An Unusual Setup to the Beginning of Fall
September 24, 2011 9:25 PM
More often than none, our weather moves from west to east, but with a cut off low over the Great Lakes, we’re in kind of an unusual set up. At the time being in the upper levels of the atmosphere, a ridge of high pressure is in place over the plains, keeping us clear and dry. To our east in the Great Lakes a cut off low is hanging out, slow to move. Typically cut off lows take some time to move out of the region. Over the next 24 hours we could see an increase in the cloud cover, depending on how far west, this area of low pressure begins to move. Notice the temperature difference on the image to your left, with cooler high temperatures around the Great Lakes and warmer temperatures west of the James River. Cloud cover makes a huge difference when it comes to predicting highs.
A cut off low is a closed low, which is cut off from the jet stream. The jet stream is what drives the weather. In the mid west, our weather is driven by the westerlies, which is what allows our weather to move west to east. The jet stream is found in the upper levels of the atmosphere, at approximately 300 mb. At the lower levels of the atmosphere it is revered to as the low level jet, which plays a key role in forecasting severe weather. When an area of low-pressure gets cut off, it has nothing to really move or steer it along. Beginning Saturday night the area of low pressure located over the Great Lakes will start to retrograde or work it’s way backwards into the Northern Plains. Which means we could see an increase in the cloud cover Sunday afternoon along and mainly east of the I-29 corridor, but for the most part it should stay partly cloudy in the Sioux Empire.
The wind flows in a counter clockwise fashion around an area of low pressure. With a low sitting just off to our west, our winds will be coming out of the east on both Sunday and Monday morning, ushering down cooler air. We’re not looking at a major cool down, but temperatures will be in the 60’s opposed to the 70’s. You can think of an area of low pressure as a pinwheel. As the disturbance begins to rotate westward, the cloud band will begin to pinwheel around it…streaming down some low and high-level clouds Sunday afternoon through Monday morning. By the mid portion of the week, a ridge of high pressure will begin to build back into the region, and the cut off low will move well out of South Dakota, allowing temperatures to slowly be on the rise. Some of us could even reach the 80’s by the mid portion of the week.
Oftentimes meteorologists' will use a satellite image to see how well the models are handling the current set up. The water vapor image below shows the cut off low just east of the Great Lakes. The second image shows clouds from Eastern Iowa onward, with clear skies in South Dakota. The following images are output from models…or what they think should happen.
GFS Model, 12Z run from this morning, for Saturday evening: shows low over northwest Illinois/eastern Iowa.
European model, from 12z for Sunday morning: shows low over eastern Iowa by Sunday AM.
By looking at the actual setup (satellite imagery) as well as the model output, it looks like the bulk of the cloud cover will stay to our east in Iowa as well as Minnesota, however we could see a slight increase in cloud cover along and east of the I-29 corridor, mainly in southeastern South Dakota Sunday afternoon, and northeastern South Dakota Monday morning.