Stormy Weekend on the Way: Update 5

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Sunday’s Storm Update

Posted 8:45pm Saturday evening

It’s been a pretty active past 48 hours across northern California as a potent jet stream has sagged south, directing multiple systems toward/into northern California, along with a very moist atmospheric river originating from the central/eastern Pacific east of Hawaii. This final system – the strongest of the three, is shaping up to be a fairly powerful mid-latitude cyclone, bottoming out down to as low as 984 – 986mb off the California coast Sunday afternoon & evening. As this low rapidly strengthens offshore, it’ll once again draw northward the very moist streak of moisture continuing to surge northeastward toward California in the warm sector of the system Sunday morning and afternoon, ahead of the cold front. A brief period of clearing may occur ahead of the storm’s cloud shield Sunday morning, allowing some surface temperature warming. Clouds are likely to quickly overspread northern California by late morning and afternoon… with frontal precipitation not too far behind.

Given the strong surface cyclone offshore Sunday, a strong southerly low-level flow will develop, transporting warm, moist surface air up the valley, which on top of perhaps a brief period of sun in the morning should allow surface temperatures to climb into the low to mid 60s across much of the lower elevations, from the inland coastal valleys to the central/Sacramento valley. This surface heat beneath increasingly cool mid/upper-level temps should boost mid-level lapse rates to around 6.5 – 7.5c (moderately steep), as well as when combined with relatively high surface moisture (dewpoints in the upper 50s to low 60s) should promote surface-based CAPEs in the 200 – 500 j/kg range on a widespread scale through the afternoon. This instability combined with strong lift and wind energy from the surface upward should promote potential for at least isolated thunderstorms embedded or ahead of the cold front during the morning & early afternoon. Behind the front, which may pass by early to mid-afternoon, a more showery/cloud-broken airmass slides overhead… potentially allowing surface temperatures to climb a few extra degrees locally and boost dewpoints further (locally, once again), and support increased CAPE values on a scattered scale from 500 – 1,000 j/kg, mainly in the Sacramento valley, delta, and coast.

0z 4km NAM's forecast ML (mixed-layer) CAPE valid at 5pm. Light green hues are values in excess of 750 j/kg, and darker greens are 1,000 j/kg+.

0z 4km NAM’s forecast ML (mixed-layer) CAPE valid at 5pm. Light green hues are values in excess of 750 j/kg, and darker greens are 1,000 j/kg+.

Fairly strong south to southeasterly low-level to surface flow combined with a WSW to SW mid/upper-level flow create decent directional shear… not quite a 90-degree angle in difference of winds with height, but not a too far off. Thus, when you add in the strength of these winds from the low to mid-levels, shear in general is quite strong, and should be supportive of rotating storms through the afternoon and into the early to mid-evening before instability wanes. Behind the front, from mid to late-afternoon onward into the evening, is when the threat for the strongest thunderstorms is expected. Given the available shear and potentially decent instability as currently modeled, some thunderstorms could strengthen and take up supercellular structures, rotate, and become capable of producing tornadoes and large hail. It’s also possible that bands of thunderstorms could form, which would be more capable of strong/damaging straight-line wind, as well as hail.

Winds even without thunderstorms will be pretty strong from Sunday morning through the evening as the potent surface low slides northward up the coast toward the Pacific Northwest, with 900mb winds in excess of 45 knots across a good chunk of the region… with locally some 50  – 55kt values being indicated by the 4km NAM. At this level, not every thing makes it to the surface… but a strong thunderstorm could quite easily drag down equivalent speeds to possibly locally higher speeds.

0z 4km NAM's forecast 850mb winds, valid at 4pm Sunday. Note: these winds are several thousand feet above the surface, thus, winds of this strength likely wouldn't directly make it to the surface without the aid of a strong thunderstorm.

0z 4km NAM’s forecast 850mb winds, valid at 4pm Sunday. Note: these winds are several thousand feet above the surface, thus, winds of this strength likely wouldn’t directly make it to the surface without the aid of a strong thunderstorm.

I’ll likely post another update Sunday morning or afternoon as the event begins to unfold with an update on thunderstorm potential.

Previous updates are below, with still relative information to this event.

Thunderstorm potential update, posted at 9pm Friday evening

An active weekend is on the way… or I suppose already underway given much of northern California has already experienced at least some rain, wind, and snow – in some cases a lot of each. Offshore lies a parade of shortwaves and developing surface lows – the first of which about 300 – 400 miles off the Oregon/northern California coast as of early Friday evening. This system will continue driving a plume of quite deep subtropical moisture into northern California through night tonight and on into Saturday morning, before much of the moisture and lift exists to the south. The next (second) wave is between California and Hawaii, and should follow this first wave into northern California later Saturday with another wave of showers… but nothing too widespread or heavy like we’ve seen much of today (Friday). The third, and perhaps most potent single system, is still out in the mid-eastern Pacific, and will be off the California coast later Saturday, and begin driving in a fairly powerful cold front supported by a potent jet streak in excess of 100 knots later Sunday into Monday.

Saturday, during most of the morning and afternoon, we’ll be between systems… perhaps allowing some breaks in the clouds to occur and warm up surface temperatures in the lower elevations, while a good amount of the higher elevations may remain socked in with cloud cover as upslope flow continues to wring out showers. Slighty cooler air aloft behind the first system combined with likely warming valley temperatures and a fair amount of moisture in the low/mid-levels should promote at least some modest instability through the morning and afternoon. Lift may be a bit sparse though until later in the way when the second shortwave arrives later in the afternoon, into the evening, which will likely end up being the best timeframe for any thunderstorms. Strong westerly upper-level flow combined with veering surface winds (though, they’ll be much weaker than Friday & Sunday) with the shortwave will promote some decent supportive shear for any storms, but as the shortwave comes inland cloud cover may increase… potentially limiting instability a bit, and keeping lapse rates fairly poor. However, at least slight instability in the form of a few hundred J/KG CAPE to work with and the strong mid & upper-level flow to promote the potential for a few thunderstorms over the course of the afternoon and evening.

Thunderstorm potential map for Saturday.

Thunderstorm potential map for Saturday.

Sunday could perhaps be the most interesting day, as a powerful sub-990mb surface low slides northeast off the California coast. Ahead of the low’s potent precipitation shield, some cloud breaks may occur Sunday morning as warm air advection occurs ahead of the cold front… which combined with cooling mid/upper-level temperatures directly following the warm air advection ahead of the front could lead to fairly substantial instability when it comes to California’s standards. For instance, the 4km NAM and it’s 12km counterpart both indicate SBCAPE values in excess of 700 – 800 j/kg across a fairly large chunk of inland northern California – with potential for 1000 – 1500 j/kg values in the valley and delta. A more conservative analysis of forecast SBCAPE generates 300 – 500 j/kg, and this analysis comes from the SREF’s mean… and usually these values increase as the event nears. In any case, both of these solutions, bullish and conservative, either one supports the idea of sufficient instability for thunderstorms to develop and sustain themselves. Strong flow across all levels, including a 50kt+ southerly 850mb jet, beneath a SW to WSW 500 – 300mb flow creates a moderately strong directional shear setup on top of strong speed shear.

0z 4km NAM's forecast (surface-based) CAPE values, valid at about 1pm Sunday.

0z 4km NAM’s forecast (surface-based) CAPE values, valid at about 1pm Sunday.

As the cold front begins to push inland Sunday afternoon into the evening, believe the increasing lift on the southeast side of the low will favor potential for pre-frontal convective development, as well as convection directly along the front where convergence is maxed. Pre-frontal thunderstorms, if any develop, would have the best potential to rotate and potentially become tornado-producing in this environment, as well as the best potential overall to become severe. Along the front, a line of thunderstorms or embedded thunderstorms could develop, and sweep through a good chunk of interior northern California. Storms along the front would have a lower chances of producing any tornadoes, certainly not a zero chance, though, but typically bands of storms like this are more known for strong winds and flooding rainfall rates. The thunderstorm threat Sunday appears that it’ll be maximized between around 2pm and 9pm, when the pre-frontal zone begins entering (~2pm) and the front exists much of the region (~9pm). As a side note, the Storm Prediction Center (SPC) of the National Weather Service/NOAA has also expressed interest in Sunday across northern California thunderstorm wise – and may even introduce severe probabilities in later outlooks.

Thunderstorm potential graphic for Sunday.

Thunderstorm potential graphic for Sunday.

Colder upper-level air moves overhead Monday, with another round of isolated to scattered thunderstorms possible. The flow Monday begins to flip more northwesterly, which typically isn’t very favorable for severe storms… but will be monitored as we get closer to the day.

I’ll bring more updates Saturday & Sunday regarding thunderstorm potential this weekend with an emphasis on Sunday it appears.

Previous post, from Wednesday:

A fairly significant precipitation event is still on the to northern California, and confidence has increased in that double-digit precipitation totals in some places are likely through early next week as a series of storms travels along a belt of very moist air. This belt of very moist air, also known as an atmospheric river, will drive precipitation amounts up significantly compared to what we may normally see from these systems without the significant plume of moisture.

Feb-4th-R

The first in the series of systems traveling along a raging west to east jet will traverse into the Pacific Northwest & British Columbia late Thursday, spreading precipitation into northwestern California through the afternoon, and into the rest of northern California early Friday. The belt of moisture ahead of the system’s cold front will remain aimed at northern California through the day Friday, in fact increasing in intensity through the day into the evening as a another system moves upstream, strengthens, and slams into the west coast late Friday night into early Saturday, intensifying lift and increasing precipitation intensity across the region.

Lift weakens as this system moves inland and the moisture plume sags southward into central and even southern California through the day Saturday; meanwhile, cooler air behind the front, still packed with decent moisture, swings inland during the day Saturday. This cooler airmass will lead to a more unstable environment across northern California during the day Saturday… thus, some convection appears possible. All layers of the jet remain aimed at northern California through Saturday, thus there’s good mid/upper-level support for thunderstorms to form and perhaps get their act together… that is, if some pockets of sunshine can break out and boost instability/buoyancy further to increase potential for more robust updrafts given the presence of decent shear. This will be looked at closer in the coming days.

Saturday's forecast thunderstorm potential.

Saturday’s forecast thunderstorm potential.

A brief break in the action appears to be a good bet come Saturday night and (very) early Sunday before the next system arrives. This next wave, by the way, will consist of a fairly deep mid-latitude cyclone off the California coast riding the jet (stream) on northeastward into the Pacific Northwest, spreading a frontal shield of rain across northern California once more by Sunday morning along the coast, and into the interior by afternoon into the evening. The main frontal band scoots southward fairly quickly compared to the previous firehose event Thursday – Saturday, but showers will remain possible through Monday as another bout of cooler, unstable air slides overhead… thus, can’t rule out more thunderstorms Monday morning/afternoon. The front will dump a decent shot of precipitation across the region to add to Thursday – Saturday’s accumulations. But after this storm, it appears we’ll be done for a while once more, unfortunately.

Wind

Fairly tight pressure gradients between the train of surface lows off the west coast and the surface high(s) in the Great Basin and southwestern U.S. combined with periodically heavy precipitation late this week, periods of gusty winds are likely… with the strongest winds expected over the sierra. The strongest winds are currently expected during the day Friday into Friday evening, as a surface low intensifies off the west coast. 850mb winds are currently forecast in the 55 – 75kt range from the valley into the sierra Friday afternoon into the evening, which also when precipitation is expected to reach peak intensity across a good chunk of the region. Thus, believe some gusts in excess of 40mph are likely in the lower elevations Friday, with >60mph in the sierra.

Wind gust forecast based off a mean of model solutions.

Wind gust forecast based off a mean of model solutions.

Orographics/terrain enhancement

Given the strong south to southwesterly flow slamming into the mountains, the higher elevations will pick up substantially more precipitation than the lower elevations. South and southwest facing slopes will pick up the most, as the flow parallels these slopes and precipitation becomes maximized. The great flow stacking up against the mountains will also create a banking effect that extends into the east part of the Sacramento valley, where precipitation amounts will be higher compared to the west side due to banking of moisture against the west slope of the sierra/foothills.

Precipitation

Essentially every major global model except for the ECMWF indicates pretty substantial precipitation amounts across northern California through Monday. A blend, or mean of these solutions including that of the somewhat less-wet ECMWF generates fairly generous amounts of precipitation, and these amounts are as follows: 2 – 5″ for much of the Sacramento valley with upwards of half a foot or so in the north end of the valley and parts of the eastern valley, 1 – 3″ in the northern San Joaquin valley, 4 – 8″ across much of the foothills & mountains, 2 – 5″ along a good chunk of the immediate, 4 – 8″ in the coastal mountains (locally higher amounts possible along south/southwest facing slopes), near the same in the northern mountains, and upwards of 8 – 12″ in orographically favored slopes inland, such as the higher elevations of Butte, Plumas, Yuba, Sierra, Nevada, Shasta, and Siskiyou counties.

Rainfall amounts based off a mean of model solutions.

Rainfall amounts based off a mean of model solutions.

Next update should be out later Thursday, or Friday if no significant changes are needed.

 

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