Skip Talbot
EF5
What an interesting looking setup this is. Lots of extreme values and lots that could go wrong with the forecast. Tornado outbreak or gusty line of showers, I think anything is possible with this one. The models have been consistently showing a very deep, negatively tilted trough, with significant height falls, an extremely potent shortwave, and bombing low ejecting over the MS/OH Valley and Great Lakes region on Sunday. A 100+ knot diverging 500 mb jet is forecast over IL/IN. A 60-70 knot low level jet looks to transport dews into the lower 60's up to central IL/IN, some runs had that moisture making it all the way into Michigan. Heating looks questionable given the saturated moisture, wide open cap, and strong LLJ that should kick up a bunch of clouds and crapvection in the warm sector. CAPE will be pretty modest as a result, which leaves us with a classic fall extreme shear, low instability setup. Dynamic forcing from the diverging jet, steep midlevel lapse rates from the approaching trough, and ample moisture advection should easily overcome limitations with instability, however.
Initiation looks to occur first over north central IL and south, possibly west in NW IL or even eastern IA. Given the cap being wide open, the moisture in place, and extreme shear, initiation looks to be very early, possibly before noon. As the cold front surges, a line of cells should erupt down the cold front, aided with lift from the diverging trough, there should be pretty solid storm coverage all the way down into KY or TN. The line should race into OH overnight with trailing cells south of the OH River in KY/TN.
This setup looks very favorable for severe weather in the form of damaging winds and tornadoes. The shear profiles will support damaging winds even with more modest looking cells. I'm not sure how extensive the tornado threat will be though. I see several potential problems with the tornado play. First, the wind profiles are nearly unidirectional along the cold front and hodographs are big straight lines. This suggests a fast moving line of splitting cells, bow echoes, and a few embedded supercells. Forcing along the fast moving cold front may kick up a nearly solid line of cells that choke each other's inflow, with the front undercutting them as well. That cold front surges quickly too. There doesn't seem to be much time between initiation and when that front blasting through. Still, given the extreme shear profiles, storms could produce right at initiation over IL, and then continue to produce from embedded supercells and QLCS tornadoes as the line advances. After dark, the tornado threat will probably continue, given that moisture advection continues and the LLJ ramps up even further than it's already extreme values. Some more discrete cells may track through KY and TN overnight posing a tornado threat. Favorable low level instability and lid strength indices suggest that surface based storms aided by dynamic forcing, could be in progress through the duration of the event, from moring into the overnight hours. I don't see a cool boundary layer keeping stuff elevated either at the start of the event in N IL or the end of it below the OH River, given the warm air advection. The other big question is what kind of prefrontal initiation we'll see. Helicity values are very impressive ahead of the cold front. While the mean flow doesn't bisect the cold front for lift downstream, it doesn't parallel it either. The main line will probably be rooted to the cold front, but if there's enough lift ahead of the cold front, we could see a few discrete tornadic supercells tracking across western IL, IN, and OH. This scenario seems like it would be most difficult to forecast though.
Given the lapse rates and moisture, the models have been fairly generous with the instability. Both the GFS and NAM have been hinting at MLCAPE approaching 1000 J/KG across IL and into IN. It's more than enough for tornadic supercells given the extreme shear values.
IL to TN could see tornadoes I think. It's going to be a question of if we get some semi discrete supercells at one end of the setup or another, or a solid line of undercut showers on the cold front.
Chasing this thing is a whole different story. Storm motion is going to be extremely fast. Cells on the main line are going to be moving NE at 50 to 60 mph or even faster. You won't be able to keep up with that unless you're staying ahead of the line on the interstate. The tornadic play on the southern end I'd consider to be almost completely unchaseable unless you're local and have a good perch, since again storms will be screaming, and through bad terrain and after dark. If you're going for the prefrontal play, you're going to have to be well downstream in IN probably, which might be the best option for local chasers there. Otherwise, I'd probably try to target initiation of the main line in north central IL, by late morning/early afternoon. The highway system there is very favorable for moving up and down a line quickly, and this might be the best time to see more discrete cells before they all fill in down the line.
One area to watch is further northwest. The GFS has been fairly consistent at keeping the surface low further south, wrapping the moisture around it, while pushing some really cold air aloft over the top of it. Negative lifted indices are spiking like some sort of quasi cold core type setup. on the MS river in NW IL and eastern IA.
http://www.twisterdata.com/data/mod...1/16/00/GFS_3_2013111600_F42_LFTX_SURFACE.png
If that materializes, these areas could see some low topped, north moving tornadic supercells, which will probably be the most discrete activity of the day unless something prefrontal goes up.
Preliminary target is Rochelle to Bloominton, IL and I plan on using either 88, 80, or 74 to stay ahead of the line until I can intercept something interesting before the lines passes me by. If the cold air target pans out, I'd pick a secondary target of Sterling, IL to Davenport, IA using 88 again to position along any cells that might go up there. I do not expect to see a tornado, unless I get extremely lucky with timing and positioning. I think is going to be a very frustrating chase given storm motion and the potential for storm mode to be predominantly linear. Tornadoes might be far and few between, or difficult to see given low, grungy bases. It's close though, so I'll give it a shot.
Initiation looks to occur first over north central IL and south, possibly west in NW IL or even eastern IA. Given the cap being wide open, the moisture in place, and extreme shear, initiation looks to be very early, possibly before noon. As the cold front surges, a line of cells should erupt down the cold front, aided with lift from the diverging trough, there should be pretty solid storm coverage all the way down into KY or TN. The line should race into OH overnight with trailing cells south of the OH River in KY/TN.
This setup looks very favorable for severe weather in the form of damaging winds and tornadoes. The shear profiles will support damaging winds even with more modest looking cells. I'm not sure how extensive the tornado threat will be though. I see several potential problems with the tornado play. First, the wind profiles are nearly unidirectional along the cold front and hodographs are big straight lines. This suggests a fast moving line of splitting cells, bow echoes, and a few embedded supercells. Forcing along the fast moving cold front may kick up a nearly solid line of cells that choke each other's inflow, with the front undercutting them as well. That cold front surges quickly too. There doesn't seem to be much time between initiation and when that front blasting through. Still, given the extreme shear profiles, storms could produce right at initiation over IL, and then continue to produce from embedded supercells and QLCS tornadoes as the line advances. After dark, the tornado threat will probably continue, given that moisture advection continues and the LLJ ramps up even further than it's already extreme values. Some more discrete cells may track through KY and TN overnight posing a tornado threat. Favorable low level instability and lid strength indices suggest that surface based storms aided by dynamic forcing, could be in progress through the duration of the event, from moring into the overnight hours. I don't see a cool boundary layer keeping stuff elevated either at the start of the event in N IL or the end of it below the OH River, given the warm air advection. The other big question is what kind of prefrontal initiation we'll see. Helicity values are very impressive ahead of the cold front. While the mean flow doesn't bisect the cold front for lift downstream, it doesn't parallel it either. The main line will probably be rooted to the cold front, but if there's enough lift ahead of the cold front, we could see a few discrete tornadic supercells tracking across western IL, IN, and OH. This scenario seems like it would be most difficult to forecast though.
Given the lapse rates and moisture, the models have been fairly generous with the instability. Both the GFS and NAM have been hinting at MLCAPE approaching 1000 J/KG across IL and into IN. It's more than enough for tornadic supercells given the extreme shear values.
IL to TN could see tornadoes I think. It's going to be a question of if we get some semi discrete supercells at one end of the setup or another, or a solid line of undercut showers on the cold front.
Chasing this thing is a whole different story. Storm motion is going to be extremely fast. Cells on the main line are going to be moving NE at 50 to 60 mph or even faster. You won't be able to keep up with that unless you're staying ahead of the line on the interstate. The tornadic play on the southern end I'd consider to be almost completely unchaseable unless you're local and have a good perch, since again storms will be screaming, and through bad terrain and after dark. If you're going for the prefrontal play, you're going to have to be well downstream in IN probably, which might be the best option for local chasers there. Otherwise, I'd probably try to target initiation of the main line in north central IL, by late morning/early afternoon. The highway system there is very favorable for moving up and down a line quickly, and this might be the best time to see more discrete cells before they all fill in down the line.
One area to watch is further northwest. The GFS has been fairly consistent at keeping the surface low further south, wrapping the moisture around it, while pushing some really cold air aloft over the top of it. Negative lifted indices are spiking like some sort of quasi cold core type setup. on the MS river in NW IL and eastern IA.
http://www.twisterdata.com/data/mod...1/16/00/GFS_3_2013111600_F42_LFTX_SURFACE.png
If that materializes, these areas could see some low topped, north moving tornadic supercells, which will probably be the most discrete activity of the day unless something prefrontal goes up.
Preliminary target is Rochelle to Bloominton, IL and I plan on using either 88, 80, or 74 to stay ahead of the line until I can intercept something interesting before the lines passes me by. If the cold air target pans out, I'd pick a secondary target of Sterling, IL to Davenport, IA using 88 again to position along any cells that might go up there. I do not expect to see a tornado, unless I get extremely lucky with timing and positioning. I think is going to be a very frustrating chase given storm motion and the potential for storm mode to be predominantly linear. Tornadoes might be far and few between, or difficult to see given low, grungy bases. It's close though, so I'll give it a shot.
