Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Fri Dec 29 2023 11:04:00 FOUS30 KWBC 291550 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1049 AM EST Fri Dec 29 2023 Day 1 Valid 16Z Fri Dec 29 2023 - 12Z Sat Dec 30 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL AND NORTHERN CALIFORNIA THROUGH SOUTHWEST OREGON... ....16Z Update... Marginal Risk area remains unchanged from the previous issuance as guidance maintains continuity in the placement and timing of the heaviest rainfall from the anomalous moisture flux along and ahead of a cold front propagating inland from the Pacific. 12Z HREF neighborhood probability of at least 2"/24-hrs depicts three areas of focus for this afternoon and evening; the first is within the King Range south of Eureka where a touch over 0.8" (Cooskie Mountain), has already fallen over the past 6 hrs with a 70-80% likelihood of at least 2" within the confines of the topography and even a 40-50% probability of at least 3" is being depicted. The second area is across the southern face of Mount Shasta and the southern hills where probabilities are actually the highest among both the HREF and ECENS/GEFS for at least 2" today, and over 3" pushing 50-60% on the latest HREF. This is due to the upslope component of the wind field as prevailing south to southwest flow will align itself against the terrain and provide localized enhancement in the precip field through the height of the event (19-02Z) before waning. The third area is along the central CA coast around Big Sur where the inland propagation of the front will induce an area of moisture convergence along the terrain south of Monterey. This area will see the heaviest rainfall between 21-06z before dwindling as the cold front moves inland and best low-level convergence shifts to the Sierra. The primary risk will be localized flooding from hours of heavy rainfall as hourly rates likely peak at 0.5-0.75"/hr with a slight chance (15-25%) of up to 1"/hr according to the latest neighborhood probabilities from the HREF, primarily within a short window as the cold front bisects each targeted location. Some ponding of roadways is possible over the urban corridors, including San Francisco and western San Jose, but the prospects of major flooding appears unlikely given the anticipated rates. Overall, the threat remains low-end within the MRGL spectrum and no further upgrades are anticipated at this time. Kleebauer ....Previous Discussion... A highly amplified trough will move into the West Coast Friday. The base of the trough will move into California today, immediate downstream of which the flow will abruptly turn southerly upwind of a large amplified ridge over the Rockies. In fact the southerly flow will reach as far north as the coast of the Arctic Ocean near the Yukon/Nunavut border. The result will be a strong cold front that will only slowly move east, but all of the associated rainfall will be directed almost due south to north in the broad southerly flow. The strong forcing will be the driver for some of the heavier rainfall rates, with orographic uplift the other primary contributing factor. Thus, the mountains of the coastal ranges will be where the greatest flash flooding concerns will be, especially those that were hit hard in recent weeks. The Marginal Risk was expanded a bit down the coast to include portions of the Southern Coast Ranges. As the front pushes inland into the Central Valley, the south facing slopes of the Klamath Mountains will be the primary focal point for the heaviest rains and hence the best chances of flash flooding. The Marginal Risk for this area remains unchanged with this update. Wegman Day 2 Valid 12Z Sat Dec 30 2023 - 12Z Sun Dec 31 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 3 Valid 12Z Sun Dec 31 2023 - 12Z Mon Jan 01 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .