Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Nov 01 2023 15:42:30 FOUS30 KWBC 011542 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1141 AM EDT Wed Nov 01 2023 Day 1 Valid 16Z Wed Nov 01 2023 - 12Z Thu Nov 02 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR WESTERN WASHINGTON STATE AND NORTHWEST OREGON... ....16Z Update... No change to the previous as the latest short range guidance has little to no deviation from previous runs. Rates will remain below 1" for a majority of the area, but a short period of heavier rain will be found across the coastal plain of OR/WA with a secondary enhancement along the southern WA Cascades down towards the vicinity of Mount Hood. Columbia Gorge area in northern OR along with the eastern periphery of Portland will be the area of interest with localized flood potential possible given the terrain and urbanized influences respectively. 12z HREF probability for 3"/12-hrs is still very high (70-90%) with even some 5"/12-hrs probabilities up around 20-30% for the southern WA Cascades. 10-year ARI exceedance probabilities are running between 35-45% over the aforementioned areas with 100-year ARI indices non-zero, but still fairly low overall (10-20%). General axis of the moisture flux for the AR is aligned from the central OR coast up into the Olympic Peninsula which correlates well to the latitudinal range of the forecast MRGL. Totals between 1-3" with localized 4-5" are forecast during the period with most of the rainfall occurring between 00-12z Thursday. The MRGL is maintained due to the risk of localized flooding and maintaining continuity with guidance and probabilistic fields generally unwavering. Kleebauer ....Previous Discussion... The latest guidance is persistent with the atmospheric river across western Washington and into northwest Oregon with the highest rainfall focused over the Cascades in Washington and Oregon into the Columbia Gorge over northern Oregon. Rain intensity ramps up around 00Z Nov 2 and maintains a continuous 0.5-1"/hr rainfall rate over terrain through 12Z. Rainfall intensity will mostly reside below the 1"/hr mark for a majority of the area, but will cause some localized flooding potential within smaller towns and more established urban settings, especially for recent burn scars and other sensitive areas. Areal averages are still expected to be in the 1 to 3 inch range with very localized maxes up to 4 inches certainly possible. The 12z HREF neighborhood probabilities of at least 3" of rainfall between 00-12z Thursday (Second half of the D1 period) is running very high (70-90%) within the southern Washington Cascades down into the valley west of Portland near Mount Hood. The Marginal Risk area was maintained with some minor adjustments to reflect the latest trend in model guidance and WPC QPF. Campbell Day 2 Valid 12Z Thu Nov 02 2023 - 12Z Fri Nov 03 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR WESTERN WASHINGTON AND OREGON... The atmospheric river will be the most active during the 12Z Nov 2 to 00Z Nov 3 period across western Washington and Oregon with a notable southward shift of the main moisture flux anticipated with higher totals reflected along the northwest and central Oregon coast with some heavier rain expected along the central Oregon Cascades. The AR signal will decay the second half of the period. The WPC forecast has additional 1 to 2 inches falling over much of the same areas as the D1 period-- yielding 48 hour totals in the 2 to 4 inch range with isolated maximums nearing 6 inches. Minor adjustments were made to the inherited Marginal Risk area to reflect the latest trends and for the compounding effect from the initial surge of moisture at the back end of D2 with a continuation into D3 before rainfall subsides. Campbell Day 3 Valid 12Z Fri Nov 03 2023 - 12Z Sat Nov 04 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent.=20 During this period a resurgence of anomalous moisture will stream onshore further south than the most recent atmospheric river, focusing over southern Oregon and northern/central California. These areas largely were not impacted by the recent moderate/heavy rainfall event thus being able to absorb at least a couple of inches without notable hazardous impacts to the region. The area to watch for soils reaching saturation, or possibly exceeding saturation, will likely be northern/central California where the PW anomalies reach triple the average for early November. The WPC forecast has much of the Pacific Northwest and northern California less than 1 with a concentration of 2/2.25 inches near the far southwest corner of Oregon/Northwest California. =20 Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!53Wud-dLmz9MHTl6PCSx9QlI25iDdqps88De3M3cTdGe= OkC1B65zuJNnX7Y3B9LXKp3Ruc0IMbqgitHeyO0i2mPd7B0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!53Wud-dLmz9MHTl6PCSx9QlI25iDdqps88De3M3cTdGe= OkC1B65zuJNnX7Y3B9LXKp3Ruc0IMbqgitHeyO0i0FWvmvM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!53Wud-dLmz9MHTl6PCSx9QlI25iDdqps88De3M3cTdGe= OkC1B65zuJNnX7Y3B9LXKp3Ruc0IMbqgitHeyO0iI7zuB60$=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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