Interpreting NexRad

Interpreting NexRad by Peter Cassidy
AirPlay First Quarter 2011

Winter will soon be behind us and our weather concerns will change from icing to thunderstorms. Last fall, at the American Bonanza Society’s annual convention, I had the pleasure of listening to Dr. David Strahle talk for two hours on interpreting NexRad. Without question, it was the highlight of the three-day convention. Dr. Strahle’s day job is running a 140-person medical imaging business in Flint, Michigan. His passion, going back to 1969, is flying. He currently flies a King Air. He is one of the forces behind NexRad in the cockpit. His efforts these days are focused on helping pilots interpret the information they have available to them. He maintains that NexRad is better than onboard radar for avoiding convective weather. Onboard radar and spherics should be used to confirm what NexRad is showing. Here are Dr. Strahle’s recommendations.

In interpreting NexRad for convective activity, it’s critical that we are looking at composite images, not base reflectivity images. Composite images show the maximum echo intensity at all radar tilt angles. XM and the ADDS website provide composite radar images. WSI does not. Most FBO weather is from WSI. Be sure you know what the radar images are based on. It may take some research to find out.

We are used to thinking about radar in terms of colors: green, yellow, red, purple. Not everyone interprets them the same. Better to use participation rates in dBz. Most displays provide this information. The following table is Dr. Strahle’s simplified rules for avoiding convective weather:

dBz Color Intensity Avoidance
20 Green Light Usually OK
30 Yellow Moderate Maybe OK
40 Red Heavy Stay 10 miles away
50 Purple Severe Stay 20 miles away

The distance for avoiding cells is calculated  from the edge of the cell green, not from the edge of the red or purple. If you are flying between two 50 dBz cells, the hole needs to be 40 miles wide.

Cells typically move at about 30 kts. Since NexRad images are about 5 minutes old by the time they get to us, we need to add 5 miles to the required distance to avoid the cell on the front side of the cell to allow for movement.

If you are flying below 10,000’ and south or behind a cell and in a stable weather situation, then the required distances for avoidance can be cut in half from 10 to 5 miles for red and from 20 to 10 miles for purple.

If severe weather is forecast for an area or if severe weather is already present in the area, stay out of all precipitation.

To tell if severe weather is forecast for an area, i.e. an unstable area, or if a cell may rapidly change, check the convective outlook charts on websites like ADDS. Unstable areas are reported as slight, moderate or high risk for severe thunderstorms. Other factors indicating instability include: cells 50 dBz or greater in intensity already present in the area, TAFs of moderate or heavy thunderstorms, a broken or solid line of thunderstorms, cell tops over 35,000’, cell movement greater than 20 kts, METAR reports of dew points greater than 50F, METAR reports of temperature-dew point spreads greater than 50F, 25 or greater lightning strikes per minute per cell. The more of these factors present, the more unstable the region.

Precipitation in stable stratiform weather will indicate high precipitation intensities because small water droplets reflect radar better then large droplets. Flight in such conditions is usually not a problem.

 The proper order to apply thought processes is: (a) Is it real or artifact? (b) Is it stratiform weather? Use your Stormscope, datalink lightning images, satellite data, and your weather knowledge to determine this. (c) Are we in an unstable environment? (d) Now apply the rules of avoidance above. Use onboard radar or ATC to verify what you are seeing on NexRad.

ATC (Center) has NexRad available under a program called WARP. It displays 30, 40, and 50 dBz composite images on the controller’s radarscope. They do not show 20 dBz precipitation. Ask ATC for help if you need verification of what you are seeing.  Approach controllers may or may not have weather available to them.

Don’t get caught between two cells that are ready to share energy. They can combine and grow very quickly.

Dr. Strahle has also developed a system for dealing with the vertical distribution of energy within cells which is just as important as the horizontal distribution of energy. This system requires access to both base and composite precipitation rates. The greater the spread in precipitation rates between base and composite images, the greater the vertical instability. I won’t say more about it as we don’t usually have access to both sets of images. Perhaps we will in the future.

What I’ve given here is the summary of Dr. Strahle’s recommendations from his handout. If you get the chance to attend one of Dr. Strahle’s presentations, be sure to take it. You will come away with a better undersanding of NexRad in the cockpit and the logic behind Dr. Strahle's recommendations. You’ll be glad you took the time and your datalink weather will be much more useful.