Saturday, August 16, 2014

Shifting upper level pattern means changes in Mid-South weather

A very cool July has spilled over into August with the first half of the month averaging 1.9 degrees below normal. This past week especially was very pleasant with below normal temps and humidity, resulting in weather that felt more like mid-September. However, the upper level trough over the eastern U.S. that was present this week and has dominated much of the summer has departed, allowing for warmer and more humid air to begin making its way back into the Mid-South.

A large trough of low pressure at the mid levels of the atmosphere (18,000') was present earlier this week, bringing cooler weather to the eastern U.S. and Mid-South.
The upper level pattern this weekend into early next week will be more "zonal," meaning air will move west to east with a lack of troughs and ridges (or hills and valleys) in the pattern. A zonal flow typically means near average conditions with weather systems moving west to east that tend not to be very strong.

A zonal pattern is evident at the jetstream level (34,000') by Sunday evening as wind flows across the country pretty much from west to east with no major hills or valleys in the pressure field across the CONUS.

In this case, we need to remember that we are, climatologically-speaking, just past the hottest part of the year. So a "normal" pattern will mean warmer and more humid conditions. In addition, we'll see increased chances of rain and thunderstorms. Since a zonal flow typically doesn't result in strong weather systems, severe weather is not expected, though summertime storms can result in strong wind gusts and brief downpours due to strong heating of the lower atmosphere and higher humidity levels. This hot, humid pattern with chances of thunderstorms is expected from Sunday through the first half of the week.

By mid-week, a large ridge of high pressure looks to build in the upper levels over the central U.S. and slowly shift east late this week. High pressure aloft generally forces weather systems north (or over the ridge) and result in above normal temperatures under the ridge.

The pattern at jetstream level (34,000') by Thursday shows a ridge of high pressure building over the central U.S (note the "hill" in the solid black lines over the Plains. Upper level ridges typically result in drier and hotter surface weather.

The result for us? A drying pattern with hotter temps. In fact, some model data is pointing to high temperatures approaching 100 by next weekend, though we've seen the same data forecast temperatures that were too warm many times this summer. Mid 90s could be a fairly safe bet though for late week into next weekend. In fact, the Climate Prediction Center's 8-14 day outlook covering the last week of August shows that this pattern may stick around, resulting in above normal temperatures for the rest of the month. The atmosphere tends to balance itself out over time, so this may be payback for a mild summer! We've only had a single 95 degree day this summer, so we're still due for some heat.

The 8-14 day outlook from CPC for August 23-29 indicates a 50% chance of above normal temperatures.

For the complete MWN Forecast for the upcoming week, click here or visit our mobile apps, linked below.

--Erik Proseus, MWN Meteorologist

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