Let's use the SPC Short-Range Ensemble Forecast (SREF) system again to take a look at this question with regards to the upcoming snow event in the upper midwest. We can measure the "variability" in the models by looking at the standard deviations (the "spread") between different models and how that changes over time.
First, here's the current SREF image for a 36 hour forecast of 700 mb heights. This would be forecast to occur at 21Z this Saturday--sometime Saturday afternoon.
|Fig 1 -- SREF 700mb mean height and standard deviation (shaded) in a 36 hour forecast for 21Z, Saturday, Dec. 11, 2010. From the SREF website.|
|Fig 2 -- SREF 700mb mean height and standard deviation (shaded) in an 84 hour forecast for 21Z, Saturday, Dec. 11, 2010. From the SREF website.|
For an animation of how this 700mb map changed between an 84-hour forecast and the current 36 hour forecast, you can go to this animation here. It's by looking at sequences of images like this that we can identify "trends" in the models. Based on that animation, here are three quick trends that I spot:
- The center of the trough axis (or cut-off low-height center) does seem to be subtlely moving westward and slightly further north as model runs got closer to the actual time. It was difficult to really see this in comparing the first and last images, but in looking at the animation, this does seem to be the case.
- As each model run got closer, the strength of the 700mb cut-off low (based on the lowest minimum height and the number of contours surrounding it) has definitely been increasing--the closer we get, the more intense the models seem to be making this storm.
- The standard deviation tends to decrease the closer we get to that forecast time--this is what we expect. Smaller standard deviations indicate better agreement between the models and more confidence in the overall forecast. So whereas two days ago it was made a lot of sense for forecasters to be talking about so much "uncertainty", now things are coming into better agreement and we're drawing stronger conclusions.
|Fig 3 -- SREF Surface low pressure center "spread" in a 36 hour forecast for 21Z, Saturday, Dec. 11, 2010. From the SREF website.|
|Fig 4 -- SREF Surface low pressure center "spread" in an 84 hour forecast for 21Z, Saturday, Dec. 11, 2010. From the SREF website.|
Like before, here is an animation of how the surface low centers map evolved from the 84 hour forecast to the current 36 hour forecast. We see that the low centers quickly began to accumulate further north and then slowly have been retreating westward to the close cluster that they are currently. We've seen slow westward retreats in both the surface lows and the 700 mb map--both of these point to the models thinking the cyclone is going to move through later and later, moving more slowly than previously thought.
We can keep trends like that in mind when looking to future runs as well. You might guess that as we get closer and closer to this event, the models may continue this slowing trend and the timing of the forecast will change. However, also remember that the closer we get to the time, the more the model "spread" or standard deviation tends to decrease. As the models come into better agreement and are more confident on a solution, that particular solution may not change as much as it used to at earlier forecast times. So while we may still see that slowing of the progression of this cyclone, it may not slow down in future model runs nearly as much as it has been.
Finally, I STRONGLY encourage you to explore the SPC's SREF website. They have ensemble graphics for nearly any variable imaginable. Also, if you're viewing a particular image at a particular forecast time and want to see what previous model runs said for that time (like I did above), just click on the forecast hour number (f36, for example) for the image at the time you want and it will load up all the model forecasts for that time from all the previous runs. Have fun!