Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Aug 25 2024 19:46:20 FOUS30 KWBC 251946 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 346 PM EDT Sun Aug 25 2024 Day 1 Valid 16Z Sun Aug 25 2024 - 12Z Mon Aug 26 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST ... ....Arizona/New Mexico and Colorado... 16Z update... The Marginal Risk was expanded a bit further west across southeast Utah and north-central Arizona to encompass the Flood Watch that was raised by the local forecast offices. The latest hi-res guidance depicts narrow bands of modest to heavy rainfall focused over this part of the region and supports the changes made to the Marginal Risk. Campbell The axis of highest precipitable water values start the period extending from eastern Arizona and western New Mexico into southwest Colorado...with maximum values at or slightly above an inch near the international border. This should help fuel additional late day and evening convection...with at least a low- end chance for rainfall rates that approach an inch per hour. Flow increases across the region as the western portion of the upper high erodes and a closed low over Nevada this morning makes its way eastward...which should help convection be progressive enough to mitigate at least some of the excessive rainfall threat. Even so...even brief heavy rainfall can produce flash flooding within complex terrain and especially around any burn scars. ....Midwest... With the upper high/warm temperatures firmly in place and apparently sufficiently strong to suppress buoyancy...and high resolution models now struggling to generate more than light showers...have removed the Marginal Risk area over portions of Iowa and Illinois. Bann Day 2 Valid 12Z Mon Aug 26 2024 - 12Z Tue Aug 27 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST US AND THE NORTHERN PLAINS/UPPER MIDWEST... ....Southwest US... 21Z update... The environment still looks favorable for isolated=20 to scattered areas of heavy rain. The inherited Marginal Risk=20 encompasses the area where there are elevated concerns for=20 localized flash flooding therefore, no adjustments were needed at=20 this time. Campbell The threat of excessive rainfall will linger into Monday across portions of the Southwest US as precipitable water values of 0.75 to 1.00 inches remain in place from extreme southeast Arizona into New Mexico. Scattered daytime storms remain possible within the plume if deeper moisture...but anomalously strong steering flow should help limit residence time to mitigate some of the flash flooding concern. As mentioned earlier...though...even brief heavy rainfall rates in a short period of time may still lead to flash flooding, and a Marginal Risk was maintained in the region. ....Northern Plains... 21Z update... The last couple of model cycles have hinted at a southern shift in the QPF axis, however there still remains some uncertainty on how progressive the nature of this system will be. There was enough support to justify trimming the northern bounds of the Marginal Risk a bit southward across North Dakota and northern Minnesota with this issuance. Campbell An upper trough over the Intermountain region will begin lifting northeastward across the High Plains on Monday...helping to sharpen a front across parts of the Upper Midwest/Northern Plains. Precipitable water values are forecast to approach 1.75 inches over portions of the eastern Dakotas prior to the arrival of the surface front and increasing divergence/difluence associated with the upper level system. The expectation is that moisture flux convergence along the front will focus convection moving into an unstable environment driven by diurnal heating that leads to locally heavy downpours. Spread lingers in the guidance with respect to how progressive the system will be and how much of excessive rainfall threat will be associated with the upper system compared to the threat associated with the convection pushing into the better instability. Kept the westward extension introduced on Saturday...but there was a consensus that some territory could be trimmed out near the international border. Bann Day 3 Valid 12Z Tue Aug 27 2024 - 12Z Wed Aug 28 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST US AND THE UPPER GREAT LAKES... ....Southwest US... 21Z update... The environment still looks favorable for isolated=20 to scattered areas of heavy rain. The inherited Marginal Risk=20 encompasses the area where there are elevated concerns for=20 localized flash flooding therefore, no adjustments were needed at=20 this time. Campbell The risk of excessive rainfall will linger on Tuesday afternoon and evening across portions of the Southwest US as a moisture plume lingers across the area and winds aloft/steering flow weakens in response to the a mid level wave moves away over the Northern Plains as it begins to fill. Model based ingredients suggest the potential for locally heavy downpours may extend a bit west of the area shown by the deterministic runs...so made a minor expansion that way. ....Upper Great Lakes... 21Z update... As noted in the Day 2 section, there has been an increasing southward trend with the QPF axis with the precipitation across the northern tier states. While there continues to be a fair amount of uncertainty on how convection evolves and how progressive there was a need to adjust the Marginal Risk area further south to reflect this trend. The southwest portion of the Marginal was expanded further into east/southeast Minnesota and into central Wisconsin and northern portions of Lower Michigan. Campbell There is a Marginal risk of excessive rainfall over portions of the western Great Lakes as the upper trough initially over the Upper Midwest continues to move northeastward during the day. Precipitable water values of 1.75 to 2 inches should already be in place before the flow aloft becomes increasingly diffluent along and north of a quasi-stationary boundary helping enhance rainfall rates locally. The operational guidance tended to the potential for a couple of inches of rain...although there was considerable north- south spread in the guidance limiting the confidence that excessive rainfall will occur at any spot. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4Je6LvIvAHUObFmEVudPu-apqA8-N83rxlWDzvCkQqnK= jPC2jV3-lOoIB-fHSUo-Xtmy8SAID8__OzsqADHMFhg-ZTk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4Je6LvIvAHUObFmEVudPu-apqA8-N83rxlWDzvCkQqnK= jPC2jV3-lOoIB-fHSUo-Xtmy8SAID8__OzsqADHM2UNIEiI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4Je6LvIvAHUObFmEVudPu-apqA8-N83rxlWDzvCkQqnK= jPC2jV3-lOoIB-fHSUo-Xtmy8SAID8__OzsqADHMDW0mqfQ$=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. 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