Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 24 2022 20:32:25 FOUS30 KWBC 242032 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 331 PM EST Sat Dec 24 2022 Day 1 Valid 16Z Sat Dec 24 2022 - 12Z Sun Dec 25 2022 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE COASTAL MOUNTAINS AND CASCADES OF SOUTHERN WASHINGTON AND OREGON... ....16Z Update... No changes made to the inherited Marginal Risk area in the Pacific Northwest. The front is approaching the coast as of the time of this writing. Thus, rainfall rates are expected to increase over the next few hours into the coastal mountains. This will be the prime time for the greatest chances of excessive rain causing flash flooding, especially in the Olympic Mountains. As the front moves inland this afternoon, the rain will both weaken and align more perpendicular to the coast. Both of these factors will greatly diminish the overall rainfall threat for the rest of today through tonight. Thus, the Marginal Risk may be able to be canceled early. For additional details, please reference MPD 1169 recently issued at https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/metwatch/metwatch= _mpd_multi.php?md=3D11__;!!DZ3fjg!_YS1r0NpzGDFxNXcuPEWNNdIscW-uTqo2A8YnagsU= AMH-k0rdh1bIKVGvg9vsLS6q9bCIBkUSleRa-f0f_h9fS75l9k$=20 69&yr=3D2022. Wegman ....Previous Discussion... A weak shortwave trough will arrive over British Columbia today. The surface wave associated with this system will advect a shallow plume of moisture into the Pacific Northwest along a weak and progressive cold front. PWAT's between 0.5-1.25 inches and anomalies of 2-3 standard deviations will support periods of heavy rain throughout the day before wrapping up early this evening. Isolated instances of flash flooding are possible, especially over the higher elevations of the Olympics and Washington/Oregon Cascades. Kebede Day 2 Valid 12Z Sun Dec 25 2022 - 12Z Mon Dec 26 2022 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE COASTAL MOUNTAINS OF WASHINGTON AND OREGON... ....2030Z Update.... Very few changes needed for this afternoon's update. Rain will be fairly light for the morning hours as an upper level front and disturbance move into the coast, largely north of the area into British Columbia, so only the southern end of the front impacts Washington State in the morning. The primary event of concern begins in the evening, and continues through the overnight, as a much stronger upper level front with an attendant surface front plow into the Washington and Oregon coasts. There is some disagreement in the high resolution guidance as to how this event will evolve, with some suggesting steady rain into the Olympic Peninsula for the first half of the overnight until a stronger front expands the rain shield south into Oregon for the second half of the night, while others having it largely dry until the stronger front moves in. Regardless, the primary threat for flash flooding remains along the Olympic Peninsula, especially along south and southwest facing mountains. Wegman ....Previous Discussion... Another weak shortwave trough will break off from an upper low and arrive over British Columbia early Monday morning. Therefore, any rain impacts associated with this system will take place later in the day 2 period (early Monday morning). Moderate to heavy rainfall may focus along the higher elevations of the Washington/Oregon coastal ranges. PWATs hovering around 1 inch will be 2-3 standard deviations above average for the region. The progressive nature of this system will mitigate any impacts from potential heavy rainfall as well. Kebede Day 3 Valid 12Z Mon Dec 26 2022 - 12Z Tue Dec 27 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!_YS1r0NpzGDFxNXcuPEWNNdIscW-uTqo2A8YnagsUAMH= -k0rdh1bIKVGvg9vsLS6q9bCIBkUSleRa-f0f_h9TO6AYMM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_YS1r0NpzGDFxNXcuPEWNNdIscW-uTqo2A8YnagsUAMH= -k0rdh1bIKVGvg9vsLS6q9bCIBkUSleRa-f0f_h9HbR62No$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_YS1r0NpzGDFxNXcuPEWNNdIscW-uTqo2A8YnagsUAMH= -k0rdh1bIKVGvg9vsLS6q9bCIBkUSleRa-f0f_h9G2FPvZw$=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) .