September 13: Not As Hot, Tracking The Tropics



We tied the Triangle’s record high temperature of 96° yesterday, but today’s temperatures will be substantially cooler. Clouds will fill in this morning and stick around all day — temperatures will stay in the 70s to the northeast of the Triangle, and we’ll only climb to right around 80° in the Triangle itself. Areas south of the Triangle will be quite a bit warmer, since the cooler air will get to you later in the day.

The humidity is still substantial, but northeasterly winds will bring a slightly drier air mass into central North Carolina this weekend. If you’re eagerly anticipating a BIG drop in humidity, you’ll probably have to wait until the middle of next week.

As the cooler air makes its way in today, we’ll see some scattered off-and-on showers. A few storms could fire up in our southern counties, where temperatures reach the mid to upper 80s. The HRRR model’s radar simulation from 8:00am through 8:00pm has a pretty good handle on the overall pattern.

We’ll see lingering clouds to start the day on Saturday, but enough sun will break through in the afternoon to warm us up to the mid 80s.

Just a very slight rain chance Saturday, but Sunday could bring us some spotty storms in the afternoon and evening. The European forecast model’s simulation from 10:00am through 10:00pm doesn’t show a washout, but be flexible with outdoor plans, just in case.

Highs in the upper 80s to around 90° will return Sunday and stick around through early next week. After that, it looks like we’ll get a break from the heat and humidity by Wednesday and Thursday.

But the long-range forecast is highly dependent on what happens (or doesn’t happen) in the tropics, which brings us to…


11:30AM UPDATE: The National Hurricane Center’s 11:00 update on “Potential Tropical Cyclone Nine” included a bunch of stuff I talked about on-air and in this morning’s original post (below):
– Eastward shift off the coast of Florida
– Hard right turn, farther from the NC coast
– Intensification to hurricane status next week

We’re not out of the woods with this one quite yet — but everything is trending in the right direction for now.

POSTED 8:30AM: Yesterday, the National Hurricane Center started issuing advisories on “Potential Tropical Cyclone Nine.” That phrasing is the official way of saying that it’s not a tropical depression or storm yet, but it’s expected to reach that status soon. This morning is still a disorganized cluster of storms to the east of the Bahamas. The storm activity is almost entirely to the east of the system’s center — that lack of symmetry will change as it evolves into a tropical storm.

The National Hurricane Center’s forecast path shows it reaching tropical storm strength (39+mph winds) tonight — the next name on the list is Humberto (silent H). It tracks through the Bahamas and then along the east coast of Florida this weekend, still as a tropical storm.

After that, the NHC forecast track curves the storm away from the coast — but the 5-day “cone of uncertainty” is very wide. The coast of North Carolina and even some of our southern counties in central North Carolina are in the cone at this point.

That’s the official forecast…now, let’s get to how it’s likely to change. High wind shear (red lines on the next map) will keep soon-to-be-Humberto from strengthening too much over the next 36 hours. But after that point, the storm gets into an area of much lower shear (green lines), giving the storm a better chance to become better organized.

Hurricane Dorian traveled a similar path under two weeks ago, but there’s still plenty of warm water available to fuel additional strengthening.

Intensity is THE hardest thing to predict with tropical systems, but it’s certainly possible that Humberto reaches hurricane strength by early next week. The odds of that go up if the storm follows a path farther offshore — to the east of the NHC’s forecast, where the storm wouldn’t interact with land to as great an extent. Most of this morning’s forecast model data — not all, but most — shows such a path, farther off the coast of Florida.

While that’s bad news in terms of eventually-Humberto’s strength, it could be good news farther down the line. While the forecast models are east of the NHC forecast, they do generally agree with the NHC’s assessment that the storm will take a right turn, away from the southeast U.S. coast. The farther the storm is to the east to begin with, the farther that right turn will take it from shore…including coastal North Carolina.

You’ll see a lot of media outlets showing a side-by-side animation of the American forecast model and European forecast model, depicting different versions of the where the storm will go. YOU WON’T SEE THAT HERE. Those are “deterministic” models — one run of one model, which gives people a false sense of certainty in one version or the other. It’s much more useful to look at “ensemble” data — the same model run dozens of times, to give us a range of possibilities and hopefully a most-likely scenario.

This is from the European model’s 51-member ensemble — the “spaghettios” plot shows us each ensemble member’s version of where the storm will be (and how strong it will be) over the next week. Overall, most show a hard eastward turn for soon-to-be-Humberto, keeping it away from the North Carolina coast.

While that’s certainly a hopeful trend, there’s a LOT that can change over the next several days. As the hurricane hunter aircraft investigate the system over the weekend, we’ll get more and more data for the forecast models to chew on. Weak tropical systems in particular are difficult to forecast, because they can do some crazy things before they get organized. I’ll repeat my assessment from yesterday: I’m not worried, but I’m watching it. We’ll have updates during our weekend newscasts, and I’ll be posting updates throughout the weekend on Twitter, Facebook and Instagram.


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