Hilary threatens America!

No, not that one, the tropical storm (soon to be hurricane) off the west coast of Mexico. After several weeks of the tropical action being in the West and Central Pacific, the areas of concern have shifted east in to the East Pacific (and less so the Atlantic, but that non-story is discussed below). And, if that’s not enough excitement, there will be math!

Tropical Storm Hilary has formed off the west coast of Mexico, and is moving in to conditions favorable for strengthening as is parallels the Baja Peninsula then makes landfall just south of the Mexico/US border early next week:

Here are NHC’s Key Messages regarding Tropical Storm Hilary (Link). On this track the biggest risk will be flash flooding across Baja and, starting Sunday, southern California and Arizona as the storm pumps rain into the desert Southwest. Economic impacts are estimated by the models at around $200 million, but depending on how much wind reaches San Diego (minimal tropical storm at this point), and rain across the Southwest, than number could go up or down a lot.

As noted previously, Typhoon Lan is dissipating as it approaches Sakhalin Island. Economic impacts across Japan are likely in the $1 Billion range, with reports of over 60 people injured, which is no surprising given the number of people exposed to storm conditions. Here’s the final track estimate …

There is paranoia over two disturbed areas in the Atlantic, and angst over a yellow blob on NHC’s outlook in the Gulf of Mexico. Presently there are two formal invest areas in the east Atlantic that the models are starting to get a lock on. Neither are expected to impact land at this point, here are the trajectory Barotropic-Advection Models. There are three BAM models that use data from the US Global Forecast System (GFS) to forecast storm motion. The neat thing about the BAM models, which have been around for a long time, is that they look at the winds at different levels – shallow, medium, and deep. Weaker storms tend to follow the shallow path, whereas a stronger storm, having more vertical structure, will tend to follow the deep path. When all three line up, that’s a good indication that the “steering” is consistent across the various levels of the atmo. Where they diverge, that’s a sign that there is sheering or conflicting steering (or the shallow steering layer is being contaminated by the storm itself – remember to cook your spaghetti with the heat of experience!) Here are the two storms with the TBAM models … neither storm looks to be a problem for land in the forseeable future (and probably not after that either)

As for the blob in the hot pool of fetid water bubbling in the Gulf of Mexico, well, the Gulf is running 3-4 degrees above normal for this time of year:

So what does this mean? The energy in the upper few hundred feet of the ocean is a primary source of energy for a growing storm. Storms don’t form when the ocean temperature is below 26.5C (~80F for those of you still measuring things in hogsheads). Normally this time of year the central Gulf of Mexico is around 30C. How much difference does it make if it’s 34C? There has been a lot of research on the relationship between the maximum potential intensity (MPI) of a hurricane and sea surface temperatures (see this link to a 2016 J. Atmospheric Sciences paper for some recent work). The Gulf temperatures are well above the range normally studied in this type of research, but given the logarithmic nature of the relationship (e^x), and the range of intensities we see in the Gulf , and so forth, we would expect a storm to be on the order of 20 knots stronger, all things being equal, between average mid-August temperatures and what we are seeing today. That’s basically a full Saffir-Simpson category! Now, before you freak out, other conditions over the Gulf are otherwise not terribly favorable at the moment. There is a broad area of low pressure, and NHC’s outlook (link) has it tagged at a 30% chance of becoming a tropical system as it drifts west. So nothing to get excited about, no “magic words” in the text yet.