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11/13/2013

Severe Weather Could Affect Part Of The Weekend! A Full Discussion Of The Risk Inside...

We will drop into the 20s tonight and I am addressing the potential for severe weather later this week. I will tell you there are some BIG changes over the next 7 days. In tonight's blog, I want to look at the severe weather threat for Sunday evening because I do think we will go in a severe risk in the coming days.

 

Storm Prediction Center Severe Weather Forecast Sunday

 

The Storm Prediction Center only puts areas that they are forecasting a 30% chance or greater risk of severe weather in their long range slight risks. Since Sunday is 5 days out, they only have the slight risk to western KY, but I am convinced that will change in the coming days.

 

SPC Categorical Severe Weather Risk Sunday

Notice SPC does have a rare day 5 severe weather risk as far east as around western KY.

 

SPC 4-8

 I can tell you day 5 severe weather risks are rare, so this is something to note.

 

Severe Weather Discussion For Sunday

 

As is the case in almost every severe weather blog I write, I want to refresh you on the ingredients I look for when assessing a severe weather risk for our area.

 

Severe Weather Ingredients

 

Forcing

The forcing Sunday will come from a very powerful upper level low and a really monster cold front.  Notice the big upper level low moving into our area Sunday evening.

 

Gfs_500_1

 

The cold front is equally impressive late Sunday. Notice the much colder blues and purples moving across Iowa during Sunday evening.

 

Gfs_850_1

 

 

Wind Shear / Energy


The wind energy Sunday night is extremely supportive of severe weather. It is powerful and there are signals that the more dangerous directional wind shear will be present. Directional wind shear occurs when the winds shift direction as you go upward in the atmosphere. Directional wind shear is supportive of rotating storms and must be monitored more closely.

 

GFS Surface Winds Sunday

Notice the winds at the surface are from the south at about 15 mph - 25 mph.

Gfs_sfc_1
GFS Winds 1 Mile Above The Ground Sunday

Notice the winds shift about 50 - 60 degrees to the west-southwest 1 mile above the ground and increase to a powerful 70 mph. This is a significant change in direction and increase in wind speed.

Gfs_850_winds 1

GFS Winds 6 Miles Above The Ground Sunday

Notice the winds continue to increase to nearly 90 mph in the mid levels and continue from the west-southwest.

Gfs_500_winds

The bottom line is the wind energy is actually very, very supportive of severe weather. The winds increase from around 20 mph at the surface to nearly 70 mph over the lowest 1 mile of the atmosphere. In addition, there is a significant change in wind direction from the south to the west-southwest over the lowest 1 mile of the atmosphere. This is indeed directional wind shear and needs to be watched more closely.

 

 

Instability


Instability is nearly a problem with every single storm that occurs. I can tell you this will be a big story on Sunday and will determine how this plays out. The normally very conservative GFS computer model is actually showing some instability on Sunday. This is very important to note because it rarely ever shows anything this far out.

 

Gfs CAPE 1

 

It should be noted that the GFS is basing this instability on about 73 degrees Sunday and the high for Sunday is still very uncertain.

 

 

My Thoughts On Severe Weather Sunday

 

The setup for Sunday is actually very interesting. The forcing is extremely strong with a powerful mid level disturbance and a robust front. Both will create tons of lift and easily support rain or storms. The wind energy is very strong and there is directional wind shear. Remember directional wind shear supports rotating storms. The last ingredient is instability and the GFS does hint that enough will be present to support severe weather. Honestly, I cannot see a way right now that the Storm Prediction Center won't extend that slight risk into our area. I think it should be in our area.

 

There remain small details that we cannot resolve at this point, so there are still questions. The GFS computer model brings rain potentially all day long on Sunday and that would lower the severe weather threat, but the EURO does not and it clearly shows a severe weather scenario in our area. Looking at the GFS, you can some storms moving through on Sunday, but the stronger storms do appear to our south on the GFS.

 

Gfs_1

 
Conversely, the EURO shows a concerning appearance. Notice the lines from southwest to northeast on the precipitation data. Those traditionally are called "supercell" lines and they indicate rotating storms producing severe weather. The EURO computer models moves those across our area.

 

EURO Precip

 

At this point, I think we should be in a severe risk for Sunday and I suspect the Storm Prediction Center will do that in the coming days. Damaging winds would be the primary threat, but if that directional wind shear stays in the data then some  tornadoes are in play. That wind field on Sunday has me concerned and if there is any instability on Sunday then we could have a problem. This is still 5 days out and nothing is set in stone. This is a forecast I will be watching closely in the coming days.

 

 

 

It is fall storm season and if you want to be one of my storm spotters, you can join me on my facebook or twitter page. Just follow the link below and click "like" or "follow".

 

 

If you ever have any question, please remember I can be reached on facebook or twitter easily! Just follow the link below to my facebook or twitter page and click "LIKE/FOLLOW"!

Comments

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I look at your blog almost every day! Thanks for all the effort you put in to help us understand the weather!

Emily, we appreciate you visiting the blog. We will dial this forecast in over the coming days.

From the teens and low 20's to possible severe weather in five days....the battle between fall and winter is here.

jeremy s, I think next Tuesday / Wednesday will be colder than this week too. It is a pretty wild stretch.

Wow, though we are in the second severe weather season of the year. We had our first few fall storms and looks like more to come with this battle of fall and winter. The NAO and AO are going negative and the PNA positive (maybe.) We will be having some interesting wild times ahead of us. Just hope the instability lacks like the last severe weather back in halloween.

Israel, I have to say I don't buy into the alphabet soup stuff at all. The AO is not predictive, but simply a look at the obvious... where the cold air is located. The NAO is the show. Notice how the warm and cold periods directly relate to NAO phase. That is the important one.

That's true. Hey Marc, I found this cool device known as the "worlds smallest weather station" that you can connect right into your phone! It can detect wind speed, wind direction, wind chill, temperature, relative humidity, and even pressure! As a weather enthusiast, I'm definitely getting one.

Sorry. Its called Shaka wind meter.

So much information. Thank you.

Savanna, we will dive in again tonight.

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