Friday, July 21, 2017

Excessive heat slowly gives way to increased rain chances


We'll start this blog with a bit of "hopeful" news as we bake in the Mid-South sauna...

This map by Brian Brettschneider shows the normal midpoint of summer for a given location. Here in Memphis, it's around July 17 according to the data. We're slowly making progress towards fall!
What this means is that the average "peak" of summer heat here in the Memphis area is now in the rearview mirror. We're on the long downhill slide towards autumn. But summer is certainly not over yet!

A sprawling high pressure system at the mid and upper levels of the atmosphere is responsible for bringing the heat the past few days, while surface high pressure just to our east has allowed Gulf moisture to pool over the region, creating a very humid airmass that has exacerbated the hot conditions.

High pressure at the mid-levels of the atmosphere (500mb or 18,000', shown above) sits directly over the region, resulting in very hot air in the Mid-South under this "heat dome." It will slowly weaken as shift east this weekend. (PivotalWx)
UPDATED 3:00PM: An Excessive Heat Warning is in effect for much of the metro through Saturday evening and Heat Advisories remain in effect through Sunday as temperatures rise well into the mid (and for today, perhaps upper) 90s. Heat indices are averaging near 110° at mid-afternoon as the humidity has cranked up as well, evident in dewpoint readings in the upper 70s to near 80.

Like yesterday, there is only a slight chance of a cooling shower today, and it certainly does very little for the high humidity values. The general trend continues Saturday, though the upper level high pressure system that has brought the "heat dome" to the middle part of the country the past few days will start to move east, allowing perhaps a few more clouds and widely scattered showers, which could work to keep the heat index below 110° and temperatures in the mid 90s instead of creeping towards the upper 90s. If you have to be outside the next couple days, please take precautions against the heat and listen to your body for these signs of heat stress:

As high pressure aloft weakens this weekend, the heat will abate just a bit, as well as allowing rain chances to increase a bit, especially during the afternoon hours. Humidity will remain high, but with temperatures still in the mid 90s through Sunday, heat indices will remain above the 105° danger level. A few afternoon t'storms are possible Saturday, then scattered thunderstorms are expected on Sunday into Sunday night.

By early next week, our high shifts further east, a trough of low pressure develops in the Great Lakes region, and another high forms in the western U.S. The overall impact on our weather is that we'll be in a "northwest flow" pattern, meaning the wind aloft will blow from northwest to southeast. This is a pattern that favors upper level disturbances to move across the region from the northwest.

A loop of the mid-level (18,000') pressure and wind pattern, every 12 hours from Friday morning through next Wednesday morning, shows the high pressure ridge over our region breaking down by early next week as a trough forms to our north, then rebuilding from the west by the end of the loop (middle of next week). (PivotalWx)

In addition, a cold front will sink into the Mid-South by Tuesday, but stall out. This combination means the likelihood of scattered showers and thunderstorms to start the week, holding high temperatures down closer to 90°. Humidity remains of course, making for a muggy forecast.

By mid-week, high pressure to the west begins building back into the region, the front lifts back to the north, and temperatures warm back up into the 90s. We will likely still see a few daily thunderstorms typical of summertime but the storm track should shift back a bit north of the metro by Wednesday or Thursday.

But as I said at the top of the post, we're slowly getting closer to autumn than we are moving into the heart of summer! Schools start in a few weeks, which hopefully means the warmest part of the year will soon be behind us!

Erik Proseus
MWN Meteorologist

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