Posts Tagged ‘outbreak’

Before we dive into tomorrow’s severe weather setup, lets begin this discussion the way I always like to, starting off with the latest convective outlook.

1/21/2012 11:30 AM CST - Day 2 Convective Outlook

The Storm Prediction Center has issued a large standard risk of severe weather, identified by the orange outline denouting a 15% risk, from Shreveport, LA eastward to around Atlanta, GA, northward to Knoxville, TN and west back towards St. Louis. Within that 15% risk zone is an enhanced risk of severe weather which is identified by the red outline. Within the red line, residents have a 30% chance of experiencing severe weather within 25 miles of their location. That’s a nearly 1 in 3 chance, so it’s pretty significant. The white, dotted line indicates the potential for higher end severe weather, in this case meaning wind gusts over 75 MPH and the potential of significant tornadoes (EF-2+). I believe the outlook may be shifted west a tad with part of the 30% zone being upgraded to a 45% probability, which indicates a ‘Moderate Risk’ from the Storm Prediction Center. There’s no doubt that a major, significant severe weather outbreak is becoming increasingly likely. The wind shear values are very favorable for severe weather and the instability values will be unusually high for January. You combine those two ingredients and you have the potential for several tornadoes, some of which could be significant, and a major wind event (derecho) with 75+ MPH winds. This event will begin in Arkansas late in the afternoon and really ramp up overnight, before a squall line forms along a cold front from Louisiana up north into Missouri. That line will sweep eastward along with the severe weather risk. If you live out in the affected areas shown on this map above or have someone that does, let them know that there could be very bad weather tomorrow night. It could be down right dangerous. Now that I’ve described the risk areas, lets begin to dive into the overall setup.

0Z NAM: 250 Millibars at 6 PM Sunday

I’m going to start off a little higher then I normally do in these discussions. The following graphics will be from the 0Z NAM for 6 PM on Sunday. This is at 250 millibars which is around 35,000 feet above sea level. I’m not even going to describe the thresholds for wind shear in this case, because all those are met and nearly doubled for Sunday. It’s not a matter of if there will be enough wind shear, its just a matter of where. At 250 millbars off the 0Z NAM for 6 PM Sunday, the base of the trough is located across East Texas and Southeast Arkansas, with the leading edge of the trough digging into Indiana and points southwest towards Eastern Arkansas. Notice the black lines, known as heights, on this map. I want to point out the top portion of the graphic. See how the lines are pointed in a southeasterly fashion? This means this storm is going negative tilt, which nearly doubles its energy. We should have at least 100 knots of wind at 250 millibars across eastern Arkansas at 6 PM tomorrow, which is very supportive of upper-level organization of updrafts.

0Z NAM: 500 Millibars at 6 PM Sunday

Looking lower in the atmosphere, specifically at 500 millibars or 18,500 feet above sea level, can you find the storm? This is a very strong system that’s going negative tilt by 6 PM on Sunday. Winds in the base of the trough located across Northern Texas are over 95 knots with a peak value of 100 knots across Missouri. For those that have been long-time readers of the blog, you’ve seen this graphic several times in my discussions. You know how we usually see the yellows (55-65 knots) at 500 knots, but you don’t normally see the red and purple values which indicate 70-100 knots. You will tomorrow! I cannot stress how amplified and strong this system will be as it pushes into Dixie Alley tomorrow night. We see these systems a few times each winter and because we don’t have the moisture/instability values, we don’t have issues. As you’ll see in a few graphics, we have unusually high instability amounts for January. Combine those with this wind shear and we could be facing a very significant outbreak tomorrow night. The fact this will be after dark just amplifies the risk. Tomorrow night is going to be dangerous across the Southeast!

0Z NAM: 700 Millibars at 6 PM Sunday

0Z NAM: 850 Millibars at 6 PM Sunday

As we look at the lower part of the atmosphere, not much changes. We still have very high wind shear at 700 millibars and 850 millibars. In terms of the specific severe weather mode, I do want to discuss the wind direction in the lowest 150 millibars (850 millibars down to the surface). When we’re looking for the potential of tornadoes, we need directional shear. That can best be described with southeast winds at the surface, south winds at 850 millibars, and southwesterly winds at 700-500 millibars. Looking at the 850 millibar graphic, we see that winds will generally be out of the Southwest. I want to focus my attention on Eastern Arkansas, which is where we’re monitoring for a potential chase on Sunday. Winds at 850 millibars are out of the southwest at about 50-55 knots, which is pretty strong. Looking from 500 millibars down to 850 millbars, we don’t see a lot of change in the direction of the wind. This is what could be described as a linear look, meaning squall lines verses a major tornado potential. However, out in Dixie Alley especially, we see this more often then not with most tornado events. The one exception I can remember is April 27, 2011 where we had southeasterly 850 millibar winds. If we had southeast 850 MB winds tomorrow, we would be facing a massive tornado outbreak with dozens of tornadoes likely. I don’t think we’re going to have that many tornadoes tomorrow, but let me show you something a little different.

0Z NAM: 925 Millibars at 6 PM Sunday

During these blog posts, I don’t normally get this detailed in terms of diagnosing tornado potential because it starts getting pretty complicated. This graphic comes from a different weather website, but is still from the 0Z NAM and is for 6 PM on Tuesday. This is actually at 925 millibars which is below 850 millbiars, and is around 2500 feet above sea level. I know it’s pretty small on this graphic, but I want you to notice the wind direction across Eastern Arkansas and points east. The wind direction seems to be coming out of the south/southeast. That would help provide the directional shear I was talking about above. Honestly, I can’t tell you if this is going to be how it turns out. However, I need to stress that if we get more directional shear, much like is shown on this graphic, the tornado potential will skyrocket. We’re walking a very fine line between a few tornadoes and a major tornado event with significant, long track tornadoes. At this point, I’m expecting several tornadoes tomorrow night with the potential for one or two to cause significant damage. If we get a backed low level jet (fancy way of saying southeast winds), tomorrow night is going to be a very active tornado night, likely the most active we’ve seen since last Spring.

Before I begin to speak on the instability, let me describe the overall scenario based off current data. A strong mid-level inversion layer, known as cap, will keep storms from developing most of the day across Arkansas and Louisiana. I’m not looking further because my focus is on the I-40 corridor and points south. By around 4 to 5 PM, the energy associated with our negative tilt trough is going to begin influencing the region by providing lift and weakening the inversion layer. We’ll likely have storms going by 6 PM. Current data suggests we’ll be dealing with a mixed storm move, both linear segments (small lines) and discrete supercells. The supercells will have the highest risk of producing long track tornadoes, but the line segments will be capable of producing tornadoes as well. By around 9 to 10 PM, we’ll have a solid, to near solid, line of thunderstorms extending along the cold front. With the extreme amounts of wind shear present and assuming instability will be sufficient (we’ll get into that next), I’m expecting a long duration, bow echo to develop. This bow echo will likely be producing widespread damaging winds, some exceeding 80 MPH. Tornadoes will also be a problem with spinups likely in the line as it moves east across Tennessee, Mississippi, and Alabama. Tomorrow night will be a night that folks in Dixie Alley want to have a way of getting weather information.

0Z NAM: Surface Based Instability Values at 6 PM Sunday (SBCAPE at 0Z 1/23/12)

Finally, lets talk about the instability values for tomorrow. When dealing with cool season severe weather events, we often have a lack of instability, which is the primary reason we don’t have to deal with severe weather. With the excellent wind shear in place, thunderstorms don’t need much instability to become organized. I generally want to see at least 750 joules per kilogram of Mixed Layer CAPE in the cool season to support a marginal severe weather risk and anything about 1,000 joules is a red-flag indicator of a potential issue. It looks like we’ll have widespread instability values over 1,000 joules across most of Arkansas by 6 PM. While I’m not posting the graphics showing the progression, these values will spread eastward overnight across Mississippi and Southern Tennessee. That amount of instability combined with the excellent wind shear will help thunderstorm organize and remain organized for a long duration, as I said above. I won’t repeat what I said above, but it does look like it’s going to be a busy Sunday Evening and Sunday Night.

Now, we are a storm chasing website, so let me detail our plans for chasing this event. Connor is going to leave Oklahoma City tomorrow morning to head out and chase in Eastern Arkansas, likely targeting near Pine Bluff based on current data. He’ll most likely have his live streaming video available, which you’ll be able to watch on our Live Video page on our website! His stream will be available for media use. Media can contact us via the information on the contact page. I’m planning to remain in Dallas tomorrow to help provide up to date coverage on our social media pages, which is great for yall because we’ll be on top of our game tomorrow! Stay Tuned, I’ll be providing numerous updates tomorrow and tomorrow night as we get underway.