August 30: Here Comes Harvey

The storm named Harvey has been the dominant nationwide weather story for the past week — first as it strengthened, then as it made three separate landfalls along the Gulf Coast, and of course as it produced devastating flooding along the way.  Harvey will weaken to a tropical depression overnight, but now it’s starting to have a direct impact on our weather.

The remnants of Hurricane Harvey won’t pass directly overhead until Friday morning, but the greatest threat of damaging weather will shape up ahead of that.  I’ll get into a LOT of detail about the forecast, but overall here’s what to expect:
– Several waves of heavy rainfall tonight, Thursday, Thursday night, and even Friday morning.
– 2″-4″ of widespread rain, but up to 6″-8″+ will be possible — certainly enough to cause flash flooding.  The target zone for that should be to the northwest of Nashville, but it will be a close call.
– Isolated tornadoes will be possible, especially southeast of Nashville — once again, it will be a very close call.
– The timing of the greatest threat of flash flooding and severe weather looks to be Thursday afternoon through sunrise Friday.

Okay, now the details…starting with tonight.  The first wave of rain (that’s falling as I type this) will lift to the north and largely dissipate…but that will be replaced by more waves of rain late tonight and into Thursday morning.  The HRRR model’s radar simulation has a good handle on that:

I can’t give you a firm “yes” or “no” regarding whether it will be raining for Thursday morning’s rush hour, but I’d plan on it.  Widespread flooding and severe weather are unlikely in this time frame.

Off-and-on showers will dominate most of Thursday — it won’t rain everywhere all the time, but keep the rain gear handy.  (I’m not sure how much good an umbrella will do, considering the breezy conditions that will develop during the day.)  The RPM model’s radar simulation shows that pattern, and the approach of heavier rain late in the day:

Keep in mind, those images represent one version of one model — there are always differences between what the computers think and what actually happens.

As I said above, the time frame for the worst weather area-wide will be from late Thursday afternoon through midday Friday…here’s the same model’s version of that:

That’s when the flash-flooding and tornado threats will be at their highest.  The Storm Prediction Center has outlined a “Slight Risk” (level 2 of 5) for roughly the southern half of the Midstate, with a “Marginal Risk” (level 1 of 5) farther north:

There will still enough rotation associated with Harvey’s remnants that we’ll have to watch for some of that rotation to make its way down to ground level.  I wouldn’t plan on hiding in your safe space for 18 hours — panic doesn’t do you any good, and tornadoes associated with land-falling tropical systems tend to be pretty short-lived.  The flooding threat is the greater concern — the National Weather Service has issued a Flash Flood Watch for roughly the northwestern half of the Midstate, in effect through Friday:

Okay, now for the complicated part.  Tropical systems are tricky to predict, especially as they move inland and start to lose their tropical characteristics.  Just a little “wiggle” in the forecast path of what’s left of Harvey could have a big impact on the weather in your neighborhood.  Here’s what I’m talking about…the National Hurricane Center’s forecast path of the center of circulation (the “remnant low” that was the eye of the storm last week) shows it traveling just to the northwest of Nashville:

But if that path wiggles just 20 miles to the east, it rearranges the placement of the greatest tornado threat AND the greatest flash flood threat:

And if the path wiggles just 20 miles to the west, the threats get rearranged in the same direction:

So while the most likely scenario is represented by the NHC’s forecast path, there’s enough disagreement among the various forecast models to make us nervous.  Either way, the weather looks nasty — it’s just a matter of which version of nasty weather you’ll have to deal with in your neighborhood.

As an example of that uncertainty, here’s the forecast rainfall from three generally-reliable models.  First the European model, which conforms more-or-less to the NHC forecast:

Next up, the BAMS model, which pushes the path just fractionally east…but that results in the heavier rain being pushed east as well:

Finally, back to the RPM model (shown in the radar simulations above) — its path is the farthest east of any model, and it shows the heaviest rain targeted on Nashville:

If you don’t like maps, check out this graph of the rainfall estimated by various members of the SPC’s short-range ensemble forecast:

That’s anywhere from 1.41″ to 9.61″ for Nashville, as a result of very subtle differences in the way the various versions of the model predict the weather!  In short…

Again, the most-likely scenario is what I started this post with: the greatest flooding threat just to the northwest of Nashville, the greatest tornado threat just to the southeast of Nashville…but that means Nashville has to worry about BOTH.  There’s so little room for error that you should definitely stay plugged-into the forecast throughout the day no matter where you live, and especially when the nastiest weather moves in late in the day.  The one thing you can be sure of is that there will be at least some changes.

Here’s the good news: our weather should improve dramatically for the Labor Day weekend…just a few lingering showers early Saturday, then we’ll dry out for Sunday and Monday:

The next chance of rain won’t last nearly as long — just a quick shot Tuesday into early Wednesday.

Social media links

Twitter: @WSMVweather, @PaulHeggenWSMV, @WSMVLisaSpencer, @WSMVDanThomas, @daphne_deloren, @NWSNashville

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About paulheggen33

Morning meteorologist for WNCN-TV in Raleigh.
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