Unusual summer pattern (but not for this year) maintains cool conditions

What a pleasant reprieve we have had the past few days from typical August conditions! Actually, after a below average July temperature-wise, August is running nearly as much below average as July was. Through the first half of the month, the average temperature has been 1.8 degrees below normal at Memphis International.  As we indicated this past weekend, a cool spell has dominated the second half of this week with many areas seeing morning lows in the upper half of the 50s the past couple of mornings, while afternoon highs have been near 80 for 3 straight days now. In fact, the official low of 60 on Thursday morning was 2 degrees above the record set 60 years ago!

Driving this cool pattern has been another large-scale trough over the Great Lakes and Mississippi Valley, similar to the setup we have experienced multiple times this summer that has contributed to cooler than normal conditions. An upper-level trough pushes the jet stream to the south and allows cooler air normally reserved for Canada to dip south under the trough. With a trough nearby, disturbances in the atmosphere ("flies in the ointment" we like to call them) can move through, triggering showers and thunderstorms.

At the jet stream level, a large trough dominates the Mississippi Valley, bringing well below normal temperatures to the eastern U.S. The jet stream (represented by the blue colors above) has been shoved well south of it's usual summertime position.

Once such disturbance is approaching the area today, moving across Arkansas and towards west Tennessee this afternoon. It will bring a slight chance of showers to the metro later this afternoon and evening, though it is so weak that it's not producing much in the way of precipitation, and certainly nothing strong. As the trough lingers, we'll see continued small chances of rain through the weekend, but no washouts and nothing to change plans over. The additional cloud cover will help moderate temps for another couple of days.

In the mid levels, the trough is evident in the east as massive high pressure dominates the western U.S.  An upper-level disturbance (lighter blues on the map) will be nearly overhead at midnight, but little in the way of precipitation is expected.

By next week, the summer ridge of high pressure will move back in and bring with it more typical August weather, though not excessively hot. Most of next week should be dry, but a few disturbances could trigger scattered showers or thunderstorms, especially to our east where an upper-level low could take up residence. Highs will be back in the lower 90s with August mugginess and warm nights in the 70s.  I guess we can be thankful for a fairly mild summer and definitely a few days this week that felt more like late September than mid-August! Find the complete 8-day MWN Forecast for the weekend and next week here (mobile link).

--Erik Proseus, MWN Meteorologist

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