« Surprise rain | Main | Changing pattern »

Fall Changes

Now that we are in September, we begin to look for fall changes. These changes are directly tied into the jet stream. During the summer, the jet stream usually retreats to the north into Canada. As fall begins the jet stream strengthens and begins to flow further south. January & February is when the jet stream reaches its peak strength & usually its furthest south. Note, the jet stream is caused by temperature differences, cold north-warm south. During the winter the colder air is further south, so the temperature gradient in the atmosphere is further south, hence the jet stream is further south.

So are we seeing any signs that the jet stream is headed south? Yes,
below are 2 upper level charts. The first one is from today. You will see that the jet stream (tigter collection of lines) is in southern Canada. The second map is a forecast for 10 days from now. Notice, how the jet stream is forecasted to sag south into the United States. This means in 8-15 days we may have our first fall cold front.

MAP 1: Upper Level Chart from September 3, 2005

MAP 2: Upper Level Chart Forecast September 13, 2005

</a>