IN DEPTH: Breaking Down the Severe Chances on Sunday

Chief Meteorologist Shane Smith dives deep into the severe weather cookbook to break down the chances of severe weather on Sunday.

Update 3:00 PM Saturday 7/26/2014:

Hey friends, Chief Meteorologist Shane Smith back in with a quick update to our severe weather threat tomorrow.  The Storm Prediction Center has upgraded parts of the region to a "Moderate Risk" of severe weather.  Here's the latest:

You can see the main area the SPC is targeting is the northern end of the Big Sandy and Kentucky River Valleys.  I'd say draw a line from Williamson, WV, to Prestonsburg, KY, west to Campton and anywhere north of that, you are in the bulls-eye for the best chance of severe weather.  

There looks like two waves of storms will impact us tomorrow.  One in the morning between 3-9 and one in the afternoon, with the best chance being after 2 PM.  The first line of storms will have the potential for damaging winds and heavy rain.  The second line in the afternoon will have that plus and additional risk of very large hail, and a few tornadoes.  

Now is the time to prepare for these storms.  Make sure you have a plan of action in place in case severe weather moves into your neighborhood.  Make sure your cell phones are fully charged, that you are ready to deal with potential power outages, and that you know where to go and what to do during severe weather.  If you live in a mobile home, you may want to be ready to move to a sturdier structure.  

We will have updates all day long tomorrow, the best way to follow those is on our social media pages: and

Original Post 11:30 PM Friday 7/25/2014:

Good Friday evening my friends.  I hope you don't mind a little heavy reading to start the weekend cause we have a very complicated and complex set up on our hands for the potential of severe weather late tomorrow and primarily on Sunday.  I'm going to break tonight's blog down into two parts, the first is just a general summary of the forecast and threats as they stand right now, and the 2nd is a more detailed meteorological breakdown of what to expect.  

Severe Weather Risk:

Right now the Storm Prediction Center has a severe weather risk area highlighted for our region both tomorrow and Sunday.  The bulk of the action looks to stay off to our north tomorrow, Sunday looks to be our better chance.  However the two events are connected and what happens tomorrow will impact Sunday... more on that in a minute.

SPC Saturday Outlook:

SPC Sunday Outlook:

As you can see the main risk Saturday is well off to our north.  However it's that same storm system, an unseasonable strong cold front that will bring us our risk on Sunday.  The leftovers from Saturdays storms will likely come through our region Sunday morning.


The question is then what happens between then and the second round of storms developing in the afternoon.  Do we see more rain and clouds, or do the skies clear and we sunshine?  If we see sunshine the atmosphere will be supercharged for the threat of severe weather.  Here's the risks as I see them now... IF we get the sunshine late Sunday morning and early in the afternoon.


That will set the stage for potent severe thunderstorms on Sunday with the main concerts being very large hail, damaging winds, heavy rain, intense lightning and a few tornadoes.  Now if we don't see the sunshine, we dial those risks back.

Here's a different look at the risks on Sunday from the Storm Prediction Center, the chances of it happening:

Now as you can see that's 30%.  That's not too high you say, and you are correct.  But that's is 30% higher than on a normal day.  On a normal day we have a next to 0% chance of storms.  Also notice the hash marks over Eastern Kentucky.  That indicates a 10% chance of "Significant" severe weather.  Significant being defined as :

  • Tornadoes rated EF2 or greater,
  • Thunderstorm wind gusts of hurricane force or higher--65 kt or 74 mph;
  • Hail 2 inches or larger in diameter.

 That is certainly enough to get my attention.

BOTTOM LINE:  There is a significant enough chance of severe weather that I am telling you about the potential risks.  IT IS NOT A GUARANTEE.  It all depends on how exactly the ingredients come together on Sunday morning.  Severe weather is a lot like baking a cake.  You have to have the right ingredients, mixed in the right way for it to come out perfect.  We are seeing the potential for the ingredients to be there and to come together, but there are limits to our computer models.  The POTENTIAL is certainly there, the question is do those ingredients get thrown together or not.


Okay so let's bust out the old severe weather cookbook to go over our storm chances.  Severe weather needs 4 key components:

*Instability- fuel for thunderstorms/the tendency for air to rise
*Wind Shear- difference in wind speed in direction with height in the atmosphere
*Lifting- A force or mechanism to lift the air up
*Moisture- More fuel

Moisture is by far the simplest to understand and measure.  It's strictly how much water vapor aka "moisture" is in the air.  The best way to measure that is the dew point temperature.  That is the temperature which if you were to cool the atmosphere down it could not hold any more moisture.  Here's a break down of what those temperatures mean for thunderstorms:

Less than 55- Pretty Dry
55-65- A little low but can get the job done
65-75- Enough moisture to cause problems
More than 75- Soupy atmosphere, more than enough

Here's a look at our Dew Point forecast for Sunday afternoon:

Yeah that will get the job done.

Next let's go back to Lifting.  We need something to cause the air to rise.  In this case it's our potent cold front that is coming in on Sunday.  It will force the air to lift.  You can see the cold air that will be driving in behind the front:

We also have a measure to look at the lift potential called the Lifted Index.  The lower the number the more lift the atmosphere will have. 

0 to -4: Some Lift
-4 to -7 : Enough Lift to Cause Problems
-8 and Lower: Extreme Lift

Here's a look at the Lifted Index forecast for Sunday:

As you can see it's enough to cause problems.

Now let's go to Instability.  Instability is the tendency for air to rise.  You need the air to rise for those storms to really pop up and get tall enough to go severe.  The main tool we measure instability is what we call CAPE, or Convectively Available Potential Energy.

0-1,500 CAPE: General Storms
1,500- 2,500 CAPE: Strong to Severe Storms
2,500+ CAPE: Severe Storms Likely

Here's a look at the CAPE forecast for Sunday:

As you can see, it's high... in fact very high.  

Finally here's a look at the wind shear, remember that is the difference in wind speed and direction with height.  The measurement we use to look at that is called Helicity.  Here's what it means for thunderstorm forecasting:

150-300: Super cells Possible
300-400: Super cells Likely
400+: Tornadic Super cells 

And here's what the forecast is looking like:

Again it's high enough to cause me to be concerned.

BOTTOM LINE:  So if all these ingredients come together properly on Sunday as the models are  depicting, we could have some problems.  I wouldn't call it a major severe weather outbreak (I reserve that term for something in the style of a March 2, 2012 event), but more of a significant severe weather risk.  Once again the big question is will the ingredients come together just right, and that is something we won't know until Sunday. The models keep bouncing everything around a little bit, so the situation is still a bit murky. Keep checking back with us as we get a better handle on the situation.

  -Shane Smith

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