Monday, November 25, 2013

Strong winds and rain in the east; snow in the northwest?

In my last blog post I talked about the blast of arctic air that was forecast to move across the country last weekend and into this week in association with a cut-off low that would slowly creep across the southern US.  Here we are on Monday and today's 500mb pattern from the ECMWF still shows that cut-off low over Texas and New Mexico.

The colder air and lift that are accompanying this storm have already contributed to snow and ice throughout New Mexico, Texas and Oklahoma.  In fact, the weather map from the Oklahoma Mesonet today shows several sites that are not able to report the wind speed or direction (the red dots on the map below) because the anemometers and wind vanes are frozen!

This storm is expected to  slowly chug eastward over the next few days before ramping things up along the east coast.  Look again at the 500mb chart above.  See that other trough in the northern Great Lakes / northwestern Ontario?  That trough is forecast to merge with the cut-off low sometime during the next few days.  One one hand, this means that the cut-off low will no longer be cut-off---back in the normal flow, it will accelerate and move on out of here.  Before it does, though, the cold air it's bringing behind it is going to run into the warm Atlantic Ocean, and that means rain--heavy rain--for the eastern US.  Here's the ECMWF forecast rainfall around 15Z Wednesday morning:

Heavy rain up and down the east coast on a day when lots and lots of people will be travelling.  Not good.  Fortunately by Thanksgiving day itself, it appears that the precipitation will be on its way out (as this storm accelerates off the coast).  Here's the ECMWF precipitation forecast for Thursday morning at 15Z:
Only light precipitation for parts of the northeast.  A lot of this near the lakes may be lake effect snow as strong northwesterly winds will occur behind this storm.  See the tight gradient in the blue contours on the above map?  Those are the surface pressure contours, and the tighter the pressure gradient the stronger the winds.

This brings up another question that I know many people are dying to hear the answer to--will the Macy's Thanksgiving Day parade balloons be able to float with this weather?  I'm told that if the winds gust higher than 35 mph then the balloons are a no-go for the parade.  So what are the models saying?

It's probably going to be close.  Our models mostly forecast sustained winds, that is, the average wind speed over a certain amount of time (usually a few minutes).  Wind gusts are instantaneous wind speeds, and the wind gusts can be significantly higher than the sustained winds.  Here is the ECMWF sustained wind forecast over New York City for 15Z on Thursday while the parade is going on:

You can see the winds really pick up speed off the coast.  But over the city itself, the bluish-greenish colors indicate winds of 15-18 knots or 17-21 mph.  Again, those are the sustained winds--gusts will be higher.  The GFS and NAM have slightly stronger sustained winds, getting up to more like 25-28 mph--again, with higher gusts.  So, as of right now, it's going to be borderline.  We'll have to watch the model runs as we get closer to get a better idea of how strong the winds are going to be.

And finally, just as a teaser for everyone out here in the Pacific Northwest, the really long range models for next Sunday-Monday have a strong cold front coming through in association with a sharp trough coming down the Pacific coast from Alaska.  Here's the ECMWF 156 hour forecast (!) of 850mb heights and temperatures for next Sunday.

A blast of cold air headed our way.  The ECMWF has some low-level instability following this cold front and is throwing out some convective snow showers in its wake.  Though we should never, ever believe models this far out into the future, for fun here is the ECMWF snowfall accumulation forecast for next Monday morning:
Two inches of snow in the lowlands east of Everett?  Eh...maybe.  Don't take my word for it yet.  But it's definitely something to keep an eye on...

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