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PostPosted: Sun Dec 21, 2008 4:42 pm 
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Location: Kansas
Plotted a surface map in DA 1.40d this morning and overlaid wind chill (F) on it. The analyzed (and plotted in the plot options) values for wind chills were about 10-15 degrees too cold. I'm not sure if this was already a reported bug, so am posting it here.

On the map below, the time is 17Z on 21 Dec. The site used for comparison is Concordia in north central KS, just east of the red square in the center of the map. On DA, the analysis (dotted blue on the plot below) and plot (value directly below the station circle) were -30F. The NWS observation listing and the NWS wind chill calculator gave -13F.

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PostPosted: Sun Dec 21, 2008 5:56 pm 
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Location: Fort Worth, TX
geophi,
The first thing that comes to mind is that DA is "probably" using the older wind chill (and I would assume, the heat index too) calculation. Using your data from 1600, here are the results using old vs. new calculations:

The other thing I notice is that using your station plot at 1600, the temp is 3 F and the wind is from the northwest at 10 knots. The calculators require mph which converts to 11.5 mph. You used the current conditions which was different (14 mph). When you use DA to plot, it utilizes the downloaded data only. So DA IS plotting correctly, using the older wind chill calculation, something else for Tim to update.
Greg Higgins


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PostPosted: Sun Dec 21, 2008 6:04 pm 
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Joined: Thu Jul 15, 2004 3:42 pm
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Sorry, I had to replace the graphic with a 17Z one because of a mistake in the overlay. But I think you are right in that the program may be using the old formula.

But on the topic of wind speeds and plots, at 16Z, while the plot showed 10 kts, the actual wind was 12 kts, which converts to 14 mph, but for the sake of plotting, is rounded down to a 10 kt barb. One would think the calculation still should have used the correct value (12 kts or 14 mph).


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PostPosted: Sun Dec 21, 2008 6:28 pm 
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Location: Fort Worth, TX
You need to understand where DA gets the data and how often it is updated. Unless you have a direct feed to your local office reporting station (or other source) and DA was capable of updating magically every time new data arrived, DA will use only what was downloaded and that data is probably x minutes older than what is provided on the NWS current conditions web page. Where did you get the "actual wind of 12 kts"?


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