Cold nights with Valley fog and frost possible into Monday. Rain and snow chances return by early next week, becoming widespread by late Monday. A series of storms will continue this threat through the week into next weekend. Mountain travel will likely be significantly impacted at times. Periods of gusty winds are are possible.
Discussion
The latest IR satellite imagery indicates an area of fog and low clouds over the Delta and the Valley from around Red Bluff southward. Some visibilities down to 1/4 mile have been locally observed at Chico, Oroville, Marysville, Fairfield, with some mist elsewhere. Sacramento International briefly saw visibility drop down to 1/8 mile, but most recently is back up to 1/2 mile. Winds have been light overnight, and this combined with mostly clear skies and wet ground has brought favorable conditions for fog development. HREF probability of <1 mile visibility shows the best probability of fog over the central Sacramento Valley this morning, ending in the early afternoon.
Low clouds could persist through much of the day in the Valley and Delta after the fog lifts. The foothills and mountains have better potential for seeing some sun today. More morning fog is expected Sunday and again Monday in the Valley.
Temperatures at 3 am PST were quite cold, about 1-6 degrees lower than yesterday. For the Valley this means temperatures already in the low to mid 30s. Areas of frost are likely across the Valley, Delta, and foothills. Freezing fog could bring some locally slippery conditions, particularly over bridges and overpasses.
Ensemble guidance remains in good agreement that cold morning temperatures continue into early Monday under upper level ridging. Sub- freezing low temperatures remain possible in the Valley, in particular the over the Sacramento Valley.
Frost is likely to develop. The mountains will see morning lows down into the teens and 20s, so be prepared for cold if doing some outdoor recreation in the mountains this weekend.
Early next week a major change in the weather pattern is coming with wet weather returning to the region. Ensemble guidance remains in agreement a cold low pressure system from the Gulf of Alaska will likely deepen off the West Coast. This system is forecast to bring periods of Valley rain and mountain snow. Timing is still uncertain, but the trend is toward a slower onset of widespread precipitation as the low deepens off the coast. Shasta County could see some light precipitation as early as Sunday, but current guidance suggests much of the area won’t see wet weather until later Monday.
Snow levels could initially start below 3000 feet, then generally above 4500 feet. Snow could begin to bring travel problems to the northern mountains starting Monday, though higher impacts are expected Tuesday on through the week. Holiday travel could be significantly disrupted by these storms, with little break in the snow during this extended event.
Extended Discussion (Wednesday through Saturday)
Ensembles and clusters continue to be in agreement with longwave troughing deepening off the West Coast next week. This pattern will bring multiple rounds of widespread precipitation to the region into the holiday weekend. Cluster analysis still shows some differences regarding the progression, depth and position of the trough.
Nevertheless, NBM guidance shows at least a 50-75% chance of exceeding 2″ of rain for some Valley locations through Saturday morning. Overall, preliminary QPF amounts are forecast to be 1 to 3.5 inches for the Valley, 3 to 6+ inches for the foothills and 6 to 8+ over the mountains over a multi-day period.
Several feet of mountain snow are possible above 5000 feet. This will likely disrupt holiday travel. Periods of gusty southerly winds are also possible next week. Stay tuned for forecast updates.