Powerful Series of Storms to Slam Northern California This Weekend into Next Week

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A major, game-changing pattern change has begun to take shape across the western U.S. as strong ridging that has promoted warm, dry weather across California for weeks essentially begins to get forced southeast, due to a large trough/upper-low overtaking the eastern Pacific. This large upper-low will position itself in the southern Gulf of Alaska, lining up a powerful westerly jet that’ll extend across the Pacific from the western U.S. to the west Pacific east of Japan. Upper-level wind speeds with these strong westerlies upwards of 190 knots north of Hawaii signify the strength of these upper-level winds, of which will help intensify storms that develop along it in the days ahead. A significant subtropical moisture plume originating from the western Pacific near the Philippines will extend out across the entire Pacific (about seven thousand miles) to the west coast of the U.S. by Friday as it combines with a secondary moisture plume feeding northeast from near Hawaii, creating a broad region of high-moisture content in the eastern Pacific from Washington to southern California, all waiting to be forced inland. A weak impulse will slide northward along these moisture plumes out ahead of the trough into northern California Friday, and begin to initiate what will be a several-day period of unsettled conditions for California.


Friday’s ‘system’ will more or less be convective showers and thunderstorms, which will make for a very interesting start to a long period of interesting weather. The cause to Friday’s showers and thunderstorms will be increasing lift ahead of the trough in the warm sector of the system — not a place California usually sees much action. You’ll typically hear of warm-sector convection in the eastern U.S., as it’s where you’ll find most severe outbreaks occur due to the high moisture content and extreme instability. The fuel for this instability will be still (relatively) very warm surface temperatures, with increasingly cold air aloft being forced overhead as the trough nears, creating cooling air with height, which in itself is instability. However, steep lapse rates can’t do much by themselves. When you add in moisture, this allows that warm surface air to have moisture content as it rises higher into the atmosphere where it’s cooler, where it can condense and form clouds and convection. Dewpoints are forecast to be in the upper 50s to low 60s across the lower elevations Friday, which is fairly high for California’s standards. These high dews combined with the falling temperatures with height equate to what would be considered with California’s standards as a large amount of instability — or about 1000 – 1500 j/kg of CAPE for those accustom to this index.


It’s not very often this amount of instability exists in northern California’s lowlands, so it’ll make for an interesting day Friday. Upper-level winds will still be fairly modest/weak due to the distance away from the trough offshore where the jet would obviously be much stronger, thus making upper-level support weak. Nonetheless, there is some modest turning of wind with height with winds at the surface out of the south to southwest, with mid/upper-level winds out of the west-southwest to west, which equates to some notable directional shear, albeit the winds at these levels being fairly weak (40 knots or less, even in the upper-levels). Lift, while increasing, won’t be very strong — but the large amount of instability should help make up at least a little for the lack of large-scale accent which would be much stronger with the trough/low’s precipitation shield and cold front. High resolution models suggest scattered afternoon convection within the belt of moisture and lift from around Shasta county south to Fresno county. With the available energy and shear, if thunderstorms arise out of this environment, some could become strong — and would be slow moving due to the fairly modest mid and upper-level flow at the time. Primary thunderstorm threats would be hail, possibly large, and very heavy rain with the moisture available. I don’t care for even risking using the wording “tornado possible” with a marginal setup, but because of the uncertainty here, I wouldn’t say it’s a zero threat of something supercellular developing. If the afternoon looks crazier come Friday morning, I’ll post a quick update to this post.

Major Storm Slams Northern California Saturday/Sunday

After Friday’s warm sector showers/storms comes the main event. Late Friday into early Saturday, cyclogenesis occurs off the California coast along the strong westerly jet pumped up with precipitable water values up around 1.5″ with a strong temperature gradient perfect for rapid intensification. Speaking of rapid intensification, it appears this low will meteorologically “bomb”, dropping somewhere around 24 – 28mb within a 24 hour period (a 24mb drop in 24 hours qualifies a surface low as a “bomb”, a big meteorological event that occurs with powerful cyclones) between late Friday and late Saturday, with the low bottoming out around 974mb off the Pacific Northwest coast by late Saturday. As the low reaches maturity off the Pacific Northwest coast, the massive precipitation shield slams into northern California, as early as Saturday afternoon. The heaviest precipitation looks to occur Saturday evening, with rainfall rates of up to a half inch an hour locally in the lower elevations — even higher in the foothills and mountains. Given the significant deepening of this low and the very powerful upper-level jet, you can imagine winds down lower being strong, too. That’ll be the case, with wind gusts of 50 – 60mph possible along the north coast, and 40 – 55mph gusts possible across much of the lower elevations of northern California including the Bay Area, Delta, Sacramento, and northern San Joaquin valleys. Gusts in the foothills and mountains will be about the same, with high-sierra/ridge top gusts in excess of 70mph possible. These winds in combination with heavy rainfall could bring down trees and tear off tree limbs, with scattered power outages likely. These winds could also be directly damaging in some cases, tearing off roof shingles and blowing around large, unsecured objects.


A moist flow continues Sunday ahead of the next system, continuing showers on a widespread scale. Instability behind the first system beneath cooler upper-level air will also increase the chance of some thunderstorms here and there, something that’ll have to be looked into a bit closer as the time nears. Cyclogenesis occurs once again along the primed westerly jet late Saturday into Sunday well off the California coast, with the low arriving to the northern California/Oregon coast by late Sunday into Monday, bringing another round of widespread precipitation and gusty winds. This low doesn’t strengthen as much as the Saturday storm, but does make it down t0 990 – 994mb and in much closer proximity than the bombed out low, as this one will impact southern Oregon/northwest California, over the previous one riding northeast into British Columbia. This storm won’t be quite as windy as Saturday’s, but will deliver another round of gusts up around 30 – 40mph on a fairly widespread scale Sunday afternoon and evening, with stronger gusts along the northwest coast (and southern Oregon’s coast). The main cold front of this system clears through by late Sunday evening with the most widespread and heavy precipitation, leaving the shower post-frontal environment overhead through the rest of the night into Monday. Wouldn’t rule out another chance of thunderstorms here and there Monday with the cold pool aloft, but the northwest flow aloft will likely limit severe chances (but don’t rule anything out this early on).


Rainfall amounts

Precipitation will remain nearly constant through the weekend into Monday across northern California, with totals adding up to anywhere from 2 – 4″ in the Sacramento and northern San Joaquin valley, 3 – 7″ in the foothills (higher amounts in the northern foothills), 1 – 3″ in the Bay Area with locally higher amounts in the Bay Area hills, 2 – 5″ in the coastal ranges, 4 – 7″ in northwestern California, 6 – 8″ in favored slopes of Shasta county, 5 – 8″ in higher elevations of Butte and western Plumas county, and a general broad-brush of 4 – 6″ along the western slope of the sierra. Orographic enhancement will be strongest along southwest facing slopes in this event.


Snow levels & totals

Snow levels will begin up around 7000 – 8000ft through Saturday afternoon, lowering a bit to near 6000ft by later Saturday night. Sunday and Monday’s system will have much more significant impacts on the sierra, as it’ll be much cooler – with snow levels during the brunt of the precipitation hoovering around 4000ft. Through late Monday, a widespread 2 – 3 feet of snow is likely above 6000 feet, with localized amounts near 4 feet possible up above 7000 – 8000 feet, boosting the sierra snow pack significantly.

This won’t be the end of the stormy weather. Models suggest a brief period of ridging Tuesday – Wednesday or Thursday, with a return of precipitation (possibly heavy once again) either Thursday or Friday into the weekend. Models are sketchy with the timing (hence the “Thursday or Friday” timeframe) and strength, but there is agreement of something toward the latter half of next week. Stay tuned for future updates on what’ll make at least March’s first quarter fairly active.

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