Moderate-Strength Storm to Impact Western U.S. Over the Weekend

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We’ve been swarmed with a handful of quite weak/tail-end systems over the last couple of weeks or so, but it appears we’ll finally see a decent system over the weekend, as the large Gulf of Alaskan low creates a favorable pattern for cyclogenesis well off the Californian coast, as warm, most sub-tropical air gets pulled up ahead of the larger low in the Gulf, creating a favorable zone of convergence to allow rapid development of a mid-latitude cyclone (well) off our coast.

0z GFS's forecast 500mb relative vorticity and MSLP, valid at 5pm Thursday, indicating the development of a wave well off the Californian coast that will soon become a deep mid-latitude cyclone (most likely, that is).

0z GFS’s forecast 500mb relative vorticity and MSLP, valid at 5pm Thursday, indicating the development of a wave well off the Californian coast that will soon become a deep mid-latitude cyclone (most likely, that is).

The upper-level flow in place will likely take this strong low into Oregon or Washington. Model disagreement exists with the track and strength of the low, with the NAM being weaker and dryer with a 990mb surface low, compared to 978mb (ECMWF) to 988mb (GFS). The strength/power of the low will be enough to draw northeastward with it a 1 – 1.3″ precipitable water plume, and even more impressive, pack a 140 – 150kt upper-level jet, and a 100 – 105kt mid-level (500mb) jet, fueling one heck of a cloud shield/baroclinic leaf most likely.

0z GFS's forecast 300mb wind and MSLP, valid at 5am Saturday morning, with a powerful upper-level jet streak noted racing ahead of the low.

0z GFS’s forecast 300mb wind and MSLP, valid at 5am Saturday morning, with a powerful upper-level jet streak noted racing ahead of the low.

The low is forecast to initially develop roughly 800 miles off the west coast Thursday afternoon, beginning around 1007mb. 24 hours later by Friday afternoon, the low will have deepened to around 990mb according to the GFS/ECMWF off California’s coast, and deepens slightly further as it heads northward towards the OR/WA coast. The ECMWF doesn’t indicate rapid strengthening until it begins the more rapid/notable northwest motion as the jet strengthens and kicks up ahead of the low as it strengthens, pulling warm, moist air out ahead of it.

Model disagreements to note: the ECMWF is considerably deeper than any other model, with the low actually bombing out off the CA/OR/WA coast between early Friday and early Saturday, with the low beginning at 1004mb early Friday, bottoming out at 978mb early Saturday – a 26mb drop, 2mb more than the 24mb criteria for bombogenesis. There’s also some notable timing differences between models, with the GFS being quite favorable for afternoon convection come Saturday behind the cyclones cold frontal precipitation band, which itself could drop a fairly decent amount of precipitation according to the two models. The NAM is considerable dryer and a bit weaker with the low compared to the GFS, and significantly weaker than the ECMWF… but then again, the NAM isn’t well know for good outcomes past about 48 – 72 hours or so.

Given the depth/strength of the surface low/mid-latitude cyclone, breezy to gusty winds could be possible ahead of and with the cold front, but all depends on the depth and track of the low. A deeper low and a more southerly track could bring some fairly strong winds to parts of northern California, while a more northerly track or a weaker low would mean a more minimalistic/non-dramatic wind event to go along with the rain. Something I’ll nail down in the next update.

At the moment, the general model consensus would support rainfall amounts in the range of a tenth of an inch to a quarter for the northern San Joaquin valley, around a quarter to half an inch for the southern/central Sacramento valley, delta, and bay area, around three quarters of an inch in the north end of the valley, jumping into the foothills with around three quarters of an inch to an inch of rain from around Sonora northward, and an inch to two inches along most of the western slope and northern mountains, and lastly about one to three inches along the north coast, from the cold front and post-frontal showers/convection lingering into Sunday. These amounts could certainly need adjusting in the next day or two, whether that’s up or down we’ll just have to wait and see.

October 22nd Map 1

October 22nd Map 2

Thunderstorm potential certainly does exist Saturday afternoon and into the evening, but all depends on timing and strength, which as mentioned isn’t very well nailed down yet, and needs a few days to stir in the models. I’ll have another post either late Thursday or Friday regarding the system as a whole, so keep an eye out for that to be posted in the next 24 to 48 hours!

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