May 25: Typical Summertime Weather, Daily Links


In this type of weather pattern, forecasting the daily weather should be easy — it’ll be warm, it’ll be muggy, and there will be a chance of showers/storms every day.  Unfortunately, it’s not that easy — because the day-to-day fluctuations in the weather pattern will produce subtle variations in HOW warm we’ll be, HOW muggy it will feel, WHERE the storms will develop, and HOW MANY of them there will be.  You know…the stuff you care about the most, especially heading into a holiday weekend.

Here’s what I mean…check out the side-by-side-by-side comparison of what the RPM, BAMS, and HRRR weather models show for our rain chances throughout the day:
I mean, that’s essentially worthless.  All of the models have struggled this week, to the point where their collective output has resembled this:
Every model starts with a set of “initial conditions” — basically, what the weather looks like RIGHT NOW.  But different models initialize differently, which means they don’t always have a good handle on where to start.  All week, most of the forecast models have vastly overestimated our rain chances right from the start — so if they’re starting with some rain, they’re more impressed with the odds of additional rain forming along the edges of rain-cooled air later in the day.  Meteorologists call this “convective feedback” — the colloquial term is, “garbage in, garbage out.”  Regardless of what you call it, the effect for forecasters is this:

So if I can’t pin down the exact “when and where” of the best rain chances, let’s fall back on some Meteorology 101 to try to narrow things down.  If you’re just trying to figure out if it will rain, there are two questions that need to be answered: will there be enough humidity, and will there be a trigger to get the showers/storms going?

Let’s tackle the first question: will there be enough humidity?  The past couple of days, the answer has been NO.  But this morning, dew points have started climbing, and they’ll hover in the low 60s most of the afternoon…at least around Nashville:
That’s not all that high on the muggy meter…
…but those numbers are high enough to make the humidity feel noticeable, and to give showers at least a chance of getting going.  Notice that the highest humidity levels will be in western Middle Tennessee.

Now, about the “trigger” to get showers or storms started — that’s the part of the forecast the computer models are struggling with.  We won’t have a strong cold front or any other kind of atmospheric boundary at ground level to provide the lift needed for storms to develop, but there will be a little ripple (a short-wave, in meteorologist-speak) in the upper atmosphere.  That doesn’t sound impressive…and it isn’t, which is why I’m leaving rain chances at less than 50%.  The effects of the short-wave will be greatest farther west, because it will be traveling along the main storm track, which is aligned farther to our west and northwest.  So that’s a couple of pieces of evidence that point to the best chance of showers and stray storms in western Middle Tennessee — the closer you are to the Tennessee River, the better chance you have of picking up some measurable rain:

In between the showers, we’ll see enough of a break in the clouds to allow temperatures to warm up quite a bit, well into the 80s this afternoon:
But because of the uncertainty in the rain forecast, there’s a lot of uncertainty in the temperature forecast.  If it rains in your back yard, your back yard will be a bit cooler.  But it will still be muggy!

More of the same in the forecast the next several days — there are some day-to-day fluctuations in the rain chances and temperatures, but this is where I come back to the general forecast: it’ll be warm, it’ll be muggy, and there will be a chance of showers/storms every day:
If you have outdoor plans, it’s time to start settling into the summertime mindset — your odds of getting rain at a specific spot at a specific time are low, but the chance of storms will be there.  Have an indoor alternative in mind (just in case!) and be ready to adjust as-needed.  We’ll be here to try to nail down the day to day specifics, the challenge of which will cause me to age like Walter Donovan at the end of Indiana Jones and the Last Crusade.  No, I’m not going to post that GIF.



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About paulheggen33

Morning meteorologist for WNCN-TV in Raleigh.
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