Glenn County Tornado Review (March 26th, 2014)

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Through the morning Wednesday, an upper level impulse traveling along a rather strong jet impacted northern California, bringing a round of showers to northern California. This upper level system was associated with a upper low off the Pacific Northwest coast, which was in the perfect position in to support decent vertical motion across northern California, along with the associated 100 – 120kt upper level jet.

Visible satellite imagery + radar mosaic from around 1:10, indicating some breaks between developing cumulus in the western central valley.

Visible satellite imagery + radar mosaic from around 1:10, indicating some breaks between developing cumulus in the western central valley.

By early afternoon, a showers subsided in the northern and central Sacramento valley, which promoted partly cloudy skies and allowed surface temperatures to warm into the upper 50s to locally low 60s. At 500mb, temperatures were around -25c in association with the upper low to the north, which combined with ambient surface hearing promoted mid-level lapse rates around 7 and low-level lapse rates in the 8 – 9c range. In addition (and somewhat more importantly), decent moisture in the low/mid-levels combined with good cooling with height allowed for SBCAPE values to rise into the 200 – 500 j/kg range from Sacramento to around Red Bluff, which is sufficient for thunderstorm development in combination with the decent upper-level support and moisture.

Given the proximity to the upper low off the PNW coast and the decent jet blasting overhead, the positioning of the low made for fantastic directional shear Wednesday. Surface winds were out of the south to southeast in the valley Wednesday, and from around 4000 – 5000ft up, they were out of the west-south-west to west, and moderate to strong in speed above 5000 – 10000ft. Surface winds themselves in the central valley Wednesday were breezy from Sacramento to Red Bluff, with gusts in the 20 – 25mph range. The great directional and low-level shear made for noticable clockwise curving hodographs, also supporting the fact that the directional shear again was fantastic.

Take a look at this vertical wind profile from KDAX, the US National Weather Service Sacramento California's radar site near Davis. The latest profile is on the far right, with the lowest wind barbs being at/near the surface and the highest barb available at/near 18,000ft, in this case. Notice the surface wind barb is out of the southeast, while winds above around 3,000 - 4,000ft and up are from the southwest to west-southwest. These veering winds with height are fantastic for rotating thunderstorms, but we need to get some sunshine in the valley for instability to be sufficient enough to support strong enough thunderstorms to make use of this excellent directional shear.

Take a look at this vertical wind profile from KDAX, the US National Weather Service Sacramento California’s radar site near Davis. The latest profile is on the far right, with the lowest wind barbs being at/near the surface and the highest barb available at/near 18,000ft, in this case. Notice the surface wind barb is out of the southeast, while winds above around 3,000 – 4,000ft and up are from the southwest to west-southwest. These veering winds with height are fantastic for rotating thunderstorms, but we need to get some sunshine in the valley for instability to be sufficient enough to support strong enough thunderstorms to make use of this excellent directional shear.

A forecast hodograph for the central Sacramento valley for Wednesday afternoon.

A forecast hodograph for the central Sacramento valley for Wednesday afternoon.

With these ingredients in place, the probability of strong to tornadic thunderstorms was moderate to high if the sun were to break through – and it did.

By noon to 2pm, the partly cloudy skies in the central valley had lasted long enough and promoted strong enough buoyancy to aid in the development of a thunderstorm in central Glenn county by 2 – 2:30pm.

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More than a hour later, the storm finally began to move, which was around 3:30 – 4pm. Once eastward progression began, the storm began rotating more noticeably, and produced it’s first viewed funnel cloud around 4:40pm.

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First funnel we and other spotters and chasers spotted around 4:40.

First funnel we and other spotters and chasers spotted around 4:40.

Around 5:30, the supercell finally crossed I-5, and produced at least two more funnels, and possibly one weak touch down. We noticed quite a bit of dust being kicked up in a field in front of us north of SR-162 between Williams and Glenn.

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Just a few minutes later, a full condensation funnel made it to the ground west of I-5, and touched off and on for 1500ft according to the NWS in Sacramento’s survey, and was also rated a EF1 given several dozen almond trees were uprooted, along with some damage to outbuildings.

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In addition to this and a bunch of other photos, here’s a video we captured of the development of the tornado, and towards the end it either weakens and fades or becomes invisible due to rain wrapping around the mesocyclone. Link to the video via YouTube is below.

https://www.youtube.com/watch?v=OceS_lUk2XQ

During this time, the supercell on radar looked amazing, with a clear hook, BWER, and some rather strong surface inflow.

In this four-panel view, you can see your standard base reflectivity up in the top left, storm-relative velocity in the top right, correlation coefficient in the bottom left, and differential reflectivity in the bottom right. In the bottom left (CC), there is a one pixel that lasted a couple frames of some low values, which which contained within the hook and associated meso, and could have been a small amount of tornaod debris being pulled up high enough to be detected by the radar.

In this four-panel view, you can see your standard base reflectivity up in the top left, storm-relative velocity in the top right, correlation coefficient in the bottom left, and differential reflectivity in the bottom right. In the bottom left (CC), there is a one pixel that lasted a couple frames of some low values, which which contained within the hook and associated meso, and could have been a small amount of tornaod debris being pulled up high enough to be detected by the radar.

Shortly after this frame, the tornado dissipated, though the storm continued to rotate into western Butte county.

Shortly after this frame, the tornado dissipated, though the storm continued to rotate into western Butte county.

Around the same time, a smaller supercell spawned several funnel clouds and one tornado in Roseville as that supercell tracked from eastern Sacramento county east into Placer county.

This event goes to show, that tornado events do happen in California, and perhaps most commonly in the Sacramento/San Joaquin valleys. The frequency seems to be one to three times per winter/spring, depending on the year.

 

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