Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Dec 05 2023 07:38:18 FOUS30 KWBC 050738 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 237 AM EST Tue Dec 05 2023 Day 1 Valid 12Z Tue Dec 05 2023 - 12Z Wed Dec 06 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE OREGON COASTAL RANGES INTO PORTIONS OF THE WASHINGTON AND NORTHERN OREGON CASCADES... ....Washington and northern Oregon Cascades, Central to Northern Oregon coast,,, No significant changes made to the previous Excessive Rainfall outlook areas across portions of the Pacific Northwest for the ongoing atmospheric river event. The surface frontal boundary emanating from the deep low across the North Pacific will begin to slow day 1 as it pushes into western Washington and far northwest Oregon. Strong onshore southwesterly to west southwesterly flow in an axis of anomalous PW values, 2 to 3+ standard deviations above the mean, along this boundary will continue to impact the Washington and far northern Oregon Cascades and nearly all of the Oregon coast range for most of the upcoming day 1 period. There is very good agreement in the latest model suite for additional rainfall amounts of 2-5" along the central to northern Oregon coast into the northern Oregon Cascades and Washington Cascades.=20 The heavy rainfall over the past few days has resulted in much above to high stream flows along with high soil saturation levels across the slight risk area, with the expected additional amounts likely to continue a flooding risk. The slight risk area corresponds well with where HREF neighborhood probabilities are high for 1, 2 and 3"+ amounts day 1. The central to northern Oregon coast range and the southern Washington Cascades are areas where the HREF EAS probabilities are also high for 1, 2 and 3"+ amounts, albeit over a smaller region than the neighborhood probabilities.=20 ....Northern Idaho Panhandle, far northeast Washington and far northwest Montana... No changes made to the previous marginal risk area across the northern Idaho Panhandle, including surrounding areas of far=20 northeast Washington and far northwest Montana. This region has not seen as much precipitation over the past few days as areas closer to the Pac Northwest coast have. Latest model guidance, however, does show strengthening southwesterly low level flow impacting this region with overall good agreement on precipitation potential. Areal average 1 to 2"+ amounts expected day 1 with isolated totals of 3-4"+ totals possible. With snow levels expected to remain very high, most precip will fall as rain. This along with snow melt will continue the threat for runoff issues.=20 Stream flow as per the National Water Model are a mixed bag across this area with below normal to normal flow levels intermixed with above to much above normal flow levels. This and soil saturation levels that are relatively low, under 50% across far northern Idaho, may mitigate some of the runoff threat. For these reasons the threat level was maintained at marginal. Oravec Day 2 Valid 12Z Wed Dec 06 2023 - 12Z Thu Dec 07 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE SOUTHERN WASHINGTON AND OREGON CASCADES INTO THE COASTAL RANGES OF WESTERN OREGON AND FAR NORTHWEST CALIFORNIA AND ACROSS FAR NORTHERN IDAHO... The day 1 atmospheric river event will be diminishing day 2 as the slow moving day 1 front become more progressive inland across the Pacific Northwest. Moderate to locally heavy rains may still be ongoing for the first few hours of the upcoming day 2 period, but they will have mostly shifted to the south and east of the expected heavy day 1 axis from central to northern Oregon coast into the northern Oregon Cascades and Washington Cascades. The expected increased inland progression of northeast Pacific height falls and the associated frontal boundary will also limit the heavy rainfall potential period, compared to the day 1 time period. This and the farther south qpf axis with little max qpf overlap day 1 and day 2 has kept the threat level at marginal for the upcoming day 2 period. The marginal risk area does fit well where the HREF neighborhood probabilities are high for 1"+ rains during the 1200 UTC Wed to 0000 UTC Thu period. Probabilities for 2"+ amounts in the first 12 hours of day 2 are small and concentrated across far southwest Oregon and far northwest California where little precip is expected day 1. This region has, however, seen some very heavy precip totals over the weekend resulting in much above normal to high stream flow as per the National Water Model.=20 ....Far Northern Idaho... A small marginal risk area was added to far northern Idaho and the immediate adjacent areas of far northeast Washington and far northwest Montana. Model consensus is for additional 1"+ precip totals day 2, with much of this falling as rain before snow levels fall by early Thursday morning. This should continue the threat of runoff issues from the day 1 time period. Oravec Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-jQZFvGQOYqC9_hJ9dB_-jDbJAohwAVJPChTlTVaKpSy= fshIyd0YBfHzvcGU8M2ar7fX19_l7nqBQFTX8vGBs5OUMXg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-jQZFvGQOYqC9_hJ9dB_-jDbJAohwAVJPChTlTVaKpSy= fshIyd0YBfHzvcGU8M2ar7fX19_l7nqBQFTX8vGByJPoVOI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-jQZFvGQOYqC9_hJ9dB_-jDbJAohwAVJPChTlTVaKpSy= fshIyd0YBfHzvcGU8M2ar7fX19_l7nqBQFTX8vGBNHVe9dc$=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) .