Thursday, February 2, 2012

Another slow-moving storm forming in the southwest

Last week I was talking about a slow moving storm that was pushing through the southern US.  This week we're looking at another cyclone, currently developing over the desert southwest, that looks to slowly push eastward into the central plains into this weekend.

Let's start by looking at the GFS model 500mb forecasts.  At the moment, the Oklahoma HOOT site where I usually get my model graphics is migrating to another server and as such their output has fallen a bit behind.  For something completely different, I decided to use San Jose State's website for today's model graphics.  Here's the GFS 500mb height and vorticity forecast for 18Z today...about now.

Without going into the details of what vorticity means, one of the things it is useful for is tracing the location of shortwave troughs aloft.  For instance, you can see that in the base of the deep shortwave trough on the left side of the map above, the vorticity values are very high--those bright yellows, reds and purples indicate strong positive absolute vorticity, which is indicative of a cyclonic (counter-clockwise) horizontal shear of the winds.  We typically see such wind motions in a shortwave trough.  Let's use this to trace the forecasted path of this trough over the next few days...

By 18Z Friday, the trough is forecast to move eastward into the high plains.
Notice there still is a streak of high vorticity values associated with this trough.  Let's go another 24 hours out.  This is now Saturday at 18Z:
By this point, the trough has become much more poorly defined--looking at the 500mb height lines (the black contours), you may not necessarily think there is much of a trough there.  But, there is still a big vorticity maximum present in the central plains.  With the right temperature structure, this means that the flow could still be unstable enough to keep developing any surface cyclones.

So lets look at the surface forecast as this storm moves across the plains.  Here's the forecast for 18Z today at the surface (about now):
Looks like an area of low pressure trying to get its act together over New Mexico.  Notice that the path of the 500mb trough we saw above tends to follow the zone where there is a stronger horizontal temperature gradient at the surface.  This is not a coincidence--the steering winds aloft are controlled by low-level temperature gradients.  Also notice the very warm temperatures throughout the southern US and the southeasterly winds blowing from the Gulf of Mexico into the southern plains.  That indicates a lot of moist, warm air moving in, setting the stage for an unstable atmosphere.

Going forward 24 hours, we see that the surface low, while not very well defined on the map below, appears to be slowly moving eastward with the shortwave aloft.

While we can see the beginnings of a cold front in the Texas panhandle and New Mexico, the cold air behind it really isn't that deep, nor is the temperature transition really that abrupt.  Since we're looking at surface temperature here and the surface temperature naturally gets colder as you go up into the Rocky Mountains, the cold air behind the front is not as cold as you might think.  Furthermore, there's a strong pressure gradient developing in eastern Colorado that could help pull air down from the high mountains out onto the plains.  As air descends down the mountain slopes its pressure increases and it warms up--another factor working against a strong push of cold air.

And now the forecast surface map for Saturday:
Notice that even though our shortwave trough aloft had become somewhat less defined in the 500mb height field, the surface low pressure center has continued to consolidate (though it hasn't deepened that much.  However, in this image things don't look that good for the low.  The cold front is weak at best, and any attempt at a warm front is having difficulty getting through the Appalachian Mountains.  Still no sign of a strong push of Arctic air down from Canada, so temperatures will probably continue to be seasonably mild for much of the eastern US.

As far as precipitation goes, we're looking at showers and thunderstorms in the warm sector of this storm, which will generally stay in the southern plains and the deep south.  Lots of stratiform rain is expected to the north and east as the storm is able to pull that warm moist air up and around the low.  Some areas may see some snow, particularly in the central plains north of the low where the air is just cold enough.  Here's the forecast 6-hour precipitation accumulations for 6Z tonight:
That strong pressure gradient I mentioned over eastern Colorado will help increase the wind speeds quite a bit in that area.  Combined with the expected snow, this has led to blizzard warnings being posted for much of the west-central plains.

Moving to tomorrow, the precipitation comma follows the low nicely.
The purple contours on these maps are the 1000-500mb thickness lines.  Often we use the 5400m 1000-500mb thickness line as a separation between snow and rain.  We see here that this line runs from western Kansas up through Iowa and into the Chicago area.  While it seems clear that places like northern Colorado and western Nebraska will get all snow, the precipitation type forecast becomes a bit more difficult the further south and east you go.

On one final note, with such warm moist air coming off the Gulf into the southern plains, the Storm Prediction Center has slight risks for severe weather in Texas and Oklahoma tonight and tomorrow.  So, we may see a few strong thunderstorms with this storm as well.

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