Sunday, July 3, 2016

Sunday evening's Independence Day weather update

It's been a hot holiday weekend with a couple of stray showers that provided very brief respites for those lucky enough to be under one. Looking ahead to your "bonus weekend day" tomorrow, the weather unfortunately won't cooperate nearly as well. However we're hopeful that most thunderstorm activity ends before sunset so that all local fireworks displays can go on as planned!

Tomorrow's increased thunderstorm chances come courtesy of an mid-level (about 18,000' up) disturbance (or shortwave trough) that will move towards the region. A cold front remain anchored to our north, which keeps the Mid-South in a hot, humid, and unstable airmass as the disturbance approaches with temperatures reaching the lower 90s by early afternoon and dewpoints in the mid 70s. That disturbance will provide the lift necessary to trigger scattered thunderstorms as instability increases, most likely from late morning through the afternoon hours.

A cold front will be well north of the Mid-South, allowing for temperatures to reach the lower 90s on Monday. This warm air, along with abundant moisture, will help provide the "fuel" for mainly PM thunderstorms as a shortwave trough approaches.

Depicted above is the NAM model's prediction of the atmosphere at about 18,000' Monday afternoon. A shortwave trough (indicated by the black curved line) will be moving into the region with positive "vorticity" (or spin, indicated by the orange/red/purple colors) providing the lift to generate thunderstorms as it moves closer.

Meanwhile, precipitable water values (PWs) will be very high, indicating an abundance of moisture in the atmosphere over the Mid-South, which will contribute to heavy rainfall from any storm that forms. Values over 2" (pinks) are high enough to cause flash flooding, especially if multiple storms traverse an area.
With expected daytime convection, hopefully it will stabilize the atmosphere some, thus as dusk approaches, activity should wind down heading into the overnight hours.  The most difficult part of the forecast is determining when that decreasing trend commences and if any leftover showers or thunderstorms persist by the time fireworks are set to glow over the Mid-South.

As for any threats from tomorrow's storms, with instability high, atmospheric moisture very high, and a bit of mid-level wind energy, a few storms could bring the threat of strong wind gusts from microburst downdrafts, as well as very heavy rain. Tornadoes and hail are not expected from this setup, though a storm or two could conceivable produce some pea-size hail. As of now, the metro is under a Marginal Risk (category 1/5) of severe weather with a few storms possibly producing severe weather. These would be most likely when instability is maximized during the afternoon hours.

To recap, a slight chance of showers in the morning quickly becomes a good chance of scattered thunderstorms (a few with strong wind gusts and most with heavy downpours) by early afternoon, tapering to low chances of a few showers or a thunderstorm by 9-10pm.

If you have outdoor plans tomorrow afternoon and evening, pay particular attention to the weather scenario and be prepared with a plan B no matter where you are. If you hear thunder, you're close enough to get struck by lightning. Quickly move the party indoors until the storm passes (we aren't expecting any all-day washouts)! Also know that loose outdoor objects could get blown away if a storm passes overhead and that localized flash flooding is possible, especially if multiple storms affect a single area.

We highly recommend the mobile app to monitor radar trends and our latest Twitter activity on storm locations/threats. In addition, StormWatch+ in the app will be a handy tool to alert you to any severe weather threats in your area as you're going about your day. Links to the app are available below.

Erik Proseus
MWN Meteorologist

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