Saturday, April 2, 2011

Thoughts on Severe Weather Sunday Night, Part 1

That shortwave trough that is forecast to move through the central United States on Sunday into Monday is well on its way--we're finally beginning to clear out here in Seattle as the trough axis (and the accompanying cold front) has moved through.  Now it's out into the mountains and headed toward the plains...

In my last post I talked about how there was some ambiguity in the models with regards to the evolution of this trough, particularly as it moved into the plains.  The models have come into slightly better alignment, but many questions still remain about the complexity and exact consequences of this shortwave.  Nevertheless, the SPC has put out a slight risk of severe weather for much of the central part of the country on Sunday (and Monday, but we'll focus on Sunday right now...):
Fig 1 -- SPC day 2 outlook for Sunday, April 3, 2011 valid as of 1730Z, Saturday, April1, 2011.  From the SPC website.
A wide area from eastern Oklahoma up through northern Illinois is highlighted.  So what's the structure of this cyclone as it stands now in the models?

The GFS and the ECMWF models are in much better agreement for Sunday night than they used to be.  Here's the GFS 500mb forecast for 00Z Monday (Sunday evening):
Fig 2 -- GFS 36-hout forecast of 500mb winds (colors) and geopotential height (contours) for 00Z, Monday (Sunday night), April 4, 2011.  From the HOOT website.
The trough axis is very positively tilted (the axis extends from southwest to northeast) as it's entering the plains, which typically means that the surface low beneath is still strengthening.  However, I am not impressed with the coherency of the mid-level jets here--there's no particularly organized jet streak on the leading edge of the trough.  The ECMWF isn't any better on this:
Fig 3 -- ECMWF 48-hour forecast of 500mb winds (colors) and geopotential height (contours) for 00Z, Monday (Sunday night), April 4, 2011.  From the HOOT website.
The patterns shown by both models are very similar, so we tend to have a greater confidence in the upper-air forecast for this time.  However, like in the GFS, the wind pattern shown by the ECMWF seems rather disorganized.  There are definitely stronger winds on the leading side of the trough, but no organized jet streak.  Since the upper-air pattern and the surface pattern are very much coupled, such disorganization aloft tends to imply somewhat disorganized patterns at the surface as well.  More on that in a moment.

If we want more proof that the models have a handle on what's going on, here's the GFS surface forecast for 00Z this evening:
Fig 4 -- GFS 12-hour forecast of surface temperature (colors), mean sea-level pressure (contours) and winds (barbs)  for 00Z,Sunday (Saturday evening), April 3, 2011.  From the HOOT website.
We can see that as that shortwave is moving out of the mountains, a surface low is forecast to be forming this evening in eastern Montana.  If we look at the surface pressure trends in observations over the past three hours this afternoon (from around 12-3 PM central time) we see this:
Fig 5 -- 3-hour surface pressure trends as of 19Z, Saturday,  May 2, 2011.  From the College of DuPage.
In the image above, solid contours indicate pressure falls and dashed contours indicate pressure rises.  The yellow arrows are wind vectors.  Note that there is a strong center of pressure falls right in the area (southeastern Montana) where the GFS is forecasting the surface low to be developing this evening.  So, things seem to be verifying, at least in the short term.  Notice also that there is an area of pressure rises over the Great Lakes region.  With pressure rising to the east and falling to the west, this is going to increase the pressure gradient across the Great Plains and into the midwest.  A stronger pressure gradient means much stronger winds out of the south at the surface. This is going to help bring a lot of moisture northward from the Gulf of Mexico--fuel for the storms expected to fire tomorrow.

I have some work to do this afternoon, but I'll continue this evening with part 2--looking at how things are shaping up closer to the surface.

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