I have noticed this problem with my WMR200 and I am wondering if others have the same (design flaw) or only mine has this problem. The problem as seen again today: the temperature at 1PM is 30 degrees F on the WMR200 display, the sun comes out from behind the clouds and the temperature on the WMR200 goes up to 40 F in ~10 minutes. I know it’s not really 40 degrees outside as the ice cycles show no drips and my old window thermometer still reads ~30 F.
Happens with mine too and I have a Lacrosse. It’s an inadequate radiation shield. You have to jin up a guard or something to keep direct sunlight off the sensor and yet not totally enclose it because then it would be a heat sink. When things warm up a bit I’m going to construct a 3 sided partially enclosed device to place around the sensor that should moderate things out a bit.
You need a Stevenson Screen, See here:
Welcome to the forums.
Unfortunately you have hit upon the biggest bugbear we seem to have as amateur weather enthusiasts.
Solar radiation is a pain in the rear and there are many options to help minimize it, if you search for ‘radiation shield’ or ‘stevenson shield (screen)’ or ‘davis shield’ you should find all you need to help you out. 
Thanks for the confirmation, I thought it was a design issue as the plastic sun shield is attached directly to the sensor.
After the snow melts, I am probably going to take the temp/humidity sensor off the OS sensor mast and move it under the north side roof; or I maybe I’ll buy another temp/hum sensor and place it under the north side roof.
Thanks Again.
The sheild on the thermo/hygr is fairly useless. This is the same with all Oregon temp sensors.
I have used a home made Stevenson screen, aspirated with a 12V 0.09A computer fan driven by a solar panel.
I am running side by side with a Davis VP2 and the temp is virtually the same in direct sunlight.
I confirm, I have mounted the temp/humidity sensor in a Davis passive radiation shield and now it is ok.
I am disappointed that the WMR200 was not designed better to block the direct sunlight from the temperature sensor housing. After all it was just introduced in Nov 2007.
Even worse, it shows the thermo / hyg mounted on the roof at the same level as the anemometer :roll:
It does however give you the ability to mount it in a different location.
I Made myself a Screen the link was posted near the beginning of this thread
I actually started off with a FARS (Fan Assisted Radiation Shield) but me being
me i had issues with humidity with it, so i decided to go the full hog and make a
Stevenson Screen, It took about a week on and off to make but the results are
amazing and it was well worth it, if you have the space then go for the
Stevenson Screen, if not then go for the FARS 