Thunderstorm potential update, posted at 10pm 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.
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.
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.
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.
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.
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.
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.
Next update should be out later Thursday, or Friday if no significant changes are needed.
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