Tuesday, July 26, 2016

A break from the heat as rain chances increase

The heat and humidity of the past week(s) is about to take a break as the Mid-South weather pattern shifts from one dominated by surface high pressure and ridging aloft to one more conducive to rain, some heavy, and thunderstorms. This pattern takes us into the weekend before high pressure starts building back early next week.

On Wednesday, weak low pressure of tropical origin moves slowly north from Louisiana into the Mid-South. As it does, moisture values increase to very high levels as lift provided by the approaching low and mid-level energy combine to produce showers and thunderstorms across the region. Though a few will be possible throughout the morning, chances increase to "likely" by lunchtime or just after in the metro. Severe weather is not expected as wind energy remains low and instability (the fuel for the fire) will be present, but not overly concerning.A few storms could produce some wind gusts, but the main threat we'll be monitoring will be heavy rain and the potential for low-end flash flooding.

Precipitable water (PW), a measure of total atmospheric moisture content at all levels, increases to near record levels by Wednesday evening, nearing 2.5" as tropical moisture moves over the region. PW tells us how much water is in the air that can be "rained out." Graphic courtesy Pivotal Weather.

On Wednesday afternoon, a weak surface low moves slowly into the region as moisture increases, bringing enhanced lift in the atmosphere that generates precipitation.  Graphic courtesy Pivotal Weather.

As the surface low moves into the region, so too does a mid-level low/disturbance (500 mb or about 18,000') with fairly strong lift (depicted by the yellow to orange colors). That should help to focus to the heavier precipitation during the afternoon and evening hours on Wednesday. Graphic courtesy Pivotal Weather.
This pattern takes us into Thursday as the low meanders to our west and washes out, but a trough of lower pressure forms over the region at the upper levels. High pressure ridging aloft is conducive to sinking air and drier, warmer weather, but lower pressures, such as those associated with a trough, foster rising air, more clouds, and a wetter pattern. With mid and upper level disturbances moving through the trough aloft, the wet pattern will continue into late week and then the weekend. Wednesday and Thursday currently have the best chances of continuous precipitation. The good news is that cloud cover and rain will keep temperatures out of the 90s for a couple days and rain will also make a dent in our recent precipitation deficit.

The GFS model forecast mid-level (500 mb or 18,000') pressure map valid Saturday morning shows the trough of lower pressures over the Mid-South as wind flow "kinks" from northwest to southwest near the Mississippi River. The axis of the trough is shown by the dashed line. The trough and energy flowing through it (yellow/orange) enhances lift, which generates showers and thunderstorms. Several of these mid-level disturbances are forecast to move through the general trough into the weekend. Graphic courtesy Pivotal Weather.
Scattered showers and thunderstorms, especially those associated with the approach of upper level energy (which can't be timed specifically this far in advance) and daytime heating, will continue to be possible Friday through Sunday. Rain chances these days are near 50%. The chance for a few strong storms increases slightly this weekend as rain becomes more sporadic and daytime heating allow temperatures to return to near 90.

NOAA/NWS predicts heavy rainfall over the next 5 days south of the Ohio River into the Mid-Mississippi River Valley. The Memphis area could easily see 2-3" of rain with some areas that get thunderstorms receiving more.

By early next week, high pressure aloft begins to re-build over the region, squashing rain chances and bringing a return of summertime heat. That process will take place gradually the first half of next week.

For those with outdoor plans the next several days, keep a close eye on the sky and StormView Radar on MemphisWeather.net and our mobile apps (links below), and have your backup "plan B" ready! We'll be keeping you updated via social media as well.

Erik Proseus
MWN Meteorologist

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