23 August 2011

Severe weather probable on Tuesday

A pretty potent shortwave will dive out of the Dakotas today and provide the area with severe weather. Storms are ongoing this morning; with a pair of MCS's in western Minnesota and in Northern Iowa. These will shift east throughout the early morning and eventually decay near or north of the area, laying down a luscious boundary that will play a role in what transpires below. 

*A "kink" in the flow resembling a shortwave on the H5 maps. This "kink" will provide the focus for today's unsettled weather. Map shows projected position of shortwave at 21z (4 P.M)*
Now with the "spark" for severe weather moving into the area at 21z, let's take a look at dynamics this "spark" will ignite. We already have a triggering mechanism in the shortwave and an outflow boundary from earlier storms. As the trough approaches the OFB will meet it near the MN/IA border. 

*CAPE values are pretty bulky in the vicinity of the trof.  Here in Northern Illinois we will be experiencing a moderately unstable atmosphere. While the OFB may not be enough to spark a couple of storms, all this heat energy will be available for storms to munch on as they chew their way toward us later tonight.*
The 0z NAM and GFS (especially GFS) isn't showing much in the way of destabilization across the area. I would imagine it would be because of the left over effect of the aforementioned GFS. Temperatures holding in the upper 70's/lower 80's while areas to the west are pushing 90. 

* 0z GFS representation of temperatures across the area. 
* 06z NAM  depiction of temperatures
The four basic ingredients to thunderstorm development are 1) instability 2) shear 3) lift and 4) moisture. The amount of each will determine the severity of the storm We already established instability with the CAPE values in excess of 3500 J/KG which is strongly unstable and supportive of severe weather. Next question is do we have moisture, lift, and shear? 

* Abundant moisture is in place by 4 P.M. coupled with a strongly unstable environment * 
 * Bulk shear values over 40 kts is more than sufficient for maintaining severe weather into the night.  Along with a strong LLJ. Supercells like to thrive in bulk shear environments in the 35-40kt range* 
*Decent helicities shifting into N. IL. Helicity is otherwise known as rotation that could make it's way into a storms updraft... the greater the value the greater the chance the storm has to rotate.* 
*From 0z Joliet area*
Every model I have seen and put faith into breaks out precip along the outflow boundary stretching from the Chicago area to Dubuque. Supercells are likely with hail to the size of golf balls and destructive winds to 70 mph. Initially the tornado potential will be there, but perhaps will INCREASE as the low level jet ramps up and acts to enlarge the curvature in the low levels. 

*At 0z the nose of the LLJ is just punching into W IL. If storms remain discrete supercells through 0z, I think we may get a couple of tornadoes. 
This set up actually reminds me of the one on July 27th. A morning MCS moved through dropping a boundary which was the focus for supercells to form in NE IA and NW IL. As of now I think by 6p the Platteville, WI area is sitting prime for supercell development. If a supercell can form and latch on the residual OFB then it will tornado and may threaten Freeport/Janesville/Rockford areas. Otherwise storms will congeal into an MCS with the possibility of producing hurricane force winds and decent sized hail into the Chicago metro. Timing I am going with is into the Rockford area by 7p, Aurora by 830-9p, Downtown Chicago by 945-10p and into northwest Indiana after 11p. Main threats destructive winds. Tornado threat best north of US 20 and west of I 39. Be aware of the situation and please visit www.weather.gov/chicago for official warnings. 

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