April 21, 2017 – Dry and warmer weather for Friday and into the weekend. The next chances for precipitation return for the start of next week.
Discussion (Today through Monday)
Upper level ridge will build over the region today and bring above normal temperatures and dry conditions for northern California. An approaching upper level disturbance off the coast of the Pacific northwest will inject slightly cooler air for the weekend, but temperatures will still remain seasonable and near normal.
Model forecasts remain in good agreement to the progression of the trough base, which will take a negative tilt and lift northward just before sliding across interior northern California. As a result, the best areas of instability and forcing will remain to the north of the forecast area. Some models have indicated for some chances for a stray shower or two for the northern mountains, but confidence in any measurable precipitation is low for Saturday night into Sunday.
On the backside of the passing trough, the main jet stream under northwest flow will return to the west, but the main dynamics associated with the upper flow will remain to the north for Sunday night into Monday. There are chances for showers mainly for the northern mountains and northern Sacramento valley, and the Sierra will also have some chances for light precipitation. Precipitation will mostly be rain, although little accumulations are possible for elevations higher than 6500 feet.
Extended discussion (Tuesday through Friday)
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Models in fairly good agreement for beginning of extended period bringing a shortwave trough through the north state Tuesday in upper zonal flow. Precipitation amounts are progged to be fairly light so impacts will be minimal. Precipitation threat has dug a bit farther south however so even the southern most CWA should have a slight threat of precipitation. Moderately high springtime snow levels will lessen impacts even more.
Precipitation threat lifts northward a bit on Wednesday as upper flow ridges a bit over the eastern Pacific. Northern zones and northern Sierra will however still remain under at least a slight threat of showers.
Models diverge fairly significantly towards the end of next week. While GFS brings continued amplification of the west coast ridge, ECMWF brings a cold low into the Pacific Northwest for a continued precipitation threat over the north state. Due to model inconsistency, can not rule out precipitation threat over the northern mountains Thursday.
Models become a bit more consistent in eastern Pacific ridging by the end of next week. All models showing upper level northwest flow aloft Friday afternoon. Most NorCal locations will likely see dry conditions with near normal highs but some overrunning showers will still be possible over the northern mountains.