That is a question I have had ask of me and I thought it was a great question since this is the start of hurricane season. INVEST 91L, has just hit Tropical Storm status and is named...BILL. Landfall 8 am this morning!!! 

INVEST... is an international designation meaning a disturbed area over water that has a water temp of 80 degrees or higher to a depth of about 150 feet that meets certain criteria that bears watching closely or "investigation". Hence...the acronym..INVEST.

Each geographic region around the world carries a basin designation. I think there are 7, but will have to look it up.Those I do remember are:Atlantic/Gulf = LEast Pacific = EWest Pacific = WIndian Ocean = BSouth Pacific = PSouth East Indian = S

So, the numbers associated with the INVEST start at 90 and go to 99 before starting over at 90. NOT likely to have 10 in a basin in a season. I do not have an answer for why not start at...01???
In the case of 91L in the Gulf...it is an Atlantic basin INVEST..."L".... 91 ...or second of the current hurricane season...remember a couple of weeks ago the Invest 90L off the East coast?  She became a named Tropical Storm... ANA. 

91L, has just hit Tropical Storm status and is named...BILL.  Bill is slated to make landfall around 8AM Tuesday morning.  Bill is currently moving NW at about 17 MPH with winds of about 50 MPH. 

The forecast....
   
   - The center of Bill is expected to make landfall in the warning area along the Texas coast Tuesday morning and move inland over South-Central Texas Tuesday afternoon and Tuesday night.   

   - Maximum sustained winds are near 50 mph (85 km/h) with higher gusts.   

   - Some slight strengthening is possible before landfall. Bill is forecast to begin Weakening after the center moves inland on Tuesday, and Bill is expected to weaken to a tropical depression Tuesday night.   

   - Tropical storm force winds extend outward up to 160 miles (260 km) from the center.   

   - The estimated minimum central pressure is 1005 mb.   

Hazards affecting land----------------------rainfall:  Bill is expected to produce total rain accumulations of 4 to 8 inches over Eastern Texas and Eastern Oklahoma and 2 to 4 inches over Western Louisiana and Western Arkansas, with possibleisolated maximum amounts of 12 inches in eastern Texas.Wind:  tropical storm conditions are expected to first reach the coast within the warning area tonight.
INVEST GRAPHIC TROPICAL STORM BILL
Tom JohnsonKTXSANAU6