|Fig 1 -- NAM 36 hour forecast of dewpoint temperatures (colors) and winds (barbs) for 00Z, Monday (Sunday night), April 4, 2011. From the HOOT website.|
|Fig 2-- Same as figure 1, but annotated with surface features by the author.|
So we see a complex suite of boundaries and low pressures in the forecast--reflective of the complex upper-air pattern we saw above. The boundaries (and to an extent the upper-level winds) will provide the lifting mechanisms, and we can see in the dewpoint maps above that there is moisture available to work with. We need two more things for severe storms--wind shear and instability. With respect to wind shear, there is an awful lot of that. The 850mb winds are forecast to be roaring out of the southwest on Sunday evening.
|Fig 3 -- NAM 36 hour forecast of 850mb winds (colors) and geopotential height (contours) for 00Z, Monday (Sunday night), April 4, 2011. From the HOOT website.|
What about instability? A good first glance at instability is to look at the surface-based convective available potential energy (CAPE). The higher the CAPE, the more the instability.
|Fig 4 -- GFS 36 hour forecast of surface-based CAPE for 00Z, Monday (Sunday night), April 4, 2011. From the HOOT website.|
So what would be stopping storms from forming and becoming severe further south? The answer is something called the "capping inversion". Normally air cools with height--it's much colder further up in the atmosphere than it is near the surface. In fact, the faster it cools off with height, the more unstable the air gets because warm air near the surface is more buoyant than the colder air above. This promotes rising motion and the formation of thunderstorms. In fact, in some ways, CAPE is a measure of how steeply the temperature cools with height (it includes other things like moisture content too...). Generally, the faster the air cools with height, the more CAPE you have and the more unstable the air becomes. The opposite happens in a "capping inversion". This is a region of the atmosphere where the temperature actually warms with height. This is bad for rising air because if it runs into air that is warmer than it is, the rising air is no longer more buoyant than the air around it and it will stop rising. Capping inversions (or, "the cap" for short) are easy to see on vertical soundings. Here's a NAM model forecast sounding for Norman, Oklahoma at 21Z on Sunday (Sunday afternoon):
|Fig 5 -- NAM 33-hour forecast sounding for OUN valid 21Z, Sunday, April 3, 2011. From the HOOT website.|
So how would we get storms when there is a cap? We'd have to break the cap somehow. There are three main ways of doing this:
- Cool the air above the cap
- Warm the air below the cap
- Have a cold front or some other strong lifting mechanism move in that can shove the surface air right through the cap regardless of its relative buoyancy.
|Fig 5 -- NAM 36-hour forecast sounding for OUN valid 00Z, Monday (Sunday evening), April 4, 2011. From the HOOT website.|
|Fig 5 -- OU/OWL WRF 48-hour forecast sounding for OUN valid 00Z, Monday (Sunday evening), April 4, 2011. From the HOOT website.|
|Fig 6 -- OU/OWL WRF 51-hour forecast sounding for OUN valid 03Z, Monday (Sunday evening), April 4, 2011. From the HOOT website.|
Notice one other thing in the last two profiles I've shown--the low-level winds pick up in speed rather extraordinary between 7 PM and 10 PM (see the wind barbs between the surface and 700mb in the two soundings above). This has to do with the nocturnal low-level wind maximum (sometimes associated with what is also called the low-level jet). The details of why this happens are enough material for another whole blog post, so I'll just say that as the sun goes down (and as that capping inversion becomes re-established), the low-level winds tend to speed up rather suddenly. This is good for severe storms as they need wind shear to maintaining rotating updrafts and produce hail, strong winds and sometimes tornadoes. However, this night time increase in winds occurs at the same time that the capping inversion shows up again, causing the storms to elevate. This is why the time right around sunset is often critical if you are looking for tornadoes--you want the atmosphere to stay uncapped so you still have surface-based storms, but you also want the wind shear to increase. Right around sunset sometimes the capping inversion hasn't quite started to creep in yet and sometimes the wind shear will really start kicking up and you'll see an increased risk of tornadoes around then.
However, with the increase in low-level winds comes an increased tendency for storms to form bow-echoes and have strong, straight-line winds associated with them, regardless of if they're elevated or surface-based. We often see an afternoon full of isolated supercells and small storms quickly congeal together into big lines of storms after the sun goes down and the low-level winds kick up.
So, in summary of all this--models are coming into agreement that severe weather is possible across much of the central US. We're virtually guaranteed to see storms further north from Iowa and Missouri into Illinois because the cold front should be moving through. The cold front is such a strong enough forcing mechanism that regardless of if there is a cap, it can force enough lift to cause some strong thunderstorms. Further south, the threat becomes more conditional in the afternoon as the models are forecasting a strong capping inversion at many places. As we saw in the examples above, models are also showing that the cap may erode enough in the late afternoon and early evening that we could see some good storm development. However, the cap is forecast to quickly become re-established as night falls, limiting the surface-based development and consequently the strong tornadic potential. However, as winds pick up around sunset, the potential for lines of storms with strong winds increases.
We'll see how it all plays out tomorrow...