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A look at Sunday's severe weather threat!

by Ben Pine

Posted on November 15, 2013 at 8:03 AM

Updated Friday, Nov 15 at 12:50 PM

Heads up!  A storm system will be arriving Sunday with wet, windy, and possibly stormy conditions.  Let's start with the basics - the main threat during the afternoon and evening Sunday will be damaging winds.  Our entire area is under a Slight Risk of Severe Storm from the Storm Prediction Center.  Scattered showers will arrive through the morning with gusty winds developing.  Even without storms, wind gusts could be over 30 mph Sunday.   Storms chances will increase through the afternoon and early evening.   The highest chance looks to be from 4-9 pm.  Follow along with the maps below for a look at the severe weather ingredients with a more in depth look.


As with any potential severe weather event, we're always looking for 3 main ingredients.

Wind Shear:  Strong winds, with increasing/turning winds with altitude.

Forcing Mechanism: Cold front, or short wave.

Instability:  Warm, moist air, cold air aloft = fuel for storms. 

If there is a balance with these severe weather ingredients a severe weather outbreak can occur, if just one piece is missing, perhaps just showers will be the outcome.

Taking a look at the maps below we start off with the wind shear factor.  The first maps shows strong winds about 1 mile up.  This is a low-level jet, which can really amp storms up and help produce damaging winds. 

The next map shows the surface winds, which are also gusty.   The maps also show how they are turning in direction.  From the south ahead of the front, and from the west/southwest behind the front. 

The next map is a combination of the wind shear and it's a measure of Helicity.  Values are from 200-400, which is a pretty good range for storm development, possible rotating storms.  This map shows us we can't rule out an isolated weak, spin-up tornado.  (More discussion below the maps)

Now, let's look at the Forcing Mechanism.  Basically, this what we need to spark the storms, often called the "trigger".  As we saw with the surface wind map, we have the cold front moving into our area during the late afternoon and evening and that is an obvious trigger. 

The big forcing mechanism is the strong short wave about 3 miles up.   Notice this dip in the flow on the map below.  Not only that, but the dip leans back to the left, making this a "negatively tilted trough".   This adds to upward vertical motion, an added trigger for storms and storm strength.  Plus, the winds up there are just plain strong, and if we tap into those, there will be some damaging wind gusts in excess of 50-60 mph.

Finally, the big limiting factor this time of year is Instability.  Like the last couple of systems, if we don't get much instability, we'll just end up with breezy showers - and that's what we want!  Right now, the instability looks similar to the last windy and wet system.  The map below shows the CAPE values, and this usually needs to be up above 500, preferably over 1,000 to get some really strong storms.  What we're seeing here is a lack of instability, with most values below 500.  While this is marginal, I have seen squall-lines develop with similar amounts of instability.  The second map shows dew points in the 60s - which is fairly juiced up for this time of year.

A squall-line of strong storms (perhaps showers), is the most likely bet for Sunday afternoon and evening with damaging winds as the main threat.  There may not be much lightning, but we'll have to watch out for some rotation.  Tornadoes will not be a significant threat, but something to just keep an eye on. 

I don't want you to be overly concerned about the severe weather threat Sunday, but be ready to head inside if need be.  Let your family know about the chances of severe weather and stay tuned to forecast updates through the weekend.   Flooding does not look like a big issue with this system.

-Ben Pine