Text from the 14 Dec Situation Report:

Sunspots Causing Communication Failures

On December 12, 2006 the NOAA observed a solar radiation storm on the Sun that produced a powerful and fast Earth-directed coronal mass ejection that is expected to impact the earth around 1:00 EST on December 14th. The result of this significant geomagnetic storm is a potential adverse effect on satellite communications, HF and VHF mid-latitude radio communications, long-line telephone systems, powergrids (potentially causing blackouts), oil and gas pipelines, as well as VLF navigation and GPS systems.
A sunspot is a region on the Sun's surface (photosphere) that is marked by intense magnetic activity. NOAA monitors these occurrences and issues Space Advisory Bulletins and Space Weather Scales,introduced as a way to communicate to the general public the current and future space weather conditions and their possible effects onpeople and systems. The NOAA Scales describe the environmental disturbances for three types of events: geomagnetic storms, solar radiation storms, and radio blackouts. They list possible effects at each level.

This solar storm occurrence is Bulletin #06-4 (severe). For the next five days we could experience HF radio communication blackouts on most of the sunlit side of the Earth for one or two hours. Outages of low-frequency signals could cause minor satellite navigation disruptions on the sunlit side of the earth.
Further information on NOAA Space Weather Scale for Radio Blackouts available at:

NOAA Space Weather Advisories, Space Environmental Center
http://sec.noaa.gov/NOAAscales/index.html#GeomagneticStorms

Federal Emergency Management Agency (FEMA)
http://www.fema.gov/