Two short waves, one big storm, Chicago blizzard?

It's been a week or so since I last posted on here.  The annual meeting of the American Meteorological Society was here in Seattle last week and needless to say I was very busy during that time.  But now that's done and my schedule can resume something more normal...

Today's National Weather Service summary map looks pretty spectacular:

Fig 1 -- NWS watches and warnings as of 2300Z, January 30, 2011.

The darker purple from Montana down through the high plains and into southern Wisconsin is all winter weather advisories.  The pinks through much of Missouri and eastern Kansas, Nebraska and Oklahoma are winter storm watches.  And what about that bright green blob surrounding Chicago?  That's a blizzard watch for the Chicago CWA.  Milwaukee is also issuing blizzard watches at this time.  I quote the following from Milwaukee's latest Winter Weather advisory:

"NEEDLESS TO SAY THIS COULD BE A HISTORIC BLIZZARD CAPABLE OF

PARALYZING PARTS OF SOUTHEAST WISCONSIN."

Pretty powerful words.  So what's the setup for this?

Right now, there is a highly amplified ridge over the west coast with broad scale troughing aloft across the eastern half of the country:

Fig 2 -- GFS 12Z analysis of 500 mb geopotential heights and vorticity on Jan 30, 2011.  From the HOOT website.

Note two individual shortwaves are evident in the west embedded in the larger-scale flow.  One is the nearly cutoff shortwave in southern Oregon and northern California.  The other is further north on the British Columbia/Alberta border in Canada.  This northern shortwave is located in the region we usually see "Alberta Clipper" type storms form.  It's somewhat unusual to have two shortwaves stacked on top of each other like that. Latest model forecasts show these two shortwaves merging together over the plains by late Tuesday:

Fgi 3 -- GFS 60 hour forecast of 500 mb geopotential height and vorticity valid 00Z, Wednesday, Feb. 2, 2011.  From the HOOT website.

Cold air in association with the northern shortwave is forecast to spill down into the plains as that shortwave moves south.  We can see a blob of colder temperatures moving down across the northern plains at 850mb on Monday:

Fig 4 -- GFS 30 hour forecast of 850mb temperatures and geopotential heights valid 18Z, Monday, Jan. 31, 2011.  From the HOOT website.  

Of course, there's already very warm air to the south in Texas and along the Gulf Coast.  As the cold air moves south, this is going to increase the temperature gradient across the central part of the country.  You can already see how quickly the 850mb temperature changes from Nebraska into Oklahoma on the image above.  This increasing temperature gradient means two things:

1. Frontogenesis is going on at the low-levels--a cold front is most likely developing from Iowa down through Oklahoma at the time above.
2. Based on those thermal wind arguments (yes, those again...), the increasing temperature gradients below translate to strengthening winds aloft parallel to the forming front.

Do we see winds increasing aloft in this area?  We do indeed see a jet streak strengthening in that area--and all the way back down into west Texas.

Fig 5 -- GFS 36 hour forecast of 300mb winds and geopotential height at 00Z, Tuesday, Feb 1, 2011.  From the HOOT website.

Quite the dynamic pattern here.  Not only can we see those two shortwaves starting to merge together, but look at how amplified that ridge along the west coast has become.  It even has a north-south oriented jet streak!  That's pretty rare.  We also have two jet streaks that are roughly east-west oriented--one somewhat associated with the exit region of the northern trough and the other associated with the exit region of the southern trough.  If we look at these in terms of our four-quadrant jet streak model:

Fig 6 -- Same as figure 5 but annotated with the four-quadrant model showing regions of convergence and divergence associated with jet streaks.

We see that divergence regions of both of these jet streaks almost "coincide" over the same region.  This is a form of a phenomenon known as "jet streak coupling", where the combined effect of multiple jet streaks can really enhance the divergence (or convergence) aloft.  Of course, with this huge amount of divergence going on aloft, the pressure at the surface is going to fall rapidly:

Fig 6 -- GFS 60 hour forecast of surface (2m) temperature, MSLP and wind barbs valid 00Z, Wednesday, Feb 2, 2011.  From the HOOT website.

There's a strong surface low forecast by the GFS to move into southern Illinois and continue trekking northeastward into Michigan.  Look at the tight pressure gradient forming across the upper midwest and into the southern Plains.  This translates to very strong northeasterly winds across the upper midwest on Wednesday.  So if this model forecast were to verify, blizzard-level winds would indeed be possible on Wednesday across northern Illinois and southern Wisconsin.  The reasoning behind this blizzard watch begins to make sense.

But what about snowfall amounts?  How cold will it get?  We'll take a look at the models again tomorrow sometime and see how things are continuing to shape up.  For now, just be on the lookout for a strong winter storm during the middle of this week.