Summer heat and humidity abates as schools resume

A busy week in the the weather world started hot, included three separate Memphis metro severe weather events, and ends with another hot weekend. However, a cold front and upper level pattern shift early this week promises more comfortable late summer weather as schools resume.

Recapping last week

The past week included two lines of storms with high wind that resulted in accumulating power outages and downed trees on Wednesday. In sum, over 50,000 MLG&W customers alone lost power between these two systems that moved through late Wednesday afternoon then again just before midnight. As utility crews were busy repairing their infrastructure, temperatures were a bit more comfortable to end the week but ramped up this weekend with Heat Advisories back in place. In addition, one additional line of storms moved through the metro about rush hour Friday morning, sporting a very photogenic shelf cloud and prompting warnings, but doing little in the way of additional damage thank goodness!

The shelf cloud Friday morning approaches Arlington in this picture by Laney Borwick.

Looking ahead

Monday will be the last day of above normal temps and Heat Advisories for most of the area as a cold front and pronounced upper level pattern change ushers in cooler temperatures and much lower humidity by mid-week that looks to last into early next weekend at least.  Let’s get into the details.

Heat Advisories and Excessive Heat Warnings in effect as of Sunday afternoon reflect the heat provided by high pressure at the upper levels and southerly flow escorting high dewpoints from the Gulf into the region.

Technical forecast discussion

At the upper levels, a large dome of summertime high pressure centered over Texas dominates the southern U.S.

Upper level (18,000′ or 500 mb) high pressure is centered over the southern Plains Sunday evening. Graphic courtesy WeatherBell

However, as we head into the week, that high pressure will retrograde into the western U.S. while a large trough develops over the eastern U.S. with pressure lowering over the Mid-South in the wake of an early week cold front. As the upper high moves west, heat will abate, dropping temps to below normal levels from mid-week into the early part of the weekend.

Upper level (18,000′ or 500 mb) high pressure moves west by Wednesday evening. Graphic courtesy WeatherBell

A good way to visualize the dry airmass that will move in behind the front is by looking at precipitable water (PW), which measures the amount of water vapor in a column of air if it condensed into rainfall and piled up on the ground. In other words, it’s somewhat a measure of humidity, only it is not just surface humidity, but humidity throughout the whole atmosphere.

The first image below shows PW early Monday evening. Values in the metro (red) are near 2″, which is roughly in the 90th percentile for this time of year (meaning only about 10% of the time is it higher than this). However, by Wednesday evening (second image below), those values drop to about 0.75″, which is near the minimum observed this time of year!  So the atmosphere as a whole will be much drier by mid-week.

Precipitable water (PW) values on Monday evening, as described above. Graphic courtesy WeatherBell.

Precipitable water (PW) values on Wednesday evening, as described above. Graphic courtesy WeatherBell

Forecast for the week

As for what to expect this week, first remember that Heat Advisories remain in effect Monday. Temperatures will again be in the mid to upper 90s with heat indices above the 105° danger level. Take frequent breaks if outdoors and keep drinking water! Below is the forecast precipitation amounts with the frontal passage. As you can see, very little rainfall is expected ahead of the front locally and that will be in the form of widely scattered thunderstorms Monday afternoon and evening.

As for the severe weather threat, we are currently in a Marginal Risk (category 1/5), which is low, but not no-existent. The main threat will be that a few storms could produce damaging wind Monday afternoon and evening, though again, not a lot of storms are expected.

As for the rest of the week, temperatures will moderate to below normal levels by Wednesday, then back to near normal by the weekend. The 60s temps in the mornings will feel great! Plenty of sunshine is expected with low dewpoints, thus heat indices will not be a factor starting Tuesday.

Erik Proseus
MWN Meteorologist

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