Monday, January 9, 2012

Relief for Texas, cool weather ahead

A cut-off low over the desert southwest is slowly churning eastward through Texas and into the deep South.  In response to the low-level cyclonic motion generated by this low, southerly winds have helped pull lots of moist air out from over the Gulf of Mexico and into Texas.  The result?  Heavy rains today throughout much of the state.

According to the Houston forecast office, they're expecting 1-3" on average for most of their forecast area with locally heavier amounts up to 5" today.  That's a lot of rain, particularly for an area that still is in a large precipitation deficit.  Here's the latest national drought monitor image (from January 3rd, released January 5th):

Much of Texas, particularly south Texas, and parts of Oklahoma are still in "extreme" to "exceptional" drought. Hopefully today's rains will help alleviate some of that...

Let's look at how the upper-air pattern is going to change over the next few days.  Here's the current 500mb picture from this morning's GFS analysis:
You can see the cut-off low centered over the El Paso area.  By Wednesday morning, the low is forecast to have move eastward across the deep South.

However, notice that there's a shortwave trough that's beginning to dig in from up in Canada.  In this 48-hour forecast the trough is centered over southern Manitoba back through Eastern Montana.  This trough is forecast to deepen pretty significantly over the following 48 hours.  The little cut-off low in the south is forecast to move up the east coast, helping to bring in warm air from out over the Gulf Stream.  As the shortwave trough digging in from Canada in the above image moves eastward, it's going to encounter the rather strong baroclinic zone (warm-cold temperature gradient) left behind by the little cut-off low over the east coast.  Remember that upper-level winds are strengthened by strong horizontal temperature gradients below.  As such, that shortwave trough really starts getting strong by Friday morning:
This could mean some pretty crazy weather in the northeast on Thursday and Friday of this week.  We'll have to keep an eye on that.

But notice what is happening to the overall upper-air pattern--a strong ridge is building over the west coast, while large-scale troughing is occurring over most of the US east of the Rockies.  This would be considered the longwave pattern, as opposed to the shortwaves we usually talk about.  Shortwaves are the smaller troughs embedded in the bigger, longwave pattern.  Here the longwave pattern has shifted to ridging over the east Pacific and troughing over the central US.  This is actually a very typical La Nina type of upper-air pattern.

And if we're getting into a typical La Nina type pattern, all of those long-term weather predictions about the type of winter to expect should start coming into line.  The pattern is forecast to become even more established by Saturday:
Still a big ridge in the eastern Pacific with troughing over the eastern US.  Notice the location of the jet stream--down the Rockies and then out eastward over Texas and the south.  Since the winds aloft are connected to temperature gradients below, the jet stream often marks the boundary between cold, arctic air to the north and warmer, subtropical air to the south.  With this large-scale troughing in place, we can anticipate much cooler temperatures across much of the central US by the end of the week.  Here's the forecast 1000-500mb thicknesses (a proxy for temperature throughout the lower atmosphere) on Saturday morning:
Anywhere north of the solid blue line is usually cold enough to support snow.  You can see that everywhere except for the southern-most tier of states is in the snow regime.  We're talking high temperatures in the teens and 20s for parts of the northern US by the end of the week.  Looks like winter is going to happen this year after all.

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