Thursday's Severe Weather Threat Is Concerning. When To Expect The Storms & The Main Threats...

Thursday's Severe Weather Threat Is Concerning. When To Expect The Storms & The Main Threats...

2/18/2014

Thursday is going to be an absolute wild ride with temperatures soaring into the upper 60s, very windy conditions, then dropping nearly 20 degrees in 2 hours. The front is a monster and it will be roaring across our area at nearly 50 mph on Thursday night. All of these conditions look favorable for severe weather and I am concerned. In tonight's blog, I want to give a timeline so you know when to expect the storms and I will talk about the main threats.

Storm Prediction Center Severe Weather Risk For Thursday

As expected, the Storm Prediction Center has placed us in the severe weather risk again for Thursday and has our entire area in the 30% chance of severe weather. I agree with the placement, but I think an upgrade is necessary.

SPC Categorical Severe Weather Risk Thursday 7 AM - Friday 7 AM

Notice SPC has a giant risk area that covers our entire area.

SPC Probabilistic Severe Weather Risk Thursday 7 AM - Friday 7 AM

Notice SPC has our entire area in the 30% chance of severe weather for Thursday making this an enhanced severe weather risk.

A Discussion Of The Thursday Storm System / Severe Threat

Severe weather happens in our area in the winter, but only a few times a winter. Since these are not all that common, I want to refresh you on the ingredients I look for when assessing a severe weather risk for our area. Remember, these three ingredients are all needed if we are to have an outbreak of severe weather in the Ohio Valley.

Forcing

The forcing Thursday comes from an powerful front that will be racing across the area and a very favorable upper level low. The upper level low is what we call a negatively tilted low and these lows provide extreme lift in the atmosphere and can overcome weaker instability, so this is important to note. First, check out the monster front late Thursday. Notice the amazing difference in temperature ahead and behind the front!

To determine the tilt of a low, we draw a line from the center of the low to the edge. If that line is up and down, there is no tilt. If that line goes from top right to bottom left, then the low is positively tilted. If the line goes from top left to bottom right, then the low is negatively tilted. Notice this low is VERY negatively tilted.

Wind Shear / Energy

The data continues to show fierce winds only a mile above the ground on Thursday. The GFS and NAM computer models show winds of nearly 85 mph just 1 mile above the ground on Thursday and that is very concerning. We also see a little of the more dangerous directional wind shear in our area on Thursday night. Directional wind shear is defined as winds changing direction with height and this kind of wind shear can result in rotating t-storms.

GFS Surface Winds Late Thursday

Notice the winds at the surface are from the south at nearly 15 - 30 mph late Thursday. The surface winds alone are very strong late Thursday.

GFS Winds 1 Mile Above The Ground Late Thursday

Notice the winds shift to the southwest just 1 mile above the ground and increase to nearly 75 knots or 85 mph! These are ferorciously strong winds for this height and we do see a direction change of about 45 degrees on Thursday as the storms approach.

Here is an interesting look at the amount of rotation potential called "Helicity" for Thursday with the actual wind vectors on the map. You can see the green or orange arrows from the south at the surface and the winds at 3 km above the ground in hot pink. These winds shift about 45 degrees from the surface to about 3 km above the ground. Helicity measures the potential for the individual storms to rotate and anything over 200 is impressive... the NAM shows over 500 on Thursday.

The bottom line is the wind energy is extremely supportive of severe weather on Thursday. We genenerally see a small change from a south wind at the surface to a southwest wind over the lowest 1 mile of the atmosphere. This is not a huge shift in direction in the lowest part of the atmosphere, but it is something we do need to note. With the low pressure intensifying so fast on Thursday, I feel confident we will continue directional wind shear in the data.

Instability

The temperatures are going to push into the 60s by Thursday and this is a big part of the equation. The latest data does suggest we will dry slot in the afternoon. What this means is that the clouds will likely break a little in the afternoon on Thursday helping push those temperatures higher. Advance Trak is showing the breaks in the clouds well on Thursday afternoon.

This means instability values could get pushed even higher. The NAM and GFS have both increased instability for Thursday with the NAM the highest. The NAM tends to have a better grasp on instability with these storms, so I will heavily rely on that data.

NAM Instability Late Thursday

The NAM has about 1,000 units for the western part of our area with up to 700 near Louisville. This is an increase in instability and another reason for concern.

GFS Instability Late Thursday

The GFS has also increased instability to near 500 units in Louisville as the storms approach and nearly 750 units for our western counties.

One interesting tidbit, the data drops the temperatures about 5 degrees from the high to when the storms pass. In Kentuckiana, the temperatures nearly never drop when it is windy ahead of a storm even if it is at night. That potentially could mean temperatures will be as much as 5 degrees higher when the storms approach lending more energy to those storms.

My Thoughts On Severe Weather Chances For Thursday

The low that will move to our west on Thursday and bring this front is what we call a "cyclogenic bomb". That means the storm intensifies at an extremely fast pace that meteorologists call "bombing out". This low is no joke. The storm system itself has extremely favorable lift for severe weather and winds that are extremely strong. Both are highly supportive of severe weather. This instability continues to rise in the data and as I suggested above, I still think it could go higher. To be honest, I am concerned.

To me, this storm system is looking more and more like it could produce a severe weather outbreak in our area. That does NOT mean a March 2nd style event, but it does mean we could see a lot of warnings on Thursday. The type of wind energy and the instability present makes the main threats damaging winds on this system. With that said, isolated tornadoes absolutely can and do occur in these conditions. In fact, the NAM is showing a very solid chance of rotating storms called "supercells" on Thursday. Notice the large orange area over our region on Thursday indicating rotating storms are a good possibility.

The latest runs from AdvanceTrak show a line of strong and severe storms moving through late Thursday. You can also see there are some "renegade" storms ahead of the line and if that occurs, these tend to be the storms that can quickly produce tornadoes. Notice the timestamp on the top right part of each image.

It is worth noting again that the main threat in this kind of a setup is damaging winds, but isolated tornadoes cannot be ruled out. Based on the latest data, I do not think a slight risk of severe weather is enough for our area and I feel we should be upgraded to a moderate risk. The intensity of the severe weather threat will be highly dependent on how much instability we see. Monitoring the amount of instability in the coming days for Thursday will be key and I will be watching closely. As it stands now, I think an outbreak of severe weather is on the table.