A Flip in the Forecast

It’s hard to think about unusually cold weather when it is so unusually warm, but that’s exactly what we are studying closely for the weekend and next week. This time of year can bring a lot of forecast flips, but the one coming up in the next seven days is about as big as they get in October!

Even when you consider the unusual warmth we have had at times this month, temperatures are still 2.2º F below average in Huntsville through yesterday. In Muscle Shoals, the month-to-date average through the 22nd is 1.3º F below average.

So the over-all forecast for a cooler-than-normal October has panned out, but as you can see, sometimes the day-to-day application of that longer-range forecast does not really make a lot of sense. In fact, we have fielded many questions about why the models missed this and that, and I’m glad the questions are coming. It’s an opportunity to pull back the curtain and let you see how we come to the conclusions we come to without using the old stand-by:

Now, on to this big pattern flip that will bring some cold air into the Tennessee Valley this weekend…

The NAEFS Ensemble from Environment Canada is usually a good indicator of how temperature patterns will evolve over a 10-14 day time frame. It’s not helpful at all for any one particular day, but it helps us frame our thinking and understand how each individual day could play out. In this case, the NAEFS shows a large area of below-average temperatures over the Southeast from October 31 to November 7th.

This can mean a lot of things. In this case, all indications are that the first few days of that period will be well-below average with a gradual warm-up toward November 7th. Remember, this is showing an average, so if you have something below average only warming to near average, the over-all trend is shown as “below average.”

Specifically, we see this flip in the pattern beginning this Friday and lasting through the middle of next week. The maps below are stereographic images looking down on the Northern Hemisphere from atop the North Pole. The United States is in the bottom of the images, and I have labeled the East and West with either “warm” or “cold.”

The magnitude of the chill around here will depend on Tropical Storm Sandy and how she interacts with a cold front moving across the East this weekend. The above model guidance is assuming that Sandy gets caught up in the front and enhances the cold air advection (movement) into the South. If Sandy escapes to the east and does not get involved in the front, it will still get cool, but it won’t get quite as cold.

Ensembles are a great way to view the potential in an either/or kind of forecast. Since our numbers on the Seven Day Forecast are assuming some of that indirect influence from Sandy, we may need to adjust a little over time as the evolution of the pattern becomes more clear.

Here are the ensemble forecast probability charts for temperatures dropping below 32º F in Huntsville and Muscle Shoals over the next two weeks:

You can see there’s some wiggle room in there for some changes. Right now, I’m leaning toward at least a good frost by Monday morning, but there is some uncertainty as to how much and how thick it would be. We update our expectations multiple times a day at WHNT.com/Weather. Check it out now and then to read the latest on our thinking!

-Jason
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