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Pre-Christmas (Dec 21-23rd) Winter Storm


Chicago Storm
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Just looking at the last several Canadian and GFS runs, seems like the general waveguide over the US isn't changing much, but changes upstream/downstream of our trough over Canada are influencing how quickly the system goes negatively tilted and can really wrap up...

GFS:

2017539226_ezgif.com-gif-maker(15).thumb.gif.a64a330ae9218dd6c06052f5dbc4797b.gif

Canadian:

1335756448_ezgif.com-gif-maker(14).thumb.gif.c817bfdace490edccaed108c64dad6cb.gif

 

The two areas I'm focused on are the lobe of the PV near Hudson Bay, which on the GFS and Canadian has gradually trended west/weaker in recent runs. This could allow heights to rise more aggressively in front of our storm, supporting a quicker wrap-up and stronger "cut". However, the other area is the lobe of the PV over northwestern Canada and how it interacts with the +PNA ridge trying to pop on the west coast. In today's 12z runs, that has trended farther west, leading to a flatter ridge. This nudges the base of the trough causing the storm a bit farther east/northeast, supporting a slightly slower wrap-up.

I feel like the trend for the Hudson Bay lobe of the PV to be weaker/farther west has been more consistent (a nod in the "amped" direction) but the trends over western Canada have not, so that could continue to go either way and lead to subsequent adjustments.

I doubt we see wholesale changes to the setup, but the exact max snow amounts and where that swath occurs can trend both west/heavier and east-northeast/lighter. A very high floor set-up as with such a strong trough closing off aloft, with Arctic air involved, a lot of areas will get strong winds and some amount of snow/blowing snow (even if it isn't a lot of snow farther east of the low track). 

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