In my last post, I talked about how a feature called the "capping inversion" would work to inhibit thunderstorm development throughout the afternoon until this cap could be broken so that air could more easily rise. Remember that the capping inversion can be seen as an area where the temperature warms with height instead of cooling with height like usual.
So how did today start out? First let's look at the sounding from 12Z this morning (around 7 AM CDT) at Norman, OK:
|Fig 1 -- OUN sounding from 12Z, April 3, 2011. From the SPC website.|
Topeka, Kansas, (which is further north) had a similar-looking sounding this morning:
|Fig 1 -- TOP sounding from 12Z, April 3, 2011. From the SPC website.|
It just so happens that both Norman and Topeka launched special soundings at 20Z, or 3 PM central time. First, here's Norman's mid-afternoon sounding:
|Fig 3 -- OUN sounding from 20Z, April 3, 2011. From the SPC website.|
|Fig 4 -- Dewpoint temperatures from the Oklahoma Mesonet at 430PM, CDT, April 3, 2011. From the Oklahoma Mesonet.|
Remember I mentioned before that one way to try and erode away the cap was for turbulent and small-scale convective turbulence to "mix out" the sharp temperature rise at the level of the cap. This turbulent mixing does other things too. In this case, it's helping to bring much drier air aloft down to the surface. Notice on Norman's mid-afternoon how the dewpoint temperature drops off considerably above the level of the capping inversion. As turbulent mixing goes on trying to even out that temperature change at the cap, some of that very dry air just above the cap is mixed down below the capping inversion and into the surface-layers of the atmosphere. By mixing in this drier air, the dewpoint temperature of the air near the surface actually decreases. So, the fact that the dewpoint temperature at the surface has been decreasing during the day is actually proof that this turbulent and small-scale convective mixing is really going on. However, it has not been enough so far to significantly weaken the capping inversion so storms can grow.
However, let's take a look at the mid-afternoon sounding from Topeka:
|Fig 5 -- TOP sounding from 20Z, April 3, 2011. From the SPC website.|
So...in summary. This morning, like we expected, a strong capping inversion existed over the southern plains and into the midwest. By this afternoon, a lot of vertical mixing has taken place. In Oklahoma, the mixing has not been enough to really erode the cap very much, and as such they're probably going to have to wait for the dryline to mix through or for the cold front to swing down to provide enough lift to push air through the cap and see storms. However, further north in eastern Kansas and northern and central Missouri, the Topeka sounding seems to indicate that the capping inversion has mostly eroded by late this afternoon. This will make it much easier for storms to form, even before the cold front swings through with its strong lift. Since storms forming out ahead of a front in the warm, moist sector can be surface-based, the potential for tornadoes and large hail producing storms is higher here with any storms that do fire off before the front comes through. This is part of the reason why the SPC's current moderate risk of severe storms is right in the region I just outlined:
|Fig 6 -- SPC Day One Convective Outlook issued 20Z, April 3, 2011. From the SPC website.|
Still waiting...and watching.