BIRMINGHAM (AP) — It has been a quiet year for tornadoes in Alabama so far, but the National Weather Service says activity could pick up in the fall.

“Don’t let your guard down,” said NWS meteorologist Jim Westland. “The fall can be very active.”

As of Tuesday, the state has had 18 tornadoes this year. Westland said that on average about 51 twisters pass through Alabama each year.

Westland, who recently authored a report on historical tornado trends, said the record low number for tornadoes in the state was set in 1993 with 19. The record high was in 2008 when more than 90 struck the state.

The weather service didn’t start keeping records on tornadoes until the early 1950s, but it wasn’t until the 1970s that many weaker tornadoes were reported.

Westland said that in recent years a slow spring has been followed by an active autumn for tornado weather.

In 2001, there were four tornadoes from March through May, but 46 struck the state between October and December. In 2002, two spring tornadoes were followed by 28 in autumn, and in 2004 there were 11 in the spring and 30 in the fall.

“A relatively quiet spring has no predictive value for the following fall,” Westland said in his report. “Generally speaking, chances are we will likely see at least some tornadic activity between now and the end of the year.”