Some of the FAA terminal Doppler radar products from a few of the locations will have up to 1 minute updates for the lowest elevation scans.  This rapid scan modes, known as “Hazardous Weather Mode” is triggered when a 30dbz echo is detected aloft within 25 miles of the radar site or if there is wind shear detected.  The following TDWR radar sites will begin to deliver 1 minute imagery when the radar is in “Hazardous Weather Mode”

TDAL       DALLAS LOVE FIELD (DAL)
TDFW       DALLAS/FT. WORTH (DFW)
TCLT       CHARLOTTE (CLT)
TCMH       COLUMBUS OH (CMH)
TCVG       COVINGTON (CVG)
TDAY       DAYTON (DAY)
TMDW       CHICAGO MIDWAY (MDW)
TORD       CHICAGO O’HARE (ORD)
TMKE       MILWAUKEE (MKE)
TEWR       NEWARK (EWR)
TJFK       NEW YORK CITY (JFK)

For more information here is the Technical Implementation Notice 12-13

http://www.nws.noaa.gov/os/notification/tin12-13faa_doppler.htm

Here is a link to the Wright-Weather.com Doppler radar sites. The sites in Yellow are TDWR radars.

http://hp3.wright-weather.com/nids.shtml