Hydrometeorological Discussion National Weather Service / California Nevada RFC / Sacramento CA 750 AM PST Mon Dec 6 2021 ...A WEAK SYSTEM WILL BRING LIGHT PRECIP TODAY AND INTO TUESDAY... ...ADDITIONAL MORE WIDESPREAD PRECIP CHANCES MID TO LATE WEEK... .METEOROLOGICAL CONDITIONS (SHORT TERM: MON AM - THU AM)... Radar imagery showing some light returns over northern CA and southern OR this morning as a weak surface low and associated cold front approach British Columbia. Observations have recorded a few hundredths of precip over the past 6 hrs. Yesterday's upper ridge has shifted slightly south and westward allowing moisture to stream over region as can be seen on current water vapor imagery. Models also show a decent moisture plume of 1"+ PW that will move through the region through Tuesday morning. This will result in scattered precip moving from NW to SE throughout the day and into Tuesday morning. QPF amounts are forecast to be generally less than a tenth of an inch for most areas and 0.10"-0.30" for the wettest spots along the north coast north of Cape Mendocino, the southern OR Cascades, and the Central Sierra. Simultaneously, an upper low will move southward just off the CA coast through Tuesday. This has allowed for some improved precip chances for the Central Coast over the next 24 hours, however, models keep the bulk of the precip from this system offshore. Just a nudge eastward may result in higher precip totals than forecast for that area. Behind this upper low, ridging will temporarily move into the region keeping most locations dry. A large surface/and upper low over western Canada and associated cold front stretching southwestward off the coast of the PacNW will bring a slight chance of some light showers along the coast of CA north of Cape Mendocino and the southern OR Cascades, but likely not more than a few hundredths. This broad area of low pressure will expand on Wednesday to cover most of the western U.S. and potentially bring additional light precip to the northern half of the region. This expansion will continue into Thursday with the front also moving through northern CA overnight Wednesday resulting in more widespread precip for CA/NV. Highest totals Wednesday night look to be over the Sierra. Uncertainty remains on QPF amounts as the GFS continues to show higher totals than the ECMWF along with a wide range of possibilities amongst both model's ensemble members. Freezing levels to trend downward today to about 7-10 kft north of SoCal by this evening before rising again temporarily from W to E later Tuesday into Wednesday morning back to 10+ kft. As low pressure expands, colder air will be ushered in resulting in dropping freezing levels starting Wednesday. Levels will lower to 3- 6 kft north of Cape Mendocino and over far northern NV by the evening. .METEOROLOGICAL CONDITIONS (LONG TERM: THU AM - SUN AM)... With the upr ridge aligned well offshore along 150W...the next s/wv trof moving from west-northwest to east-southeast across the region will bring widespread light to moderate precip on Thursday. The cold front will already be traversing northern CA during the morning hours with the best precip focused on the length of the Sierra and points eastward across the southern two-thirds of NV (mainly south of I-80). Precip is also expected to reach down across southern CA with the best amounts focused on southwest and west facing slopes. Highest totals will generally be in the range of 0.25- to 0.75-inch. In terms of freezing levels...look for these to fall to approx 2500- to 4500-feet from I-80 northward...5000-to 6500-feet for central CA and central NV...and about 7000- to 10000-feet across southern CA and southern NV during the bulk of the precip. This s/wv trof will slide east of the region on Friday with lingering precip tapering off during the morning hours. Flow will turn northwesterly as an upr ridge axis moves toward the west coast and eventually cross the region early on Saturday. The next moisture plume will take aim at the Pacific Northwest on Saturday with the southern extent of this feature clipping northwest CA and southwest OR...which will bring light precip into early Sunday. Beyond the forecast period...an upr trof will shift across the northeast Pacific and dig off the west coast. This feature looks to bring unsettled conditions across the west coast into early next week...at least. This is well represented in the CPC 8-to-14 Day Precip Outlook...which shows a higher likelihood of above normal precipitation across the west coast from Dec 13 through 19. |