Embedded Files
When forecasting for one or two days in advance, the location of upper-level jet streams and troughs is critical. In the module on model errors, I mention that water vapor imagery was particularly useful in locating these features. Here's how to do it.
Not all troughs and jets show up on water vapor images, but when they do, they're usually easy to identify, especially if you compare the image with an upper-level map or a tropopause map. Here is a water vapor image of the Pacific Coast. Among the prominent features are a dark, curved swath (labeled "A"), representing a region of especially dry air, and a long boundary between dark and light (labeled "B"), representing a boundary between moist and dry air. Often, the curved dark areas represent troughs in the upper troposphere, while the straight boundaries mark the core of jet streams. If that's what these are, we can use the images to track the location and evolution of these upper-level features.
To identify them, compare the water vapor pattern to the twelve-hour forecast from the Eta model, valid three hours after this satellite image was taken. This forecast map includes upper-level wind speeds along the tropopause; the same technique works with the more conventional 300 mb and 500 mb maps. The curved feature marked "A" can be seen to correspond to the edge of a particularly strong upper-level trough, associated with unusually cold temperatures. Meanwhile, the feature marked "B" corresponds to the subtropical jet, and is associated with a core of strong winds extending from the Pacific into Colorado. If care is taken to compare the locations of these two systems, the accuracy of the 12-hour Eta forecast can be evaluated.
Report abuse