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.
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):
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:
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:
Moving to tomorrow, the precipitation comma follows the low nicely.
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.