[Updated] Strong Storms Possible Friday

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As expected, the Thursday system allocated some isolated thunderstorms across the lower elevations of northern & central California with some modest lift and weak instability to boot, however shear wasn’t too shabby and some storms tried to rotate out near the Davis area early this afternoon before weakening and consolidating into a cluster of heavy showers as they moved east. Anyhow, storm number two is about to pound the region overnight into Friday morning with another bout of moderate to locally heavy rain just in time for the early morning commute. The front will be relatively quick moving though, as a powerful 180 knot upper-level jet streak ferociously roars under the deep cyclone off the Pacific Northwest coast nudging things along fast.

NAM’s forecast 300mb winds & heights, valid at 4pm Friday, depicting a very strong jet streak nosing into California that extends deep into the eastern Pacific.

This will allow the cold front to be out of northern California and into central & southern California by mid to late-morning (north to south, respectively). Behind the front will be a pretty active cold pool environment with plenty of bands of convection moving ashore, being supported by the aforementioned low. 850mb temperatures under 1°C will sweep overhead through the day behind the front, overlapping surface temperatures in the lower elevations such as the valley and coastal areas in the upper 40s if cloud cover is widespread, to mid-50s if cloud cover becomes broken and sun is able to shine. If the latter occurs, which some convection-allowing, high resolution models suggest, we’ll very much so have enough instability to support some low-topped strong storms by California’s standards.

By mid-afternoon, surface winds should be jetting up the Sacramento and San Joaquin valleys out of the south, and could be even further backed out of the south-southeast in some central and eastern Sacramento valley areas. Meanwhile up around 500mb the flow will be out of the west-southwest, creating a nearly 90-degree angle of wind direction turning with height. With that said, hodographs exhibit excellent curvature in in the low to mid-levels, creating the classic small hooking signature that accompanies most of northern California’s severe events. To folks who know a bit about thunderstorms, we’re talking shear — and it’s looking excellent directional shear wise. Speed shear isn’t all that strong due to the jet streak being offshore and directed into southern California, but but southern California gains in shear we (northern California) gains in instability via a broken cloud cover environment vs. the south’s widespread frontal cloudiness (although they’ll still have a decent shot at strong storms).

4km NAM’s forecast sounding for central Sacramento county, valid at 4pm Friday.

Surface convergence will be notable along the west side of the valley as westerly winds jetting inland from the coast will likely mark the start point for most of the storms Friday afternoon. Some subtle mid-level convergence down the median of the valley could help storms that develop further west recharge as they run into additional lift deeper in the valley, additionally, as the westerly flow aloft scoots storms eastward at a relatively slow pace. Now, given we have all our bases covered for the most part (instability, shear, and lift): how legitimate is this setup?

Not too shabby; it isn’t perfect given the front won’t be super sharp with widespread clearing directly behind it, potentially inhibiting stronger and more widespread surface heating — but that will be the only major factor limiting the setup. Models do suggest some clearing, and in places where this occurs for a few hours some strong to severe thunderstorms will be possible, and I personally would say the southern and central Sacramento valley look to have the best shot. The San Joaquin valley could end up seeing some strong to severe storms too, depending on initiation.Enough shear will exist for stronger storms (if they develop) to rotate and exhibit supercellular characteristics Friday afternoon and early evening, meaning there will be a shot at all severe thunderstorm hazards from large hail to a tornado or two.


Initial update, written Tuesday evening:

California has experienced a brief break in storminess over the holiday weekend into the beginning of this week, however from here on out we’ll remain in a very active & stormy pattern. Off the western U.S. coast lies a very strong westerly jet extending across the Pacific, encompassing two primary troughs/developing upper-lows. Each of these upper-lows along the jet is being enhanced by mid and upper-level jet streaks in excess of 150 knots in the mid-levels, and >180 knots in the upper-levels. These fierce jets will round out each trough, strengthening them on a dynamic scale, meanwhile transporting some high-moisture vapor plumes from the subtropics, stretching them out for thousands of miles across the Pacific well past Hawaii. Colder air also exists with the storms down the line along the jet, meaning each following storm will likely be colder than the previous, making for a very snowy week ahead for the mountains in addition to a very wet week for the lower elevations. Additionally, each of these storms will be driven by deep surface lows and strong pressure gradients fueling strong low-level jets with and ahead of cold fronts, promoting strong winds at the surface with each storm.

The first storm along the nose of this moisture plume and fierce jet will slam the west through the day Wednesday into Thursday morning with widespread precipitation and strong winds. Appears a vort-max will be the driving force behind strong convergence and lift along the front, all of which parented by the primary low in the Gulf of Alaska. The vort-max of this system will crash directly into the northern California coast Wednesday evening right behind the cold front with the heaviest precipitation rates and strong winds. Well ahead of the vort-max, wide-spanning precipitation shield will begin to move overhead of coastal areas after 5am north of Fort Bragg, and into the bay area by after around 8 – 9am.  Inland, the valley and sierra will begin to see precipitation fill in after 10am to noon from north to south. It appears the precipitation shield associated with the developing vort-max will stall out for a several hour period across essentially all of northern California with continuing moderate to locally heavy precipitation through the afternoon hours. The cause to the stall will be the vort-max itself strengthening offshore and lifting northeast, anchoring the cold front and it’s associated precipitation band in the same area for a while until the whole system starts to move eastward again.

GFS’s forecast 500mb winds (fill) and heights (contour), depicting the first in a series of troughs rolling into the west. Image is valid at 4pm Wednesday, when precipitation will be widespread and ongoing.

Snow levels with this storm will start out around 5000 – 6000ft given the initial firehose of subtropical moisture along the moisture plume helping fuel such a large precipitation shield, but begin to lower Wednesday afternoon and evening as the moisture plume sags south and colder air cuts into the precipitation band. By later in the day, snow levels will hang out around 3000 – 4000ft along the western slope of the sierra, and as low as 2000ft in the northern mountains. Precipitation turns showery and primarily in the higher elevations early Thursday morning behind the cold front, however the cold air sliding overhead combined with remnant lift from the overhead trough will equate to the chance of convective showers across the region.

Another vort-max develops in the cold, post-frontal environment and begins to move ashore Thursday morning, bringing inland another wave of widespread showers. Given this wave will be in the classic cold pool environment, I expect essentially all of this precipitation to be convective in nature, with some thunderstorms also looking to be a good bet — especially if there’s a bit of sunshine for surface heating inland ahead of it Wednesday morning. Currently looks like the severe threat in northern California is near non-existent except along the coast where I wouldn’t rule out a waterspout or two with rotating storms nearing shore. After this second, weaker vort-max exists to the east we’ll get some shortwave ridging to help neutralize precipitation for the most part Thursday evening.

This weak ridging rises into our area ahead of the next storm strengthening offshore, featuring a deep sub-980mb mid-latitude cyclone about 600 – 700 miles off the Pacific Northwest coast. This system will be riding along and strengthened by the fierce 180-knot upper-level jet roaring in from the eastern Pacific, making up for the fact this system won’t have quite a solid moisture tap as the first storm. A strong vort-max at the surface will once again ride along with the cold front of this second storm, and attempt to close off into a surface low of its own right off the northern California coast once again, similar to the prior storm. Precipitation will begin to fill into northern California beginning with the coast after around 10pm Thursday, and inland after 1 – 3am. Snow levels with this storm will start out as low as 3000ft (locally down to 2000ft, mainly in the northern foothills) along the western slope of the sierra, and remain around this level through the duration of the main period of frontal precipitation into early Friday afternoon. In the northern mountains and western coastal mountains, snow levels will likely hover around 1000 – 2000ft — but could drop into the Redding area later in the afternoon Friday or Friday evening if showers continue.

Once again: the GFS’s forecast 500mb winds and heights, valid this time at 7am Friday as the second, colder upper-low drives its cold front and associated precipitation shield through the region. The offshore and very strong mid-level jet streak (with winds in excess of 130 knots at this level) is what aided in the strengthening of this system as it was well offshore.

The cold front to the late Thursday/Friday system will clear northern California by late morning, with the cold pool quickly taking over to keep us showery during the afternoon and early evening Friday. Something models don’t suggest a whole lot of currently is instability Friday afternoon, and I’m wary of that. The reason to the low modeled instability is cold surface temperatures due to cloud cover. If there is to be some widespread clearing between the onset of post-frontal showers moving ashore and the back edge of the cold front, this system would create a wonderful setup for strong thunderstorm potential. For now given the widespread clouds forecast it looks like a very low chance of robust storms, but your run-of-the-mill pulse storms could drop some hail and generate some lightning before they weaken due to lack of decent instability to thrive on.

While the second system exists the region Saturday allowing us to dry out for briefly during the day, the next system arrives off the west coast and strengthens quickly. There remains some model differences with this system regarding potential for a secondary surface low development off the California coast along the cold front, but in-general this system will pack another punch of moderate to heavy precipitation just in time for Sunday. This third system will be driven by that same deep cyclone off the Pacific Northwest coast, however it strengthens quite a bit more between Friday and Sunday as it absorbs a younger, still maturing wave cyclone riding the strong jet westward. After it absorbs the new low, it sends westward the new low’s moisture plume it worked hard for in the north-central Pacific north of Hawaii. I’d like to point out that those model differences do play a pretty big role in the timing and overall strength of this system — so I’ll post another update late this week before the third and final storm in this barrage greets us Sunday to give a more descriptive overview of it.

Each storm this week is slated to drop a half inch to a bit over an inch of rain in the lower elevations, with one to three inches of liquid QPF in the higher terrain and along parts of the coast. Total rainfall amounts through the week of anywhere from 2 – 4 inches in the valley, delta, and bay area, with 3 – 6″ along other favored coastal regions and the lower foothills and mountains inland. Favored slopes as well as the northern coastal mountains and central coast could see 5 – 10″ of liquid precipitation through the rest of the week, with locally higher amounts possible. This liquid totals mean feet of snow with each storm in the mountains, and by the time we’re through Sunday, parts of the western slope could have a fresh 6 – 8 feet of snow on top of an already healthy layer of snow, with similar totals in the higher terrain of the northern mountains. 1 – 3 feet of snow falling in a 24 hour timeframe with each of these storms in combination with strong winds (more on this below) will mean severe travel impacts in the mountains including all major highways and interstates from I-5 in the north, to I-80 and Highway-50 in north/north-central California. Periodic road closures are almost certain, with potential for long-term closures during periods with the most extreme snowfall rates and strong winds.

All three of these storms will slam northern California with not only heavy precipitation, but strong to locally damaging winds given strong surface fronts and tight pressure gradients due to deep surface lows driving each of the storms. Peak winds with storm one will occur Wednesday afternoon and evening, with gusts upwards of 45 – 55mph likely in the lower elevations — however some coastal areas could see gusts up to or locally in excess of 60mph depending on exact low placement and strength of vort-max. The second storm sliding through Friday will bring peak winds into the region during the morning, with similar gusts in comparison to the Wednesday storm. Sunday’s storm is currently modeled to have the strongest winds out of the parade, with potential for damaging winds across the region. Current guidence suggests peak gusts anywhere from 50 – 60mph on a widespread scale, with some places along the coast seeing a shot at gusts well in excess of 60mph — potentially hurricane force if the ECMWF is correct with it’s placement of a deep cyclone off the Oregon coast. Each of these storms and their strong wind in combination with heavy precipitation will not only pull down trees, cause power outages, and cause other damage related to falling trees — but once again cause periods of blizzard conditions in the mountains.

Several more inches of rain on top of an already saturated soil will lead to quick runoff and spikes in streams and creeks, bringing with these spikes in levels the threat of flooding. Rivers will once again swell, but at this moment I don’t expect as many rivers to surpass flood stage. The Sacramento river in the northern Sacramento valley will flood in several locations, however, and should be monitored by anyone living near it on low land. Additionally, anyone living near creeks and streams should be aware of flood potential be monitor levels during and after periods of heavy precipitation.

I’ll have more updates this week as each storm shows us their act and we learn more about each of the following systems. Stay tuned, stay dry, and once again: stay safe.

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