Closed Low Formation Possible This Weekend

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It’s been a very warm, occasionally windy passed few weeks across California, and much of the southwestern U.S., while the central U.S. has been in a cooler pattern compared to normal.

Looking ahead to this weekend, a trough is forecast to dig into the Rockies/Great Basin. At the same time, a weak upper level (cutoff) low will meander towards the southern California coast late in the weekend as well. On Sunday, the trough begins to split, sending a fairly strong upper level jet streak westward towards the Pacific coast. As the strong jet streak associated with the trough makes it’s way over the ocean, the cutoff low well off the southern California coast becomes, for the most part, transferred into the trough as one large cutoff low fueled by a moderate strength polar jet, and a second jet streak max, origin I’m not entirely sure of… possibly a hybrid of a polar/sub-tropical jet blasting southern California and Baja CA/Mexico.

cutoff low formation

cutoff low formation

The GFS indicates the cutoff low deepening offshore later Saturday, to a surfaced base low Saturday night/Sunday morning. As the low deepens, lift and sub-tropical moisture influx becomes stronger. Precipitable water values of 1 – 1.3″ are depicted off the S CA coast, however at the moment it appears we in northern California will have to work with .5 – .8″ values, which isn’t too moist but it’s certainly not dry.

As soon as Saturday night, bands of lift rotating inland around the low, combined with the moisture indicated, some elevated instability, and some weak to moderate jet stream level wind influence. Showers and some thunderstorms are currently forecast to develop over the sierra Saturday afternoon by all major models (ECMWF, GEM, GFS), with some solutions actually indicating some elevated nocturnal instability to be sufficient for valley convection after sunset Saturday, continuing through early Sunday. Models come into better agreement for more widespread convection Sunday afternoon in mountain areas (sierra, western slope, coastal mountains, and potentially in portions of the foothills). Again, after sunset Sunday another threat of convection exists as diurnal capping/inversions weaken and elevated instability/lift/moisture allow valley convective initiation.

Something very important to note is that this is all based on what models currently indicate. Models have trouble with at least half of cutoff lows as we get closer to the event… and given how complex this event looks, it is certainly a possibility. There are differences among models, however overall agreement suggests that this cutoff low does develop… however track, duration, and precipitation chances themselves are all different per model. At the moment, the event does look like quite interesting given how boring things have been, and it certainly bears watching and observing how models handle the situation as we near the weekend. If current solutions continue, we’ll likely be dealing with a few days of scattered showers and thunderstorms across California, starting Saturday, lasting through Wednesday.

This pattern continues through Wednesday, with convective chances remaining widespread.

The cutoff low weakens through the week, but remains stuck over the southwestern U.S. due a trough being located in the eastern U.S. and a ridge in the middle of the country… basically a “rex” block.

I will try to post an update by Friday depending the way models look.

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