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March 1, 2021 – Dry and mild weather through Friday. Rain and mountain snow return over the weekend.

Discussion

Satellite imagery shows the upper trof along 135W deepening with short-term modeling indicating a large/deep closed will track Swd along 130W before turning Ewd and moving across Socal Wed/Thu. This will nudge the Wrn States ridge axis Ewd into the 4-Corners early this week. Mild Pacific air will remain over Norcal through Wed before cooling during the latter half of the week. Max temps will trend about 5-10 degrees above normal through Wed before trending cooler on Thu and into the weekend. The GFS has out-performed the ECMWF with this pattern, capturing the synoptic pattern days ahead of the ECMWF which didn’t capture the pattern until last Fri (2/26).

As the upper low tracks across the Srn half of CA on Wed/Wed nite, there is at least a chance of moisture/instability in the NE quadrant of the upper low reaching into our zone 69, mainly near the Sierra Crest. Have introduced some low PoPs to account for this possibility. An increase in cloud cover over much of the Srn half of our CWA is likely Wed/Wed nite as the upper moves inland.

5H cluster analysis for Days 3/4 (Thu/Fri) would indicate the potential for a deep trof to develop over the Ern Pac resulting in a change to a cooler/wetter weather pattern this weekend. IVT plume forecasts indicate a weak AR making landfall this weekend. Increasing onshore flow ahead of the deepening trof should begin the cooling trend on Thu.

Extended discussion (Friday through Monday)

Pacific frontal system approaches NorCal Friday, spreading widespread precipitation inland Friday night or Saturday. Model differences exist with timing of this system, but ensembles lean to the more progressive solution.

Snow levels are expected to be around 4000 to 6500 feet. Showers diminish Saturday night as system exits the area. Drier weather expected on Sunday. Another Pacific storm looks to impact the area early next week, however model differences with timing again exist with this system.