Slight Risk: 5/20/14

The Storm Prediction Center has issued a Slight Risk today. Actually, they issued two, but I’m mostly interested in the westernmost area.

5-20-14 spc

Associated with the western Slight Risk is a 2% Tornado Threat Ring:

5-20-14 torn

SPC is calling for another scattered thunderstorm day with low tornado probabilities across the northwestern Great Plains.

There is shear across the target area, according to the area soundings. North Platte, NE is showing 19 kts surface to 1 km low level shear, and 48 kts for the lower 6 km, so that is enough shear to support supercells today. There are similar numbers at Denver, CO; Rapid City, SD; Aberdeen, SD; and Bismarck, ND.

Surface analysis shows a cold front approaching from the north, currently bowing through South Dakota. This boundary could be the impetus to get a few storms to fire. South Dakota and Wyoming show surface winds facing directly from the frontal boundary. I would feel better about the storm potential if the surface winds were blowing towards the frontal boundary, which tends to favor discrete supercells versus winds that travel along the boundary.

5-20-14 raw

The 0Z Wednesday (tonight) NAM surface plot shows a dryline, with a bulge in it across Kansas. It is not in the target area, but I might want to remain a little north of that bulge anyway. This may be the place to watch over the next few days.

5-20-14 surf

The 0Z Wednesday NAM 850 mb plot is showing Cold Air Advection (CAA) across Nebraska. This is due to the cold front passage, and will likely limit severe weather over the next few days for this area. I think the threat area will drift south for tomorrow and Thursday.

5-20-14 850mb

The 0Z Wednesday NAM 700 mb plot is showing some rising air in the Nebraska panhandle. This doesn’t seem to be due to Warm Air Advection (WAA), and the 500 mb plot shows Negative Vorticity Advection (NVA) which would also limit storm potential. I need to learn more about the 700 mb plot- everything I am seeing says there should be falling air there instead of rising air- NVA, CAA, and general downslope (albeit a slight gradient).

5-20-14 700mb

The 0Z Wednesday NAM 300 mb plot shows the trough in the jetstream to the west. There is a jetstreak over the northern part of the target area- but once again, I am confused, as the area of convergence aloft (sinking air) is occurring over the target air.

5-20-14 300mb

Overall, I’m not sure what is happening. If I was already out chasing, I would opt to hang out at the northern edge of the Slight Risk area, and watch for vertical development.

It seems like there are several parameters that point to sinking air instead of rising air. A little bit of sinking air is a good thing when the area is socked in with cloud cover. The sinking air will help to evaporate the clouds and let diurnal heating take over. However, the visible satellite imagery shows that there is not any cloud cover- other than the cloud streets that are forming across parts of the Slight Risk area.

5-20-14 1830z

I hope you enjoyed my post!

The Storm Prediction Center forecast and soundings are from the The satellite imagery is from and the upper air charts are from Unisys – Weather. The radar imagery is from the National Weather Service.


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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