Stuck Between Systems; Precip Possible Sunday – Monday

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Northern California currently sits between two general systems: a ridge in the southwestern U.S. that has a significant amount of sub-tropical moisture rotating around it, and a cool and fairly deep upper level low off the Pacific Northwest coast.

At the moment, northern California is getting blasted by dry air between the monsoonal airmass to the south and a cooler low off the PNW coast. This dry air will shift northward as increased monsoonal moisture from the south works its way northward as the upper low slowly slides further east increasing forcing and lift across northern California.

At the moment, northern California is getting blasted by dry air between the monsoonal airmass to the south and a cooler low off the PNW coast. This dry air will shift northward as increased monsoonal moisture from the south works its way northward as the upper low slowly slides further east increasing forcing and lift across northern California.

Beginning Sunday, lift and upper/mid level forcing from the upper level low combined with a surge of sub-tropical moisture pushing northward into northern California will increase the chance of some precipitation across the area, especially in the higher elevations where the southwest flow will favor orographics.

Precipitable water values in excess of 1.4 to 1.5″ streaming into northern California between the systems won’t take much to be extracted from the atmosphere. Bands of lift/energy rotating around the PNW upper low will push through northern California beginning Sunday morning, continuing through Monday.

Models indicate potential showers possible anywhere, including in the valley, with the best chances in the mountains, due to the pretty good orographic flow as stated above.

Graphic depicting the two systems we're trapped between.

Graphic depicting the two systems we’re trapped between.

Instability isn’t forecast to be very notable, but steepening lapse rates will support a convective precipitation mode mixed in with possible stratiform cloud or precipitation bands. It should make for an interesting combination. Cloudy, somewhat moist/humid and showery pattern will continue into Monday for the lower elevations, and into Tuesday in the mountains.

While instability is expected to be weak across the region, somewhat better instability (though still weak) is expected to arise in the mountains Sunday and Monday afternoon. This may support a better chance of thunderstorms… however I wouldn’t completely rule out a thunderstorm (or a few) along the eastern side of the valley (where the upslope flow up the sierra begins).

Models differ quite a bit with possible QPF through the 48 hour period… some generating .25 – .50″ along the western slopes and locally .10 – .15″ in the valley, while some are nearly dry with under a tenth of an inch across the board. With the amount of moisture present, I’d personally say a wetter solution is possible, but highly depends on the strength of the bands that push across our region.

Fires continue to burn across the area, and while rain would be very beneficial, increased between systems in the mountains combined with potentially isolated thunderstorms could increase fire behavior unpredictability.

Moisture slides south of our area after Monday, and along with it our precipitation chances.

This low is expected to remain nearly stationary off the PNW coast until late in the week when the upper level ridge finally budges southwest and allows the trough and associated upper level low passage eastward.

00z GFS's 300mb winds and heights valid for Friday night indicating the upper low finally exiting eastward from off the Pacific Northwest coast.

00z GFS’s 300mb winds and heights valid for Friday night indicating the upper low finally exiting eastward from off the Pacific Northwest coast.

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