I’m currently keeping
a close watch on the small but noteworthy possibility of a winter event
occurring across parts of the South after January 15th.
If you strictly look analyze model data at the mid and upper levels of the atmosphere, you’d be tempted to believe that the pattern is complete garbage for wintry weather probabilities farther south. However, the GFS model has a ridge building into the Gulf of Alaska and Alaska and a trough downstream of that ridge just off the West Coast. Then, it has a broad ridge over the central U.S with another trough over New England. Overall, the mean-layer flow is pretty zonal across the central U.S. and Southeast. Ridging usually means warmer weather, but how is this different?
You must consider the
placement of these features aloft, and what will occur at the surface in
response. I drew some arrows to show the overall flow with a pattern like this.
Essentially, winds aloft go up and over the ridge, and winds also go around and
under the base of the western trough. As a result, confluence occurs over
western Canada. In response, surface high pressure develops over western
Canada, and if the high moves into the U.S., it funnels cold, Arctic air into
the U.S. due to the clockwise flow associated with high pressure in the
If a southern stream shortwave were to become embedded within the zonal flow and precipitation were to occur as a result, then it’s not entirely impossible to get some wintry precipitation (ice or snow) across parts of the Southern Plains and Southeast with this type of pattern.
Keep in mind that this
pattern needs to keep showing up in the models. Any minor changes in the
placement of these mid to upper level features can greatly change the outcome
of a forecast. I do believe there’s value in using pattern recognition to
consider the different possibilities using model data. Even if nothing occurs,
it’s still good to learn about what would happen under a given pattern.
Matthew Holliday (Curriculum Vitae - Resume) is a graduate of the University of Oklahoma, where he completed a B.S. in Meteorology and a B.S. in Geographic Information Science. He completed his master's degree in meteorology and climatology at Mississippi State University and is currently pursing a Ph.D. in Earth and Atmospheric Sciences.