The P-3 Orion (pictured above) usually flies the same altitudes as the WC-130J Air Force Hurricane Hunter aircraft do.
Surface to about 12,000 feet on reconnaisance missions. Enroute altitudes are much higher.

The NOAA 49 referred to in the Plan of The Day is a Gulfstream 4-SP jet aircraft. Reconnaisance flight levels up to 45,000 feet are common.
See the link to the NOAA Aircraft Ops site.
http://www.aoc.noaa.gov/aircraft.htm

The WP-3s are NOAA 42 and 43.
More info here:
http://www.aoc.noaa.gov/aircraft_lockheed.htm

The U.S. Navy also operates P-3 Orions for weather reconnaisance, anti submarine and anti smuggling missions.
http://www.navy.mil/navydata/fact_display.asp?cid=1100&tid=1400&ct=1

http://en.wikipedia.org/wiki/P-3_Orion