Showers and morning fog today, atmospheric river event likely starting Friday

December 3, 2019 – Unsettled pattern may bring some showers today and Wednesday. Another winter storm will approach Thursday before moving across the area on Friday into the weekend bringing an increase in precipitation and wind.

Discussion

Partly cloudy skies cover the region early this morning with only isolated showers indicated on radar. Patches of thick fog have developed across portions of the southern Sacramento Valley and northern San Joaquin Valley with visibility down to around 1/4 mile in spots at times.

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Relatively dry day expected across interior NorCal today as the closed low remains just north of 30N/130W. An increase in shower activity is expected south of I-80 on Wednesday as the system tracks inland across SoCal.

The next winter storm approaches the West Coast Thursday from the northeastern Pacific. This is shaping up to be another impactful winter storm that will bring major mountain travel headaches Friday into the weekend with periods of heavier precipitation and gusty southerly winds.

Extended Discussion (Saturday through Tuesday)

Another atmospheric river event will likely impact the region late this week into into Sunday morning. Mid-level heights accompanying the closed low are running around 525-dm although the system will fill as it approaches the Oregon coast. Relative to climatology, such heights will run about 1.5 to 2 standard deviations below average. Ample subtropical moisture is forecast to stream ahead of the advancing cold front with precipitable water values around an inch or so. Enhanced moisture coupled with the usual orographic enhancements will support moderate to heavy precipitation over the region late this week through Sunday morning. Significant mountain travel impacts are expected during this timeframe, particularly through the Sierra passes as snow levels will be around 5,500 feet Saturday. If you must travel, keep an emergency supply kit in your car including a first aid kit, food, water, flashlights, and blankets in case of an emergency.

QPF amounts indicate 1.25 to 2.50 inches in the Valley, and 2 to 4.50 inches over the foothills/mountains. Heavy mountain snow is also anticipated, with the latest forecast indicating 2 to 4 feet of snow at pass levels. Gusty winds over higher terrain will likely lead to whiteout conditions at times. Windy conditions are also expected in the Valley, with the ECMWF ensembles mostly showing 30 to 40 mph during this timeframe. Latest deterministic and ensemble guidance keeps precipitation over the Sierra continuing through at least Sunday afternoon. Then, dry weather returns early next week as ridging builds in.