Welcome to December 2011. After a wet Saturday, the weather is taking a turn for the cold. A cold front has moved through our area Sunday morning (Dec. 4) but more importantly is the shift in the upper air wind this week.
One of the things forecasters look for in deciding the near-future trend in temperatures is the North Atlantic Oscillation (NAO). It tracks the fluctuation in pressure between a polar low pressure over Iceland in the North Atlantic and a subtropic high pressure over the Azores (off the northwest coast of Africa). The value of the NOA varies between negative and positive values and can flip-flop between the two every couple of weeks.
Based on the long range numerical prediction computer models such as the GFS, the NOA value is positive now and is expected to remain positive for the remainder of the week. This means a stronger polar low and weaker subtropical high. This typically results in a colder-than-average weather pattern over the upper Midwest and the Great Lakes.
Below is the chart for the forecasted 500 miilbar upper air flow (a height above the ground that corresponds to approximately 18,000 feet) for Friday, December 9, 2011. Note the deep trough, or dip, in the upper flow and a strong polar low pressure wave to our north. The upper flow plunges cold air from central Canada into Wisconsin.
From the GFS global numerical prediction model, showing the forecasted 500mb flow over North America for Friday, December 9, 2011.
The NAO is expected to become less positive after 7 to 10 days from now and this could mean we lose our grip on the arctic air and begin to warm a little. The forecasted 500 millibar flow for Monday, December 19, 2011 is shown below. Note how the flow over the upper Midwest and Wisconsin has become more zonal (west-to-east), meaning the cold, arctic air will be pushed farther north and our temperature will begin to moderate.
From the GFS global numerical prediction model, showing the forecasted 500mb flow over North America for Monday, December 19, 2011.
Long-range computer models like the GFS are not infallible, but they do show a general trend. Going out beyond 7 to 10 days, the skill level of these models is decreased. However, it is something we consider when looking at long term trends in our weather.
If the cold temperatures of the next 6 to 7 days get you down, hang in there. The odds are good tht a warming trend may be a little over a week away.