Radar, Lies and Videotape Part IV
UPDATE: I corrected the use of "evaporating" to "evaporating or sublimating" below (thanks Jim).
ORGINAL POST:
We've talked before about how radar maps, especially "composite" maps (which are made out of data from several NEXRAD radars) can lie and say that snow's over you when it's actually evaporating (or sublimating [WikiPedia] before hitting the ground.
And in these situations I often suggest that you consult the local NEXRAD image, or the government's official airport observations, which are generally more accurate at saying whether or not it is precipitating.
Today we must look at the false positives from those airport stations -- when they say precipitation is occurring but it is not.
But first, the radar is lying today, and we've caught it red-handed.
The composite radar shows it snowing in State College at 10:35 AM while the Local NEXRAD loop shows it's still not snowing -- even at 12:30 PM. You can see the "hole" forming around State College because the radar looks up and out, seeing snow at the same height all around. Meanwhile here on Earth it's still dry outside.
NOW ONTO THE FALSE POSITIVES:
Before watching the observations screens this morning, I thought that this was a fairly rare condition. But now I'm not so sure. No fewer than three stations in eastern Pennsylvania have reported precipitation this morning when it's clear from the local NEXRAD images that none is present.
Generally precipitation is reported as falling at the point where it is observed, but manual observations are allowed to put a qualifier of "Vicinity" ahead of report where precipitation can be seen within 10 miles [more info], for example VCSH* would decode to "showers (not specified if rain or snow) in the vicinity." But in this case, the stations are automated (the A02 in the raw obs means "automated station"). So they are claiming that precipitation is falling where they are.
Check out these two maps from 11 AM and 12 PM today:
There's no way that Light Rain (R-) or Light Snow (S-) is falling at these stations. Today's NEXRAD loops from Philly (11 AM | 12 PM) shows nothing at all. Sure, occasionally NEXRAD radar misses the lightest drizzle or snow flurries, though and I might buy that if it were cloudy there, but skies were clear ahead of the clouds approaching this afternoon, as shown by the observations themselves (RAW | DECODED) and the Visible Satellite Animation. Still, one station brazenly admitted on NOAA's website:
Clear and Rain? Not gonna happen.
By the way, you'll also notice a "Light Drizzle" (L-) down in North Carolina on the 11 AM obs image. Same story -- nothing going on there either as the NEXRAD animation and visible satellite loop prove. That's impressive, four bogus observations within two hours. Let's spend some tax money to improve these government sensors!
*VCSH will appear as a "W" on AccuWeather maps and decoded METAR text because they are limited to two-letter codes.
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