Explainer: Tropical Storm Alberto

CHARLOTTE, NC (WBTV) - Late in the day on Saturday, nearly two weeks before the official start of hurricane season, a disturbance off the Carolina coast spun into the first Tropical Storm of 2012.  It's small, it's relatively weak, but a closed area of low pressure with 40 mph winds technically counts.

It's unusual to see preseason storms, but it happens.  The vast majority of storms form during the official season, which spans June 1 to November 30, and even then storms are typically seen between the months of August and October, when ocean waters are at their warmest.

So what happened?  Let's go back to Friday, when what would become Alberto was just a small area of low pressure that had broken away from a larger low, then headed southward.  It's path took it right along the Gulf Stream, a current that brings very warm ocean waters Northward from the tropics.  It's the reason that our beaches are nice and warm, unlike the California current that brings cold water from the northern Pacific down along the west coast.

In order for tropical systems to form they need very warm ocean water.  In this case, the water was just above the threshold of 80° we need for tropical formation.

There is also a range of wind shear, or change in wind direction with height, that is needed for storms to form and keep going.  Too little and they won't get started, too much and they'll be ripped apart.  A rule of thumb with tropical cyclones is that the shear must be 20 knots (just under 25 mph) or less.  If shear is less than about 10 knots, disturbances will not often develop.  This is generally when we see an upper level trough nearby interfering with its development.

See to right: Image showing effects of vertical wind shear on tropical systems.

Upper level winds near Alberto are creating wind shear within acceptable levels, between 15-20 knots.

Even with conditions conducive to tropical development, Alberto is a very shallow, very small and weak storm expected to have little or no impact other than a higher threat of rip currents.  And it's day in the sun is about to come to a close.

Alberto is beginning to ingest dry air, like poison to hurricanes.  Also, it's moving into an area of slightly cooler ocean water as it starts looping back around.

Map showing Sea Surface temperatures as of Monday morning to right.  Notice how temperatures have dropped below the 80° threshold.

Earlier heading Southwest, it's now taking a more Northeasterly track.  As a longwave trough approaches (bringing with it our next cold front) it will speed off to the Northeast and likely die out completely by Thursday.

The Pacific has also seen an early start this year, with Tropical Storm Aletta forming before the Pacific hurricane season started on May 15th.  This marks the first time in recorded history when we've seen a tropical storm form in both the Pacific and Atlantic basins before the official start of hurricane season.