The National Weather Service is currently in a multi-year upgrade of their WSR-88D weather radars. This technology upgrade is called Dual Polarization radar, and is sometimes referred to as Dual-Pole technology. (Learn more from Nexrad Now Issue 20 – Dec 2010)
“In the spring of 2003 the National Weather Service Office of Science and Technology tasked the National Severe Storms Laboratory with providing data collection and analysis to support a WSR-88D dual polarization decision briefing to the NEXRAD Program Management Committee. The Joint Polarization Experiment (JPOLE) project was designed to evaluate the engineering design and data quality of the polarimetric KOUN WSR-88D radar and demonstrate the utility of polarimetric radar data and products to operational users. The NSSL JPOLE experiment demonstrated the performance improvements dual polarization provided in rainfall estimates, hydrometeor classification, and general data quality.” (From NOAA ROC)
The NOAA Radar Operations Center has created a status upgrade map in PDF form that shows the weekly progression of the upgrade process. When the upgrade is occurring, the radar site will be down for roughly two weeks. As of 9/30/2011, there are 7 sites across the US that have received this upgrade including Vance AFB, OK; Wichita, KS; Pittsburgh, PA; Portland, OR; Phoenix, AZ; Morehead City, NC; and the new radar at Langley Hill, WA.