hi, i have got a mail reply from Foreflight. The following is their mail reply

ForeFlight receives most US weather
from the National Weather Service. Our radar and satellite maps come
from WDT, which provides the best
and most accurate composite radar
data and satellite data for pilots. http://www.wdtinc.com/pages/ radar_mosaics Note that composite takes into
account the radar returns at all
altitudes, so it may differ from a
NEXRAD image you may also be
viewing. Take a look at that and let us know if
you have any questions -- we're
happy to help! Janessa Barber
ForeFlight Pilot Support
ForeFlight LLC