Severe Weather Threat Friday Feb 24th 2017

SPC SWODY2 or “Day 2 severe weather outlook”.

Well, when I wrote my initial post about a week ago I didn’t think severe weather would be possible for my home town area for a few more weeks. Evidently, and as is tradition, I was wrong. The Storm Prediction Center (SPC) in Norman, Oklahoma has issued an Enhanced Risk for much of the Ohio valley.  I’m going to go through and break down some of the information I have been looking at over the past few days and try to explain what is going on. First, a little background.  SPC is a part of the National Weather Service (NWS) operational meteorology units. Think of these guys as the “rock stars of severe weather”.  This group of meteorologists are some of the most skilled severe weather forecasters in the country.  They spend their days forecasting severe weather for the entire country, as opposed to specific regions like the office I work at. I am fortunate enough to call some of these people my friends, as their office was located inside the National Weather Center.  This was the same building I went to school in. SPC  uses a variety of tools to communicate their message to the public, core partners (Emergency management +Government), and even other meteorologists. The most common is the Severe Weather or Convective Outlook.  This product, displayed above, shows geographical areas where severe weather is most likely to occur. It breaks down the severe threat based on confidence and impact into 6 categories, each quantifying a different threat level. The graphic below the list breaks down the threats in more detail.

  • General Thunderstorm
  • Marginal
  • Slight
  • Enhanced
  • Moderate


SPC Risk Levels. We are in a Day 2 Enhanced Risk.

Now that we have gone over some of the background e can dig into the details of this event. We are in a Day 2 Enhanced risk  (SWODY2 ENH). What that means is that there is an elevated threat of severe weather for Friday. Its not very common to see a risk like that especially in February. This typically happens only a few times per year usually in mid to late spring. However, sever weather is possible when ever the ingredients line up. Which is exactly the case this time. Speaking of ingredients lets talk about what makes severe storms.

To get a severe thunderstorm you need 4 ingredients.

  • Moisture.
  • Instability.
  • A source of lift.
  • Wind Shear.

Moisture quite literally means water in the air. We all have experienced humidity during the summer that makes it feel like 1 million degrees. It is extra water vapor that has been evaporated into the air.   The more water vapor the air contains the more moist it is.

Instability and moisture go together hand in hand.  Instability is the tendency of a system to accelerate from rest in order to find equilibrium. Think of a pencil balanced on its point.  That system is inherently unstable. even the slighted nudge from a gentle breeze pushes the pencil out of balance, causing it to topple over. The system had potential energy and that energy was used and converted until the pencil, which is now lying on the table, no longer had any energy.  The system is now in equilibrium and the pencil is “stable”. The same thing happens in the atmosphere.  I’m not going to dive too heavily into the thermodynamics, but water contains energy in the form of latent heat. When phase changes occur, energy is used or released.  Think about how you have to heat up a pot of water to make it boil into steam. It takes a tremendous amount of energy to do that. That energy doesn’t disappear, it is stored in the vapor. When that vapor is condensed that energy is released in the form of “Latent Heat”. The same thing happens when you freeze water, energy is released.  You see where I’m going with this. When air is warm and contains lots of water, it hold lots of energy. When the air hold lots of energy, it is unstable.  That energy wants to move towards equilibrium. It does this by going up. When air moves up higher it cools, expands, and releases that energy by condensing and freezing water. This is called convection. This is the basis for thunderstorms.  They are giant heat engines that equilibrate instability in the atmosphere. That’s why you see those big tall puffy white clouds before thunderstorms. Air is rising because of instability.

But, there’s a catch.  You need a trigger to make that air go up.  That’s where lift comes in. Imagine the gust of wind I mentioned knocking over the pencil.  You need the slight push to tip the system out of unstable equilibrium.  Now imagine another gust of wind coming from the opposite direction as the first.  When the meet head on where do they go? They cant gun into the ground, the can only go up. The “convergence” of the wind forces lift. This is called mass continuity. The same thing can happen if winds above the ground are moving in opposite directions also called “divergence”. If all the air moves out of one spot, lower pressure would form. Air would have to rush up from below to fill that void.  This is what causes lift required for thunderstorms. We find this lift along things like cold fronts. These are the triggers for storms because they force unstable air to rise.  When it rises a certain amount it continues on its own until all the energy is gone.

So now we have a thunderstorm.  But how do we keep it alive? Well we need to maintain the three ingredients we had before, but throw an extra one in there. Wind shear is the change in speed and direction of thee wind with height. Winds 10’s of thousands of feet above you can be moving in different directions and different speeds (usually much faster).  This causes shear to develop. Shear is important because it helps keep thunderstorms alive, and gives them special properties. Remember that a thunderstorm is just a  rising column or air. But there are also sinking currents of air containing rain and the air after its used up its energy. If that falls back through the rising air it will slow it down and kill the storm! Wind shear helps blow that sinking air away from the rising air keeping the storm alive. Special cases of wind shear can also help a storm spin by adding rotational energy to the rising air.  This strengthens the storm immensely and can transform it into something called a supercell. This is why you hear meteorologists constantly talking about wind shear and tornadoes.

So now we have a basic knowledge of storms and how they work. We are going to look at some weather models to determine what ingredients we have. The latest run which finished about 7 pm my time is the 00Z run and will be used. Lets look for moisture. A good way to do that is to look at dewpoints, or the temperature at which dew would form. Right now we have temperatures in the lower 60’s and dewpoints in the middle to upper 50’s across much of the area.  While this ist bad for February, its not overly impressive. The North American Model (NAM) indicates that we will see some increase to near 57-58 degrees by tomorrow afternoon. This should provided limited but sufficient energy for thunder storms.

Dewpoints and temperatures from sfc analysis plot

Dewpoint forecast for 4 pm on Friday. Greens and blues mean more moisture.

SO we have our first check mark. We have moisture.  We generally like to see 55+ degree dewpoints for thunderstorms.  This time its marginal but sufficiency. Next we can take a look at Instability or the energy the storms will have. A good way to look at this is to use something called Convective Available Potential Energy (CAPE). This measure the amount of energy available for  convective storms per unit mass (per kilogram) of air. Generally 500-1000 is considered weak 1000-2000 moderate and 3000+ j/kg strong. This image for 4 pm shows roughly 800-1100 j/kg of CAPE. This is week but more than enough to get things moving up quickly. We can also look at something called the lapse rate. This tell us how quickly temperatures cool with height. The faster they do that the more unstable the atmosphere. Typically the very steep lapse rates are found across Oklahoma Kansas and Texas.  They come from the high Rocky mountains to the west.  Here, the sun heats the air at a higher elevation, as it moves over the lower plains, you are  left with a very steep lapse rate above the surface.  This is known as an Elevated Mixed Layer (EML) It is not very common that the EML makes it all the way out to Ohio. When it does we often see strong instability develop. The other thing the EML does is “cap” the atmosphere. Think of this warmer air a loft as a lid on a jar. It keeps things closed, until you heat the jar and blow open that lid.  We can visualize the cap by looking at something called Convective Inhibition or CIN. CIN is the opposite of CAPE and tell us how much the air doesn’t want to rise.  While it may seem counter intuitive, having a cap is actually a good thing for strong storms. It keeps the number of storms down so they don’t have to compete for the same resources. Cap strength usually looks like this. 0-150 j/k is weak, 150-300 is moderate  and 300+ is strongly capped. The 4 pm time frame shows weak to almost fully uncapped conditions across eastern Indiana and western Ohio. This is another check mark. Now we need to look for our trigger.

CIN where white is 0 joules per kilogram

Sure enough it looks like we have one. This image shows sea level pressure, temperatures, and winds. An area of low pressure (the big L) is located near Chicago with a cold front (drawn in blue) trailing down across western Illinois and Indiana. A warm front (red) was located near the Indiana Michigan border. WE have plenty of boundaries to choose to initiate our storms.  Drawn in yellow is something special. A sublet wind shift can be seen where south westerly winds meet more southerly winds. You can also the the Isobars (Black lines represent constant pressure) slope to the south.  This is an indication of a pre-frontal trough. This wind shift can initiate storms ahead of the cold front. Pre frontal troughs are not as strong as cold fronts.  But they can still be important. The weaker lift helps the storms remain relatively isolated, which again is a good thing for them. So we can check off our 3rd ingredient. We have multiple trigger mechanisms.

Last but most importantly, we need to check for wind shear. We can do this by taking something called a point sounding.  Imagine taking a knife and cutting a big slice of the atmosphere and looking at it from the side.  You can get a sense for how everything changes a you go up and down much like looking at the layers on a cake. This is a super helpful tool. The point selected is about an hour north and west of Cincinnati. This one plot called a SKEW-T Log P can give a tremendous amount of information. For now, focus on the right side with the red circle. At some point ill do a post on how to read some of the other plots. This circle is highlighting the winds at each level the the atmosphere. The long lines indicate the direction in which the wind is blowing from. The dashes and triangles perpendicular to the long lines tell us the speed. Each dash represents 5 nautical miles per hour. The longer dashes represent 10 kts and the triangles or flags mean 50 kts. From this we can see that the winds are turning and increasing in speed as you go higher up.  This is wind shear, and plenty of it!! We can now check off our 4th ingredient.

Check out the wind shear

So we now have all 4 ingredients we need for severe thunderstorms.  This is why the SPC has highlighted the area for a risk. Lets now take a look at some more complex things. I will be using a lot of technically terminology in this part. Google is your friend if you don’t know what something means.

Using some of the higher resolution Convection Allowing Models (CAMS) we can get a sense of where the storms will form and what type they will be. 4 km NAM below shows two bands of convection developing across Indiana. The secondary band is along the cold front and should promote a risk for mainly damaging winds and possibly some small tornadoes.  The primary band is located along the surface pressure trough and will be favorable for supercellular development given increased low level shear. All Hazards including large hail damaging winds and tornadoes will be possible. The primary band has the potential in my opinion to be the most severe, however it is conditional if storms develop.

4 km NAM reflectivity and UH.

Updraft helicity (amount of spin inside each storm correlates well with tornadoes )tracks are somewhat weak but there is a consistent signal that some rotating storms (ie supercells) will be possible. This means potential for tornadoes.

Storm Rotation potential. Updraft hellicity.

With this information in mind, it is likely that there will be a sizable sever weather event across the area tomorrow. Damaging winds, a few tornadoes, and some large hail will be possible. I would expect storms to form across central and North Eastern Indiana in the afternoon and move east through the day and into the evening hours. Storms will be possible through midnight with severe weather still likely. Be alert for the weather and check with local meteorologists as new information becomes available Friday. Check out these link below for some of the graphics and places you can find more information.

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