Good Evening! This is pretty interesting, thought that I'd share it with you. You hear me speak of not very powerful pressure systems. Here's a powerful system for you. Look at the loop in the Jet Stream by New England. Notice how close the Isobars (the white lines showing the dip of the Jet Stream. These are lines of equal pressure in the atmosphere, and if they look like lines of a topographical chart of land formations, you're right. Those lines on a land chart show lines of equal elevation - these are like that and lines of equal pressure, and it shows the pressure gradient) are together. This is a 300 MB chart, which means the it's 30,000 feet above Sea Level. It's a common altitude for jet liners to fly at. This is 8 days from now:
Now, look what happens a couple of days later:
A new Low Pressure center is created, and again, (I've been talking about these for a couple of weeks now) it is a Cut-Off Low. But, look at this next map:
Up in Canada above Lake Superior, you see the center of the Low, shown by a L with numbers below it. The numbers are the lowest pressure of the system, in Milibars. 877.8 millibars is equivalant to 25.92 inchs of mercury (29.92 is standard sea level pressure). Now - this is a storm. This is hurricane strength type storm. And the cut off low is a massive storm in it's own right. Anyway, that's tonights gee whiz thing. You don't see this sort of thing very often. It's worth watching to see what this thing does. And I want to you think about this storm and the hugh number of Isobars, when I refer to pressure systems with little power. Power equates to number of Isobars, and how close these are together. The closer together, the greater the number of Isobars, the stronger the Low pressure center, and the stronger the winds associated with the system.
This is a progression over two days of the cut-off low formation in the SW that is bringing the rain to the SW area:
As you've probably seen in the past, once the Cut-Off Low is formed, it no longer follows the Jet Stream, and has been the case with the last several, they stick around in the area where they are formed.
Now, let's look at 4 days from now in the Midwest Jet Stream:
See what I'm talking about. Look how far apart the Isobars are. This is a stable atmosphere. Very little lifting action, very little turbulence to aid in thunderstorm development. This is my basis for disagreeing with the extended forecasts. Now, can we still get thunderstorm development. Sure. Unequal heating of the Earth's surface is enough to cause updrafts, and get moisture high enough in the atmosphere to cool it to the Dew Point (where the air in the the atmosphere condenses and causes visable moisture - clouds, rain, fog, etc) which can then fall as rain.
This is Tuesday's map. The purple is showing possible moisture - lots of coverage area, very little in the way of large areas of precip:
The darkest purple areas are showing 0.175 inches of rain. Whoop-de-ding. The green spot in E TX and LA is 0.50 (1/2 inch) of precip. Now again - this doesn't mean that a big thunderstorm can't get going and dump on you. But with the stability in the Jet Stream atmosphere, these are the type of storms that should generally die after sunset, because there is no more lifting mechanism to push the air mass where it gets cold enough to cool it to the Dew Point, and the storms die.
Anyway, there ya go. My lesson for tonight. Rain for the Midwest? Some - Scattered, and not very much.
Jen
