Another Severe Weather Threat Wednesday: Inside the Forecast

On Sunday, the Storm Prediction Center’s Day 4 outlook placed our area in a threat for severe weather this Wednesday, and it was continued in their Day 3 outlook issued early this morning – referring to this Wednesday (1 March). Of course, after enduring a round of severe weather on Saturday, I know many of you may seem anxious at yet another threat and wonder if it will be as bad? The short answer: possibly. I have mixed feelings about the threat on Wednesday as there are signs pointing to both a more significant event and a limited severe event. There are three basic things when forecasting severe weather we look for: a lifting/forcing mechanism (like a front), low level moisture (high humidity in the lowest ~1500 ft of the atmosphere), and wind shear (wind changing direction/increasing in speed with height). Based on the model data available, we have the ingredients, but how they come together is the question. Let’s break this all down then…

 

Unseasonable Warmth & Low Level Moisture

More unseasonable warmth will be with us on Wednesday as temperatures soar again into the 70s. An 80+ observation is also possible, but may be difficult due to cloud cover.  In addition, SW winds will push in plenty of low level moisture. Surface dew points should sit around 60°, which for February is plenty sufficient for moisture. Remember, Dew Point is the temperature at which the air would be completely saturated (rain/fog would be present as the water vapor would condense to liquid).

In terms of Thunderstorms, warm air is also very important, because to get good upward vertical motion (aids in precipitation/thunderstorm formation), rising air should be warmer than the surrounding environment. Why? Warm, moist air is very light. If a rising parcel of air is warmer than the surrounding environment, it will rise on it’s own (you can think of a parcel of air like a bubble of air). Naturally, the parcel will cool as it rises and will eventually reach equilibrium with the environment, but as long as it remains warmer, it will rise and gain upward momentum. This becomes the storm’s energy – and we call this CAPE, or Convective Available Potential Energy. Higher CAPE values typically mean stronger storms. Values over 500 can produce severe weather, but 1000+ is more optimal. For reference, on Saturday, CAPE was around 1000.

The following diagram is what’s called a Skew-T/Log-P diagram, or a sounding. It’s basically a vertical profile of the atmosphere, allowing us to see how temperature, moisture, and wind change throughout the atmosphere. The red line below is the environment temperature, and the white-dashed line is the temperature of a rising parcel of air. Notice how that line is to the right of the red line – it means it stays warmer than the environment. The area in between these two lines from the time the parcel becomes warmer until it reaches the same temperature as the environment, is what’s called the CAPE region, and it takes some calculus to figure out what the CAPE is – thankfully, computers do that for us. CAPE values are forecasted to be in the 900-1000+ range, which is  sufficient for severe weather, especially in February/early March.

Also notice how the green line separates from the red line between about 5,000ft and 10,000 ft. The green line is the dewpoint line, and this separation is an indication of dry air. This layer of dry air is commonly associated with severe thunderstorms and strong winds. As thunderstorms form and the moisture condenses it forces the surrounding air to cool as it fills with moisture. This creates a rather cold layer of air – and cold air is heavier (more dense) and wants to sink. If the air is colder than the environment it gains momentum as it sinks. Combine this with the thunderstorm’s downdraft and you can get some very strong winds to push down to the surface.

Upper Level Winds & Wind Shear

One of the concerning aspects of this severe threat is the presence of a very strong band of winds around 10,000ft over our area. The models are currently forecasting winds at this level to be over 75 MPH. The downward motion of air in a thunderstorm can take this momentum in the atmosphere and mix it down to the surface. This aids the threat for damaging winds.

Going back the skew-t diagram, also notice how the little wind barbs (I’ve outlined them for you) turn clockwise with height. The surface winds are out of the southwest, but become more westerly with height – this is directional wind shear. The presence of directional shear means the tornado threat on Wednesday will not be zero, and will be present. I doubt we’ll see an outbreak of tornadoes but an isolated tornado or two can’t be ruled out.

 

Against the Severe Threat

So after reading all that, you’re probably thinking severe weather is a good bet, right? Well, there are things that can limit the severe threat, and they will be present Wednesday as well. First of all, most of Wednesday will be fairly cloudy. Sunshine is an important aid to severe weather as it helps with daytime heating and warms the low levels (which helps them to rise). The unseasonable warmth may help to overcome this, but you’d be surprised how much sustained cloudiness can hinder a severe threat, even with sufficient warmth and low-level moisture. Cloud cover has busted days which looked like a real nasty severe weather event was going to unfold – of course, that a good thing. (White = greater % chance of cloud cover)

The other thing that is not ideal is the upper level trough. On Saturday, the trough was much sharper and had a negative tilt to it (the trough axis orients NW to SE).  The upper level trough for Wednesday is forecast to be much more broad and is neutrally tilted (axis orients N-S). Negatively tilted troughs typically produce more significant severe weather events – as the orientation of the trough creates a stronger temperature gradient. However, the other dynamics in play may help overcome this.

Finally, the timing of the storms will come into play. Right now, it looks like late afternoon/evening is the time when we will see the storms roll through. If the storms come in earlier or later, they won’t have the full day time heating to work with, and won’t have as much energy to work off of.

Final Thoughts

Right now, it looks like we will see another round of severe thunderstorms. Looking at the data, all modes of severe weather are in play-Damaging winds, hail, and an isolated tornado. The biggest threat, as usual will be the damaging wind potential. How significant of an event this will be is still to be determined, and we won’t really know this until Wednesday as we see how the day progresses and these ingredients come together. For now, stay tuned to the forecast and just be prepared. It’s not time to worry, just yet.

-Andrew

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