Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 01 2023 19:57:46 FOUS30 KWBC 011957 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 356 PM EDT Fri Sep 01 2023 Day 1 Valid 1949Z Fri Sep 01 2023 - 12Z Sat Sep 02 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG THE GULF COAST AND OVER THE SOUTHWEST UNITED STATES... ....Gulf Coast through Florida Peninsula... 1945 UTC Update -- Have expanded the Slight Risk area across central and west-central FL based on the latest observational and mesoanalysis trends, along with the robust HREF exceedance probabilities from the 12Z run. Hurley Previous discussion... A mid-level low across Louisiana early this morning will help channel deep Gulf moisture northward along the Gulf Coast region over the next several days. Early morning TPW product shows the greatest PW axis extending across the Florida Peninsula (2-2.5") with near 2" right along the immediate Gulf Coast. Over the course of today and tonight, expect the southerly deep layer flow to bring those higher moisture values northward while with peak heating today, there should be widespread to numerous showers and thunderstorms along the central/northern Florida Peninsula through much of the Panhandle into southern Mississippi and Alabama. The deep moisture and high instability (upwards of 1500-2000 J/kg MUCAPE) will support localized 2-2.5" hourly totals for a few hours this afternoon into the evening and a few locations may pick up 3-5" totals over the period particularly across portions of the FL Panhandle and Peninsula. Given some sensitivity due to heavy rainfall from Idalia and potential for localized heavy rainfall today, the Slight Risk was only minimally changed ....Southwest United States... 16Z update... The latest guidance is showing an uptick in QPF and rain rates over the desert areas of southern California and over southern/southeast Nevada. The western bounds of the Marginal and Slight Risk areas were adjusted westward to reflect this trend. The best potential for some of the highest totals and risk for flash flooding remains near the Junction of CA/NV/AZ to southwest UT. Campbell The synoptic setup will favor heavy rainfall and flash flooding across much of the Southwest U.S. today/tonight as anomalous moisture lifts northward the region. This is aided by a large closed low positioned over Pacific Northwest that is expected to settle toward southern Oregon by early Saturday morning. An early morning analysis also showed a subtle shortwave embedded in the flow across portions of Arizona, helping to fuel the nighttime convection across Arizona into southern Utah. This activity will continue lifting northward through this morning, spreading across much of Utah this morning where locally intense rain rates may bring an early start to the flash flood potential, though the expansive cloud cover may limit the intensity of the rain rates some this morning. In the wake of the passing shortwave this morning, clearing skies will help build an axis of higher instability across southern Nevada, western Arizona, and southwest Utah, with upwards of 1000-2000 J/kg of MLCAPE expected. This instability along with the much above normal PW (1" to locally 1.5"+ across Arizona) will help drive another round or an uptick in the shower and thunderstorm activity in the region, especially after about 18-20Z. The deep layer mean flow orienting nearly parallel to the expected storm motions will favor some repeating rounds. Multiple areas of intense convection will be likely. Based on the 00Z hi-res guidance and the 00Z HREF probabilities, localized hourly totals 1"+ will be possible, especially across portions of southern Nevada, southwest Utah, and western Arizona. Isolated total amounts of 2-3" will be possible (3" HREF probs approach 40-50 percent in spots). A Moderate Risk was considered for portions of southern Nevada, southwest Utah, and western Arizona and could be needed for the later morning update if trends in the higher QPF and more intense rain rates for this afternoon/evening continue. For now, the risk was nudged toward the higher end range of the Slight Risk category and some localized significant flash flooding will be possible, especially for the vulnerable/susceptible slot canyons, dry washes, and other low lying locations. ....Northern California and Southern Oregon... 16Z update... The latest runs of the deterministic and hi-res guidance is signaling higher amounts further south over the northern portions of the Sierra Nevada Range. Areal averages of 0.50 to 1+ inches with isolated maximums over 2 inches possible therefore the southern bounds of the Marginal Risk area was expanded southward. Campbell A closed mid-level low developing over the region today will help bring locally heavy rainfall to parts of northern California and southern Oregon. Lower heights will drive steeper lapse rates and some potential for instability that could result in locally intense rain rates in excess of 0.5"/hr. While for most areas the rainfall will be beneficial, recent burn scar areas will be susceptible to flooding impacts and the Marginal Risk area was largely unchanged from the previous issuance. Taylor Day 2 Valid 12Z Sat Sep 02 2023 - 12Z Sun Sep 03 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST UNITED STATES... ....Arizona to southeast Nevada and northward into Utah... 21Z update... Minor adjustments were made to the Marginal and Slight Risk areas but in large the pattern and level of threat for excessive rainfall remain the same. Campbell Another round of mainly diurnally driven convection is expected Saturday afternoon/evening across portions of the Southwest into the Intermountain West in response to the mid-level closed low over California helping to channel anomalously high moisture northward. Compared to Friday, the moisture axis isn't as deep though instability is expected to top out around 1000-1500 J/kg MLCAPE especially across western AZ, southwest UT, and portions of southern Nevada. This should support some localized intense rain rates greater than 0.5-0.75"/hr at times with the strongest cores. Based on the 00Z guidance, localized 1-1.5" totals will be possible and combined with the expected amounts today/Friday, some greater sensitivity may be present so the Slight Risk still looks reasonable and covers where 2-day totals may exceed 3" in spots over some more vulnerable locations. ....West Coast... Closed mid-level low is forecast to remain nearly stationary over the course of the period, generally across northern California and southern Oregon. The lower heights contributing to steeper lapse rates combined with at least some higher than normal moisture and potential for marginal instability should support some stronger showers and perhaps thunderstorms capable of producing locally intense rain rates greater than 0.5"/hr. This may lead to a few localized flooding concerns particularly for the terrain areas of northern California. ....Gulf Coast... 21Z update... A minor westward expansion of the Marginal Risk was made for southeast Louisiana where there was increase in QPF potential with the latest guidance. Campbell The axis of deep moisture, characterized by precipitable water values in excess of 2-2.25", will remain along the Gulf coast, aided in part by onshore flow provided by mid level low positioned over portions of eastern Texas and western Louisiana. The best instability will remain generally confined to the immediate coast where locally 1-3" of heavy rainfall will be possible and may lead to isolated/localized instances of flash flooding for mainly urban and low-lying locations. Taylor Day 3 Valid 12Z Sun Sep 03 2023 - 12Z Mon Sep 04 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST UNITED STATES... 2030Z Update... Overall, made only minor adjustments this cycle. Pereira Previous Discussion... ....Arizona to southeast Nevada and northward into Utah... There will be a continuation of strong northward moisture flux along a channel between an anonymously deep trough along the west coast and a mid/upper high over the Plains. This should result in another round of diurnal convection along and near the higher terrain...although the guidance is not quite as bullish with amounts as they were on Saturday apparently in response to the southern end of a Pacific trough swinging through late in the period helping to usher at least some of the moisture out of the area. The placement of the maximum QPF over northern Arizona into southwest Utah and a small portion of nearby Nevada is very similar with the placement of the highest QPF on Saturday...with combined Day 2 and Day 3 QPF from WPC adding up to 2 inches or more in spots. That warranted maintaining the placement of the Slight Risk with few adjustments. ....Gulf Coast... On-shore flow continues to keep showers and thunderstorms along the Gulf coast...with most of the models depicting maximum QPF immediately along the coast or offshore. An upper low that was much better defined at 250 mb and with a considerably weaker reflection at 500 mb is forecast to lift north and west during the period. While the reflection of the low is quite weak at the surface...thinking was that Gulf moisture will not be drawn northward as much and that the potential for excessive rainfall can be covered by a Marginal risk for the time being. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8vmyX155JzJ_A8b6nsdZfEPgT9kSI8MelIiA_VOu2ewW= AltsnCGO0miK0BFrPm2baLyxmo5GGkm4VIZ8vt4gmnOGjjo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8vmyX155JzJ_A8b6nsdZfEPgT9kSI8MelIiA_VOu2ewW= AltsnCGO0miK0BFrPm2baLyxmo5GGkm4VIZ8vt4gwkcdIyM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8vmyX155JzJ_A8b6nsdZfEPgT9kSI8MelIiA_VOu2ewW= AltsnCGO0miK0BFrPm2baLyxmo5GGkm4VIZ8vt4gB3ePcbg$=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) .