Monday, January 31, 2011

Quick Comparison of This Event with the January 1999 Blizzard

In reading a lot of the forecast discussions from WFOs across the upper midwest, I noted several references comparing this coming winter weather event to the blizzard of January 2-4th, 1999.  Though I apparently lived right through the middle of it, I honestly didn't remember this particular event.  So, I thought I'd see what all this hubub was about and quickly, qualitatively compare the two events.

The initial synoptic setup is remarkably similar.  Take this morning's 12Z 500mb analysis from the GFS:
Fig 1 -- GFS 500mb geopotential height and wind analysis at 12Z, Monday, January 31, 2011.
Note the two features I noted in my last post that will combine to produce this powerful storm--a shortwave digging out of southern Alberta into nothern Montana and another in the desert Southwest over Arizona.  Compare this to an archived 500mb analysis from 12Z on Friday, January 1st, 1999.
Fig 2 -- NOAA Difax 500mb N. America geopotential height analysis from 12Z, Jan. 1, 1999. 
There are indeed some eerie similarities between the two analyses.  There were also two shortwaves analyzed before the 1999 storm, though these were slightly further east--one was coming out of southern Saskatchewan while the axis of the other was in New Mexico.  Though these features are slightly further east, the forecast for the current event will quickly catch this week's shortwaves up and even speed past the timing of the 1999 event.  But more on that later...

Back to this current event, twenty-four hours later (12Z on Tuesday morning), the GFS forecast has a well-defined, if somewhat east-west expansive, trough across the central part of the country with embedded short waves.
Fig 3 -- GFS 24-hour forecast of 500mb geopotential height and winds valid 12Z, Tuesday, February 1, 2011.
Comparing this to the 1999 event 24 hours later, we see that the analyzed 500mb trough didn't seem to have these embedded shortwaves and was more north-south oriented.  This is an important difference--the lack of tilt in the 1999 event slows down the progression of the trough and associated surface features.
Fig 4 -- NOAA Difax 500mb N. American analysis of geopotential height valid 12Z, Saturday, Jan. 2, 2011.
Even with these differences, however, both analyses feature at least some form of trough axis moving through central or eastern Texas with an attendant jet streak around the base of the trough.  Both of the exit regions of the jets would lie in the mid-Mississippi valley, enhancing the divergence aloft there.  But the timing and orientation of these two features is still different.  Here's the surface pressure forecast for the GFS at 12 hours later--00Z on this Tuesday night.
Fig 5 -- GFS 36-hour forecast of 500mb geopotential height and winds valid 00Z, Wednesday, February 2, 2011
Compare that to the surface analysis from the 1999 event at 24 hours later--12Z, Sunday, Jan. 3 1999.
Fig 6 -- NOAA Difax surface analysis valid 12Z, Sunday, Jan. 3, 1999.
BOTH analyses have a deep surface low located in the Memphis/southern Illinois region.  However, the GFS (and most of our other) model forecasts bring that low there a full 12 hours faster than the January 1999 event.  By the time we move the full 48 hours out in the GFS forecast for this current event, the low pressure center at the surface has already moved into Indiana and begun to occlude.
Fig 7 -- GFS 48 hour forecast of surface pressure and temperature valid 12Z, Wednesday, Feb. 2, 2011.
We don't see the occlusion on the 1999 storm until sometime before 72 hours out:
Fig 8 -- NOAA Difax analysis of surface pressure (and fronts) for 12Z, Jan. 4, 1999.
So by the time this low occludes, our current forecasts for the storm this week move things along 12-24 hours faster than the 1999 storm.

There are cold air intrusions in both events, however the observed 20-25 degree Fahrenheit morning lows in Oklahoma by the time of occlusion in the 1999 event are outstripped by the single-digit lows being forecast by the GFS for this week (NOT that we trust the GFS surface temperature forecasts...).  This hints at a stronger baroclinic zone and much colder air behind the front.  This becomes important because if we look at snowfall totals from the 1999 event--
Fig 9 -- NOAA Difax analysis of observed snowfall on the ground at 12Z, Jan.3, 1999.
--we see that while the upper midwest saw large amounts of snow (17 inches at O'Hare in Chicago on that list to the right...), the snow amounts taper off to the southwest.  Only a few inches were reported at places in Missouri and none in Oklahoma.  Contrast this with the blizzard warnings extending all the way into central Oklahoma with the event tomorrow--12-18 inches of snow are expected in parts of Oklahoma.  From this, we have to conclude that there is more moisture being forecast with the current event and/or the temperatures were colder further south, supporting more snow all the way into Oklahoma.

Interestingly, according to records at Wiley Post Airport in Oklahoma City, on January 2-3, 1999, snow and rain were both reported in the observations, but no measurable accumulation of either seems to be recorded.  The high temperature also hovered in the low to mid 30's with overnight lows in the low teens after the event. Not near the cold weather we're looking at according to current forecasts...

So, in conclusion, there are some general similarities in the evolution of both the 1999 blizzard and this week's event.  However, this week's storm will move much more quickly, have much colder air behind it, and bring wintry precipitation further south.  Up north, however, the proxy seems to be pretty good--if a storm of that intensity (and perhaps even more limited moisture) in 1999 could bring 17 inches of snow to Chicago, projections in that range would not seem to be out of the question for the upper midwest with this week's storm.

Finally (on a slightly technical note), I just want to leave with a cross-section image generated on the College of DuPage website from this evening's soundings.  This cross section runs from Albuquerque in the west to Nashville in the east.  We can see that "warm conveyor belt" starting to ramp up with that bulls-eye of moisture (the green contours) in an area of southerly winds to the east.
Fig 10 -- Cross section from Albuquerque, NM (left) to Nashville, TN (right) showing contours of potential temperature (red), mixing ratio (green), theta-e (yellow) and wind barbs valid at 00Z, Tuesday, Feb. 1, 2011.  From the College of DuPage website.
And so it begins...

Also, for all of you reading this at the University of Washington--I will be giving tomorrow's weather briefing in 310C at 12:30 PM and will be going into a fair amount of detail about this event during the presentation.  Feel free to come by...

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