the rainfall shield where some better instability is present. Hourly rainfall totals as high as 1-1.5" are possible, both where these heavier convective elements materialize, and along favored upslope terrain within the strong southerly flow. The 18Z HREF neighborhood probabilities of exceeding the 100 yr ARI peak between 00z and 06z tonight...with a near 100% chance across most of the High risk area. Overall the event is evolving as forecast, thus little to no changes were needed to the ongoing ERO areas. Please see our recent MPDs for the most up to date mesoscale information regarding the flash flood threat across the region. A second area of concern is across portions of central ID, where orographics and some embedded heavier convective elements may also result in a localized flash flood risk tonight. MPD #954 describes that threat in more detail. Chenard Day 2 Valid 12Z Mon Aug 21 2023 - 12Z Tue Aug 22 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS OVER PORTIONS OF THE WEST... ....2030z Update... No major changes to the previous forecast across the western US. A general expansion across northern periphery of the previous issuance with the largest movement being an expansion of the SLGT more into SW MT as HREF neighborhood probs lend credence to some higher rainfall rates over the ID/MT border, likely due to terrain influences. We'll be monitoring the area along the ID/OR border as deep moist convergence between Boise and Pendleton is prevalent across all guidance with a general QPF maximum co-located on the eastern flank of the remnant circulation of Hilary once it reaches area latitude. Have added a MRGL across south Texas as guidance indicates an slightly earlier arrival of the lead wave moving across the Gulf. Deep moist axis extending from the western Gulf into the S TX coastal plain indicates a period of heavier rainfall potential at the back end of the D2 ERO with most guidance indicating 1-1.5" QPF max within a 6 hr time frame between 06-12z Tue. Given the conjunction of heavier precip rates in the short time frame with forecast totals, wanted to make sure the area is covered for flood threats before the bulk of the precip field makes an appearance during D3. Kleebauer ....Previous Discussion... Portions of the West...=20 Hilary's low-level circulation fades rapidly at the beginning of the period, and precipitable water values fall in suit. However, its 750 hPa circulation is on the move northward from western NV to just west of the ID borders with OR and WA. Areas of 1.5-1.75" values remain, mainly across southwest AZ and southwest ID. For southeast ID and western MT, this is remarkably anomalous and in record territory. The main focus appears to be a front/convergent boundary at 850 hPa just ahead of Hilary's 750 hPa wind circulation which is strongest during the first half of the period (Monday morning and afternoon). The guidance is insistent on 2"+ amounts, so the Slight Risk from continuity in this area looks good.=20 Farther south across the Great Basin and Southwest, we return back towards an environment that resembles this past Friday, though with greater sensitivity due to heavy to extreme rainfall which occurs prior to Monday morning. The Slight Risk remained generally unchanged as the QPF doesn't fully tell the tale this day; some effort was made to try to include some aspect of Sunday's impacts into the Monday period and include more sensitive areas near Death Valley CA and across central NV/the US 50 corridor. It's possible that this risk area may need to be expanded or upgraded once the impacts of Sunday's potentially record rainfall become apparent.=20 =20 =20 In and near the Western Great Lakes...=20 The 12z and 00z ECMWF show the potential for heavy rainfall in this region over the top of strengthening ridging across the Plains/Midwest, the usual spot for the formation/track of an organized convective cluster. A warm front with up to 40 kts of low-level inflow causes precipitable water values to eclipse 1.5" in spots/at times. The strong veering in the low-level inflow suggests convective progression, so the main concern would be if a west-northwest to east-southeast oriented convective band formed in this area or if multiple convective rounds occurred. Hourly rain totals to 2" and local amounts to 4" would then be possible, close to what the ECMWF guidance advertises. Northwest WI, the Door Peninsula, and the northwest Mitt of MI have been wet with spots of 200-300% of average rainfall during the past week, so there should be some sensitivity there. Otherwise, issues would be restricted to urban areas.=20 =20 =20 Southeast Louisiana...=20 Now that there is an area of rotation to follow in the Florida Straits, the guidance that keeps heavy rainfall offshore the LA coast appears more believable. Coordination with LIX/the Slidell LA forecast office led to the dropping of the Marginal Risk area. =20 Roth=20 =20 Day 3 Valid 12Z Tue Aug 22 2023 - 12Z Wed Aug 23 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE WEST & SOUTH TEXAS... ....2030z Update... Minor adjustments made across the western US with general expansions around the SLGT over the PAC NW in coordination with latest forecast and ensemble guidance. Have expanded the SLGT over south Texas to include more of the Rio Grande Valley as deep-moist axis and mid-level ascent from the tropical disturbance traversing the area will generate an expansive precip field as it moves west-northwest across the lower Rio Grande into northern Coahuila. A combo of ensemble and deterministic guidance indicate a swath of 2+" potential extending from just south of Del Rio down towards Zapata and points east, aligned well with the forecast 700mb RH/Omega field where low-mid level ascent would be maximized. As stated in the previous forecast, will have to monitor the evolution of the disturbance closely as a more organized tropical complex would add a greater probability of flash flooding potential. Kleebauer ....Previous Forecast... Portions of the West... Across the interior Northwest/Northern Continental Divide, a convergence zone left in the wake of Hilary's 750 hPa circulation is expected to focus convection early on as precipitable water values fade from 1-1.5" early in the period while converging 15-25 kt winds are seen across the area. CAPE should rise to 1000 J/kg which would allow for convection with potentially high rain rates, most likely in the terrain in and near the Blue Mountains and portions of the ID Stovepipe. Continuity here was left alone as it appeared reasonable. For the Great Basin, deep layer southerly flow occurs between Hilary's 750 hPa low and the warm core ridge across MO. This keeps precipitable water values unusually high for the Great Basin, in the 1-1.25" range.=20 Continuity had a Marginal Risk here, and that still generally looks good. There is a heavy enough of a rain signal in sensitive areas of southwest UT, which should have seen rainfall of previous days, to raise a Slight Risk. In and near Michigan... A possible convective complex from the western Great Lakes is expected to slide into the Mitt of MI this period, poleward of a warm front. A high amount of CAPE under the warm core ridge to the southwest advected in by west to west-southwest flow of close to 40 kts is expected to fuel and organize the activity. The low-level flow is veering, so there should be some convective progression. The concern would be if a training band developed or more than one round of heavy rainfall occurred. At the moment, the 00z ECMWF is most bullish with the potential, but the ingredients appear to support it. The past week has been wet, with areas of the southern Mitt of MI receiving 300%+ of their average precipitation. Introduced a Marginal Risk area here since the 00z ECMWF conceptually makes sense. South Texas... A tropical disturbance currently moving through the Florida Straits appears to develop while moving into South TX this period -- NHC currently has this system with 50% odds of becoming a tropical cyclone. Inflow at 850 hPa rises to 35+ kts, which is above the mean 850-400 hPa wind, which would lead to precipitation efficiency. Effective bulk shear would be high enough for training bands. A decent percentage of the guidance, most notably the 00z NAM, 00z UKMET, and 00z Canadian Regional, bring 5"+ amounts in a rather short time frame (within ~6 hours) across portions of South TX. Since this is a small system moving relatively quickly, if a tropical cyclone formed, areal average 3-5" with local 7" amounts would be the rule. As this is still in the formative stage, the ongoing QPF is on the low end of the 3-5" range. The Western Gulf Coast has been bone dry and the area has high FFGs. Precipitable water values of 2.5"+ with 2000+ J/kg of CAPE would potentially foster hourly rain totals to 3" in training bands or potentially near the center of the tropical low. That would overwhelm the high FFGs within two hours. Issues are expected to be mostly within urban areas. Given the above, a Slight Risk was raised for South TX, despite some lingering model spread on the latitude of the heavy rain band in South TX.=20 Coordination with the EWX/Austin-San Antonio TX office kept the Slight Risk out of their area. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-gFfD5tZua4IZVa41XJl2V5XFx7M-91GUgwErpv0KCHT= zZ-n5UtrB0ZKvkBkAyK-L-5VmgsbRvRBvbT9hxPiBeENX-Y$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-gFfD5tZua4IZVa41XJl2V5XFx7M-91GUgwErpv0KCHT= zZ-n5UtrB0ZKvkBkAyK-L-5VmgsbRvRBvbT9hxPiDATs6hA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-gFfD5tZua4IZVa41XJl2V5XFx7M-91GUgwErpv0KCHT= zZ-n5UtrB0ZKvkBkAyK-L-5VmgsbRvRBvbT9hxPiVVfc59k$=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) .