EIA Map
The EIA recent wrote an article reminding motorists of the phase in or switch over to summer gasoline as it fast approaches.

The article appears on the EIA's website and summarizes what the switch over is, why it occurs, and other pertinent information that motorists should know.

As much as we've covered summer gasoline, many Americans believe it still to be a myth. A myth it is most definitely not! Why? More stringent air pollution standards have come into play with the 1990 Clear Air Act.

One of the largest problems or issues with summer gasoline is logistical as various communities have slightly different laws covering what type of gasoline must be used. Maintaining supply of each one of these blends can be a challenge early in the season as refineries build up supply of each blend.

The EIA's article goes on to say:
Refiners are currently switching to make summer-grade gasoline ahead of the May 1 compliance date for refiners and product terminals. Summer-grade gasoline has a lower volatility than winter-grade gasoline to limit evaporative emissions that normally increase with warm weather and cause unhealthy ground-level ozone.
Volatility is a measure of how easily a liquid (or solid) will change into a vapor. For gasoline, it is measured by Reid Vapor Pressure (RVP). The higher the RVP, the more volatile the gasoline. While the Environmental Protection Agency (EPA) requires lower-volatility summer gasoline, aside from any government regulation, gasoline's RVP must be limited to ensure that the fuel does not vaporize in the fuel system. If it does, the engine can stop running.
It costs refiners several cents per gallon more to make summer-grade gasoline, compared with winter-grade fuel, which is part of the reason that retail pump prices can rise in the summer.
Additionally, in parts of the country that require cleaner, reformulated gasoline (RFG), such gasoline must meet even stricter limits on volatility (see map). The EPA, for example, requires use of RFG in high-smog areas to reduce smog-forming particulates and pollutants. States or regions also have gasoline quality requirements; California, for example, has stricter requirements than the federal government.
For logistical reasons, the transition to low-RVP gasoline happens over the course of several spring months as temperatures rise and to facilitate lowering the RVP of remaining inventories of winter-grade gasoline. The federally mandated dates for summer-grade gasoline and reformulated gasoline, where required, are May 1 to September 15 for refiners and terminals, and June 1 to September 15 for gasoline retailers. In California switchover dates are earlier, and summer-grade gasoline must be in use for a longer period.