Veteran's Day thunderstorms set up a cool end to the week

We've been updating the MemphisWeather.net Facebook page daily the past few days with information on the potential severe weather event that is due to arrive tomorrow afternoon and evening (Wednesday/Veteran's Day). It's time to put it in blog format, nail down some details, and add some "meat to the bones." (As long as they're pork ribs with some Sweet Baby Ray's sauce on top!) The good news is that the threat (of severe weather, not an empty plate of baby back ribs) has diminished somewhat over the past few days.

Let's start with this afternoon's Storm Prediction Center outlook for Wednesday (as of Tuesday evening):

SPC severe weather outlook for Wednesday. The western half of the metro is within a Slight Risk (category 2 of 5) area, while the eastern half is under a Marginal (category 1) zone.

Track of the weather system responsible for storms

The strongest storms now look like they will pass to our north, closer to the center of the large low pressure area, which will move from the Front Range of Colorado tonight to central Iowa by tomorrow evening, then continue northeast into the Great Lakes region Thursday. Typically, the highest chances of severe weather are closer to the track of the low where the best upper level dynamics (wind energy and diverging air aloft, discussed below) exist and where forcing (or rising air) from a cold front are stronger. The track of tomorrow's low will be well to our north.

NOAA's forecast of the track of the surface low pressure area moving through the central U.S. on Veteran's Day

The wind field

However, the Mid-South still resides in an area where atmospheric ingredients will come together to produce the potential for strong to severe wind gusts in thunderstorms on Wednesday evening. Starting with the wind fields over the region, surface wind will be out of the south gusting to 25-30 mph. At about 5,000' (850 mb for those of you who are weather weenies), wind tomorrow evening will be out of the southwest at about 50 knots (near 60 mph). This turning of the wind with height (south to southwest) and quickly increasing strength of the wind field is referred to as wind shear and, when paired with other severe weather ingredients discussed below, would be enough to produce tornadoes.

NAM (North American Model) forecast wind at 5,000' Wednesday evening showing wind out of the southwest at 50 knots (60 mph) over Memphis (gold star).

At about 18,000' (500 mb), the metro is positioned under a southwest wind at close to 80 knots (90+ mph). Once again, this wind is supportive of strong to severe thunderstorms. The strongest wind at this level is rotating around the bottom of the low pressure area, or over Missouri. Finally, at the jet stream level - about 30,000' up (or 300 mb) - the strongest wind (over 160 mph) is located from the Ozarks into central Missouri, while over the metro it is about 100 knots, or 115 mph out of the west. The position of the core of the jet stream wind is not in an optimal position relative to the Mid-South for the best severe weather potential, but the wind is definitely strong enough to support strong storms. The best area for severe storm development and maintenance is in the right rear quadrant of the fastest wind (or over Oklahoma in this case), since the wind in this region diverges, or spreads out, in this area, promoting rising air which is necessary for the strongest storms.

NAM (North American Model) forecast wind at 18,000' Wednesday evening showing wind out of the southwest at 80 knots (90 mph) over Memphis (gold star). 

NAM (North American Model) forecast wind at 30,000' Wednesday evening showing wind out of the west at 100 knots (115 mph) over Memphis (gold star). The strongest jet stream wind is over southwest MO.

Moisture and instability

So, the wind field is favorable for at least some strong to severe storms. What about other necessary ingredients? One surface-based parameter we look at is dewpoint, which regular readers of the blog will know is a measure of the amount of moisture in the air, irrespective of the temperature. Below, the North American Model (NAM) forecasts surface dewpoints to climb to the mid 60s by 9pm tomorrow evening. This amount of moisture is more than sufficient for thunderstorms (typically we look for values above 60° F).

NAM model of surface dewpoint at 9pm Wednesday evening, courtesy PivotalWeather.com. Mid 60s dewpoints are more than sufficient for thunderstorms, especially when combined with temperatures near 70. Memphis is the gold star.

Finally, we also look at the amount of instability in the atmosphere. Instability is determined by the temperature profile of the atmosphere. The greater the difference between the warm low levels and colder mid and upper levels of the atmosphere, the higher the instability and the more likely air parcels are to rise quickly when pushed upwards by, for instance, a front. Instability is where we start to see a fairly significant hindrance to the threat of severe weather. Despite a strong front and plenty of wind, instability is meager. The values of a commonly used instability index, CAPE (Convective Available Potential Energy) at 9pm Wednesday night are shown below. Ideally, we would want over 1000 J/kg (the higher the better for severe storms), but in this case, it appears CAPE values will remain below 400-500 J/kg.

NAM model of CAPE (Convective Available Potential Eneregy), or instability, at 9pm Wednesday night, courtesy PivotalWeather.com. CAPE values are meager and are a negative factor in the formation of severe storms.

Putting it all together, the Storm Prediction Center seems to have a good handle on the severe weather threat for Veteran's Day. The best scenario lies well to our north with several, but not all, ingredients present in the Mid-South.

So what do we expect and when?

A breezy day Wednesday start dry with temperatures climbing into the mid 70s by afternoon. As the front approaches, a few showers are possible during the late afternoon hours. During the evening, most likely between about 7-10pm, a narrow and fast-moving line of heavy showers and thunderstorms should move through the metro. Sporadic wind gusts to 60 mph are possible in these storms but widespread severe weather is not expected. With storms moving quickly, the impact period will be fairly brief and, despite some downpours, flash flooding is not expected. Tornadoes are a non-zero threat, but are also not expected, nor is hail.

Behind the line, clearing will rapidly take place by Thursday morning as significant drying occurs throughout the atmosphere. As the low tracks into the Great Lakes, steady northwesterly wind will continue Thursday with temperatures about 10° cooler than Wednesday (in the mid 60s). Friday, and especially Saturday, mornings could see our first fairly widespread frost of the year as cool high pressure builds in behind the cold front. High temperatures will top out near 60° each day as it feels much like autumn!

Erik Proseus
MWN Meteorologist

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