Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Oct 31 2023 19:56:52 FOUS30 KWBC 311956 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 355 PM EDT Tue Oct 31 2023 Day 1 Valid 16Z Tue Oct 31 2023 - 12Z Wed Nov 01 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent.=20 =20 Campbell/Kleebauer Day 2 Valid 12Z Wed Nov 01 2023 - 12Z Thu Nov 02 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR WESTERN WASHINGTON STATE AND NORTHWEST OREGON... ....20Z Update... Very little deviation from the previous forecast as the atmospheric river signature is most prevalent across western WA into northwest OR with highest rainfall totals focused over the Cascades in WA/OR into the Columbia Gorge over northern OR. 12z HREF neighborhood probabilities of at least 3" of rainfall between 00-12z Thursday (Second half of the D2 period) are running very high (70-80%) within the southern Washington Cascades down into the valley west of Portland near Mount Hood. Rainfall intensity will mostly reside below the 1"/hr mark for a majority of the area, but a continuous 0.5-1"/hr rainfall rate over terrain will cause some localized flooding potential within smaller towns and more established urban settings. GFS/ECMWF combo has come into line with highest forecast Integrated Vapor Transport (IVT) centered over Northwest OR with extension into central WA state by the end of the forecast period. IVT indices point towards a borderline weak to moderate AR signal which correlates to the lower threshold of FFG exceedance potential within the impact zones. This helped maintain continuity in the previous forecast cycle. Kleebauer ....Previous Forecast... An atmospheric river will be making its way inland during this period, bringing beneficial moisture to western parts of Oregon while moderate-to-heavy rainfall for northwest Oregon and western Washington. Much of coastal Oregon and the Willamette Valley are experiencing lower than normal river levels, so the start of this first atmospheric river will largely be absorbed into the soil. Recent burn scars scattered about the Washington Cascades will increase the threat for localized flooding concerns with this anomalous plume of moisture. PW values of 1.5 inches will be nearly double the normal for early November by 00Z then another surge brings it to being tripled the average. The Marginal Risk area that was already in effect had minor adjustments made to reflect the latest trend in model guidance and WPC QPF. Campbell Day 3 Valid 12Z Thu Nov 02 2023 - 12Z Fri Nov 03 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR WESTERN WASHINGTON AND COASTAL OREGON... ....20Z Update... The first half of D3 will encompass the most active period within the forecast as the tail end of our atmospheric river will continue across western WA and coastal OR. A south shift of the main moisture flux is anticipated with higher totals reflected along the northwest and central OR coasts with some heavier rain expected along the central OR Cascades. Additional totals of 1-2" will be common within the Columbia Gorge into the adjacent terrain of the Cascades, adding to the higher totals that will have occurred through Thursday morning. Forecast vapor transport is expected to reach peak by the front end of the forecast period with slow degradation of the signal by the mid to late part of Thursday. The primary MRGL for D3 is mainly for a compounding effect from the initial surge of moisture at the back end of D2 with a continuation into D3 before rainfall subsides. GEFS IVT interval forecast shows a sharp decline after 18z Thursday likely leading to a forfeiture of the MRGL risk for the later updates once we reach D1. In any case, storm total rainfall of 2-4" will be common with local amounts of up to 6" over northwest OR where longevity of heavier precip is forecast to linger the longest. Kleebauer ....Previous Discussion... The focus of the highest rainfall will shift to the Cascades as the atmospheric river surges further inland to the Intermountain West while amounts along the coastline somewhat decreases. Rainfall from the previous day will likely have increased soil saturation thus maintaining an elevated threat for excessive rainfall and localized flash flooding. Western Oregon will likely begin to have localized flooding concerns after reaching/exceeding beneficial amounts. The latest guidance depicts areal averages of 2 to 3 inches along the Cascades and 1 to 2 inches along the coastline. A Marginal Risk area covers much of the same locations highlighted in the Day 2 Marginal with a southern expansion along the Oregon Coast and Oregon Cascades. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8QSirJoYUJqYZWpLjPxnqq2ria0uCo2y8vHZSo-vBUgR= pgPninuD9y_0OTPOqb7cx2J1U4P9MYGdAYiYlsCLmq2NlRI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8QSirJoYUJqYZWpLjPxnqq2ria0uCo2y8vHZSo-vBUgR= pgPninuD9y_0OTPOqb7cx2J1U4P9MYGdAYiYlsCLCS9VzbA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8QSirJoYUJqYZWpLjPxnqq2ria0uCo2y8vHZSo-vBUgR= pgPninuD9y_0OTPOqb7cx2J1U4P9MYGdAYiYlsCLlv8uAPg$=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) .