Showers & Thunderstorms Friday & Saturday, Wind as Well

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A cold, but fairly moisture starved upper-level low/trough will dig into northern California Friday, bringing with it increasing rain showers, possible thunderstorms, and mountain snow showers through the day Friday and into Saturday. The inland trajectory of the majority of the system’s components will greatly limit available moisture due to a lack of a tap to an external moisture source, thus this system is basically relying on the little moisture it transports into the area and any moisture hanging around locally in the low-levels (which isn’t a whole lot given the offshore flow lately). Nonetheless, sufficient cold air aloft combined to generate instability, lift provided by a vorticity max expected to slide across northern California later Friday, and a fairly potent mid/upper-level flow should be enough to generate scattered to widespread showers through the day Friday, likely reaching peak coverage between 3pm and 9pm.

Partly cloudy skies across the lower elevations Friday should allow surface temperatures to climb into the upper 50s to mid 60s depending on location and duration of sunshine. These fairly warm temperatures beneath a -20 to -25c 500mb temperatures associated with the upper-low/trough should promote some modest instability by the afternoon into the early to mid-evening. Appears a core of 7 to 7.5c mid-level lapse rates will begin sliding overhead from the mid/late-afternoon through the evening, which combined with ongoing and leftover surface warmth from the day (once we get into the evening) should get instability sufficient enough for some thunderstorms, specifically from around Chico to Modesto in the valley eastward into the adjacent foothills & mountains, as a little vortmax slides inland and across the valley, and where convergence looks to be best between onshore winds on the west side of the valley and delta colliding with a more northerly flow in the north end of the valley venturing southward. This flow also interestingly creates a bit of an eddy between Sacramento and Yuba City – at least if physics of the 4km NAM bode well. Best timing for thunderstorms appears to be about the same timeframe as given in the previous paragraph for max shower coverage (3 to 9pm).

0z 4km NAM's forecast 700mb vertical velocities and 10-meter wind vectors (directions), valid at 4pm Friday. The areas of strong vertical motion (pinks, purples, and dark greys & blacks) from the Chico area to the Sonora area are likely little convective cells/storms.

0z 4km NAM’s forecast 700mb vertical velocities and 10-meter wind vectors (directions), valid at 4pm Friday. The areas of strong vertical motion (pinks, purples, and dark greys & blacks) from the Chico area to the Sonora area are likely little convective cells/storms.

Northwesterly mid & upper-level flow events typically aren’t the best for strong or severe storms, as surface gradients are often similar – meaning little shear. This time, however, given the somewhat westerly to southwesterly surface flow from around Yuba City to Stockton during the afternoon and early evening beneath the mid and upper-level northwest flow, there is some directional shear, just at different directions than we normally see. In fact, right around Sacramento these two directions create about a 90-degree angle between the two wind directions at the different layers. However, once you get above above 500mb (approximately 18,000ft), the flow goes to, well, crap with a paralleled flow from 500mb up to 300mb. This means, if any strong thunderstorms were to develop, they’d likely need to stay low-topped, below 20,000ft in order for updraft rotation to remain intact and not town up from the parallel (not continually turning) flow as you go higher up. Another downside to the setup is the moisture, as mentioned. Dewpoints are expected to stay below 50 degrees in most cases. Nevertheless, there is sufficient shear and instability for some more robust/weakly rotating storms mainly between Yuba City and Stockton, though cannot totally rule out anything interesting as far north as Chico.

12km NAM's forecast hodograph late Friday afternoon a bit south of Yuba City. The clockwise curvature in the first sixth or so of the graph depicts decent directional shear, but beyond that it isn't too great.

12km NAM’s forecast hodograph late Friday afternoon a bit south of Yuba City. The clockwise curvature in the first sixth or so of the graph depicts decent directional shear, but beyond that it isn’t too great. Image: TwisterData.

To cut straight to the point thunderstorm wise, while some ingredients are there for some strong storms Friday afternoon/early evening from the central Sacramento valley to the northern SJ valley, some also aren’t or aren’t totally complete, keeping severe potential to more of a slight possibility rather than a chance, or more or less likely. If any thunderstorms do develop in this region, can’t rule out some somewhat larger than small hail, gusty winds, and as stated a slight possibility of some funnels or perhaps a tornado. Certainly not a chance to freak out over, but definitely worth mentioning.

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Showers will remain possible through Saturday, with thunderstorms once again becoming a possibility as things warm up a bit through the day between clouds. There’ll be more in the way of widespread weak to modest instability Saturday as the core of the upper-low/base of the trough slides overhead – thus likely more in the way of convective showers and thunderstorms developing through the afternoon and more widespread than Friday, but little mid/upper-level flow aloft will limit the strong/severe storm threat significantly. Lots of hail is a good bet with most of the strong showers or thunderstorms Saturday, though, given pretty steep lapse rates and even cooler temperatures aloft to help even the lowest-topped storms capable of a bit of hail.

Snow levels look like they’ll begin around 5000ft Friday, and lower to about 4000ft Friday night, hovering around 4000 – 4500ft through the day Saturday. However, stronger showers or thunderstorms, especially if slow-moving or storms continue moving over the same area, lowering the temperatures, snow levels could locally drop to perhaps 3500ft. Snowfall amounts through early Sunday look to range from little to nothing in some parts of the mountains/sierra (mainly in the north), to 4 to 8″ along the west slope and sierra around the I-80 corridor southward, with local amounts up to a foot possible, mainly at the highest elevations. Rainfall amounts will be fairly limited in the lower elevations for the most part due to the very showery nature of the system, however areas where showers or thunderstorms do venture over could get anywhere from a few hundredths to a few tenths, all depending on the coverage and intensity of showers/storms.

North to northeasterly winds begin to pick up across the region by later Saturday, continuing into Sunday as the trough departs eastward, with ridging not far away compressed against the back side of the trough. Looks like the strongest winds will be confined to the west side of the valley and along west to southwest facing slopes in the foothills and mountains, mainly north of Sacramento. Gusts up to 30 – 35mph appear possible in these areas before winds ease up late Sunday in time for Monday.

Another system may slide through by the middle of next week, but right now it doesn’t appear to be much of a impact itself, but the winds behind it may.

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