Crozet Weather Almanac: November 2016


Radar Is a Great Tool But Not Perfect

Weather radar images are seemingly everywhere you go these days.  The ubiquitous looping imagery is on television screens in restaurants, the gym and even the doctor’s office, not to mention that it’s a click away on your home computer or smart phone.  This widespread access makes it much easier to decide whether to grab the rain coat or cancel the swim meet.  Just a few short years ago, the data was much harder to find in a timely manner in the pre-smart phone era.  Before the internet, you had to wait forever for the data to appear on TV.  Before World War II, radar data didn’t exist at all.

Like many innovations, radar is a direct byproduct of World War II.  Nothing quite speeds along technology like the prospect of being annihilated by a powerful enemy.  In the 1930s, a handful of countries developed radar under great military secrecy.  Short bursts of radio energy were timed on an oscilloscope which could pinpoint both the direction and distance of a target.  The technology was essential to Britain’s survival in the air war with Germany.  When looking for enemy aircraft, rainfall often cluttered the view and complicated the military purpose.  The use of radar for weather analysis and forecasting was a purely accidental byproduct.

After WWII, the older, surplus radars were passed from the military to weather offices around the world.  Improvements were rapid as the machines were calibrated for rain detection rather than aircraft.  This greatly aided short term forecasting, especially for severe storms such as tornadoes.  Eventually, the data found its way into computerized weather forecast models.  Weather forecast models are only as good as their starting point.  “Garbage in, garbage out” (GIGO) is probably the biggest limitation to weather forecasting.  So, the better you can approximate the starting conditions in a weather model, the better the resulting forecast.  Radar is very useful at diagnosing the starting conditions.

Another great innovation to radar is incorporating the Doppler effect.  This allows detection of the relative motion of storms.  The United States rolled out a network of modern Doppler radars across the country in the late 1980s.  The motion component of Doppler greatly improves the ability to detect rotation within a storm which significantly improves lead time for tornado forecasting.

As great as weather radar is, there are drawbacks.  The big three drawbacks are coverage, snowfall, and the chaotic nature of rain cells themselves.

Coverage by radar has several problems which affect Crozet.  The earth is round so a radar beam gets higher and higher above the ground the farther out it goes.  Beyond 80 miles, the radar beam is too high to detect the kind of drizzle we sometimes get here on easterly winds.  Three different radars cover us but all are far away.  We are 92 miles from the Washington, DC area radar, 120 miles from the radar in southeast Virginia, and 115 miles from the radar near Blacksburg.  That is close enough to see all the big storms that soar high into the atmosphere, but sometimes, a very persistent light rain can fall and all three radars show nothing.

The distance from the radar is also problematic for snowfall detection for the same reasons as drizzle.  Snow clouds are often low and close to the ground and therefore hard to detect at a distance.  Also, snow has a much lower reflectivity so it’s harder to “see” on radar and harder to detect differences in snowfall intensity.   In mountainous areas, especially in the western USA, radar is almost useless in snowfall situations due to the combination of poor radar coverage, mountain interference, and low snowfall reflectivity.

The final problem with using radar data is that it changes so quickly, especially in thunderstorm conditions.  Often, a line of storms is moving one direction but individual storms within the line are moving a different direction.  Individual thunderstorms cells are always forming and dying and have a life expectancy of just 30 minutes.  Sometimes, we will see a storm 10 miles away but realize it is almost no threat because it is moving slowly or dying.  Other times, just a couple of tiny echoes 50 miles away can signal imminent danger to the trained forecaster.  The bottom line is that simple extrapolation of a radar loop is not very reliable and can make you look foolish.

So, what radar should you use?  Almost all the data comes from the same NOAA radars so the only issue is how it is displayed.  For our smart phones, Heidi and I prefer RadarScope.  It isn’t free ($10) but you get what you pay for.  The app is quick and easy to read and can be customized to your liking.

November 2016 Crozet Weather
November 2016 Crozet Weather

November Recap…

Heidi and I both rank November as the 12th best month for weather.  December is colder and darker, but it has Christmas and the chance of snow.  Remember, if you don’t like snow, you are just old.  November rarely has much snow and this year wasn’t even close.  Almost nothing fell from the sky until the last two days.  The average high was 64, which is way above the normal of 57.  Ironically, low temperatures were slightly colder than normal because we had sunny warm days and clear, chilly nights.  Nine nights dropped below freezing. Overall, November was much nicer than usual.


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