Severe Weather Likely On Friday ... Main Threats / Timeline In This Blog!
We have a powerful cold front that will be moving across our area late Friday and this is going to be a problem. There are a lot of severe weather parameters in place on Friday and I think we are headed toward a number of severe weather warnings on Friday. Let's dive in.
Storm Prediction Center Severe Weather Risk On Friday
For the last two day, I have talked about the severe weather risk on Friday. I am glad to see SPC has decided to increase severe weather probabilities and included much of our area now in the slight risk.
SPC Categorical Severe Weather Risk 7 AM Friday - 7 AM Saturday
Notice that all of our area except Adair county in Kentucky is now under a slight risk of severe weather.
SPC Probabilistic Severe Weather Risk 7 AM Friday - 7 AM Saturday
Notice SPC has much of our area in the 30% chance of severe weather. This is a huge increase over the 5% they had in their forecast yesterday.
Severe Weather Discussion
When I analyze the severe weather risk for our area, there are three key items I look at. I assess the wind energy, instability (fuel for t-storms), and the forcing to fire the storms. When all three of these items are present, then we see severe weather.
The forcing for the storms on Friday will be a powerful front that is moving across the area. Earlier this week it looked like the upper level low would not phase with the front taking away some of the energy from this storm. It now appears the upper level low will move right across our area late tomorrow evening.
The forcing is more than sufficient for severe storms.
In the summer, instability is almost never an issue. The storms look like they will arrive mid / late evening on Friday, but the instability looks like it will still be more than sufficient.
Instability Friday At 8 PM
Notice the instability is near 2000 units at 8 pm on Friday. This is actually solid instability.
Instability Friday At 1 PM
Notice the instability is near 1300 units at 8 pm on Friday. While the instability is waning tomorrow night, I think this should still be enough to support severe weather if there is sufficient wind energy.
The instability also looks sufficient to support severe weather on Friday / Friday evening.
Wind Energy / Shear
When I analyze wind energy, I look for a few things. I try to find if the winds are going to change direction as you go up in the atmosphere because this "directional" shear can be more dangerous. The second thing I look for is wind flow near 35 knots / 40 mph in the mid levels of the atmosphere as the bare minimum to support an organized severe weather threat. We will start by looking at the direction of the wind at the ground to determine if there is any directional change in the wind with height. I can tell you the winds above the ground will be southwest tomorrow.
Winds At The Surface At 8 PM Friday
Notice the winds are out of the southwest on Friday, so we are talking about only "speed" shear.
Winds 1 Mile Above The Ground At 8 PM Friday
Winds 6 Miles Above The Ground At 8 PM Friday
Notice the winds at this height are about 30 knots / 33 mph in Louisville late tomorrow, but go up to near 50 knots / 55 mph in the northern part of our area. This is definitely supportive of severe weather.
Winds 6 Miles Above The Ground At 11 PM Friday
The winds are not just marginally supportive of severe weather, but well within the range I look for to support severe weather.
My Thoughts On Friday's Severe Weather Risk
The forcing is good late Friday, there will be moderate instability, and the wind energy is favorable for a line of severe t-storms. The forcing along cold fronts normally promotes linear storm development or squall lines as we call them. The wind energy shows no real directional change with height, so we are looking at speed shear which is most supportive of damaging winds as well. This means the main threats late Friday are damaging winds, hail, and frequent lightning. I suspect we will very likely see a watch issued and many warnings late tomorrow. The probabilities and placement SPC has currently are exactly as I would place them. I give them credit for updating to a good forecast.
The storms look like they will be in the western part of our area near 8 pm - 10 pm, so they could affect Friday night football. The storms should be in Louisville from 11 pm - 1 am. Finally the dying line of storms will be in the southern and southeastern part of our area from 1 am - 3 am. AdvanceTrak has a good timeline for the storms and shows the storms weakening near the end of the slight risk area.
The bottom line is you need to be prepared for some severe storms late tomorrow for southern Indiana and likely northern Kentucky. Heavy rain will accompany these storms, so more beneficial rain is heading in our direction.
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