What a difference 6 hours makes

I illustrated NCEP Model Performance with verification statistics for the 18Z GFS forecast cycle because 18Z corresponds to noon CST--near the peak daily temperatures over the Continental Unites States (CONUS). However, that may have been misleading or only told a partial truth.

Let's look at the statistics for 18Z again.

18Z analysis cycle GFS temperature bias for forecast hours 0-168, compared to conventional upper air soundings. Operational GFS on the left and experimental GFS on the right.  Scale [-1.0, 1.0] C

Note the data scales.

Bias between upper air stations and the 48-hour GFS forecast for the 18Z cycle.  Bias statistics computed over ~2,750 observations.

Now let's look at the same visualizations for the 12Z data analysis/forecast cycle.

12Z analysis cycle GFS temperature bias for forecast hours 0-168, compared to conventional upper air soundings.  Operational GFS on the left and experimental GFS on the right.  Scale [-0.5, 0.5] C

Notice that the temperature bias scale is reduced from [-1.0, 1.0] to [-0.5, 0.5] C? The bias reduction is less dramatic than a factor of two.

In the next graph, the scale doesn't change, but the peak of the bias is reduced from ~4.0C to ~2.5C at the tropopause.

Bias between upper air stations and the 48-hour GFS forecast for the 12Z cycle.  Bias statistics computed over ~135,000 observations.

What causes that difference? The graph on the right gives a clue.

The horizontal scale gives the total number of observations used to compute the statistics. At 18Z, there were ~2,750. At 12Z, there were ~135,000. More radiosondes are released near 12Z than any other time of day (with 0Z coming in a close second.)

Links:

• NCEP EMC Mesoscale Verification Statistics (includes GFS)
• NCEP EMC GFS Verification Statistics (select among 00, 06, 12, 18Z forecast cycles)
• What's the difference between GFS and FNL?
• Analysis, forecast, reanalysis--what's the difference?
• NCEP Model Performance