Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 06 2025 19:47:40 FOUS30 KWBC 061947 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 247 PM EST Sat Dec 6 2025 Day 1 Valid 16Z Sat Dec 06 2025 - 12Z Sun Dec 07 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Cook/Wegman Day 2 Valid 12Z Sun Dec 07 2025 - 12Z Mon Dec 08 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Orrison Day 3 Valid 12Z Mon Dec 08 2025 - 12Z Tue Dec 09 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE PACIFIC NORTHWEST... The 06Z/12Z model guidance suggests a slightly more suppressed=20 axis of greater IVT magnitudes impacting the Pacific Northwest on=20 the front end of the upcoming atmospheric event for the region. The NAM/GFS/ECMWF solutions though still support very strong warm air=20 advection with enhanced IVT magnitudes reaching 800 to 1000+ kg/m/s across the western WA and northwest OR coastal ranges with the=20 first wave that arrives in the 12Z/08 to 00Z/09 time frame. As=20 strong surface low pressure and associated shortwave/jet energy=20 crashes into British Columbia, the southern flank of the attendant=20 cold front will slow as it is crosses WA state and this should=20 allow for a relatively longer persistence of greater IVT values=20 across southern WA and northern OR going into the 00Z to 12Z/09=20 time period.=20 The deeper layer moisture fetch will have a trans-Pacific origin=20 that extends down to just west of Hawaii, and satellite data would suggest a likelihood for tropical and subtropical moisture feeds=20 into this atmospheric river event which will drive very efficient and high rainfall rates that may reach well into the 0.50" to 1"/hour range near the periods of max IVT and deeper layer ascent. The coastal ranges in particular across southwest WA and northwest OR=20 along with the west-facing slopes of the Cascades farther inland=20 should see enhanced rainfall totals that reach as high as 4 to 6+=20 inches which will heavily aided by deep layer orthogonal flow=20 relative to the terrain. Given the subtle southward shift in the axis of greater IVT=20 magnitudes and overall higher rainfall rates/totals for the=20 northern parts of the Olympic Peninsula and the northern WA=20 Cascades, the Slight Risk for this area was trimmed just a tad, with very little to no change elsewhere. This is consistent with=20 the overall 06Z/12Z guidance QPF footprint. Regardless, the wet=20 antecedent conditions and heavy rainfall totals are expected to=20 drive concerns for locally enhanced areal and riverine flooding,=20 with urban flooding also likely to become a concern. A threat for=20 landslides will exist, especially toward the end of the period and thereafter as additional upstream atmospheric river=20 activity/moisture transport arrives with more heavy rainfall. Orrison Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9aFOFjXyp5fO2yGDbmAc8SgvmIuD4o_8YXvCRfBrrsRc= Wnn6gPdSZxHnwDSD66SCcJaPG3_6KNsEaRgA3xFh9myQhxI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9aFOFjXyp5fO2yGDbmAc8SgvmIuD4o_8YXvCRfBrrsRc= Wnn6gPdSZxHnwDSD66SCcJaPG3_6KNsEaRgA3xFhRlT2zME$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9aFOFjXyp5fO2yGDbmAc8SgvmIuD4o_8YXvCRfBrrsRc= Wnn6gPdSZxHnwDSD66SCcJaPG3_6KNsEaRgA3xFhunykWzE$=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 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .