Evening update on tonight’s severe storm potential

UPDATED Thursday, 9:15pm:

The Storm Prediction Center has dropped the Moderate Risk, but the entire Mid-South is still in a Slight Risk of severe weather overnight. The prime reason for the Moderate Risk was due to the possibility of supercells late this afternoon and evening that posed a hail/tornado risk. Those are obviously not going to materialize at this point.

However, earlier forecasts still apply overnight.  As the front moves closer, storms are expected to break out across the area and continue for possibly several hours. We expect storms to occur after 11pm (maybe even midnight) and will last through 4am or so. A squall line will move through between 1-3am most likely and that would bring the best chance of severe to damaging wind (60-70 mph) across the region.

These types of systems are also known to generate small spin-up tornadoes that are brief and typically weak.  So, in the event of a Severe Thunderstorm Warning, know that these types of spin-ups  can occur with little warning. They are very hard to pick up and quickly warn for as Tornado Warnings and can lift before the warning is even issued. We expect a watch to be issued within an hour or two for the metro, most likely a Tornado Watch, even though the primary threat will be damaging wind.

Overnight, you need a way to be alerted to severe weather that 1) affects you, but 2) not if it doesn’t (for instance, the other side of the county). We strongly recommend a NOAA Weather Radio (which satisfies #1, but not #2) and StormWatch+ in the MemphisWeather.net mobile app. Read the last paragraph of the original post below for why we believe so strongly in StormWatch+ and why it is a must-have.

One note regarding StormWatch+ though… for iOS users (iPhone, iPad): if you have your device silenced, muted, on vibrate, or on “Do Not Disturb,” StormWatch+ will NOT override that setting. Apple does not allow it. So if you want alerts, don’t use these settings tonight!  Android users on silence or vibrate will not get alerts, EXCEPT Tornado and Severe Thunderstorm Warnings, which we have set to override your silent setting. We think they are that important. You may set a “Quiet Time” in the app if you don’t want to hear the audio overnight.

We’ll continue nowcasting throughout the night to keep you informed and safe. Be prepared, not scared, and know that if thunder wakes you up, we’ll be hear to let you know what’s happening!

ORIGINAL POST Thursday, 3:45pm:
Earlier thunderstorms across AR are diminishing to showers, a mini-line of storms along I-40 on the west side of the metro fell apart quickly this morning, and all is quiet in the metro as I type this about 3:30. So where are all the storms we were supposed to get today?

How thunderstorms are like a pot of boiling water 

Most of you have boiled a pot of water on the stove (if not, your momma ought to be strung up for letting you out of the house [ok, that was unnecessary; Facebook has me on edge today…]). Picture that pot of water – in this case a clear one so you can see into it, like the one above – starting to bubble away as the heat rises under it. We have to leave the lid on in this case.

The water in the pot is the moisture, one ingredient necessary for thunderstorms. The heat from the burner is instability, another necessary ingredient for storms that typically is the result of heating of near-surface air but which can also result from colder air high in the atmosphere blowing over warm (unstable) air. Lift is the third ingredient necessary for storms, typically provided by a front, upper-level disturbance, or other mechanism that starts the air rising.  (It’s also very hard to mimic with a pot of water, so here’s where the example starts to fall apart…)

Anyway, if you leave the lid on the pot, the steam (rising air) created is trapped.  The same thing happens in the atmosphere.  A temperature inversion (rising air with height, rather than cooling air which is more typical) is what we call a cap – or the lid on the pot. Today we have most of the ingredients for storms, except for a lifting mechanism, and we have a cap at about 5000′. Thus, no storms! However, with a good deal of instability and plenty of moisture in place, if the cap were eroded, storms would likely erupt and could become severe fairly quickly due to the additional ingredient of atmospheric shear, or wind energy, which is also present.

So what am I saying? Most ingredients are in place, but with a cap and no lift, we don’t get storms.  This is what we call a “conditional” threat of severe weather. The severe weather is conditional on storms actually forming.

So when do I think all of the ingredients come together?  Tonight

Here’s what we think will happen:

The cap weakens some, but with no source of lift, the next several hours should remain mostly dry. A few showers are possible, and perhaps even a couple of thunderstorms, but until the dynamics (lift and wind energy) of the main weather system to our west move closer, we’ll stay warm, muggy, breezy… and dry.

However, after dark, the cold front moves closer, wind energy increases, and cooling aloft creates additional instability, resulting in thunderstorms forming or moving in from the west. We don’t know exactly when this will occur, but our best guess is after 10-11pm. Due to the strong dynamics of this potent weather system, some storms are expected to be severe. The main threats will be damaging wind or hail, though any storms that form in the early stages especially will pose a risk of tornadoes as well.

A heart-shaped (awww….) Moderate Risk remains in effect for tonight. Be prepared for overnight severe weather as storms roll in ahead of a potent cold front.

After midnight, probably between 1-3am, we expect a squall line will move across the metro along or just ahead of the cold front.  The primary threat with the squall line will again be damaging straight-line wind of 60-70 mph.  Hail is a much lower threat and tornadoes are not likely, but could spin up in the squall line. These types of tornadoes usually are fairly brief and weak, but objects don’t care if an 80 mph straight-line wind or EF-0 tornado with 80 mph wind hit them. They receive the same damage!

Following the squall line, rain and some thunder will likely continue for up to a couple of hours. By dawn, most all of it will be east of the metro and moving away as drier, cooler air filters in. By Friday afternoon, the sun will be shining and temperatures will remain in the 60s with a gusty northwest breeze.  Saturday looks very pleasant, then another chance of rain moves in Sunday afternoon into Monday.

How to prepare:

You’ve gotten a reprieve today. Take advantage of it. Prepare your safe place this evening before bed in case you need to get to it quickly overnight. Have your NOAA Weather Radio or StormWatch+ in your MemphisWeather.net mobile app set up and device charging/charged. Having flashlights handy (with extra batteries) would be a good idea. Have your shoes by your bed, bicycle helmets for the kids in your safe place, and photo ID close by. Here are some more safety tips:

Don’t have the MemphisWeather.net app, or haven’t activated StormWatch+?

What are you waiting for? Severe weather season is upon us!

StormWatch+ allows you to program multiple locations to receive instantaneous watches and warnings for when the National Weather Service issues them. It also allows you to choose the types of alerts you want to receive and will sound “wake-me-up” audio so that you can sleep in peace – but only if the locations(s) you programmed are in harm’s way! Not the other side of the county, or when the sirens go off, but when the NWS has put YOU in the polygon. All of this for about 1/3 the cost of a NOAA Weather Radio, plus it’s portable!  Learn more at StormWatchPlus.com or MWN.

We’ll be nowcasting throughout the event tonight on our social media channels below. If you have a question, ask. And remember, “be prepared, not scared.”

–Erik Proseus
MWN Meteorologist

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