Sometimes making the extended forecast is a relatively straightforward process. For example, last weekend I called for 94 degrees on Friday. The models suggested 90 degrees, but the upper air pattern looked a little warmer so I trended my number in that direction. As luck would have it we hit 94 yesterday afternoon. Truth be told, the long rang predictions don’t usually go that smoothly…and today is one of those days. A difference has developed between two of our main computer models for next week, and that’s caused me some headcaches while putting together the seven day. The general weather story goes like this early next week: an upper level trough will move from the Central Rockies to the Great Lakes and as a result, surface low pressure and a cold front will try to push into Middle Tennessee around Tuesday. That’s all fine and good, but now the WRF model and the GFS model have a difference of opinion on the specific timing and placement of that front. The WRF is the slower solution; it develops a closed upper low over the Plains on Tuesday and that would cause the surface front to stall off to our west. If the WRF solution verifies, our rain chances will stay slim until Tuesday and Wednesday. The GFS is more “progressive” and brings the upper low to the Great Lakes as planned, which carries the surface front into the area late Monday through Tuesday and gives us decent rain chances both days.

It’s a small difference, but it has ramifications through the week and impacts the entire seven day forecast. The GFS model develops a “cut-off” upper low pressure and brings it toward Middle Tennessee on Wednesday, Thursday and Friday. That would mean cooler temperatures, but it might also mean clouds and some scattered showers through the end of the week. I was a bit surprised that the GFS would develop a “cut-off” low this time of year, but that idea is also reflected in the European model so I’ll give it some credence. That solution (an upper low near the Mid-South) would also impact the tropical forecast. Let’s say that Tropical Storm Ernesto (currently in the Caribbean) continues toward the Gulf of Mexico by the middle of next week. If an upper level low were in place over the Mid-South, there would be a weakness in the subtropical ridge and Ernesto would be drawn toward the northern Gulf Coast. Last week some of the long-range guidance suggested Ernesto could travel toward the Texas coast, so this is a change in the models that we’ll have to watch closely. There’s always something to wonder, ponder and doubt when it comes to the long range forecast!

This entry was posted on Saturday, August 26th, 2006 at 8:33 am and is filed under Forecasts. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.