Apparently the Weather Prediction Center agrees, because in its Tuesday night update on the system, it called Alberto a “tropical depression” for the first time.
It’s not every day you see a tropical depression over Indiana, and the forecast is for it to remain a tropical depression even as it crosses the Lower Peninsula of Michigan into Canada.
Storm classifications depend on what’s going on at the upper levels and inside the storm. There are two basic types of cyclones — the ones that come through in the mid-latitudes with warm fronts and cold fronts, and the kind that develop in the tropics over the ocean, which can become hurricanes. The former is nontropical, the latter is tropical, and you can also have weird hybrids in between — something we call “subtropical.”
Classifying cyclones can be tricky to diagnose in real time when a storm is a hybrid. Forecasters at the National Hurricane Center and the Weather Prediction Center have had to make these operational decisions for Alberto. After they’re over, all storms undergo a lot of scrutiny, and it is possible that researchers will find Alberto had been a tropical cyclone since before it made landfall. If that is determined to be the case, Alberto will forever be known as a tropical storm, and we can forget all of the “subtropical” nuance.
There are complicated model-based diagnostics that meteorologists can use to determine where on the cyclone spectrum a certain storm falls. Using a combination of dynamics and thermodynamics, a cyclone can be deemed deep or shallow, symmetric or asymmetric, warm-core or cold-core. A classic tropical cyclone is a deep, symmetric, warm-core system. Alberto was indeed in that region of the diagrams from May 27 to 30 but is moving back toward something that’s more subtropical.
Regardless of its technical classification, similar hazards still exist. Alberto has brought a lot of moisture out of the Gulf of Mexico. Extreme rainfall totals have been logged in Florida, Alabama, Georgia, western Tennessee and western Kentucky.
By Thursday, upon reaching Canada, Alberto will transition again, one last time. It will become a totally nontropical cyclone.
It’s rare but not unprecedented to have a tropical storm or tropical depression tracking over Tennessee, Kentucky and into Indiana. The map below shows tracks of 11 other known storms that did the same thing. Of those, four made landfall at less than hurricane intensity — like Alberto.
What makes Alberto special is that none of the other storms occurred before June 1. In that regard, Albert is a first.