Severe Weather 101

Detecting Winter Weather

Satellite images are very useful tools for determining cloud patterns and movement of winter storms.  By looping a series of satellite pictures together, forecasters can watch a storm's development and movement.

Radar is critical for tracking the motion of precipitation and for determining what kind of precipitation is falling.

The NWS's dual-polarized radars send electromagnetic wave fields at a 45 degree angle, rather than just horizontally.  As these angled fields bounce off an object and are received back at the radar, a computer program separates the fields into horizontal and vertical information.  This 2-D snapshot now gives forecasters a measure of the size and shape of the object.  With this information, forecasters can clearly identify rain, hail, snow, ice pellets and even bugs.  If they know what type of precipitation is falling, they will make more accurate estimates of how much to expect.

Doppler radar can show the wind direction too, which is helpful when forecasting near mountains and large bodies of water.

If the radar shows wind blowing up the mountain (upslope), forecasters know that automatically, one of the ingredients is in place of the development of precipitation: lift. If the radar shows wind blowing over a large section of a body of water (fetch), then they know that another ingredient is present for the formation of precipitation – moisture.

Radar velocities can help identify the location of cold fronts because there is usually a sharp change in wind direction and will show up as a on Doppler radar.

What we do: NSSL was a leader and major contributor to the scientific and engineering development of dual-polarized weather radar and now installed on the NOAA NWS weather radars. Dual-polarization radar can clearly identify rain, hail, snow, or ice pellets inside the clouds.

NSSL scientists are developing algorithms that will produce estimates of whether the precipitation is falling in liquid or frozen form, or if the precipitation is reaching the ground. NSSL's Hydrometeor Classification Algorithm (HCA) uses dual-polarization technology to automatically sort between ten types of radar echoes including big raindrops and hail. This helps the forecaster quickly assess the precipitation event and better forecast how much will fall.

NSSL's Meteorological Phenomena Identification Near the Ground (mPING) project also collects data on types of precipitation. Volunteers are invited to submit reports of what is actually falling to the ground at their location.  This data is used to refine radar algorithms that detect hail and other frozen precipitation.

NSSL's Severe Hail Verification Experiment (SHAVE) collected data on winter precipitation by making phone calls to the public.  The data was used to refine radar algorithms that detect hail, and other frozen precipitation.

IPEX – The Intermountain Precipitation Experiment studied winter weather across northern Utah to develop a better understanding of the structure and evolution of winter storms. During January and February 2000, scientists made detailed observations of several large storms including one that produced three feet of snow. They also made unprecedented measurements of electrification and lightning in winter storms and the first dual-Doppler radar analysis of a cold front interacting with the Great Salt Lake and surrounding mountains. Researchers used data gathered to validate precipitation estimates from Doppler weather radars located at high elevations, to improve computer-based forecast models used in mountainous regions, and to study terrain-induced precipitation events and interactions that produce lake-effect snow bands.