Wednesday, October 16, 2013

Dry weather to continue...but for how long?

Dry weather to continue...but for how long?

A strong ridge of high pressure currently sits directly off the coast of Washington as seen on the 500mb height (around 17-18k ft above ground) and satellite image below. Ridges of this strength tend to persist for extended periods of time and this one will be no exception. This time of year, we can be fairly certain the ridge will keep dry weather in place, however the longer they persist, the more prone the area  is to the formation of low clouds and fog due to growing temperature inversions. If low clouds and fog form, they can have a dramatic impact on the temperatures especially if the fog persists through most of the day. Typically that does not occur as readily in October as it does later in the year, but that's for a later discussion. For now lets focus on how long this ridge could persist. 

Here's what the pattern looks like for next Wednesday off the GFS model. Notice the ridge is still in place; in fact it moves very little from its current position. (the shading shows moisture levels at 500 mbs, green=moist, white=drier). While the Inland Northwest will likely stay dry and relatively mild, the same can't be said for the Great Lakes region.  Notice the very deep low that forms over Upper Michigan. If this were to verify, it's likely that area (and especially just east of there) could see a major snow event as temperatures will certainly be cold enough. Not for our area...yet.

The latest GFS run finally breaks the ridge down by late next Friday or Saturday...and that's when we might take our turn at dealing with late fall or early winter like weather. Temperatures just off the ground will be well below freezing...while moisture associated with the low will be plentiful. So the big question is will this model verify and will our bout of dry, mild weather come to a sudden end?

If you play the odds game and give some merit to the images below, you might not break out the snow shovels ready quite yet. The upper left image simply is another representation of the image above. The lower right and left panels show what happens when we take the initial model run and introduce small perturbations or changes. We call this ensemble forecasting. This is a useful tool for determining how much confidence to place in any one model run. Generally speaking the farther out we are in time the more uncertainty there is in a forecast. This shows up quite well in the lower right panel. Each of the lines corresponds with a different model run or perturbation while the red line is the GFS. When the lines are closely clustered (such as over Mexico and the Gulf States) the forecast confidence is generally high. Over our latitude though, there is huge amount of variation or uncertainty. That doesn't help with the forecast process significantly, however we can then take all the perturbations and average them resulting in a mean forecast. Theoretically there is a better chance of the mean verifying than any of the individual perturbations or the lone GFS (that's not to say that they can't verify though!). That is what is shown in the lower left portion of the image below. In this case, the mean is suggesting the ridge will remain strong through at least next Saturday, however it does hint at a slight retrogression (westward shift). If that verifies it could introduce some cooler air from the north, but certainly not to the extreme that's being advertised from the lone GFS run.  

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