It is expected that the current temperature shown in the panel will be based on actual measurements of the temperature from nearby sources such as airports, and not based on the forecast. Even if the measurement is 10 miles away, it will be better than what is currently being reported for the Midwest in USA. I live in Kansas City, MO and the current temperature that's reported is often 5 to 15 degrees off. It seems to match the forecast given even if the forecast is incorrect. For example, now it says 34 degrees fahrenheit even though the actual temperature is 49 degrees fahrenheit and other sources, such as the weather channel, accurately report 49 degrees fahrenheit. The auto-detected latitude, longitude, and altitude were very close. I made slight adjustments according to what I could determine about my precise location, but that just made things worse (it said 39 degrees before my adjustments).

Also, it is expected that the current temperature will be for the past 15 minutes, or half hour, or at most the past hour. Instead, it is reporting what the temperature will be in a 3 hour window indicated it's not based on recent measurements but is just part of the original forecast.

I will copy and paste from http://ubuntuforums.org/showthread.php?t=2147611 some information provided by somebody in IL having the same issue: 

"Re: Xfce "Weather Update" plugin very inaccurate. Any alternatives?
I'm having the same problem, but I think those offering help don't quite understand. The problem is with the reporting of current temperature and conditions, NOT the forecast. I work in Arlington Heights, Illinois, USA, close to the Chicago Executive Airport (PWK). In Xubuntu 12.04, the weather app pulled data from the local airports, so the current temperature and conditions were always perfectly reported. In 13.04 the temperature is off by 5F to 10F most of the time, though on some rare occasions it comes to within 1F. The other problem is that the conditions are frequently misreported, such as when the app reports that it is overcast and raining, but in reality the sky is blue and the sun is shining brightly (right now at 0400 UTC it reports that it is raining, but it is only overcast and dry).

"At first, I thought my coordinates and altitude might have been incorrect. So, I verified the information from multiple sources and input the data directly. It actually made things a little worse. Next, I put in the coordinates for the Chicago Executive Airport , which is the site of a NOAA weather station used by aviation. This made things way worse. I'm not sure where the Norwegian Meteorological Institute is getting their data from for my area, but something is clearly very wrong, either in the way the geographical coordinates are handled, or in the Norwegian Meteorological Institute's data sources.

"If anyone wants to verify this for my specific location:
* In the Xubuntu weather app, enter "Arlington Heights, Illinois" for the location (default values from app: latitude 42.081268, longitude -87.980066, altitude 696 feet)
* On the web, go to http://forecast.weather.gov/MapClick....9015&site=EAX - this is the Chicago Executive Airport weather station, about 3 miles straight east from my office over very flat terrain, so the weather is essentially the same. The current conditions are in the first major section of the page. This is what the Xubuntu 12.04 app used to get its data.

"At the moment temperatures are close, but tomorrow when it warms up significantly, you should see a divergence.
Last edited by redhorse; May 28th, 2013 at 04:17 AM.

I believe this is a widespread problem for this plugin in the Midwest of the USA.