Prioritize...
When you've finished this page, you should be able to describe the impacts of important 500-mb features, such as shortwave troughs and mid-level jets on CAPE, CIN, and vertical wind shear. More specifically, you should be able to describe how shortwave troughs "prime the atmosphere" for deep, moist convection by altering lapse rates (which has implications for CAPE and CIN).
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When forecasters assess the "big picture" in making a forecast, it's important to assess conditions throughout the entire troposphere. Therefore, forecasters frequently look at surface conditions (and forecasts), as well as those at 850 mb, 700 mb, 500 mb, and either 300 mb or 250 mb. Seasoned forecasters know what signs to look for at each of these levels as they relate to the possibility of deep, moist convection developing.
To get started, we're going to look at 500 mb, a level that you examined extensively in your previous studies. You should be very familiar with the features we'll be focusing on (namely, shortwave troughs), but now we're going to tie some of your past knowledge in with how these features can effect the environment for deep, moist convection. As you are about to see, shortwave troughs have important impacts on vertical motion and lapse rates that can make the environment more favorable for deep, moist convection.
Impacts of 500-mb Shortwave Troughs
For surface-based thunderstorms, synoptic-scale lift typically boils down to low-level convergence (along surface fronts and mesoscale boundaries) and upper-level divergence. Of course, there are other forms and scales of lift that can get air parcels to the LFC (orographic lift, for example). Here, however, I only want to address synoptic-scale lift in this section, and, in particular, divergence downwind (usually east or northeast) of a 500-mb trough. For starters, review this animation tracking a parcel through a shortwave trough, which I hope you recall from your previous studies. The schematic below serves as a supplement for the animation. Any way you slice it, there's divergence downwind of a 500-mb shortwave trough because air parcels moving east from the base of a shortwave trough lose some of their spin (absolute vorticity decreases) by expanding their surface areas. Sound familiar? This upper-level divergence, in concert with low-level convergence, encourages upward motion, which promotes local cooling.
Of course, the animation and schematic above are highly idealized. Shortwave troughs often don't look as neat and tidy as they do in these idealized graphics. Take, for example, the 500-mb pattern shown by the 12Z run of the NAM on February 2, 2016, valid at 18Z that day. The feature that probably jumps out at you right away is the closed low and strong vort max right over the center of the country. That closed low marks the core of a longwave trough, but embedded within that longwave trough are several shortwave troughs.
A couple of those shortwave troughs are marked by the X's over northern Mexico. But, there's another shortwave trough over New Mexico that is not marked by an X. Do you see it? The very small closed contour (darker yellow shading) over Central New Mexico also marks a subtle vort max and shortwave trough, even though it lacks the classic "X." The details of the positions and intensity of shortwave troughs (even subtle ones) is of great interest to mesoscale forecasters, because those details determine where (and how strong) the divergence is.
To get an idea of what I'm talking about, check out this 17Z Rapid Refresh analysis of 500-mb heights (black contours), vorticity (dashed contours and color-filled regions), and differential vorticity advection (blue contours) from February 2, 2016 (one hour before the NAM forecast prog above was valid). You can think of differential vorticity advection as a proxy for upper-level divergence. Note that the strongest divergence is located in a band from eastern Nebraska through Iowa and into Illinois, to the northeast of the strongest vort max (over Kansas). However, there are lots of other pockets of differential vorticity advection (divergence) over the Southern Plains due to the details of the vorticity field. Note that our subtle vort max over New Mexico was creating some weak divergence just to its east, as well.
On this particular date, SPC had outlined an enhanced risk of severe weather in their Day 1 Convective outlook, but the greatest risk area wasn't related to the strongest divergence at 500 mb. It was farther south, in the path of those more subtle vort maxes, where periods of weaker divergence aloft would occur throughout the day as those weaker vort maxes rounded the base of the longwave trough as it crawled eastward. In other words, the severe weather threat isn't always where the strongest lifting at 500-mb is!
So, why is synoptic-scale lifting and its associated cooling pivotal to the development of deep, moist convection? It reduces (or removes) CIN! For starters, a typical vertical profile of temperature associated with the presence of CIN often shows a stable layer in the lower troposphere (on the interactive tool below, note the stable layer just above 850 mb). You may think of this stable layer as relatively warm air. To see what I mean, recall that, within a stable layer, temperature sometimes increases with increasing altitude (a temperature inversion), stays constant with increasing altitude (an isothermal layer), or decreases rather slowly with height. If you look closely at the idealized sounding, the stable layer just above 850 mb is helping to create CIN (shaded in pink).
Now, let's follow the evolution of the stable layer (a weak inversion, in this case) as a 500-mb shortwave trough approaches. Remember, there's upper-level divergence east of the 500-mb shortwave. Assuming a surface boundary also lies to the east of the 500-mb shortwave (which is a reasonable assumption), then there's also some low-level convergence to help the cause. At any rate, there's upward motion and cooling in local columns of air that extend up to 500 mb on the eastern flank of the 500-mb shortwave. To simulate this lifting and cooling, drag the shaded layer of air upward in the tool above.
Above the well-mixed boundary layer, the temperature sounding shifts to the left (the air cools aloft) in response to the upward motion associated with divergence ahead of an approaching 500-mb shortwave trough, which reduces CIN and lowers the altitude of the LFC.
What happens to CAPE when there's cooling from upward motion? To focus solely on this process, we assume that the surface temperature and dew point hold steady. The leftward shift of the temperature sounding above the boundary layer means that CAPE also increases (positive area increases). When you look at the temperature sounding at lower altitudes, note that the depth of the well-mixed boundary layer increases with time. That's because cooling (via upward motion) at the top of the boundary layer promotes local mixing so that the depth of the well-mixed layer expands.
The bottom line is that synoptic-scale lift reduces CIN, and increases CAPE. So, as a developing mesoscale forecaster, you should think of divergence east of a 500-mb shortwave trough as a way to "prime" the troposphere for deep-most convection. Indeed, a 500-mb shortwave does not really "trigger" thunderstorms. It simply makes the environment more conducive for thunderstorms to develop because it helps to reduce CIN.
Height Tendencies and Lapse Rates
Upward motion, however, isn't the only way that 500-mb shortwave troughs can make the environment more favorable for thunderstorms. In addition, falling 500-mb heights ahead of the shortwave trough tend to go along with decreasing mid-tropospheric temperatures (recall that the cores of 500-mb closed lows and shortwave troughs are mid-level pockets of relatively cold air). In other words, cooling at 500 mb usually helps to destabilize the middle troposphere. A 500-mb shortwave trough tends to cause mid-level lapse rates to increase with time (the lapse-rate tendency is positive).
As we get even closer to the core of an open 500-mb shortwave trough (or a closed low), 500-mb heights noticeably "fall" (decrease) over time as the system moves eastward, and mid-level lapse rates steepen even further. To see an example, check out the image below showing 500-mb heights (black contours) at 21Z on July 14, 2010, and the 12-hour height tendencies leading up to that time. Treating 500-mb lows and troughs as mid-level pockets of cold air, it stands to reason that 500-mb temperatures typically decrease as these cold pockets approach. Assuming there's some solar heating and / or some low-level convergence to get surface air parcels to the LFC, showers and thunderstorms can also develop closer to the core of the 500-mb low or open trough.
Note the pocket of height falls to the east of the 500-mb shortwave trough centered near the Saskatchewan / Manitoba border at 21Z. In the 12 hours leading up to 21Z, heights had been falling (and mid-level lapse rates increasing) out ahead of the trough, with the biggest height falls marked by the blue shaded region just southeast of the closed low.
The mid-level cooling and steepening lapse rates associated with 500-mb troughs acts to boost CAPE values, favoring stronger updrafts as long as parcels are able to reach the LFC. So, 500-mb troughs can prime the atmosphere for deep, moist convection through reducing CIN via synoptic-scale upward motion, and through boosting CAPE from mid-level cooling. To see all of these processes in action, check out the Case Study video below.
Mid-Level Jets
Impacts on CAPE, CIN, and lapse rates aren't all forecasters think about when they evaluate the 500-mb pattern. They also look for "mid-level jets" (zones of fast winds at 500 mb). For example, the 12Z 500-mb RUC analysis from July 14, 2010 (same date as the height tendency map above) shows a 500-mb speed maximum over the Upper Middle West that shadowed the vigorous shortwave trough.
Why are mid-level jets like this one important? They tend increase the vertical wind shear in the layer from the ground to six kilometers. Remember that the 500-mb level tends to be around 5,500 meters, so when relatively fast winds exist around that level, vertical wind shear tends to increase in the layer between the surface and six kilometers. Regardless of any changes in wind direction, the fast flow in the middle troposphere that goes along with a mid-level jet means there is usually a marked increase in wind speed between the surface (where friction slows the wind speed) and the middle troposphere.
Now that you've seen how 500-mb shortwave troughs can prime the atmosphere for deep, moist convection, and mid-level jets (which sometimes exist in concert with 500-mb shortwaves), please take some time to view the Case Study below, which details how the arrival of the 500-mb shortwave trough and mid-level jet that you just saw helped to spur deep, moist convection (and severe weather) over the Upper-Mississippi Valley.
Case Study...
You've already seen a couple of examples on this page from July 14, 2010. To tie together the concepts covered on this page, and see how upward motion, the arrival of cool air in the middle troposphere, and a mid-level jet can work together to prime the atmosphere for organized deep, moist convection, check out the video below.
Video Transcript: July 14, 2010: The Big Picture Contribution at 500 mb
While 500-mb shortwaves can prime the atmosphere for the development of thunderstorms, the details of exactly where and when they form are largely determined by surface boundaries. We'll tackle that topic shortly, but first we need to lay some more foundation for how the upper-level pattern can interact with topography to help create surface boundaries. Read on.