The following information was posted to the Surly LHT owners email list by a forecaster with the National Weather Service. I am posting this here with his permission.
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"When thunder roars go indooors" just
doesn't woork for those of us on long bike rides, hikes, etc.

The NWS has also thrown out the 6 mile rule. Really if you can hear
thunder, you are in danger. To measure lightning distance, use the
fact that the speed of sound is roughly 1000ft/sec. Measure time
between flash and boom, then divide by 5 to get miles.

But better than this, be situationally aware. Get a forecast before
you go out, especially if you will be out for many hours or days. I
think you will be hard pressed to beat the day to day forecasts from
the NWS http://www.weather.gov/ Click on the map to get your local
office. Then you can click on their map to get your local area. There
is a different forecast for every 2.5km or 5 km, depending on the
office.

But when you are out, keep watching the sky.
It takes 3 ingredients to build thunderstorms: lift, instability, and
moisture. Lift can be produced by mountains, cold fronts, differential
heating, or in Florida's case sea breezes.

As for moisture and instability, watch the sky for any cloud that has
vertical growth, especially in the morning:
cumulus http://www.usatoday.com/weather/wcumulus.htm
altocumulus castellanous (ACCAS) http://tinyurl.com/23xmvmk
cirrocumulus http://mmem.spschools.org/grade5scie...rocumulus.html
If you see the cumulus, towering cumulus, or ACCAS before 11 AM,
chances are you have sufficient moisture and instability for afternoon
thunderstorms.
Adjust your behavior with that in mind.

And you don't have to be underneath a thunderstorm to get struck by
lightning. Lightning bolts off the sides or tops of thunderstorms can
strike many miles away from the base and are some of the most powerful
lightning stikes.

The NWS recommends that if you can hear thunder, you are in danger.
And the 30 rule says, stay under shelter until 30 minutes after you
last hear thunder. I can say that if I followed that rule, I wouldn't
be out for most summer afternoons or early evenings. Lightning safety
is about risk analysis and informed choices. To be perfectly safe, you
need to very carefully sit on your couch. Of course there are many
risks associated with that behavior too!