Storm to Impact Northern California on Halloween

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Ridging will keep northern California dry through the middle of the week, however, by the middle of the week a powerful upper-level low in the Gulf of Alaska fueled by a 150kt upper-level jet entering the back side of the low begins to carve out a potent trough off the western U.S., which will begin to slide inland by Friday, which, unfortunately for some, is Halloween.

12z GFS's forecast 300mb winds and 500mb heights (contours) valid Friday morning (at about 8am).

12z GFS’s forecast 300mb winds and 500mb heights (contours) valid Friday morning (at about 8am).

As the trough digs off the west coast, the strong pull of the upper-level jet (in excess of 100 knots when it begins blasting aloft overtop northern California on Friday) draws in a narrow, but moist band of subtropical moisture, with precipitable water values in excess of an inch, and max values upwards of 1.2 to 1.3 inches along the coast. Right behind the narrow moisture band lies the front, which should provide enough lift in combination with the available moisture and upper-level support to bring through an at least modestly held-together frontal precipitation band. Ahead of the trough’s axis the flow in the mid (and upper levels) will be generally out of the southwest, thus southwest facing slopes will likely see enhanced precipitation through the day Friday.

It currently appears the front will slide through the northern mountains/valley by mid to late-morning, and pass through the Sacramento, SFO, and I-80/Highway 50 corridor by mid to late afternoon. Behind the front a somewhat unstable/showery environment takes over, continuing scattered precipitation through the night Friday, though not widespread. Showers may continue into the afternoon Saturday as well, though for the most part transitioning into the higher terrain.

For trick-or-treaters Friday evening conditions will be quite variable. Some locations could be periodically wet while some could be dry. If the sun pops out behind the front in any valley locations, instability could be boosted and allow thunderstorm development, generally along the coast and in the valley/foothills. In general it would probably be best to carry rain gear when out and about Friday evening, even if it’s dry initially. Regarding the thunderstorm potential, shear is currently forecast to be pretty meager, though there’s still a lot of time for models to change up.

October 27th 2

Currently looks like around a tenth to a quarter inch of rain is a good bet for most of the lower elevations, with upwards of half an inch from the SFO/bay area southward, and east of Sacramento where precipitation banks up against the foothills and western slope. One to two inches of liquid is a good bet for the western slope, northern mountains, and northwest coast, with around half an inch to an inch in the foothills.October 27th 1


Snow levels during the day Friday will begin around 7000 to 8000 feet, but as the cold front nears and passes they will drop off quite a bit… perhaps to around 5000ft Friday night, and perhaps nearing 4000ft Saturday morning, though precipitation will be spotty. It does appear as though that elevations over about 5000ft could pick up a couple inches of snow, with 3 – 6″ more likely above 6000ft, and upwards of 8 – 10″ at the highest elevations, which would be the most from a single storm than we’ve got from all of these (for the most part weak) systems over the last month or so.

October 27th 3

Wind looks quite minimal from this system. A few days ago it appeared there’d be potential for gusty winds with a surface low off the Pacific Northwest coast, but because that surface low is essentially gone or expected to be significantly weaker, surface pressure gradients are pretty weak, thus winds in the upper-levels shouldn’t get carried down very far.

Things look like they’ll dry out after the Halloween storm at least through several days of November. The GFS has been advertising some powerful storms out around November 10th, but that’s a long shot for weather, even if there’s been relatively decent consistency. I’ll certainly keep an eye out to see if it’s a continued trend, and if other models pick up on it when they become in range.

That’ll do it for this system… for now at least. If anything changes, I’ll either update this post or post an entirely new one by Thursday.

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