Forecast winds and pressure from the GFS model for next weekend. Don’t pay too much attention to where this system is — it will surely change over the next week. (windy.com)

The 2018 Atlantic hurricane season officially begins June 1, but we’re seeing signs it might start earlier than that.

Forecast models have consistently been hinting at some sort of tropical development in the western Caribbean next week. Just what exactly comes out of it is nebulous, but it could affect Florida next Friday or Saturday. After that, it would probably track north and bring a deluge to the Eastern Seaboard.

If something does manage to form, the first name on this year’s list is Alberto. Coincidentally, the last time Alberto appeared on the list was 2012, and it formed on May 19.

This thing that we’re talking about, though, doesn’t even exist yet. It’s just something that’s popping up in the forecast models. It’s a large-scale gyre in the atmosphere that, over the course of a couple days from Wednesday to Saturday, gradually consolidates in the southwestern Caribbean Sea and drifts north toward the Florida Peninsula.

The best way to see it is an animation of forecast winds and moisture (precipitable water — a measure of how much water the atmosphere is holding). This is just a recent example from a single run of the U.S. GFS model; other models have a less-defined low pressure but a similar evolution of precipitable water.


The details are very hazy at this point. They vary among the models and between model runs — as expected for something a week away — but the existence of this feature is at least consistent. Pulling from the most recent available runs from four models, these maps show the low-level wind speeds valid next Saturday-Sunday. Do not pay attention to the details; they are guaranteed to change.


850mb wind speed and heights from four global models: GFS, FV3, ECMWF and CMC. (tropicaltidbits.com)

Combining ensembles from three different agencies and calculating a “formation probability” yields a 20 percent chance in the 5- to 10-day period in the western Caribbean.


Tropical cyclone genesis probability in the 5- to 10-day period based on ensembles of three global models. (NOAA/GFDL)

Even if it fails to organize into a tropical cyclone, it will still be a large blob of tropical moisture capable of producing lots of rain. The cumulative rainfall forecast through next Saturday evening is extremely wet in South Florida. Fortunately, that area has been in the midst of the moderate to severe drought, so the rain would be welcome.

If this storm-to-be is named, it would join other recent preseason named storms: Arlene 2017, Alex 2016, Bonnie 2016, Ana 2015, Alberto 2012, Beryl 2012, etc.