Northwest Florida Weather (12/5/2016) - Another Severe Weather Threat

The Convective Outlook from the Storm Prediction Center (SPC) showing the risk of severe weather across the state of Florida. This map is valid until 6 AM CST Tuesday.

For the second time within a week, we are talking about the risk of severe weather. The Storm Prediction Center has placed all of Northwest Florida in a "Slight Risk" of severe weather, meaning that scattered severe thunderstorms are possible. This is a level 2 out of 5 (5 being the highest). Recall that last Wednesday, we had a slight risk of severe weather in our area. That day, we had two tornadoes in Okaloosa County. Just because it's a "slight risk" does not mean that severe weather won't happen. You need to be on guard. Let's talk about the set-up a bit.

Surface analysis showing observations, frontal positions as well as high and low pressure systems across the south central United States. This map is from the Weather Prediction Center (WPC) valid at 9 AM CST.

At 9 AM CST, the Weather Prediction Center (WPC) analyzed a stationary front from a low pressure center south of the Texas/Louisiana border northeastward to near Panama City Beach, Florida, and into southwestern Georgia.  From this low pressure center, there is a cold front extending southward through the western Gulf of Mexico. As this low strengthens and moves northeastward, the stationary front will lift northward as a warm front, bringing warm, moist air into all of Northwest Florida. This will help set up a severe weather threat this afternoon and into the overnight hours.

4 km NAM showing sea level pressure isobars (solid black lines), 1000-500 mb thickness (dashed red and blue lines), and 6 hour accumulated precipitation (shading). Fronts, highs, and lows, drawn in by me. Valid from noon CST Monday through 8 AM CST Tuesday.

The cold front, according to the 4 km NAM, is forecast to clear all of Northwest Florida by mid Tuesday morning. We will have to watch any storms that develop south of the warm front, since they will have the best environment for isolated tornadoes to develop. With the cold front, we may see a broken line of thunderstorms that will bring a risk of damaging winds and isolated tornadoes. Let's look at a couple forecast soundings.

Forecast sounding for KVPS (Eglin AFB, FL) valid at 00Z (6 PM CST). Focusing on the left half of the image, the solid red line is the temperature profile through the atmosphere. The solid green line is the dew point profile through the atmosphere. The pressure is labeled on the left in millibars (mb), decreasing with height. The temperature is labeled on the bottom in degrees Celsius. Note, the temperature contours (dashed white lines) go up and to the right. Winds are on the right hand side in knots. Several parameters are listed below. Image courtesy of pivotalweather.com

This sounding from the 4 km NAM model shows a (near) saturated atmosphere from 500 mb down to the surface, meaning that the model is likely predicting rain at this time. Let's go over some of the things that will aid in severe thunderstorm development at this time:

• Abundantly moist atmosphere
• Okay shear. Not really all that impressive, but enough.
• High (for this time of year) instability above the surface.
• Warm air advection (lifting mechanism).
• High storm relative helicity (SRH) - helps with rotation.

Okay, so, what's the catch? Well, the near surface layer. If you notice the temperature profile near the bottom of the chart, it is steeper than the temperature contours, meaning that the atmosphere is warming as you increase in height. This is opposite of what typically happens (temperature generally cools as you increase with height in the troposphere). This warming is called an inversion, and it essentially acts as a lid for the atmosphere, meaning that it is increasingly more difficult for air to rise above this lid, and cannot produce surface-based convection. That's why the SFC CAPE is only 98 in the above image. But if you look at the mixed-layer (ML) CAPE, it's over 1,000. The mixed-layer takes into account a layer of the atmosphere, which varies based on the program used, and uses that as the source of air parcels. So, it's possible that some of the storms could be, what we call, elevated, meaning not originating from the surface. However, this does not inherently decrease the severe threat. Since the inversion is rather shallow (only up to 980 mb), there are two possibilities.

• Rising air may be able to overcome the shallow inversion (especially since it is a rather weak inversion), or
• This inversion is caused by rain-cooled air (remember, we said that the sounding showed that it may be raining at this time).

Either way, the sounding's algorithm spits out a "possible hazard type" of "marginal tornado." I like to use this as a worst-case scenario. Thus, the worst thing that the atmosphere is capable of producing with this environment is a weak tornado.

Same as above, except valid at 06Z (12 AM CST) tonight

In this sounding (valid at midnight tonight), there is not much left of the inversion mentioned above, meaning air can readily rise from the surface. As a result, the SFC CAPE is now 716 in this sounding. However, the storm-relative helicity has decreased since the winds don't turn as much with height. Thus, this still results in a marginal tornado threat.

Same as above, except valid at 3 AM CST Tuesday.

This sounding shows very similar things as the previous sounding. Lack of a surface inversion, plenty of moisture, but winds are stronger above the surface (now near 50 knots). This increases the amount of shear and also increases the storm-relative helicity (which takes speed and direction into account). Thus, the "possible hazard type" has now changed to "tornado." So, what does this mean?

The severe weather threat does not end once the sun goes down. In fact, it may increase overnight tonight. I strongly urge you to have multiple ways to receive warnings. Since most people nowadays have smartphones, that is one way to receive warnings.

Check on your phone to see if you have your Wireless Emergency Alerts (WEA) activated. These will send a notification to your phone (it will vibrate and emit a loud tone) whenever a Tornado or Flash Flood Warning is issued. It is geo-located according to cell phone towers. It will wake you up in the middle of the night and alert you to what's going on. Also, if you have any weather apps on your phone, check to see if they have an alert mode. You should turn that on too. But, what if your phone is dead?

You should have this one anyways. A weather radio! This one is very important to have. They're $20 at most places and very easy to program. This one will send out very loud tones (loudest I've ever heard) when a warning is issued for your county. Remember to get batteries for your weather radio though! TV and social media re good as well, but can be unreliable, especially in the event of a power outage.

Either way, you should have at least two different ways to receive warnings, especially during the overnight hours. Let's face it. The last thing you're thinking about when going to bed is the weather. But in this case, it needs to be one of the first things you think about. Overnight tornadoes are extremely dangerous. It's dark outside so you won't be able to see the tornado until it's too late. You need to be able to be woken up in the case of threatening weather and seek safe shelter as soon as a warning is issued.

Do not rely on tornado sirens! Sirens are meant for those who are outdoors and are not meant to be heard indoors. Plus, sirens are not required in all cities or counties. They are one of the most unreliable methods of warning.

So again, stay aware and stay safe tonight. You need to have multiple ways to receive warnings. And before you go home from work or school this evening, why not pick up a weather radio and some batteries? It can save your life.