Thunderstorms Today, Very Windy Thursday – Saturday

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Updated discussion as of 10:30AM…

It was a wet night and morning across northern California, and we’re not done yet. As of 10:30AM, a shortwave was just off the SFO coast, and strengthening quickly as it moves inland.

Satellite image from around 10:30AM indicating a band of cooling clout tops with a shortwave coming inland.

Satellite image from around 10:30AM indicating a band of cooling clout tops with a shortwave coming inland.

As this shortwave comes inland, slightly colder air aloft will trickle overhead, steepening lapse rates a bit and supporting more notable instability. The RAP/HRRR indicate surface CAPEs getting into the 500 – 800j/kg range between 1pm and 6pm this afternoon/early evening… along with lifted indexes in the 0 to -4c range. In addition to this moderate instability (for California standards), the shortwave coupled with a developing upper low over northern California (which will drop south into central/southern California through Thursday and Friday) will increase surface wind backing in the valley out of the south to southeast, and with the westerly flow in the 850 – 500mb range, favorable directional shear sets up for rotating thunderstorms. Wind speeds throughout these levels aren’t expected to be very strong, though, with perhaps 15 – 25kts of effective shear and 30 – 35kts of 500mb bulk shear. This can be just enough to get a thunderstorm updraft rotating, and even enough to see some mini-supercellular thunderstoms.

The HRRR currently agrees with this thinking, developing some pretty strong thunderstorms from the Sacramento area to Yuba City between the 2pm – 6pm time frame. Showers and thunderstorms are expected to have the most coverage from San Francisco to Fresno, however south of Highway 50 shear and instability isn’t as great. Thus, I believe the highest threat of strong or potentially severe thunderstorms exists from south Butte county to central Stanislas county.

16z HRRR loop indicating strong storms developing in the Sacramento to Yuba City area.

16z HRRR loop indicating strong storms developing in the Sacramento to Yuba City area.

Given the aforementioned parameters, if any strong thunderstorms develop today they could become capable of producing some hail, very heavy rain, gusty winds, and potentially funnel clouds or a brief tornado or two, mainly in the highest threat region in the graphic below.

Threat regions plotted out for today.

Threat regions plotted out for today.

—Tuesday night’s discussion—

Showers and/or thunderstorms may linger into the evening Wednesday before the back edge of the jet and moisture plume sag south into southern California, and the system cuts off.

Things will remain dry from Thursday through the weekend, however, as the system cuts off and slides down the coast into southern California, a strong northeast barrier jet sets up over northern California beginning late Thursday, continuing through Friday into early Saturday. 700mb winds over the sierra late Thursday into Friday are forecast to top 50 – 60kts from Highway 50 north, and as winds accelerate down the western slope of the sierra, 850mb winds are forecast in the 50 – 70kt range. Winds also get gusty in the Sacramento and coastal mountains as well, with perhaps some 30 – 40kt surface gusts (35 – 45mph) elsewhere away from the western slope of the sierra and the central and northern foothills. In the western slope of the sierra and foothills north of Highway 50, I could see 40 – 50mph gusts being quite common, with 60 – 75mph winds very well possible as well.

00z NAM's forecast 850mb winds for Thursday evening indicating some strong 45 - 65kt winds at the level in the northern foothills/western slope of the sierra.

00z NAM’s forecast 850mb winds for Thursday evening indicating some strong 45 – 65kt winds at the level in the northern foothills/western slope of the sierra.

Personal idea of potential surface wind gusts.

Personal idea of potential surface wind gusts.

These winds, especially in the mountains, could be strong enough to cause structure damage, pull down trees, cause issues with high profile vehicles, and of course pose risk for some power outages.

Besides the couple days of wind, we’ll remain quite through the early to middle part of next week. There are some indications of some sort of system that could affect our region perhaps on Thanksgiving, but models are all over the place for that time frame and I wouldn’t count on anything yet.

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