Friday, February 25, 2011

Visualizing Forecast Uncertainty

Earlier posts this week have discussed the uncertainty in forecasts of front trough and snowband position for this evening.  Here's a nice loop showing a sequence of forecasts from the NAM beginning with the 84 hour forecast from 1800 UTC 22 Feb and ending with the 12 hour forecast from 1800 UTC 25 Feb (today), all valid for 0600 UTC 26 Feb (11 PM tonight).  Meteorologists sometimes refer to this as "DModel/Dt".  Notice how the position of frontal trough and precip band jumps back and forth, from as far south as central Utah to near the Utah-Idaho border.

Now, if you think that now we are dealing with a shorter lead time (24 hours or less), so presumably the forecast is more certain, think again.  The target area has narrowed, but for 1200 UTC 26 Feb (5 AM Saturday morning) the NAM puts the front right on top of Salt Lake City with heavy precipitation to the north, whereas the GFS puts the front roughly over Provo.

NAM Forecast Valid 1200 UTC (0500 MST) 26 Feb 2011
GFS Forecast Valid 1200 UTC (0500 MST) 26 Feb 2011
So, the frontal snowband remains, but the precise positioning remains uncertain.  It's going to be an interesting night.

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