NEXRAD: How Scientists Spot Hurricanes Before They Pancake Entire Cities

You know how you know about Hurricane Irene three days before it hits land and ruins the East Coast? Doppler. Specifically, the NEXRAD Doppler system that's been tracking severe weather across the US of A since 1997.

Doppler radar systems work by blasting out a short pulse of energy that scatters when it hits an object. When it does, a small amount of that energy bounces back towards the radar. The system listens for and records that ping (about 1300 times a second). That data is then compiled into mosaics that depict the amount of precipitation as well as the wind patterns.

The NEXRAD (NEXt-generation RADar, though technically it's the WSR-88D, Weather Surveillance Radar, 1988, Doppler) system consists of 159 of these high-resolution weather stations, networked together. Each individual weather station is able to track precipitation within about 80 nautical miles of itself and intense precipitation within 140 nautical miles. Site locations were chosen to provide the maximum amount of overlap were one of the towers to be knocked offline by an wind-borne Yugo.

And, unlike the tracking system it replaced, NEXRAD's antennae are not directly controlled by the user. A strong emphasis was placed on automation with this system. It continuously refreshes a 3D database of measurements within its scan zone. These zones are preprogrammed based on the weather type—slow scans, updating every 10 minutes or so, for calm weather and fast scans, updating every 5, for inclement weather.

Since the system can detect not only changes in the speed of the wind, but its direction as well, they are able to more accurately predict the conditions the precede a tornado. This ability allows meteorologists to provide faster and better warnings to the public. In fact, since the network was completed in 1997, expected U.S. fatalities and injuries from tornadoes have dropped 45 percent and 40 percent, respectively. The number of tornadoes warned about jumped from 35 percent to 60 percent and the lead-time before it touches down increased from 5 to 9.5 minutes.

The National Hurricane Center at the Florida International University, also uses the system in tracking storms (though not on such short notice). Whenever a depression appears in the Atlantic or Eastern Pacific oceans, they will track it and issue advisories—especially if it's threatening land—every six hours until the storm passes or dissipates. Public advisories are issued more often when the storm threatens land.

[NexRAD Wiki - National Hurrican Center Wiki - National Hurricane Center - National Sever Storms Lab - NOAA - NOAA Doppler FAQ]