Significant Pattern Change Looming; Major Fall Storms Targeting Northern California

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What do a powerful typhoon (by name of Songda) and the west coast have in common? The answer is preparing to begin blasting into the west coast: a powerful westerly jet that extends across the entire northern Pacific, connecting the western Pacific tropics, Japan, and China to the west coast of the United States. This potent belt of upper-level winds is racing across the Pacific in excess of 170 knots in the strongest part of the westerlies (making this belt of the strongest winds a jet streak). These strong westerlies, transporting moisture from previously super typhoon Songda and other nearby tropical convective elements, will wrap around a large upper-level low situated in the Gulf of Alaska, and become a extremely conducive region for rapid and potentially explosive cyclogenesis. The amount of warm, tropical moisture traveling along the westerlies combined with cold, unstable air circulating around the upper-low in the Gulf of Alaska will provide the available thermal gradient and moisture availability for hurricane-force cyclones to explode in the eastern Pacific off the west coast.

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GFS’s forecast 300mb wind and heights, valid Friday afternoon as the first system begins to exit the region, while a new, very strong jet streak ramps up in the west-central Pacific. This powerful new jet streak will fuel the next system, all the while the west coast of the United States and Asia are essentially connected via a high-speed meteorological rail line.

This pattern is something we’d more or less expect to see during the heart of winter — but it can indeed happen this early, for example back in mid-October 2009 the remnants of typhoon Melor pounded the west, especially northern California, with exceptional rainfall and powerful winds. In this scenario, we’ll be dealing with 3 strong to medium-strength storms. The first of which will target the west Thursday & Friday — here in California, mainly late Thursday throughout Friday. This first system will develop from wave (~998mb) to mature cyclone (~976mb) within a period of 24 hours, equating to explosive cyclogenesis, or simply put a (meteorological) “bomb”. While the low itself will drive itself into the Pacific Northwest, it’s powerful cold front combined with the potent moisture plume & very strong westerly jet will send a band of moderate to heavy frontal precipitation throughout the region.

Eastern Pacific

The first storm’s frontal precipitation band will begin to enter the northwest part of the state Thursday afternoon, making it into the northern Sacramento valley and northern foothills around midnight to 3am. The central and southern Sacramento valley will begin to see precipitation from the front by 3 – 5am, continuing through 8 – 10am. While the precipitation band will thin out as it drops further and further south (likely to dissipate by or before it reaches Fresno), it’ll remain fairly robust down until it reaches the northern San Joaquin valley — with upwards of a quarter inch an hour rainfall rates in the Sacramento region, with half inch rates possible in the central and northern end of the valley from the center of the valley eastward into the foothills. These rainfall rates on a widespread scale will extract all the excess oil on roadways and make for interesting travel during the Friday morning commute.

Behind the front during Friday afternoon, precipitation becomes showery in the moist post-frontal environment as the large upper-low in the Gulf of Alaska directs clusters of energy and convective cloud cover into the region. At this point, it’s hard to say if there will be a threat of widespread thunderstorms Friday afternoon due to the question of cloud cover. Sufficient instability is progged, but stronger low-level buoyancy via direct sunshine and surface heating would be required to get strong enough updrafts for organized thunderstorms to develop. At the very least, I’d expect a few isolated or scattered storms to develop/impact the coast through the day Friday and into the evening hours ahead of the next system rapidly taking shape offshore.

California's Setup

The second storm will develop from open-wave to hurricane force, mature cyclone within a matter of <24 hours. This second cyclone will slam into the Pacific Northwest coast once again late Saturday into Sunday, and develop from wave to its strongest point between Friday afternoon 800 – 1000 miles off the west coast, to Saturday afternoon just off the British Columbia/Washington coast. The GFS (the United States’ global forecast system/model) suggests a 33 millibar drop in pressure within this 24 hour time frame, which overshoots the threshold (a 24mb drop over the course of 24 hours) of a meteorological “bomb” by 9mb. This powerful cyclone will be backed and aided by one of the most powerful westerly jet streaks I’ve seen forecast in some time; a nearly 200-knot streak extending for over a thousand miles, from the west coast into the central Pacific. The powerful cyclone’s wind field will deliver storm-force winds to the west coast from northern California to British Columbia’s coast, while locally in interior northern California, gusts up to 50mph are possible in the northern Sacramento valley. Further south, the pressure gradient doesn’t hold up quite as much, but some gusts up to 35 to perhaps 40mph cannot be ruled out as far south as Sacramento. Additionally, there’s always potential for such a strong cyclone’s track to shift as models try their best to remain stable handling such meteorological extremes.

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GFS’s forecast 850mb wind, surface pressure (blue contours), and 300mb heights (magenta contours), valid Saturday morning depicting a hurricane-force low off the Washington coast with a surface pressure around 955mb.

Precipitation from the second storm will be fairly similar in strength compared to the first system, with moderate to locally heavy rain (especially in the higher terrain) beginning Saturday afternoon in the north and evening in the I-80/Highway-50 corridor and Bay Area. A moist onshore cyclonic flow will continue behind the front through the night into Sunday to keep showers scattered to widespread at times as pieces of energy rotate in along the fierce westerly jet continuing to firehose into the west. Wouldn’t rule out some isolated thunderstorms Sunday afternoon in the cool/unstable onshore flow supported by those fierce upper-level winds. Next system doesn’t reach maturity but will be a potent shortwave, and it is expected to slide through Sunday night into early Monday, increasing precipitation intensity and coverage across the region again.

Precipitation

Precipitation amounts through Monday will be very impressive for mid-October, and would be impressive through the rest of winter in fact. Rainfall totals in the lower elevations will range from 1 – 2.5″ along the coast from Point Arena south, with 3 – 6″ north of Point Arena into southern Oregon. Inland coastal mountains could see anywhere from 5 – 8″ north of Point Arena, while favorable west/southwest slopes throughout northwest California and southern Oregon could receive a foot to 16 inches of rainfall when trying to factor in orographic lift.

Inland and in the lower elevations, the Bay area into the southern Sacramento valley looks set to pick up 1 – 2″ of rain, with the central and northern Sacramento valley likely to receive anywhere from 2 – 4″. In the mountains, the western slope of the sierra and foothills will easily see a widespread 3 – 6″, with upwards of 10 – 12″ in higher elevations of Butte & Plumas counties, and 8 – 12″ in north-central Shasta county. Snow levels will stay relatively high through Saturday, but the final wave Sunday will be cool enough to perhaps dump a foot or so of snow above 6000ft, with around 2 feet possible in the central high sierra.

Strong ridging should build in after Monday, drying us out for what could be a while once again.

Showing 2 comments
  • Ameya

    Do you think there might be some NCFRs that form with these systems?

    • wxtracker15

      I wouldn’t be surprised if the late Saturday and Sunday night/Monday morning storm roll in with some NCFRs. The late Saturday storm would be most capable given the deep cyclone and extreme speed shear available coupled with the exceptional column moisture.

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