Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Dec 25 2022 20:27:10 FOUS30 KWBC 252027 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 326 PM EST Sun Dec 25 2022 Day 1 Valid 16Z Sun Dec 25 2022 - 12Z Mon Dec 26 2022 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF COASTAL WASHINGTON AND NORTHERN OREGON... ....16Z Update... No changes were made from the inherited. Light to occasionally moderate rain continues to move into the Oregon and Washington coasts this morning. The rain is likely to become more intermittent and lighter into the afternoon and early evening. However, after that, a strong front with attendant upper level forcing will careen into the coast, causing a rapid increase in rainfall intensity and coverage all over the Pacific Northwest west of the Cascades, with some rain spilling over east of the mountains as well. A south to southwesterly oriented low level jet will favor those facing slopes, which will most dramatically impact the Olympic Mountains and Peninsula where 2-3 inches of rain are expected on already very saturated soils. Further south into the southern Washington and Oregon coastal ranges, 1-2 inches of rain are forecast, so chances of flash flooding should be a bit lower there. Wegman ....Previous Discussion... A pair of surface waves will bring a couple of rounds of moderate rainfall to the coastal ranges of the Pacific Northwest on this Christmas day. The first round of rainfall will arrive with a weak cold front this afternoon. The second round of rainfall will arrive early Monday morning as a stronger cold front supported by an upper-level shortwave trough moves into the region. PWATs will remain modest at around an inch throughout the 24 hour period. Rain rates will remain relatively low as well at around 0.1 in/hr at their peak. The heaviest rainfall footprint is expected to occur over the Olympic mountains where temperatures are expected to remain above freezing. Kebede Day 2 Valid 12Z Mon Dec 26 2022 - 12Z Tue Dec 27 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER COASTAL PORTIONS OF SOUTHERN OREGON AND NORTHERN CALIFORNIA... ....2030Z Update... No significant changes were made with this update. A few areas along the highest peaks in northern CA were trimmed with the expectation that most of the precipitation that falls here will be in the form of snow, precluding any flash flooding threat. Otherwise the meteorology remains the same. A front that moves into the PacNW tonight will continue moving inland and dissipating through the day Monday. Onshore flow will keep light rainfall ongoing into the coastal mountains behind the front. Then a much stronger low will barrel into the WA coast Monday night. A rapidly strengthening upper level shortwave trough will accompany the low. These two teaming up will result in unidirectional westerly flow throughout the atmosphere into the southwestern OR coast and the coast of northern CA. Here, perpendicular flow and tremendous forcing could result in rainfall rates as high as 1.5 inches per hour against the coastal mountains. 4 to 7 inches of rain are possible for the 24-hour total, but most of that will happen overnight Monday night. According to NASA Sport soil imagery, soil moisture going into this event is around average. This means the area can handle some rainfall, but rates that high are going to struggle to handle so much water all at once. Expect 2-4 inches of rain in the area just north of the Bay Area, and also through central and northern OR up to the Olympics. Forcing will be decidedly lesser in these areas as the flow turns more southerly with time. Wegman ....Previous Discussion... The slight risk area inherited from day 3 remains mostly unchanged. An atmospheric river event is expected to unfold from the southern Oregon coast to northern California on Monday. Pacific moisture will continue advecting into the Pacific Northwest on Monday morning and afternoon as Sunday's dying cold front pushes into the interior Northwest. PWATs will remain modest, though, around 1 inch. Rain rates will be between ..1-.25in/hr from the coast to the northern Cascades through Monday afternoon. A mid-latitude cyclone supported by a closed upper-level low and a 50-65kt low level jet will kick out the previous system and arrive over the Pacific Northwest early Tuesday morning. A potent moisture regime will follow and overspread the coastal ranges of southern Oregon and northern/central California. Rain rates between 0.25-0.75in/hr are likely early in the morning with the highest rates expected near the Oregon/California border. Moderate to heavy rainfall is likely to make its way into northern portions of the Sierra with this second round of precip, but temperatures should remain warm enough to support rainfall. Flash flooding will be possible over burn scars, urban areas and slot canyons. Kebede Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-Waedwkc67FuPbvnqNhG4Lmvamj46fugXlsi3NPDcpNN= pjGyAdfCSB7Rgl5ONyL7WfwJEsd0XwmDe3d88RIcZ_r7nJE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-Waedwkc67FuPbvnqNhG4Lmvamj46fugXlsi3NPDcpNN= pjGyAdfCSB7Rgl5ONyL7WfwJEsd0XwmDe3d88RIceK2A8mA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-Waedwkc67FuPbvnqNhG4Lmvamj46fugXlsi3NPDcpNN= pjGyAdfCSB7Rgl5ONyL7WfwJEsd0XwmDe3d88RIc0yZ3IVE$=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) .