Saturday, December 11, 2010

Central US Blizzard

The cyclone expected to hit this weekend is definitely hitting--with quite a bit of force.  Here's the current US watches and warnings map for this evening:
Fig 1 -- NWS watches and warnings as of 327Z, Dec. 11, 2010.  From the NWS website.
That large red area in the middle is all blizzard warnings--a huge area that includes the entire state of Iowa.  The pinks are winter storm warnings and the purples are mostly winter weather advisories.  Almost the entire midwest is under some kind of winter weather alert.  Pretty amazing.  So how did our ensemble model forecast from two days ago do?

Well, here's the actual surface analysis from 21Z this afternoon--the time we were looking at in the ensemble forecasts in my last blog post:
Fig 2 -- Surface analysis from 2100Z, Dec. 11, 2010.  From the HOOT website.
Here the low pressure center was analyzed over northeastern Iowa.  I'd say that's on the western and southern edge of the area where all the surface lows were clustered in the 36 hour ensemble forecast.  So the thought I mentioned yesterday about the low continuing to slow down seems to have verified--looking at the model trends appeared to be useful in this case.  However, the models had also been trending toward moving the surface low further north and we see that it didn't head as far north as the ensemble had been indicating.  So in that case, the model trends didn't exactly help. Since how far north the low would be has radical implications on the rain/freezing rain/sleet/snow divisions, this is a major forecasting issue.

A quick look at the 700mb map from 00Z tonight--
Fig 3 -- 700mb analysis from 00Z, Dec. 12, 2010.  From the HOOT website.
And we see a 700 mb low that is indeed cut off--so the strengthening trend in the ensembles seems correct.  However--this is at 00Z--three hours after the 21Z 36-hour forecast we were looking at in the ensembles.  In that particular forecast image, the 700 mb low-height center was over northeastern Iowa at 21Z.  But here we see that the 700 mb low height center was analyzed over far western Iowa at 00Z--three hours later!  This tells us two things:

  1. The upper air pattern (and, it seems, the surface pattern as well) is moving more slowly than the models had been indicating.  This means adverse conditions may linger longer over areas that didn't expect to see them for so long.
  2. The upper air pattern seems to be lagging more than the surface pattern.  Remember from my previous posts how a stacked low--one where the upper-air trough is directly over the surface low--represents a cyclone that is no longer strengthening.  When there is a separation, particularly a westward tilt in the low with height, then the cyclone is still continuing to strengthen.  In our case here, the more the 700 mb and the rest of the upper air pattern are lagging behind the surface low, the deeper and stronger the surface low will get.  Rapid cyclogenesis (strengthening of the surface low) can mean very, very strong winds and lots of vertical motion--two ingredients that can make a bad snowstorm even worse.
Looking at the surface map above (in figure 2), we can already see the very tight pressure gradient on the western side of the surface low.  Tight pressure gradients mean stronger winds and we're already seeing some 35+ knot sustained winds across parts of the northern plains.  As this storm moves eastward, these winds will move eastward as well.  "Wrap around" snow behind the low combined with cold temperatures and these strong winds definitely justify the blizzard warnings out for such a large area.

So how much snow has fallen so far?  It's difficult to tell in the middle of the event.  The Iowa Environmental Mesonet has compiled a map of snow reports so far in that state:
Fig 4 -- Objective analysis of snowfall amounts from local storm reports as of 320Z, Dec. 12, 2010.  From the Iowa Environmental Mesonet
The first wave of snowfall behind the cold front had "relatively" small amounts of snow associated with it--all those light blues and dark blues are in the 1-2 inch range, with some isolated higher amounts further south.  But now the "wrap around" snow is beginning to move into the northern part of the state--and those are 8+ inches of snowfall up in the northwest corner of the state.  Those bands that dumped that snow are now moving through the central part of the state.
Fig 5 -- Regional base reflectivity radar mosaic from 0348Z, Dec. 12, 2010.  From the NWS website
So it's going to be a long night for people in Iowa, Wisconsin, Illinois and Missouri as these bands of snow with very strong winds move through.  Don't travel tonight if you live in areas expected to see heavy snow unless you absolutely have to.  And even then think twice.  This is going to be quite the punch.

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