Weather Nerd

Hello, Dolly!

After teasing forecasters for a week, Invest 94L has finally organized into Tropical Storm Dolly. Winds are at 45 mph, and Dolly could be a Gulf hurricane in a few days. I’ll post more soon.

(Posted via cell phone.)

UPDATE: The NHC issued its first advisory on Dolly just 45 minutes after issuing its final advisory on Bertha — advisory number 70! Bertha stuck around for a remarkable 17 1/2 days, but is now finally, officially extratropical (meaning it has lost its “tropical characteristics,” i.e., its warm core and symmetry).

That means we can now focus on the two Atlantic-basin storms: Cristobal and Dolly.


(Image credit: NASA. Hat tip: Dr. Jeff Masters.)

Cristobal is brushing the Outer Banks of North Carolina at this very moment. You can watch its progress on both long-range and short-range radar. The storm’s ability to intensify — or to drop more than spotty rainfall amounts on the drought-stricken region — has been hampered by dry air getting sucked into the circulation. Mild strengthening is expected tonight or tomorrow, but by then, Cristobal will be moving away from land. (And by the time Cristobal approaches Nova Scotia, it will most likely have been absorbed by an approaching trough of low pressure.) Alan Sullivan grouses, “There has been far too much hoopla about this trivial, marginal system.”

The bigger story is definitely the new kid on the block, Tropical Storm Dolly. As I mentioned previously, the tropical wave that spawned Dolly has been teasing forecasters and weatherbloggers for days, as it originated in the Cape Verde region and then trekked across the Atlantic and Caribbean, repeatedly threatening to develop but never quite doing so — until this morning. Now, finally, we have a 45 mph tropical storm on our hands, and it’s expected to strengthen.

The National Hurricane Center says Dolly will only intensify slightly between now and when it makes landfall in the Yucatan Peninsula early tomorrow morning, in part because of wind shear caused by a nearby upper-level low pressure system. But that system is weakening, and Dr. Jeff Masters thinks Dolly could get pretty strong, pretty fast:

The upper-level low that prevented Dolly from developing a closed circulation [until now] has weakened considerably over the past day, and will continue to weaken and slide southwestward out of Dolly’s way today. This will create a low-shear environment for Dolly to intensify in. With very warm waters of 28.5°C ahead of it extending to great depth, Dolly is expected to intensify right up until landfall tonight over the Yucatan Peninsula. It is unlikely Dolly has time to reach hurricane status today, but a 60 mph tropical storm is possible tonight at landfall.

Dr. Masters mentions the warm waters, and indeed, the portion of the western Caribbean that Dolly will traverse between now and its Yucatan landfall is the most conducive ocean for hurricane development that this storm will encounter for the remainder of its life, according to the Tropical Cyclone Heat Potential map, which considers both the surface temperature and the depth of warm water. Here’s a rough drawing of Dolly’s expected track, superimposed on that map:

dolly-warm.jpg

As you can see, Dolly isn’t expected to pass over truly superheated Gulf waters of the sort that produced monster hurricanes Katrina and Rita in 2005. Nevertheless, that turquoise-blue water is plenty warm, so there will be ample opportunity for strengthening after Dolly clears the Yucatan. The NHC’s initial discussion — whle predicting only that Dolly will reach 70 mph winds, the highest tropical-storm strength — acknowledged that it’s “certainly possible” there will be a hurricane in the western Gulf of Mexico in about three days’ time. Dr. Masters writes that Dolly “will probably have at least two days over the warm waters of the Gulf,” which “may allow Dolly to intensify into a Category 1 hurricane.” The Houston Chronicle‘s Eric Berger also mentions “the possibility the system could strengthen rapidly over the warm Gulf of Mexico waters.”

That said, let’s remember that hurricane intensity forecasting is very difficult — especially when it comes to guessing if and when rapid intensification might occur. So don’t put too much stock in these early predictions. The best approach is to wait and see what happens once Dolly reaches the Gulf and re-organizes itself after its expected weakening over the Yucatan.

The official forecast calls for landfall Thursday morning near the Texas-Mexico border. However, there is uncertainty as to both the exact location and the timing — particularly the latter. The NHC writes: “THE FORWARD MOTION OF DOLLY IS LIKELY TO BECOME SLOWER WHEN IT IS OVER THE GULF OF MEXICO. . . . [THE COMPUTER MODELS] DISAGREE GREATLY ON HOW LONG IT MIGHT TAKE FOR DOLLY TO MAKE FINAL LANDFALL.”

Here’s a look at the current computer-model “spread”:


(Image credit: Colorado State.)

Residents of the Houston area should note that “CLP5” — the turqouise line taking Dolly toward them — is not a real computer model, but essentially a “dummy” model, based purely on climatology and “persistence,” against which the real models are tested. In other words, pay no attention to it.

Oh, and one last thing. In case four tropical storms by July 20 isn’t enough for you, we may soon have a fifth. Dr. Masters notes that “three of our four reliable computer models for are predicting a new tropical depression will develop off the coast of Africa 3-5 days from now.” The Weather Channel’s Dr. Stu Ostro wrote of that storm earlier this morning: “the next disturbance over Africa . . . looks like it’ll head into the tropical Atlantic during the next couple of days as a spinning low pressure system, not just a tropical wave. At least temporarily it too has an impressive signature on the imagery — looking like something typically more likely to be seen there in mid-August than mid-July.”

Hmm… should I start calling it “proto-Edouard” already?