Cold Trough to Dive Into Great Basin Monday/Tuesday

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It’s been a long, dry, and fairly warm month thus far, and while widespread precipitation still doesn’t appear likely, it seems we should finally see a shift in the position of the trough this weekend, which should allow a fairly cold trough to dive into the Great Basin early next week.

Graphic depicting the most likely solution for Monday into Tuesday's trough.

Graphic depicting the most likely solution for Monday into Tuesday’s trough.

Models differ quite a bit on how the trough will evolve, with the GFS & ECMWF keeping the system contained to once main piece in the Great Basin, while the GEM is the wettest, most aggressive model indicating the main upper low strengthening and cutting off directly over northern California and off the coast, tapping into a better offshore/sub-tropical moisture source from the cutoff low that’s already offshore (see image below).

IR satellite image from around 10:30pm tonight, Wednesday. You can clearly point out an already existent and pesky cutoff low well off the California coast.

IR satellite image from around 10:30pm tonight, Wednesday. You can clearly point out an already existent and pesky cutoff low well off the California coast.

For now, I’ll go with the majority (ECMWF/GFS), which would support some sierra snow showers Monday into Tuesday morning, and much cooler temperatures across the region. Monday, highs in the valley will only make it into the low to mid 70s, perhaps a 10 degree drop from Sunday. Monday night into Tuesday snow levels may drop down to around 4000ft wherever precipitation occurs. If the ECMWF/GFS are to be correct, a dusting to 2″ of snow is possible… however, locally 3″ is possible if any wetter solution were to validate besides the GEM solution.

In addition, the pressure difference between the strong ridge off the British Columbia coast and the trough in the Great Basin currently looks like it will set up a period of gusty north  winds through the region Monday into Tuesday.

The cutoff low currently offshore will move eastward this weekend, dumping a good bit of rain a couple hundred miles offshore, but the low will retreat westward again early in the week as the trough dives south. It looks like this cutoff low may remain offshore through at least late week, but if the GFS is right, it could push inland into Baja California by Thursday. The ECMWF has a more interesting solution, with the cutoff low remaining off the central CA coast until the weekend when a trough in the Gulf of Alaska starts to enhance the low at the end of the run, at least according to the 12z run (00z was not out yet as of 10:30pm).

The cutoff low piece of the forecast is still pretty fuzzy and will likely change over the next week or so, thus we’ll have to hold off until early this (coming) week to get a clearer picture and see if it will have a notable affect on our weather in northern California.

If any piece of the forecast changes significantly over the next few days, stay tuned for additional updates/posts.

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