Sunday, November 13, 2011

Update on Weekly Outlook...

I don't normally make an "update" to the weekly outlook that I post on Saturday's, but rather make updated posts throughout the week on individual elements that were covered within the outlook.  I felt that an overall update was warranted in this case because 2 major facets of the outlook have undergone some rather dramatic shifts in the last 24 to 36 hours.

The first, and the most certain of the 2, has to do with the Texas through Northeast rain event in association with an upper level weather disturbance coming out of the Southwest.

The disturbance has slown by about 24 hours, and will not likely come out in full force until Monday night or early Tuesday.  The good news is that this will allow low-level moisture to build up over a larger portion of Texas, which could result in heavier rain further Southwest than earlier forecast.  

The latest run of the GFS model accumulated precipitation forecast is shown below:

As you can see, the model is now forecasting 1 inch or more of rain (1 inch begins with the lightest blue shading, as per the scale on the right hand side of the image) from portions of westcentral and northwest Texas on to the Northeast.

So, when it has to do with this early to mid week rain event, you can push the timing aspects that were covered in the original weekly outlook out about 24 hours, and everything else will hold in place (other than the potential for more generous rain further West in Texas - which would certainly be welcome).

Stay tuned for another detailed update on this rain event, particularly having to do with Texas, on Monday.

Now, on to the less certain portion of the update - which deals with the movement of cold air into the U.S. this week.

The last few model runs have indicated that at least a piece of the cold air we talked about in the weekly outlook will break loose earlier than expected, and potentially make a run even further South than earlier expected (although I did point out that we would need to keep an eye on that part of the forecast).

Take, for example, the latest GFS model forecast of the departure from normal temperatures valid at 6pm CST for the period Monday through Thursday of this week (watch the progression of the cold air - blue and green shaded area from Southwest Canada into the U.S. during the period):

If the above forecast verifies, then colder air (though not the coldest of the cold - see further below) will sweep Southward much earlier than originally expected, possibly reaching the central Plains of Kansas and Oklahoma as early as Tuesday night or early Wednesday.

Granted, this is not the coldest of the cold air that we originally talked about, and the models still project that even colder air will move into the U.S. beyond this next weekend, into the week of Thanksgiving, but this will be cold enough to feel the difference (and some 15 degrees or more below normal in many areas).

Is this just a computer model "flip-flop", or is this the real deal?  Time will tell, but as I pointed out in the original post, cold air this dense and heavy can only hold on in one place for so long.  It appears that the models are coming around, rather abruptly, on just how long a period of time that will be, with at least a piece of it being forecast to break loose rather quickly this week.

Watch for another more detailed update on this situation on Monday as well...

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