Very Powerful Storm to Slam California Through Friday

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Thursday afternoon update, as of 1:45pm

It’s been a wet, windy day across northern California, as a powerful mid-latitude/baroclinic cyclone rapidly developed off the California/Oregon border, intensifying an already strong cold front and associated precipitation band across the region. Winds have weakened, and didn’t quite get as strong as expected – but general gusts in the 40 to 50mph range in the valley have caused enough problems, with a few 60+mph gusts in the north valley and even more in the higher terrain and coast. Pressure gradient is relaxing as the surface low heads northeast up the Oregon/Washington coast and gradually weakens… however, this does not mean precipitation is (close to being) done. In fact, quite the contrary. Precipitation will slowly diminish along the north coast and northern mountains… but from the north valley southward, precipitation is generally expected to continue at least through early to mid-evening. Heavy precipitation will remain possible as long as precipitation continues given the strong upper-level support and long fetch of subtropical moisture still feeding the front.

An interesting feature offshore that models don’t seem to be picking up very well is a rapidly cooling band of clouds that models don’t seem to be handling all that well. While the southern end of this will head into southern California tonight and Friday, the northern half will slide into northern California this evening, at least keeping precipitation going through then, but perhaps increasing intensity depending how strong it ends up getting. Given precipitation still has upwards of 8 – 12 hours of continuation ahead, especially from the I-80 corridor south, the flood threat will be increasing as rain continues. Rock/mudslides and burn scar debris flow potential will also increase through the evening, and even some time after precipitation wanes.

Colder/unstable air moves overhead Friday, with showers continuing, and possibly some thunderstorms if breaks in the clouds allow some surface heating to occur. Little upper-level wind directly under the trough will mean slow moving storms, and bringing localized flood potential, especially if training of convection occurs.

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IR satellite imagery from 1:00pm Thursday. Notice the rapidly cooling cloud tops offshore ahead of the surface front – where convection appears to be developing rapidly given cloud tops are cooling at rates upwards of 10c per hour.


Quick late-evening update, as of 11:20pm Wednesday

As expected, rapid/explosive cyclogenesis is occurring offshore. Minimum surface pressure analyzed around 983mb at the present time via mesoscale analysis. Could be a tad lower. Models still showing some issues as to how deep it gets through the morning – but currently believe 972 – 974mb isn’t out of the question, though, 0z GFS had higher pressure up around 977 millibars or so. A trailing cold front can be noted on satellite imagery to the south of the low, which will strengthen through the night and should become more in-tact with the low itself by morning, increasing dynamics and frontal band strength significantly. The front is still likely to stall somewhere as the offshore trough deepens and stretches out the front, but where exactly and for how long is still in question, but will be monitored through the day to see where/if/when it does.

IR satellite imagery via GOES-west from about 11pm - with clear developing cyclone off the California/Oregon coast.

IR satellite imagery via GOES-west from about 11pm – with clearly visible developing cyclone off the California/Oregon coast.


 

Wednesday afternoon update

A powerful, once in multi-year storm is in its beginning stages about 800 miles offshore of the northern California coast this Wednesday, and will slam into the coast tonight through Thursday as one of the highest-impact events since the 2008/2009 storm season.

December-8th-M2

Original Wednesday afternoon post

Offshore lies a raging westerly upper-level jet in excess of 150kts, transporting very moist air across the Pacific, most of which originating from the Hawaii region, while some was entrained well back in the western Pacific, from ex-typhoon Hagupit. Along the strong jet, strong temperature contrasts and a vast amount of moisture to work with is allowing the beginning stages of what will be rapid cyclogenesis to occur along the fierce belt of strong upper-level winds. This low will rapidly strengthen tonight as it makes a run for the California/Oregon coast, potentially even “bombing” out – as forecast surface pressure indicates a potential starting minimum pressure around 995mb, dropping to what some models indicate sub-975mb by early Thursday. For those who have no idea what I mean by “bomb-out”, a meteorological “bomb” is when a surface low drops 1mb per hour for 24 hours (i.e. rapid strengthening). While this low wouldn’t drop 24mb over that long time period, it could drop 20 – 25mb in under 24 hours.

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This strengthening may not only be supported by the very strong meteorological dynamics at play – but as well as one potentially key/interesting oceanic factor: the anomalously warm sea surface temperatures (SSTs) off the western U.S. coastline – especially along the California coast, where SSTs are generally depicted in the 55 – 62 degree range (cooler further north, of course). These SSTs are upwards of 2 to 4c warmer than normal, with average SSTs usually in the low 50s. These few to several extra degrees of warmth will provide low-level fuel for the cyclone to tap into… warmer than most have to work with when compared to many, many years in the past. For instance, the National Weather Service in San Francisco/Monterey mentioned looking back as far as the 1980s; SSTs so anomalous couldn’t be traced back that far.

Analysis of sea surface temperature anomalies.

This powerful surface low will track toward the northern California/Oregon coast tonight, then curve north and ride up the Oregon/Washington coast through the day Thursday. As this deep low slides past the CA/OR border through Thursday morning, the very tight pressure gradient between the deep surface low and a surface high in the four corners region – with the tightest gradient directly over northern California. 900 – 850mb winds ahead of the front in excess of 70 knots will, when combined with heavy frontal precipitation, likely mix down surface gusts in the 50 – 60mph range on a widespread scale across the northern California Thursday morning – with the strongest winds expected from Stockton northward. Gusts upwards of 70 – 75mph cannot be ruled out from about Sacramento northward, with the best potential of 70mph+ gusts from Chico north to Redding. Winds of this magnitude/strength are easily enough to take out trees and tree branches, and when combined with heavy rainfall atop relatively moist soil, the potential for downed trees increases even further as soil around root systems becomes increasingly softened/loose. Timing for this strong/damaging winds is generally expected between 5am and 2pm, weakening rapidly from north to south behind the front as mid/low-level winds cuts off and strong mixing from heavy frontal precipitation wanes.

Local-Wind-Gust-Map-fixed

Frontal precipitation, which will be heavy to say the least, will begin along the northern coast through the afternoon, and along much of the coast north of the Bay Area overnight. The cold front’s intense precipitation band will spread deeper inland early Thursday morning, into the north/central valley by early to mid-morning, and roll into the Sacramento/Bay Area during the late morning and early to mid-afternoon. Pre-frontal rain should spread well inland overnight, though, ahead of the main cold fronts heavier band, and should generally remain moderate in nature, with the heavier rain holding off until Thursday morning through evening.

Precipitable water values up to around 1.4″ inland combined with intense large-scale lift and strong upper-level divergence/dynamics support the idea of extremely heavy rainfall across much of northern California along/ahead of the cold front – with very heavy snowfall above 7000ft or so in the high sierra until snow levels drop off Thursday evening. The strong dynamics at play will likely support the development of what’s known as a NCFR (narrow cold front rain band) along or near the leading edge of the cold front’s precipitation band. This squall-like feature may or may not be powerful enough to support lightning, but extremely heavy rainfall with this feature will not only lead to rapidly increasing flood potential, but will also likely drag/mix down some of the strongest winds of this event. This intense band will, along with the main precipitation band, slide through northern California during the morning in the north, and afternoon/evening in the south.

Precipitation may cut off along the northern coast by late afternoon to early evening as the trough digs southward – however this may slow or stall the frontal precipitation band somewhere in northern California, keeping moderate to heavy precipitation going through the evening, and possibly into the overnight hours, continually increasing flood potential along creeks and streams, as well as urban flooding due to poor drainage, and likely continuing/increasing the mud/rockslide threat in the foothills/mountains, as well as debris flows near previous summers’ wildfire burn scars.

Storm-Threats

Precipitation should become more showery late Thursday night/early Friday, as the cold pool begins to move aloft behind the front. Snow levels are forecast fall off to around 5000 – 5500ft by early Friday morning, with heavy snow continuing as moist/upslope flow continues. Colder air aloft combined with a large amount of low-level moisture and probably at least some scattered sunshine Friday will support some modest instability – and given the limited upper-level flow due to the base of the trough being located directly overhead, any showers and thunderstorms that develop Friday will be relatively slow movers. The lack of decent mid/upper-level winds will preclude a severe threat, but couldn’t rule out some hail if any storms do develop, aside from heavy rainfall of course atop already soaked ground… which would boost localized flooding issues once more.

Rainfall amounts, while in general they should be uniformly heavy, will be somewhat tricky with this event given the potential for the front to stall. At the moment, there is generally good agreement the front will stall out between Sacramento and Redding, dumping much heavier totals over this swath of the state over the several extra hours it sits overhead. A blend of solutions would generate the following: 2 – 5″ across much of the Sacramento valley, with 1 – 3″ in the northern San Joaquin valley. 3 – 6″ across much of the foothills and west slope of the sierra, with upwards of 7 – 9″ in orographically favored slopes, especially in Butte/Plumas and Shasta counties, with 2 – 5″ along much of the coast… with 4 – 8″ in favorable terrain/hillsides along the coast.  Locally higher amounts are possibly anywhere – especially if the front does indeed stall.

Personal rainfall forecast updated as of 5:30pm. In general, higher amounts were implemented along the coast, parts of the foothills, and other orographically favored regions.

Personal rainfall forecast updated as of 5:30pm. In general, higher amounts were implemented along the coast, parts of the foothills, and other orographically favored regions.

Snowfall amounts could be in excess of 2 – 3ft over 7000 – 8000ft/along sierra ridges… with several inches down to 5000ft possible. Extremely heavy snowfall combined with very strong winds Thursday afternoon into the evening as snow levels begin to fall will lead to hazardous high-sierra travel. Snow levels may be above most passes during the afternoon… but by evening passes may start seeing snow, which would become quite heavy as snow levels continue to lower.

Saturday should see conditions dry out, but a weak system with some more precipitation is expected Monday, with a potentially unsettled pattern continuing into the rest of the week… but at the moment no real significant storms compared to this system are expected. I’ll probably post an update over the weekend looking over the systems expected next week.

Note: I’ll probably update some parts of this post this evening, including rainfall/wind forecast graphics.

Showing 2 comments
  • Jerry

    I would,appreciate if you can add date on your blog. I checked your blog once in a while and can never be certain which storm you are talking about. Is this the one just passed or,the one that is coming?

    Thanks
    Jerry

    • wxtracker15

      Hi Jerry,

      On the homepage, under the title of each post is a date the post was initially uploaded. I do update some specific posts instead of posting a brand new one, so sometimes the post may be newer than the date that shows up.

      For this current storm (impacting Feb 6th – 10th), take a peak at this post:
      http://blog.norcalweather.net/stormy-weekend-on-the-way/

      Thanks for visiting!

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