NORTH TEXAS (CBSDFW.COM) – If you tuned in to Meteorologist Garry Seith’s weather report Monday morning, you heard him say this “could be” the day we hit 100° at DFW Airport.  Garry went with a high temp of 99° — one degree shy of the infamous mark.  The question has come up:  How do we meteorologists decide when it’s this close to go with 99° or 100°?

First of all, meteorology is (spoiler alert) an inexact science.  Therefore, even the best meteorologist, using the most up-to-date technology will still struggle with the exactness of weather.  For example, we can give a general idea of where and when storms will fire later in the day, but it’s nearly impossible to give a specific address as to where this will happen.  The same holds true with forecasting the exact temperature, on one thermometer in all of North Texas.

All that said, there is scientific reasoning behind the numbers you see on the CBS 11 Storm Team forecasts.  On a day when 100° is in reach, we consider several factors.  The first (and most important) is “Will we have abundant sunshine?”  The answer today is yes. According to our charts, our computer models, and the tried and true method of looking out the window (yes this is something we should do daily).  Now let me take a break to emphasize something I’ve heard before by older and wiser meteorologists…we forecast the conditions that cause the weather…which means that if one of those conditions is off a bit or we miss it greatly, the entire forecast’s accuracy is in danger.  So will it be sunny today? Looking at the conditions we have available to us now…yes!

Another vital factor in temperature forecasts: how strong and from what direction is the wind blowing?  Usually, in order for DFW and most of North Texas to hit the century mark, we need to have a south to slightly southwest component of our wind.  Wind that blows from a little west of true south is sinking slightly off the higher plateaus of the West Texas and the Hill Country.  Whenever air sinks, it is compressed and thus heats rapidly.  A slightly southwest wind is usually aided by an approaching cold front to the north.  The front helps to shift the winds ahead of it to south/southwest and thus we often see our highest temperatures during the year, right before a cold front pulls through.

We also look of course at the actual data that our various computer models are spitting out.  Today, our RPM model, a very reliable shorter-term guide, is showing us right on the threshold of triple digits.  We refer to the RPM as “FutureSky Forecast” and as you can see below, we have our south wind, an approaching cold front and dry-line, and plenty of sunshine for later today’s weather simulation.

(credit: CBS 11 Storm Team)

Another good thing to look at is climatology.  What was the weather like in the past when we did hit 100°?  Do we have the same make-up in today’s atmosphere to replicate?  Last year, we hit 100° for the first time at DFW on June 13.  The low temperature that morning was 78°.  We had a similar south/southwest wind and plenty of sunshine.  Today’s low temperature at DFW has been 78° as well.  Whenever we start the today that warm, it makes it that much easier to get the mercury up to 100°F.

So why then are we going for 99° instead of 100°?  In a situation like this when we’re talking about a difference of one degree, it ultimately comes down to intuition of the meteorologist.  Forecasting is very much an art…based on science of course, but also less concrete ideas of experience, history, and that good ol’ “gut feeling”.  Would any of us be able to feel the difference between 99° and 100°?  I dare say “no”.  But there is something psychological about that number “100” and given the summer of 2011, we are all at least a little wary of seeing that show up on the temperature map later this afternoon.