Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Dec 25 2022 08:37:05 FOUS30 KWBC 250836 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 335 AM EST Sun Dec 25 2022 Day 1 Valid 12Z 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... 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 0.1in/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... 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 Valid 12Z Tue Dec 27 2022 - 12Z Wed Dec 28 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE COASTAL RANGES OF NORTHERN CALIFORNIA AND SOUTHERN OREGON... ....2030Z Update... Rain that began with a strong front moving into the coast at the end of the Day 2 period Sunday night will continue through Monday and Monday night. The cold front associated with a North Pacific low will continue pressing southward into the northern CA coast during the day on Monday. Meanwhile a second stronger North Pacific low taking a more southerly track will approach the Washington coast Tuesday morning. This second low will have quite a bit more upper level support than its predecessor. Thus, the heaviest rain is expected overnight Monday night, as an attendant shortwave slams into the northern CA coast. The inherited Slight and Marginal Risk areas were largely unchanged. Somewhat lower snow levels along the higher, more inland Cascades down to northern CA will result in more snow there, so small trimmings were done to both areas to account for this. Otherwise the guidance for this storm has been incredibly consistent and so very few changes were needed. Wegman ....Previous Discussion... Anomalous moisture will focus over portions of southwestern Oregon and northern California down to northern parts of the Bay Area late Monday night into Tuesday. A 50-75kt low level jet, PWATs of 1-2 inches and anomalies of 3-4 standard deviations may cause flash flooding over burn scars, urban areas and slot canyons. The highest rain rates within the slight risk area will reach around 0.5in/hr early Tuesday morning before the heavy rain threat shifts south along the associated cold front. The relatively progressive nature of this potent system should proved to be a mitigating factor when considering potential moderate risks of excessive rainfall. Elsewhere, more diffuse rainfall will impact parts of the coastal ranges of the Pacific Northwest, Cascades, Bay area and northern Sierra Nevada. The heavy rainfall threat shifts southward along the California coast heading into mid-week. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5f_2w50_H_QYo_FYNeA36KIFy_QcsQLr1yTBRf-oha8x= 9iGOwXB8aJ7beFARKQhxYc5PDPoZVBslBwuAWwyE4emK-vY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5f_2w50_H_QYo_FYNeA36KIFy_QcsQLr1yTBRf-oha8x= 9iGOwXB8aJ7beFARKQhxYc5PDPoZVBslBwuAWwyEWf5OudI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5f_2w50_H_QYo_FYNeA36KIFy_QcsQLr1yTBRf-oha8x= 9iGOwXB8aJ7beFARKQhxYc5PDPoZVBslBwuAWwyE91ReCWc$=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) .