|Fig 1 -- SPC Day 1 convective outlook as of 12Z, May 11, 2011. From the SPC.|
|Fig 2 -- SPC Day 1 convective outlook as of 1630Z, May 11, 2011. From the SPC.|
The answer lies in the timing of convection with this surface low--and what convection in the early morning does for the next day.
Take a look at Norman, Oklahoma's sounding from 12Z this morning:
|Fig 3 -- KOUN sounding from 12Z, May 11, 2011. From the SPC.|
- By having a capping inversion, it ensures that storms can't just fire off everywhere--only in unique places where there is enough lift to overcome the capping inversion can storms form.
- Having the capping inversion in place allows the surface to heat up (and possible more moisture to advect in) throughout the day, creating an even more unstable environment so that if that instability is released in the late afternoon, the convective development is particularly explosive. With no capping inversion in place, as soon as the surface started heating at the beginning of the day things would quickly become unstable and storms would fire off without taking advanatage of a full-day's worth of heating.
Also, in the above sounding, notice that I drew a yellow circle around the layer right at and above the capping inversion. Notice how dry the air is right here--the dewpoint temperature (shown by the green line) is much less than the actual air temperature, indicating dry air here. Now, this can be a good thing for severe thunderstorm formation as dry air aloft tends to contribute to stronger downdrafts and more damaging winds at the surface (but that's for another blog post). But, for now, just keep this feature in mind.
This morning, as the sun came up, the visible satellite image showed this:
|Fig 4 -- GOES-E visible satellite image from 1316Z, May 11, 2011. From the HOOT website.|
Mid-level winds this morning were generally out of the southwest, as shown in this GFS analysis from 12Z this morning:
|Fig 5 -- GFS analysis of 700mb winds (colors) and height (contours) for 12Z, May 11, 2011.|
|Fig 6 -- KOUN sounding from 17Z, May 11, 2011. From the SPC.|
You can see in the sounding above that that's exactly what happened--not only has that circled layer become moister, but the temperatures have cooled down considerably. In fact, they've cooled so much that the capping inversion (which was a layer where temperatures warmed considerably with height) is no longer visible--the cooling due to the cloud water has removed the cap!
Now remember what I said before--to get supercellular types of storms, you want the capping inversion to linger throughout the day, at least into the middle of the afternoon, to allow instability to build and to prevent storms from firing up everywhere. But now it's late morning and the cap seems to have completely disappeared over Norman due to this cloud intrusion. The result? Here's the radar two hours later at 1938Z:
|Fig 7 -- NEXRAD base reflectivity radar mosaic for 1938Z, May 11, 2011. From the NWS.|
One interesting facet of these storms is how they have fired and propagated ahead of the convergence along the main dryline/front. Here's the surface analysis from the SPC around the time of the radar image above:
|Fig 8 -- SPC surface analysis of dewpoint temperature (color shadings), temperature (red contours), mean sea-level pressure (black contours) and wind (barbs) for 19Z, May 11, 2011.|
However, the storms at that time were more in west-central Oklahoma--slightly ahead of the dryline. Look at the the temperature contours (the red lines) in western Oklahoma. See how underneath the storms it's much colder than it is elsewhere? In fact, while temperatures are in the mid-80s in eastern Oklahoma, underneath the storms it gets down into the mid-60s. This is a result of all that cold, downdraft air falling with the rain underneath the storms. This zone of colder temperatures underneath the storms is often referred to as the "cold pool".
Here's a map from the Oklahoma Mesonet from one hour later. The storms have moved further east, and you can see that there's a wide swath of colder temperatures underneath them:
|Fig 9 -- Air temperature at 2m from the Oklahoma Mesonet at 5:25PM CDT, May 11, 2011. From the Oklahoma Mesonet.|
One interesting thing about cold pools is that they can provide a lifting mechanism for storms to propagate into areas even when there is a capping inversion present. Think about the leading edge of the storms and their cold pool--it's like a miniature cold front. Downdraft air from the thunderstorms can blast out in front of the storms and provide convergence and lift as it runs along. This allows storms to keep going as they track along with the leading edge of their cold pool. So, if a bunch of storms can establish a decent cold pool, they don't need to have a front or other convergence around to provide lift--the leading edge of their cold pool can provide them with their own lift. It's a lot like the squall-line and bow-echo dynamics I discussed in a previous blog post--convergence along the leading edge of their cold pools are what keep them going.
So...even though the moderate risk got cancelled, there still have been a lot of severe storms today, including in the convergent zone along a warm frontal boundary in the upper midwest. This slow-moving cyclone will continue to track across the country over the next day or two, bringing even more chances of severe weather.