Groundhog day (Tuesday) looks like it is going to be an active one in our area. I feel pretty confident that we will see some severe weather in our area on Tuesday, so I want to discuss the timeline for the storms and the main threats in tonight's blog. If you are just looking for my thoughts, I have a "My Thoughts" section at the end of the blog to just get to the main points in this blog.
Storm Prediction Center Severe Weather Risk For Tuesday
The Storm Prediction Center (SPC) has had a good beat on this storm system for a while. We have been talking about their risk for days and they have us in their higher end slight risk of severe weather for Tuesday. On TV, I don't show the "enhanced" risk because it can be rather confusing, but in the blog, I can explain it a bit better.
SPC Categorical Risk Of Severe Weather Tuesday
Notice SPC has our entire area in their slight risk for severe weather on Tuesday. We do not show the "enhanced" risk on TV because we feel it can get very confusing to have 6 thunderstorm classes, but in the blog I have highlighted their enhanced risk to near the Ohio River.
SPC Probabilistic Risk Of Severe Weather Tuesday
Notice SPC has a 15% chance of severe weather for their slight risk area on Tuesday with a 30% chance covering their enhanced risk area. In year's past, the 30% area was still considered slight risk so I just want to make that distinction.
An Analysis Of The Severe Storm Ingredients In Place For Tuesday Night...
To get organized severe weather, we traditionally need specific ingredients present. It has been a little while since we discussed severe storm risk, so I want to remind you of the ingredients I look for when analyzing a severe threat.
The forcing late Tuesday comes from an absolutely powerful upper level low that will move across our region. The low is absolutely a beast with more than sufficient forcing for storms in our area.
The second part to the forcing is the cold front and it is solid. Behind the front, we will see temperatures drop about 20 degrees so it will be a more than sufficient to fire storms late Tuesday.
The bottom line is the forcing is easily enough to fire storms on Tuesday evening.
The wind energy ahead of this storm is incredibly strong. We also do see some change in the wind direction with height which makes this type of wind shear "directional" wind shear. Directional wind shear can enhance the severe threat in certain situations and does open the door for isolated tornadoes. It is indeed important to note when directional wind shear is present.
As always, I will start at the ground and move upward into the atmosphere. The winds start out at the ground from the southeast late on Tuesday at about 15 - 25 mph and shift to the south-southwest / increase to nearly 70 mph just 1 mile above the ground. They are incredibly strong winds.
NAM Winds At The Surface Late Tuesday
Notice the NAM computer model shows southeasterly winds late Wednesday at about 15 - 25 mph. These are solid surface winds.
NAM Winds 1 Mile Above The Ground Late Tuesday
Notice the NAM computer model increases the strength of these winds a whole lot to nearly 70 mph just 1 mile above the ground. These are seriously strong winds. We can also see a change in the wind direction to the south-southwest or a shift of about 70 degrees. This is definitely directional wind shear.
The wind shear is EASILY supportive of severe weather on Tuesday and it is important to note this is directional wind shear.
Winter severe setups almost always have tons of wind energy and not very much instability. Tuesday falls exactly in that traditional cold season threat. The instability in virtually every computer model is just not very high at all for our area. Not very high and non-existent are very different though. The NAM shows more instability than the other models, but I will say this is a very known bias of the NAM.
NAM Instability Late Tuesday
Notice the NAM computer model still indicates about 750 units of instability in the storm time frame late Tuesday. This seems maybe a touch high, but I will say it is possible considering some will get close 70 degrees on Tuesday.
GFS Instability Late Tuesday
Notice the GFS computer model has only about 250 units of instability in the storm time frame late Tuesday which is about 1/3 of the NAM's numbers. This is not enormous instability values, but also not non-existent.
The bottom line is that some ingredients are clearly present in force late Wednesday, while others are marginally present.
My Thoughts On Severe Weather Chances Tuesday
The forcing to fire storms late in the day on Tuesday is superb and there is no questions we will see t-storms late Tuesday. The wind energy or shear is very powerful on Tuesday. Winds increase to nearly 70 mph just a mile off the ground and that is very strong. The winds are so strong through the column of the atmosphere on Tuesday that the NWS has posted a Wind Advisory for our area Tuesday and Tuesday night. Winds at the surface could gust to 45 mph outside of the t-storms.
Continuing to look at the wind energy for Tuesday, we see noticeable change in the wind direction as we go upward in the atmosphere indicating this is "directional" shear. That is an important distinction because directional wind shear can enhance a severe potential slightly and does introduce a risk of tornadoes. The instability in the winter is always the limiting factor, but it does appear enough instability will be present to support severe storms. That means we have all 3 ingredients needed to support severe weather in our area.
There is a variable called the "supercell composite" which can give us an idea if rotating t-storms are likely and if the important variables point toward severe weather. You can clearly see our area has a favorable signature for severe weather on Tuesday evening with very solid supercell composite values.
AdvanceTrak shows the cluster approaching our area late Tuesday and moving across our area Tuesday night. As you look at the data, you can see the window opens around 6 pm and closes around 1 am. It is pretty obvious that AdvanceTrak clearly shows a line of strong to severe storms late tomorrow. You will notice the timestamp on the top right part of each image.
The bottom line is that some severe storms will likely occur in our area late Tuesday and Tuesday night. I think the window for severe storms opens in our western counties around 6 pm and ends in our eastern counties at 3 am. Louisville should fall in the middle of that timeframe. I think damaging winds, small hail, and isolated tornadoes are all on the table. This is not a high tornado threat, but we absolutely do see quick tornado spin-ups in this kind of scenario in our area.
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