It looks like we could get a brief tease of autumn later this week as a respectable cold front moves into Texas. It doesn’t look like the cooler air will stick around long, and you’ll hardly be breaking out the parka, but perhaps the light jacket.
Upper-air analysis from the 300 mb pressure level (about 30,000 feet above sea-level) this morning shows a trough digging into the Northern Rockies. To the west of the trough axis, we see a pocket of strong winds. This pocket of strong winds is called a “jet streak”. The positioning of this jet streak upwind from the trough axis typically means the trough will amplify. Why? Let’s go down that rabbit hole!
300 mb analysis valid 1200 UTC September 24, 2018 (image source: SPC).
Geostrophic wind: The flow of air that results from the pressure gradient force (air response to spatial variations in pressure) and the Coriolis force.
Coriolis Force: The apparent rightward turn (leftward in the Southern Hemisphere) of an air parcel due to the earth’s rotation.
Ageostrophic wind: A component of the total wind that is not accounted for by the pressure gradient and Coriolis force (i.e. caused by “something else”). This is a slightly watered down definition, but is sufficient for our purposes.
In the 300 mb plot above, the two features to note are the trough axis (solid black line) and the jet streak (red circle). The jet streak is upwind from the trough axis with the trough axis to the left. I know what you’re thinking: that’s right, not left, on this map! Left/right in meteorology correspond to the reference frame of the flow, so the air is moving from north to south, so left is east and right is west (in this particular location).
Yes, there’s a lot going on in the graphic above, but hang with me here. In this particular conceptual model, the wind is blowing from west to east, so north is the “left” side, and south is the “right” side. The entrance region is where the air enters the maximum (west side) and the exit region is where it exits (east side). We can plot acceleration vectors (red arrows) that indicate how the wind speed and direction are changing with time. This is why on the east side of the jet streak, the acceleration vectors are pointing west (the winds are slowing down, or put another way, are becoming less westerly and more easterly with time).
Through a lot of math, we can show that the ageostrophic wind (blue arrows) points to the left of the acceleration vectors. The result of all of this is that the ageostrophic wind is blowing from south to north on the west side of the jet streak, and from north to south on the east side. The strongest ageostrophic wind is where the largest acceleration is occurring (i.e. along the west-to-east black line). The vectors above are to scale in a relative sense. You may have noticed the words “convergence” and “divergence”. Where you see convergence happening, this is where the ageostrophic wind is “piling up”. By piling up, I mean the wind is slowing down in the direction of flow. Think of it this way: if you are driving 60 MPH behind someone going 30 MPH, you will eventually run into them (convergence!). Where you see divergence, the opposite is true. The wind is spreading apart (you are driving 60 MPH in front of someone going 30 MPH). Where convergence occurs (in the upper parts of the troposphere, which is where this is all happening), there is sinking motion. Where there is divergence, rising motion occurs. This is the opposite of what occurs at the surface, but that’s beyond the scope of this post today (if you really want the full explanation, see this article from Luke Madaus at the National Center for Atmospheric Research).
Air speeding up or slowing down results in ageostrophic wind. That is, wind that is not the direct result of pressure gradients and the Coriolis force.
The ageostrophic wind blows to the left of the acceleration.
The local variations in ageostrophic wind cause air to “pile up”, resulting in convergence and thus downward motion in the left entrance region and right exit region. Conversely, the air “spreads out”, resulting in divergence and upward motion in the right entrance region and left exit region.
So why is all this important? Let’s go back to that 300mb analysis. The left exit region is located directly over the trough. This means that there is going to be rising motion (mass evacuation) within the trough, meaning the trough will get deeper, causing heights to continue falling. Low heights aloft correspond to cooler air within the air mass. The grand result of this entire process is that cold air that is bottled up over the Canadian Prairies will be forced to spread south. And we can also arrive at the conclusion for the opposite case using the same logic. If the jet streak is downwind of the trough, the trough will be underneath the left entrance region (convergence), and will weaken.
For those of you that want a simple rule to remember, just remember that if the strongest winds in a trough are upwind (typically west) of the axis, the trough will amplify. Conversely, if the strongest winds are downwind (typically east) of the axis, the trough will deamplify.
GFS forecast surface temperatures valid 7 PM CDT Wednesday, September 26, 2018 (image source: Pivotal Weather).
The GFS model does indeed show that cooler air making it into Texas by Wednesday evening, albeit much warmer than it is over Canada right now. We will have to get shorter days and some snowpack over the Great Plains and Canadian Prairies before we really start getting some cold air down here, but I hope this explanation will help you to better understand just how we get cold fronts to move south. Perhaps next time, we’ll talk more about where these jet streaks actually come from. I’ll leave you with a simple yet beautiful video showing the quaking aspens already turning in Yellowstone National Park. Make sure to turn on the sound!