Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 01 2023 00:34:59 FOUS30 KWBC 010034 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 834 PM EDT Thu Aug 31 2023 Day 1 Valid 01Z Fri Sep 01 2023 - 12Z Fri Sep 01 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHEASTERN AND SOUTHWESTERN U.S.... ....Central-North Florida into southern Georgia and southeastern Alabama... Now well west-southwest of Post-Tropical Idalia, subtle upper divergence, deep-layer ascent, and low-level frontogenesis within the right entrance region of a 70-90kt jet streak over the Mid Atlantic (confluence north of the western Atlantic upper ridge) is maintaining scattered convection past sunset over parts of the Southeast. Surface and mixed-layer CAPE trends are actually positive close to the surface front (along and on either side of the FL/GA border, which is rather a-typical following the loss of diurnal heating. Continued (albeit weakening) synoptic support overnight across a thermodynamically-rich environment (PWs 2.1 to 2.3" with mixed-layer CAPEs currently ~1000-1500 J/Kg) will allow for a continuation of scattered storms with a localized flash flood threat, especially given the recent heavy rainfall along and near Idalia's path. The latest (18Z) HREF shows spotty/isolated probabilities between 40-60% of >2"/hr rainfall rates through 04-05Z. ....Western U.S.... 0100 UTC Update -- Based on the latest observational and meso-analysis trends, have expanded the Marginal Risk area across central-eastern portions of AZ and south-central UT. Enhancement within the monsoonal pattern has commenced as a deepening longwave trough over the west coast and a building upper ridge center over the Plains works in tandem to start drawing low to mid-level moisture over the Southwest U.S, mainly Arizona into Utah. While modest...the forecast values of instability is more than sufficient to produce late day showers and thunderstorms from the southern border up through the Mogollon Rim into UT. Locally heavy rainfall within any convective development can pose a threat for flash flooding, especially near any terrain or urbanized corridors.=20 Hurley/Bann Day 2 Valid 12Z 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... 2030Z Update... Made mostly minor adjustments based on the 12Z guidance and WFO input. Adjustments included a small westward expansion of the outlook areas in the Southwest. PW anomalies and HREF consensus supported bringing the Marginal and Slight Risk areas farther west to cover a greater extent of southern California, southern Arizona, and eastern Nevada. In the Southeast, expanded the Marginal Risk to include the Atlantic coast. Deep moisture (PWs at or above 2 inches) pooling along a cold front sliding south across the peninsula will support the potential for heavy rainfall, with localized flooding possible, especially along the urbanized corridor.=20 Pereira Previous Discussion... ....Gulf Coast... A mid level closed low is forecast to develop over the Gulf coast region as height falls aloft develops a weakness in the sprawling upper level high at/above 300 mb. The circulation at the surface and low levels is weak...but enough to induce low level on-shore flow along the Gulf coast. As a result...on-shore flow will be induced which draws deep moisture northward during the day. Both the 31/00Z runs of the NAM and GFS have precipitable water values at or above 2 inches being drawn across much of Alabama and eastern Mississippi by 02/00Z. Given the magnitude of the precipitable water and moisture transport...locally heavy rainfall amounts and rainfall rates are expected. The models offer a range of solution in terms of placement and amounts...with the NAM being more aggressive with highest rainfall amounts well east of other guidance and the GFS was a northern outlier. For now...tended to favor a consensus approach in placement. Maintained the Slight risk where there could be some sensitivity about run off where model QPF overlapped rainfall from Idalia. Confidence remained below average this forecast cycle. ....Southwest United States... Maintained the Slight Risk area along and near the higher terrain from Arizona to Utah....where moisture gets channeled northward efficiently between an anomalously deep trough along the west coast of North America and a pronounced upper high over the Plains. Precipitable water values in excess of 1.5 inches initially over southwest Arizona get drawn towards the higher terrain over central Arizona by 850 mb flow of 15 to 20 kts by late afternoon...with values in excess of three-quarters of an inch along and near the terrain into Utah by late afternoon. There was a subtle shift westward in placement compared with guidance from 24 hours ago...but the overall pattern still sets the stage for some of the storms to be locally heavy rainfall producers.=20 With the guidance showing some westward shift...opted to expand the existing Marginal and Slight risk areas westward while maintaining the eastern boundaries with minimal change given the consistent moisture flux across the region due to the large scale synoptic pattern. Of particular concern would be for flash flooding in sensitive slot canyons and dry washes. The placement of the WPC Slight Risk covers the corridor of highest model QPF and along the axis of the particularly strong moisture vapor transport shown by the NAEFS and EC ensembles. Bann Day 3 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... 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!-Y8rAYL9B2RbvNO8Ub9s_txlivQYxcUMuWdfxw-RfRGB= mUaqEWfZVFOb0LmsktCt3ormNrOX61YI251flolq-NcvvUw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-Y8rAYL9B2RbvNO8Ub9s_txlivQYxcUMuWdfxw-RfRGB= mUaqEWfZVFOb0LmsktCt3ormNrOX61YI251flolqsP0irqM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-Y8rAYL9B2RbvNO8Ub9s_txlivQYxcUMuWdfxw-RfRGB= mUaqEWfZVFOb0LmsktCt3ormNrOX61YI251flolq1H2k5jM$=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) .