27 September 2011
The folks at the NOAA Environmental Visualization Lab have a nice long loop from the GOES water vapor channel out today. It shows the huge cutoff low that has been stuck over the Midwest for several days. I’ve been dealing with this in my day-to-day forecasting as well, and these things can be very tricky to forecast. These are what meteorologists call a cold core low and they are more intense aloft than near the surface.
There is an old saying that “there is almost always a surprise under a cold core low” and there are several reasons for this. One reason is the cold air aloft associated with them that can cause a rather weak thunderstorm to produce a lot of hail, or even an unusually early burst of snow that wasn’t forecasted. Every synoptic weather forecaster (including me) can tell you of busted forecast horror stories, and I guarantee you they will be related most of the time to a cold core low!
The NWP (numerical weather prediction) models tend to handle them poorly and once they develop the atmosphere acts like it’s in a train wreck with everything coming to a halt or at least slowing down. In other words, the normal movement of weather systems is greatly slowed and you tend to get a long spell of the same type weather even if you are well away from the low. This is happening right now with unusually cool and cloudy weather in the Midwest but just the opposite weather is covering the Rockies with sunshine and warm weather day after day!
Update: Scott Bachmeier up at CIMMS has also posted some incredible imagery using the McIDAS software that includes the MODIS 1 km images. They are gorgeous!