Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Oct 31 2023 00:51:06 FOUS30 KWBC 310050 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 850 PM EDT Mon Oct 30 2023 Day 1 Valid 01Z Tue Oct 31 2023 - 12Z Tue Oct 31 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent.=20 =20 Chenard Day 2 Valid 12Z Tue Oct 31 2023 - 12Z Wed Nov 01 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent.=20 =20 Wegman Day 3 Valid 12Z Wed Nov 01 2023 - 12Z Thu Nov 02 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR MUCH OF WASHINGTON STATE FROM THE CASCADES WEST... ....2030Z Update... Only a few minor changes were made to the Marginal Risk area with this update. Much of the OR Cascades were removed with this update per collaboration with PQR/Portland, OR forecast office. Much of this atmospheric river's rainfall in OR will occur after 12Z Thursday, thus occurring in the Day 4 period. Further, much of coastal OR and the Willamette Valley are experiencing lower than normal river levels, so the start of this first atmospheric river is likely to be beneficial rainfall for the area and largely absorbed into the soil. Meanwhile further north into WA, several burn scars from this past summer are in the Washington Cascades, which pose a higher localized threat for flash flooding. The Marginal Risk area was largely maintained with a few minor tweaks in these areas in coordination with SEW/Seattle, WA and OTX/Spokane, WA forecast offices. The 12Z Thursday cutoff to the Day 3 period will likely split this event about in half...so rainfall rates will generally be increasing up until the peak of the event around 12Z Thu, followed by a slow decrease in rainfall rates. It's likely if there are flooding impacts outside of burn scars, that they will manifest more in the Day 4/Thursday time frame. It's worth noting that a second roughly equally strong atmospheric river will follow in rather quick succession into the coastal Pacific Northwest in time for the weekend, which could result in more widespread impacts due to the brief time in between atmospheric rivers. Wegman ....Previous Discussion... Shortwave energy along with a plume of PW values of 1.25/1.5 inches approaching the Pacific Northwest will move inland during this period, spreading rainfall across the coast and points inland. This moisture surge will be 2 to 2.5 st. deviations above normal for early November, and with orographic enhancement, the rain may be moderate to heavy across western portions of Washington and Oregon. Recent moisture has increased soil saturation across the region and with areal averages of 1 to 3 inches forecast local areas of flash flooding may arise. A Marginal Risk area is in effect for portions of the Pacific Northwest Coast and east into the Northern Cascades. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_Nsr43oPkPcfEYYrgOSytszcM0BFx8soG3b_CL2ltPdc= 0sLuJezCdVAVafy_VICQBp4oidPfZGuFgvRG-bog3TgSXUQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_Nsr43oPkPcfEYYrgOSytszcM0BFx8soG3b_CL2ltPdc= 0sLuJezCdVAVafy_VICQBp4oidPfZGuFgvRG-bogEiu5qRI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_Nsr43oPkPcfEYYrgOSytszcM0BFx8soG3b_CL2ltPdc= 0sLuJezCdVAVafy_VICQBp4oidPfZGuFgvRG-bognZBFU_w$=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) .