January 29, 2019 – Low chances for precipitation over the mountains Tuesday through Wednesday. Light precipitation possible most areas Thursday. Wet weather expected Friday through the weekend, with the potential for significant mountain snow.
A weak shortwave trough is currently dropping southeast through Northern California. Clouds cover much of the forecast area and a few very light showers may reach the ground especially over the mountains but rain or snowfall amounts should be insignificant. Some light valley fog has developed over night but cloud cover should keep it on the light side as well. Upper trough axis shifts to the southeast of the forecast area by late this afternoon so precipitation threat will shift to the northern Sierra by this evening. Upper ridge axis moves quickly across the state tonight and Wednesday morning so dry condition and some sky clearing is likely. Reduced clouds and light winds will allow for at least come valley fog again Wednesday morning.
Another Pacific low pressure system is forecast to push towards the coast Wednesday afternoon bringing a slight chance of rain and higher elevation snow to the coast range. Models in good agreement in dropping the low center southward down the coast Wednesday night. Forecast currently has a precipitation risk over the entire CWA Wednesday night but anything more than very light precipitation will be limited to areas west of the Sacramento and San Joaquin valleys closest to the low center. Even in these areas, precipitation is not expected to be heavy and impacts from this system should be minimal as the low moves into southern California on Thursday.
Upper ridging will bring dry conditions Thursday night most areas but some light warm frontal precip may ride over the retreating ridge and into areas north of about Chico. Next Pacific cold front pushes on to the coast during the day Friday. Precipitation increases Friday afternoon and night. Snow levels lower enough on Friday and Friday night to start impacting Sierra passes so mountain travel hazards are likely going into the weekend.
Extended Discussion (Saturday through Tuesday)
The stormy pattern continues as a powerful mid/upper low approaches the San Francisco Bay Area early Saturday. 500-mb height comparisons of this system indicate its strength sits around 2 to 2.5 standard deviations below climatology given sub-540 dm readings. Strong low-level warm advection courtesy of a 50 knot low-level jet will allow widespread precipitation to fall over the region on Saturday. The 06Z/00Z GFS show localized 6-hour amounts nearing the 0.75 to 1 inch range indicative of the heavy precipitation potential. Would not be surprised to see isolated thunderstorms to spawn given steep lapse rates accompanying the cold, upper low.
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The only notable limiting factor is the low-level flow somewhat parallels the Sierra which could cut back on orographic enhancements. However, the strength of the system itself should be more than enough to allow for substantial multi-day amounts across the Sierra and Southern Cascades. Overall snow levels should generally sit around 5,000 to 6,000 feet with this event. Besides the precipitation, some guidance suggests an inherent risk for locally gusty winds on Saturday, particularly as shown by the 06Z GFS. A deep surface low offshore would foster the development of gusty southerly flow. 925-mb winds sit around 40 to 50 knots which would be a concern if these were to mix down to the surface. This aspect of the forecast will need to be monitored in future model cycles.
A brief lull in the wet weather is possible this weekend, but it will likely be short lived with the next system arriving early Sunday. An even colder upper low dropping south-southeastward will keep conditions unsettled to conclude the weekend. The 06Z GFS/00Z ECMWF show the system paralleling the coast before sweeping inland over Central California by Sunday evening. Snow levels should continue to drop reaching the 3,000 to 4,000 foot range with accumulations potentially reaching the foothills. Further interests are with the 00Z ECMWF as it depicts an upper low diving down from British Columbia on Monday evening with 700-mb temperatures dropping down to the -15C to -18C range. Given the 06Z GFS shows a broad ridge building over the eastern Pacific, much uncertainty remains. However, a gradual consensus for a ridge exists by Tuesday (Feb 5) onward favoring a turn to drier weather. Generally speaking, temperatures should be slightly below normal although numbers could be a tad lower than forecast on days with extensive cloud cover and precipitation.