Weather Blog

The beginning of the month of August was relatively quiet in the North American weather market. There were a couple of tropical features, but they were marked by their languid movement (Debby) or hit small landmasses, away from the American shores (Ernesto). The severe season has been slowed as well. Debby was the only real tornado producer of the month to date.

This will be changing over the next several days, as is often the case as summer changes to fall. Not only should the Atlantic basin start getting moving — there are two curious looking features on the long range model guidance — but with the change in air mass coming with the change of season, there should be enough of a conflicting air mass to produce strong thunderstorms on a regular basis, at least for a few weeks.

This isn’t atypical. There is usually a secondary severe season in the autumn, though cold air is typically more forceful than warm air. As a result, the cold can more easily sweep warmer air away, while warm and cold do battle for longer in the spring. Even though the fall season is shorter, the storms can be quite powerful.

Such was the case in the Great Lakes early this week, where strong straight line winds brought damage to the Twin Cities in a couple of rounds, and then carried on through Wisconsin and eventually Michigan. Consider this the opening salvo of the Fall season.

Severe weather is much more challenging to predict at longer ranges than tropical weather, at least in terms of trends, so we don’t know exactly what to expect more than a few days out, but we do have the short range outlook, which is looking askance at the wave moving through the Dakotas today. Severe weather is possible today and tomorrow, and may carry through across the Great Lakes again as the weekend approaches. Eventually, it will work south again before winter finally seizes the scene.

Thus far, the biggest tropical story in the Continental US surrounded the remnants of Beryl after it had done it’s worst in the Caribbean, and Debby, which was more notorious for lingering in the southeast than for being particularly strong. Now, Ernesto took a swipe at Bermuda and has left us with nothing to monitor in the Atlantic Basin.

Saharan sands are blowing off of the African continent, and getting into the trade winds, inhibiting cyclonic development. The NHC foresees no tropical development for at least the next 48 hours, and the GFS – by no means meteorological gospel – doesn’t really show anything in the area until around Labor Day, nearly two weeks from today.

That’s a real slow stretch in a season that was promised to be very active, particularly since the peak of hurricane season is typically in early September. It seems like this might be a precursor for saying that the season may not be as busy as we believed at the outset, but it is not. Even with the data we have on hand, and access to the freshest short term modeling, the Hurricane Center’s latest outlook came out, and didn’t go easy on the number of tropical storms.

There will still be a week or so from the end of the GFS run to the traditional “peak” of the season. After this peak, the oceanic temperature doesn’t immediately fall off. The season begins in June, and has been creeping earlier. As a result, the season still goes another couple of months, until November. Plenty of time, and with the very warm Atlantic, plenty of fuel still left for storms.

Enjoy the last bit of summer, but remember that by Labor Day, things may be getting pretty hairy in the Caribbean once again.

Undoubtedly, in the last 10 years or so, there has been an increase in hurricane headlines. Part of this is because of our own unconscious bias. The US went for about a decade without a landfalling hurricane, which is pretty much absurd, and now that the rate of American landfall is ticking up, it feels like it is happening all the time.

And of course, with alarm bells ringing on the increasing heat in the ocean, we are acutely aware that the hurricane season is longer than it ever used to be. Sure, there are more large hurricanes making landfall in the US recently, and the season has become longer over the last couple of decades, but this doesn’t fully translate the issue.

As I said, we had a long lull after Katrina, where the US was spared, so an uptick in landfalling storms was always going to seem worse than it had been in recent memory, even if it is more a correction to what is a normal landfall rate. I think the misperception, however, is that we are having a bunch of strong storms regularly arriving on US Shores, when that is not really the case.

One thing is true about tropical storms is that they are being infused with more moisture, and as a result are bringing more rain. Every storm you see now has a greater tendency to produce flash flooding. This is partly being seen in Debby, where parts of the Carolinas and Georgia have been inundated. Debby wasn’t a particularly strong storm, but she was sure a soaker.

This is where the longer season comes into play a bit. As we know, the transition seasons of spring and fall tend to have the most dramatic weather over the continental United States. The jet dives south and brings active cold and warm fronts that make life a bit more interesting for everyone. Historically, the hurricane season is most active in September, just when things get more interesting with subtropical weather, and these cold fronts usher remnant hurricanes off into the north Atlantic.

In August, the dog days of summer, there are no big jet troughs or cold fronts, and storms are left to dawdle. Debby has been such a danger because she’s not been forced offshore. Harvey was an August storm, and he had the same issue 7 years ago. Dawdling. Storms are carrying more moisture, and early season storms especially are less likely to be kept moving, and flash flooding is on the rise with tropical storms. Expect this trend to continue for the long term.