Getting the most out of radar in winter
Weather or Not, Inc.
The Internet has become the "do it yourself" answer for many public works departments (PWD), particularly when it comes to weather forecasting. Google "weather" and you'll get approximately 225,000,000 website options. While many of these do not directly address the concerns of the average snow boss, free information remains alluring for many watching and waiting for the next storm. The best way to keep this free information from costing you in the long run is to learn how to use it correctly. We'll examine a few easy ways in which to do that.
Nearly every time someone says they use the Internet to get their weather, they're only referring to radar. PWD employees augment the radar with Road Weather Information Systems (RWIS) during the winter. Let's get specific on how best to utilize these systems and outline other pertinent data pieces that will improve operational and fiscal efficiency when snow and ice threaten.
Radar works by sending out short pulses of microwave energy into the atmosphere. When the microwave pulse comes into contact with precipitation, a portion of the energy is reflected back to the radar antenna. The amount of energy that is reflected back to the antenna is directly proportional to the volume of precipitation detected. Graphically, this reflected energy (backscatter or Z) is represented by color contours in units of decibels of Z (dBZ).
Detecting precipitation is done in two modes: clear air mode and precipitation mode.
Clear air mode. Clear air mode is highly sensitive to smaller particles in the atmosphere. Its advantage is in detecting very light precipitation. For snow amounts that will accumulate less than 1", clear air mode is a terrific indicator. Many PWD find the very small amounts of snow to be quite frustrating and hazardous particularly when their roads have not been pretreated.
The best way to tell if a radar image is in clear air or precipitation mode is to look at the color scale. Because of its greater sensitivity, clear air mode detects and displays returns of small amounts of backscatter energy or dBZ. Therefore, the color scale runs from -28 dBZ to +28 dBZ. Each radar image will have a color-coded bar located next to the image to let you know the intensity of the precipitation.
Assuming that the layer of air from the surface through approximately 5,000 feet is at or below 32? F for snow to fall, the following guidelines can be used when attempting to forecast those pesky but potentially dangerous light snow events.
16 dBZ: Light Snow could accumulate over a period of a few hours
20 dBZ: near or greater than 1/4" per hour
24 dBZ: near or greater than 1/2" per hour
28 dBZ: near or greater than 1" per hour
35-40 dBZ: near or greater than 2" per hour
Precipitation mode. On the other hand, precipitation mode detects and displays returns of a much larger magnitude, and its scale ranges from 0 to 80 dBZ. Once you see 35-40 dBZ, you can expect snow rates of approximately 2" per hour.
Ground truth is the key to keeping radar in perspective. This can't be stressed strongly enough! RWIS and National Weather Service (NWS) data must be reviewed before any winter maintenance decision is made. Many PWD have blown their snow budgets by the radar picture that enticed a snow boss to bring in a crew unnecessarily.
More than one site
Two heads are better than one. On that, most would agree. The same goes for radar sites. While regional radar gives you the "big picture," individual radar sites offer the best clarity. A single radar site is most accurate within 40 miles of the radar dome. Know where that dome is. For example, Kansas City's radar is really in Pleasant Hill, MO, which is roughly 30 miles from downtown Kansas City. The radar that covers Boston, MA is located approximately 35 miles southward, in Taunton, MA.
Overall, you can get the best estimate from radar by using more than one site. Topeka, KS radar can be helpful when looking at radar echoes approaching Kansas City, MO. Be careful when utilizing radar as the Topeka, KS radar is located 90 miles west of Kansas City and the radar beam angles slightly upward. Therefore, the Topeka radar will only detect precipitation echoes at or above approximately 11,000 feet over the Kansas City area. This technique should not be used for calling in crews but may be an early indicator of where snow will fall next. In time, higher dBZ returns over the same area may be showing increased saturation of the atmosphere which may alert you to an early snow warning.
All of this assumes that you are reading pure radar. Many commercial companies and TV stations boast that their radar depictions do not include ground clutter. While this may make the picture less confusing, it may also eradicate necessary data which keeps PWD from being surprised by those very light snow events. The lighter radar returns also allow you to more accurately define a true ending time for precipitation.
Example: The benefit of analyzing neighboring single site radars. Monitoring Topeka's radar as well as eastern Kansas RWIS and surface observations, meteorologists were able to give early heavy snow warnings to southern Kansas City metropolitan cities and counties. Mid-level winds indicated snow would arc through southern Kansas City. Topeka's radar loop identified which cities would get a fast 6" of snow versus northern Kansas City municipalities which only received a light dusting (see graphic above).
Timing is everything
Never assume. Always look at the clock to see how old your radar picture is. Server delays, radars that are "out for maintenance" for a few hours, and other numerous potential technical difficulties can cause the radar picture to be old, which can translate to an expensive decision being based on false information.
Most commercial weather companies use the raw data from the NWS radar sites. If your usual radar source is delayed, try your local TV stations. Often they are up and running when the NWS sites are down. When viewing TV station radars look for (a) their location and (b) how their sensitivity differs from your usual source.
We all wish efficient winter maintenance decisions were simply one click away. The federal government and many meteorological firms have spent millions of dollars toward that end. Neither science nor technology is there yet. So if you want to be your own meteorologist, take the time to learn the complexities of atmospheric science and the tools that technology offers. If not, I can assure you there are experienced weather forecasting services that will gladly and expertly do the job for you so you can concentrate on the many other aspects of keeping roads safe for the traveling public.
Sara Croke can be reached at (913) 722-3955 or firstname.lastname@example.org.
How radar tips you:
How radar tricks you: