Monday, December 8, 2014

Ridges and troughs: signs of fluctuating temperatures this week

It was a rather pleasant early December day in the Mid-South, but don't get too comfortable as changes come quickly. It's the time of year that variations in the weather pattern can come frequently and we have a classic pattern setting up that favors quick swings in temperature as the atmosphere tries to figure out what season it is.

The Mid-South will get wedged between an upper level trough (or lower pressure) to our east and an upper level ridge (or higher pressure) to our west for most of the work week. This will result in what is called "northwest flow" for the area, in which the upper level flow of air is from northwest to southeast. This pattern is usually quite progressive, resulting in large oscillations in temperatures due to frontal passages and increased precipitation. In this case though, the pattern looks a lot drier, with a shortwave embedded in the flow only able to bring cloud cover (and perhaps a few sprinkles) to Memphis Wednesday into Wednesday night.

Upper level map showing height anomalies, indicative of ridges and troughs.
The Mid-South wedged in between the two, resulting in northwest flow.
Now, lets talk about those temperatures swings. First we'll start with a cold front that is coming through the Mid-South tonight, which is associated with a weak upper-level trough that makes its way through the area and then strengthens to our east, thus strengthening the northwest flow. This will bring cooler temps south for the next couple of days, resulting in lows near freezing Wednesday morning and highs in the mid to upper 40's for the middle part of the work week.

Surface temperature anomalies Wednesday morning showing temperatures about 8 degrees below average for the Mid-South.
These below average temps won't stick around too long though, with a building upper level ridge to our west that will slowly make its way into the Mid-South. We'll start feeling the effects of that ridge by Friday as above average temps settle in for the weekend (high temps in upper 50's and lower 60's).

Surface temperature anomalies during the day on Saturday. A drastic difference just 3 days after the previous map.
Looking into the longer range, there is fair agreement that temps will stay above average heading into next week. The Climate Prediction Center agrees, with their probability forecasts showing upwards of a 50% chance of above average temps all the way through the 3rd week in December. This does not mean that we won't see periods of cold, just that above average temperatures are more likely for the majority of the time.
The Climate Prediction Center forecasting over a 50% probability
that we see above average temps from December 14th-18th (early next week).
Below are the expected highs and lows for this week and as always, you can get the full forecast here. Also follow us on Facebook and Twitter for daily details on the forecast.

William Churchill
Social Media Intern

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