Severe Weather Today: 12/11/14

The Storm Prediction Center (SPC) has issued a Marginal Risk for several areas on the Pacific Coastline.

Associated with the Marginal Risk are 2% Tornado Threat Rings on the California Coastline.

The full disk visible satellite imagery shows a thick band of clouds extending towards the moisture source along the cold front of this system. Notice that the cold front affecting Oregon and California is the same cold front that is affecting Hawaii.

The 12Z upper air sounding from Oakland, CA shows a wet sounding throughout the entire column. The high dewpoint (53F) and the high precipitable water (1.33 in) indicate that there is a lot of water that is suspended in this system, waiting to precipitate out.

There is a lot of moisture moving into the region. Just for fun, I compared the upper air soundings from Yuma, AZ, spaced one hour apart. They must be doing some specialized test to launch an upper air sounding every hour. The precipitable water climbs. At 14:00Z, it was 0.48 inches. It progresses to 0.52 inches, 0.54 inches, 0.62 inches, and is now at 0.68 inches.

The deep-layer shear is impressive in these Marginal Risk areas. In Oakland, CA, the deep-layer is at 57 kts. In Salem, OR, the deep-layer is 65 kts. At Medford, OR, the deep-layer shear is 97 kts, over twice what is required to sustain rotating storms!

Turning to the NAM and its synoptic charts, the deep trough off the Oregon coast is visible at all levels.

At the 300 mb level, we can see that there is a powerful jetstreak over the Oregon coastline. While this jetstreak is on the eastern side of the trough axis, there are more jetstreaks on the western side, meaning that this trough is not going to attenuate much today.

At the 500 mb level, there is a whole train of Positive Vorticity Advection (PVA) ready to affect the San Francisco Bay Area. This PVA will increase the lift in the area.

I hate using the word “extremely.” I will use it today to describe the Upward Vertical Velocities (UVV) forecasted at the 700 mb level
for central California. They are extremely strong.

The 850 mb chart shows strong convergence at this level. Notice how winds spinning around the low pressure system merge with winds coming from the desert southwest? All of this air has to go somewhere. It’s not going down into the earth, so it has to rise.

Flipping through the severe weather observations and indices, there is nothing spectacular to report. The Convective Available Potential Energy (CAPE) is small. Storms that form today will be due to synoptic features and forcing rather than buoyancy.

The high shear shows that any storm that forms could quickly become severe. The low-level shear is large as well, which will contribute to high wind speeds and potentially wind damage. However, the high precipitable water and low CAPE lead me to believe this system will produce a ton of rainfall, and its primary threat will be flooding and mudslides.

Thank you for reading my prediction.

The Severe Weather Outlook is from the Storm Prediction Center website.
The satellite imagery is from the NOAA Satellite and Information Service.
The surface observation and upper level charts are from Unisys Weather.


About highplainschasing

This blog is about my tales in storm chasing. My name is Seth Price and I am an instrumentation instructor at New Mexico Tech. My amateur radio call sign is N3MRA.
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