Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Dec 26 2022 19:45:01 FOUS30 KWBC 261944 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 244 PM EST Mon Dec 26 2022 Day 1 Valid 16Z Mon Dec 26 2022 - 12Z Tue Dec 27 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR COASTAL PORTIONS OF SOUTHERN OREGON AND NORTHERN CALIFORNIA... ....16Z Update... No changes were made to the inherited. The guidance remains in very good agreement that the main event as far as heavy rainfall goes really will be late this afternoon through tonight in the Slight Risk area in northern CA & far southwestern OR. The rain ongoing at the time of this writing across western WA and far northwestern OR is weakening. The rain approaching the northern CA coast may become heavy at times, but the forcing for this rain is transient, and therefore not expected to last more than a few hours at those heavier rates. The forcing that will cause the later rain to become heavy at times is still several hundred miles from the West Coast, and notably weaker than it is expected to be tonight. Otherwise, the details described in the previous discussion below remain valid. Wegman ....Previous Discussion... A shortwave trough will weaken as it drifts into British Columbia today. A potent upper-level low will move into the Pacific Northwest region tonight, kicking off a relatively brief but impactful atmospheric river event. A dying cold front with weak onshore flow will spread rain showers across the coastal ranges of the Pacific Northwest this morning before mostly diminishing this afternoon. A stronger low pressure system will move into the region early this evening, bringing with it a deeper plume of subtropical moisture. PWATs of around 1.25 inches are likely with this moisture regime. Rain rates as high as 0.75in/hr and a 50kt low level jet will contribute to the slight risk of excessive rainfall over the coastal ranges of southern Oregon and northern California. Relatively moist/susceptible soils will provide favorable conditions for flash flooding of burn scars and urban areas. High snow levels will certainly allow for heavy rainfall/flash flooding potential along the windward side of the Cascades. Kebede Day 2 Valid 12Z Tue Dec 27 2022 - 12Z Wed Dec 28 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER COASTAL PORTIONS OF CALIFORNIA AND SOUTHERN OREGON AND FOR THE WESTERN FOOTHILLS OF THE SIERRA NEVADA OF CENTRAL CALIFORNIA... ....2030Z Update... In coordination with the STO/Sacramento, CA and HNX/Hanford, CA forecast offices, the previous Marginal Risk area for the western foothills of the Sierra Nevada Mountains of central CA has been upgraded to a Slight Risk. The primary point of uncertainty has been snow levels in this area...but there is good agreement with the latest guidance that snow levels could start off this event on Tuesday above 10,000 feet, when most of the precipitation is expected to fall. Snow levels will fall back to between 4,000 and 5,000 feet by early Wednesday morning, ending the flooding threat. This cooler air moving in will also coincide with most of the heaviest precipitation shifting south out of the area and weakening. The California Nevada River Forecast Center stated that despite the very high snow levels, much of this rainfall should be absorbed by the existing snowpack. The other Slight Risk area for the entire Pacific coast from southwestern OR to Oxnard, CA remains unchanged with this update. The greatest rainfall totals are expected in the mountains between the Bay Area and Oxnard. While folks north from the Bay Area will get less precipitation...the flooding threat will be ongoing at the start of Day 2 from the excessive rain tonight. Wegman ....Previous Discussion... Anomalous moisture associated with a fast moving atmospheric river will glide down the California coast and Sierra on Tuesday. Moderate to heavy rainfall (rates of 0.1-0.25in/hr) will spread down the state of California where a slight risk of excessive rainfall is in effect. Orographic enhancement and forcing from a cold front will allow for higher amounts over the foothills of the Sierra and heavy snowfall at higher elevations, but a marginal risk will suffice in these areas for now. PWATs near 1.5 inches (4-5 standard deviations from normal) will lead to instances of flash flooding from the Bay area early Tuesday morning to Los Angeles County later in the afternoon. Remnant moisture will spread across the Pacific Northwest, generating heavy snow in the Cascades and modest rainfall over the coastal ranges and Cascades foothills Tuesday morning, where a marginal risk is in effect. Kebede Day 3 Valid 12Z Wed Dec 28 2022 - 12Z Thu Dec 29 2022 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF ARIZONA... ....2030Z Update... No major changes made with this update. In a collaboration call with the PSR/Phoenix, AZ, FGZ/Flagstaff, AZ, and TWC/Tucson, AZ forecast offices held this afternoon, it was agreed that the Marginal should remain as is...with the expectation of some snow at the highest elevations and lack of hourly rainfall rates needed to induce flash flooding. Wegman ....Previous Discussion... The flash flooding threat on Wednesday will remain temporally confined to the morning hours over portions of central and southern Arizona, as remnant moisture from the coastal atmospheric river event pushes through the region along a strong cold front. The latest 00z Euro has come into better agreement with the slower 00z GFS with respect to the arrival of another east Pacific shortwave trough. Thus, heavier rainfall totals are currently forecast to remain off of the northern California/southern Oregon coast on Wednesday. PWATs may hover around an inch with anomalies between 2-3 standard deviations in southern/central Arizona Wednesday morning before quickly shifting southward and diminishing. The marginal risk area is cutoff on the northeastern edge along the Mogollon Rim and White Mountains, where moderate snowfall is expected. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-aCEFbVRuXUfMkCdNhRZCl7uNePzfcPgAZyt30HVqH7l= v8vZlTewpcL5VCEXcksrES7AtnFA7OlXU44h2AV_0mU2IPg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-aCEFbVRuXUfMkCdNhRZCl7uNePzfcPgAZyt30HVqH7l= v8vZlTewpcL5VCEXcksrES7AtnFA7OlXU44h2AV_qy_CsTk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-aCEFbVRuXUfMkCdNhRZCl7uNePzfcPgAZyt30HVqH7l= v8vZlTewpcL5VCEXcksrES7AtnFA7OlXU44h2AV_anwlA-g$=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) .