ABOUT NWS/NOAA RADAR IMAGES @ FLOODGAP GOPHER GROUNDHOG
updated 20 April 2021

Please read ths important document first to get the most out of this data.
Report wonkiness and fawning missives of love to gopher@floodgap.com .

WHERE DO THESE IMAGES COME FROM?

These images are based off public domain data from the National Weather
Service of the US National Oceanic and Atmospheric Administration. Images
are watermarked not because they are copyrighted (they aren't), but to
discourage people from using our overworked CPU to do the work for them for
free. The NOAA/NWS makes this data available freely and easily; please
visit www.weather.gov and find out how.

HOW DO I USE THE SERVICE?

To select an image, pick the radar station closest to your desired location.
Although some radar stations may appear to overlap, some areas may actually
be out of range for a given station by distance (see below).

Images take a couple moments to generate if they were not already cached, so
please be patient. If you get an error message, the data may not be currently
available or we could not contact the NWS data store; please try again in a
few minutes.

WHAT IS ON THESE IMAGES? HOW CURRENT ARE THEY?

Images contain base reflectivity data for a particular station, based on
the most current radar data available within the last 10 minutes; check
the date stamp on an image for the exact time the observation was taken.
Once generated, the image is cached for 20 minutes to reduce server load.

Maps also include cities, highways and county borders to aid the viewer
in location, but should not be used as geographically accurate navigation.

CURRENT RADAR INFORMATION MAY NOT BE TIMELY ENOUGH FOR IMMINENT DISASTERS.
Do not rely upon them to determine if a dangerous weather situation is
in progress or is developing. Use your local radio or television stations
to determine if a local weather warning will affect you and what to do.
The Emergency Alert System will sound for important events you should know
about, if your station carries such alerts.

HOW DO I READ THE RADAR PLOTS?

These plots are based off the NWS NEXRAD WSR-88D base reflectivity data, using
Doppler radar to detect and measure the reflection of radar pulses bounced
back from air objects (usually precipitation) that they hit.

Base reflectivity indicates the intensity of the echo back to the detector.
It is measured in dBZ or "decibels of Z," where Z represents the reflected
signal. A higher dBZ indicates more reflection is occurring, and implies that
there is increased precipitation in the way. Base refers to the single angle
used for the radar antenna, which in this case is at 0.5 degrees above the
horizon.

COLOUR:
Typically, light rain is indicated on these plots by colours at or "higher"
than green (such as green, yellow, red and so on). "Higher" colours indicate
higher intensities of precipitation, and extremely reflective objects such as
hail make even more intense signal colours. Grey and other low dBZ signals may
be from various local reflections which are not necessarily meteorological.

RANGE:
The range of the radar plots is usually 80-140 nautical miles (approximately
90-160 land miles, or 150-260 kilometres). This means not all areas of the
United States are covered by radar, and not all areas indicated on the map
may in fact be reachable by the radar station requested due to geologic
interference or other conditions. This range corresponds to what the NWS
calls a "Short Range" or "SRng" plot, and was selected because of its
higher resolution.

Within the first 20 nautical miles (app. 25mi/37km) of a radar station
there may be "ground clutter" noise caused by local reflection, as well as
flying birds or airplanes that may cause transient echoes and point targets,
and on some images an artifactual "radar sweep line" may even seem to appear.
Although data filtering is employed on the final render to diminish these
kinds of spurious signals, they may still appear on maps and the viewer
should keep the possibility in mind.

WHY BASE REFLECTIVITY?

Base reflectivity takes only one measurement, i.e., at the lowest 0.5 degree
elevation. This is the lowest "base" measurement and is most correlative of
what is happening on the surface, but underestimates high atmospheric
precipitation which may predict intense storms at longer distances. Composite
reflectivity views, on the other hand, take the highest echo intensity from a
collection of measurements taken at a variety of angles. At the higher angles,
this allows the beam to reach very high altitudes and can be very useful for
large storm cells, but may not necessarily match prevailing ground conditions
and may completely miss some distant areas of low altitude precipitation.

Because the base reflectivity is the most useful for surface observation, we
have chosen this data for Floodgap's radar imaging system as it is likely to
be the most relevant to the largest number of viewers.

SERVICE CHANGE NOTE

The previous NWS radar service also had an overlay for local severe
weather warnings which we incorporated as an additional aid to users.
Unfortunately this service was discontinued as of December 2020 and the
new current warning API is not easily adapted to our rendering pathway,
so current NEXRAD imaging will show radar data only until further notice.


Please send in all questions and concerns to gopher@floodgap.com .