Monday, December 27, 2010

This Friday's trough and January 7th, 2008

After a few days off to celebrate the holidays, it's once again time for me to look at the weather.

Of course, in my days off, a powerful nor'easter swamped the east coast with rain and heavy snow.  Blizzard conditions were experienced in New York City.  It was quite the event.

But I kind of missed that.  So, instead I'm turning to the potent trough that models are indicating should move through the country late this week.

What trough is this?  Here's the 96 hour GFS 500 mb height forecast for Friday morning--
Fig 1 -- 96 hour GFS forecast of 500 mb geopotential heights and winds for 12Z, Friday, Dec. 31, 2010. From the HOOT website.
Now that's a very deep trough for this time of year--the minimum close contour is at 5280 meters and is all the way down over Kansas.  Based on this cyclonically-curved jet streak pattern, one might imagine a deepening surface low pressure center somewhere near Kansas City, Missouri, with a trailing cold front back through central Kansas and western Oklahoma (underneath the jet streak).  And, based on the GFS surface forecast for that time, you'd be pretty accurate with that imagination--
Fig 2 -- 96 hour GFS forecast of surface temperature (shaded), sea-level pressure (contoured) and winds (barbs) for  12Z, Friday, Dec. 31, 2010. From the HOOT website.
 We can also see a warm front in the above image extending northeastward from the surface low pressure center through eastern Iowa and into southern Wisconsin.  Superficially, this reminds me of the surface synoptic-scale setup from the first week of January in 2008:
Fig 3 -- HPC surface analysis from 15Z, January 7, 2008.  From the SPC Severe Thunderstorm Event Archive.
At this time, there was also a surface low near Kansas City with a warm front extending into southern Wisconsin and a cold front trailing into western Oklahoma.  With this particular event, several tornadoes were reported later that day in the lower Mississippi valley in addition to strong EF3 tornadoes that moved across northern Illinois and southern Wisconsin--on January 7th.  Not what you'd expect from the weather at this time of year.

However, there are some noticeable differences between these two events (or rather, from this theoretical GFS forecast and this event two years ago).  First, the surface temperatures are rather different.  In the GFS forecast, northern Illinois is only in the upper 30s in the morning, where as in the map from January 7, 2008, the dewpoints (and consequently low temperatures) were in the 50s--so not only is it much cooler, there also doesn't seem to be as much moisture.  In fact, looking at the GFS dewpoint forecast for the same time:
Fig 4 -- 96 hour GFS forecast for dewpoint temperature (shaded) and winds at 12Z, Dec. 27, 2010.  From the HOOT website.
We see that dewpoints are only in the 40s (which is still high for this time of year, but not as high as it was in 2008) in much of Missouri, Oklahoma and Arkansas with the really deep moisture confined to the far south and gulf coast.

However, in looking at the 500mb chart from 12Z, January 7th, 2008, we see more differences:
Fig 5 -- Objective analysis of 500mb heights, winds, and temperatures from 12Z, January 7th, 2008. From the SPC Severe Thunderstorm Event Archive.
It's immediately obvious that that the 500mb pattern looks rather different.  There is no cutoff contour of low heights like we are seeing in the GFS forecast.  Winds are more out of the southwest over the warm frontal zone in Iowa and southern Wisconsin as opposed to the more southerly winds being forecast in the same area by the GFS for the end of this week.  This may seem like a small difference, but changes in directional wind shear can have a huge impact on the ability of any storms that form to rotate.

We can also look at the upper air temperatures to get a hint at possible instability.  In figure 5 above, on January 7th, 2008, the 500mb temperatures over the surface baroclinic zone (over the fronts) ranged from the -10 to -15 degree range in the lower Mississippi valley to around -20 degrees Celsius over northern Illinois and southern Wisconsin.  Looking at the GFS 500mb temperatures forecast for this Friday shows surprisingly similar numbers:
Fig 6 -- 96 hour GFS forecast of 500mb temperatures (shaded), geopotential height (contoured) and winds for 12Z, Friday, Dec. 31, 2010.  From the HOOT website.
Those temperatures line up rather well.  This would theoretically say good things about the potential instability of the atmosphere, as colder temperatures aloft tend to imply steeper lapse rates.  However, remember that our surface temperatures are around 15 degrees cooler or more with significantly less moisture in this forecast than it was on January 7th, 2008.  That does not make it a good environment for being able to get surface-based convection.  Forecast soundings agree with this, still showing a deep isothermal layer (which is a stable layer) near the surface even after all the heating that will go on during the day on Friday.
Fig 7 -- BUFKIT GFS 108-hour forecast sounding for KRFD for 00Z, Jan. 1, 2010.
 We can see that we're saturated well up to 700 mb, which does imply a good rain event.  Wind directional shear actually looks all right near the surface.  But that isothermal layer near the surface and lapse rates that in general really aren't that impressive would seem to inhibit any good, deep convection from going on.  But we'll see...

Anyhow, just thought I'd do a quick run down comparing this long-term forecast to the January 7th event after noting the striking similarities in the surface patterns.  However, as we can see here, just because the conditions look similar from being viewed one way doesn't mean that they are similar in other ways.  We'll still see unusually warm temperatures throughout much of the middle of the country and probably some thunderstorms somewhere this Friday.  But at this point it doesn't look like quite the severe weather event we had two years ago.

Things could change in the models, though.  And they most definitely will.  96-hour forecasts are still way, way, way out there and there's no doubt that that GFS forecast will change in the days to come.  Already there's disagreement with the ECMWF model.  Here's the ECMWF 500mb chart for the same 96-hour forecast:
Fig 8 -- 96 hour ECMWF 500mb forecast winds (shaded) and geopotential heights (contoured) for 12Z, Dec. 31, 2010.  From the HOOT website.
The ECMWF model is slightly slower than the GFS with this trough--note that the main jet streak is over central and western Kansas instead of eastern Kansas.  It also brings the jet streak further north--now we'd expect to find a surface low somewhere near Sioux Falls, South Dakota, with a trailing cold front over western Kansas and the panhandles of Oklahoma and Texas.  The trough itself is also deeper--note the minimum closed contour is still 5280 meter (like it was in figure 1 above) but this closed contour extends over a much, much larger area.  So there are definite and distinct differences.

We'll just have to wait and see how this one all comes together.

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