Holy Cold Snap, Batman!

For fun, I decided to check the 10-day GFS model this morning just to see if there was anything interesting on the horizon.  I must admit, I didn’t think I would see this in the 7-10 period:

850 mb Temperatures valid the evening of Monday, Sept. 21

850 mb Temperatures valid the evening of Monday, Sept. 21

850 mb Temperatures valid the evening of Tuesday, September 22

850 mb Temperatures valid the evening of Tuesday, Sept. 22

850 mb Temperatures valid the evening of Wednesday, Sept. 23

850 mb Temperatures valid the evening of Wednesday, Sept. 23

850 mb Temperatures valid the evening of Thursday, Sept. 24

850 mb Temperatures valid the evening of Thursday, Sept. 24

The images show temperatures at 850 mb (a pressure surface about 1 mile aloft).  The temperatures at this level reflect the temperatures near the surface.  Thus, if temperatures cool at 850 mb, you can expect temperatures will also cool at the surface.  Surface temps will be warmer than 850 mb, however.

As you can see from this series of images (which I actually saved this time), the eastern portion of the US is in for a cold snap, especially in the Great Lakes region.  Meanwhile, it appears above-normal temperatures will visit the West coast.  (Note: the 850 mb surface is actually underground in many locales of the western US.  Thus, temperatures at 850 mb are somewhat useless in the mountain regions.  That said, the coast of CA is near sea-level, obviously, and this forecast indicates above-average temps along the West coast.)

Forecasted jet stream for the morning of Wednesday, Sept. 23

Forecasted jet stream for the morning of Wednesday, Sept. 23

In this setup, the jet stream aloft shows a ridge of high pressure in the West and a trough of low pressure in the East.  This comes as no surprise given the 850 mb temperatures.  What is interesting about this setup is that it usually doesn’t occur during El Nino events.  At present, the El Nino in the Pacific is still somewhat weak and this may be a short-lived cold snap in the US.  Still, it is interesting to note that an El Nino should produce a relatively flat jet stream that does not allow for cold air to dive southward out of Canada.

I checked some forecasts for the areas in question.  Accuweather is forecasting temperatures to fall about 10 degrees below normal in the eastern part of the US starting in the Upper Great Lakes around Wednesday and continuing southeast through the Carolinas by the end of next week.  We can get a better handle on the strength of this forecast by looking at the ensemble forecasts from the GFS.  Ensemble forecasts are just that – an ensemble of forecasts.  The same model is run 20 times with slight variations each run to test the sensitivity of the forecast.  If each solution varies widely, it’s difficult to trust the model forecast.  On the other hand, if the 20 solutions are generally close together, you can have greater confidence in the forecast.  Here is the spaghetti chart for the forecasted jet stream pictured above:

Ensemble forecast for the jet stream on the morning of Wednesday, Sept. 23.  All 20 solutions are represented.

Ensemble forecast for the jet stream on the morning of Wednesday, Sept. 23. All 20 solutions are represented.

While it appears there’s some uncertainty about the ridge in the west, the model is fairly confident about the trough in the east.  There is, obviously, still some uncertainty, but for 204 hours away, it’s a pretty solid forecast.

We’ll probably look at a specific city in the affected region for our next installment of “fun with extended forecasts” in the next day or two.  If anyone has any personal requests, let us know in the comments sections and we’ll take a look.

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