The above forecast depicts the initial cold front advancing into Texas by Sunday morning, however the really cold, arctic air lags further behind in the Northern Plains - but not for long. Take a look at the same forecast, except valid 24 hours later, at 6am CST Monday, January 10th:
As you can see, the really cold air makes a big move Southward beginning Monday, such that by Tuesday much of the central third of the nation and Midwest will be socked into the bitterly cold airmass.
The computer forecast models are also suggesting that this won't be a short lived event, either. In fact, most show that cold air will continue to spill Southward into the nation throughout much of next week. The screen images below (click to enlarge) are the GFS model surface forecasts valid at 6pm CST for each of the days Tuesday through Saturday of next week (respectively).
Take note of the thin blue line across west-central through North Texas beginning Tuesday. That is the 0 degree celsius temperature line (which is 32 degrees F, or freezing). Note how it sinks south across much of central and into south Texas and then remains there through Saturday:
GFS Surface Model Forecast Valid 6pm CST Tue 1-11-11
GFS Surface Model Forecast Valid 6pm CST Wed 1-12-11
GFS Surface Model Forecast Valid 6pm CST Thu 1-13-11
GFS Surface Model Forecast Valid 6pm CST Fri 1-14-11
GFS Surface Model Forecast Valid 6pm CST Sat 1-15-11
If the above verifies, much of the central and northern two-thirds of Texas wouldn't even get above freezing during the daylight hours for the 2nd half of next week! You can trace the freezing line across the remainder of the nation in each image and see where your locale comes in on the cold weather train.
The blue lines to the North of the 0 degree line are -10 C (+14 F), -20 C (-4 F) and -30 C (-22 F), respectively, to give you an idea as to where the really, really cold air is forecast.
I need to dig a little further, but taking a quick look at the records for the nearest reporting station to my home, which is the Austin Bergstrom Airport, it appears as though it will have been 28 years (1983) since we will have had as long of a cold stretch as we're potentially looking at for next week. If the above model forecasts verify, all kinds of records are going to be broken across central & south Texas, as well as many other parts of the nation.
I'll continue to monitor this situation and update you regarding both the cold air invasion and the chances for any significant wintry precipitation as the situation unfolds throughout the coming days.