[NLRS] Weird MSP Radar plot. What is this?

[Doug Reed]

Here is the definitive answer.....

>Date: Mon, 01 Jul 2002 15:36:00 +0100
>From: "Todd Krause" <Todd.Krause@noaa.gov>
>Organization: NWSFO Chanhassen
>Subject: Re: [NLRS] Weird MSP Radar plot. What is this?
>
>Hi Doug...
>Please feel free to relay this to John.
>
>Unfortunately unless I see the radar loop at the precise time, one cannot go
>back and see what it had been showing at the time.
>
>At about 10 pm CDT Sunday, I do recall seeing a northeast-southwest oriented
>line progressing southeastward across the Twin Cities.  If that is what
John is
>referring to, it was called a "fine line," which is a line of echoes on the
>radar, but no precip.  Usually fine lines indicate the passage of a front,
>outflow boundary, towering cumulus.  Last night, it indicated the front
weakly
>dropping southeast through the metro.  The front was so weak by that point
that
>nobody would have noticed any worthwhile change in temp or dewpoint.
>
>When we have the radar in "clear air mode" (i.e. "sensitive" mode during
which
>the ground clutter is typically orange, yellow and red), the radar circles
>around at a much slower rate than when we are looking at showers and
>thunderstorms.  In "clear air mode", the radar takes 2.5 minutes to
complete a
>full 360 degrees, whereas in "precip" mode (when you see blue as the ground
>color clutter), it goes completely around in 20 seconds or 1 minute.  In
precip
>mode we need it to go around alot faster since thunderstorms change so
rapidly
>and we need to have the radar complete its 360 degrees quickly to keep up
with
>the storms.
>
>So when it's in "clear air mode", it's taking much longer to scan the
>atmosphere.  That's when it can pick up subtle changes in the atmosphere,
and a
>fine line typically indicates the presence of differences in air density on
>either side of the front or outflow boundary or towering cumulus.  On one
side
>of the front there is more humidity than on the other side.  And the radar
can
>pick that up.  It's extremely helpful to forecasters, since fine lines can
>develop into a line of storms, providing there is enough lift, humidity,
>instability, etc.
>
>Hope this helps.
>Todd
>todd.krause@noaa.gov