Friday, October 3, 2014

Long Range Lookout: Persistent Cold May Lead to Early-Season Snow

Upcoming spells of rather persistent cold may bring about the risk for some snowfall as we push into October.

The image above shows 500mb height contours from the Canadian model ensembles, valid about four days from today. In this graphic, we see a strong cold weather signal evolving over the United States. Suppressed ridging looks to form over the Western US, as the arcing motion of the contour lines show. Storminess over Alaska and western Canada look to keep a lid on that ridging, but we then see a body of strong ridging shooting up into Greenland, near the top-right corner of the graphic. It is this ridging that permits a strong upper level low to shift south from far northern Canada, buckling the jet stream south and allowing cold air to flow south into the United States. The depressed 500mb height contours confirm this proposition.

The potential for snow comes about from that stormy Gulf of Alaska, where pieces of energy may break off and surge into North America, possibly plummeting into the Plains and giving precipitation to the Central/East US.

Fast forward five days, now at the 9-day forecast window, and we still see a prevailing cold-weather signal from the CMC ensembles. Per the image above, the Canadian ensembles are expecting the strong ridging near Greenland to break down, eventually transitioning to zonal (not 'wavy', generally quiet weather) flow in the northern Atlantic. The West US now provokes this cold air potential, as that aforementioned suppressed ridging starts to build north into Canada. This change from a suppressed ridge to a building ridge is reflected well by the tilting of the depressed 500mb contours from east to west, giving the Upper Midwest and North Plains some chilly weather, sparing the East.

The cold weather signals continue, even out into the Day 14 outlook. The image above shows each individual member of the GFS ensemble system, and its forecasted temperature anomalies about 13 days from today over North America. The big panel at the top of this image shows the average temperature anomalies from all of these individual members, and per this average, we can see the slightly below-normal temperature anomalies pushing south from Canada, possibly continuing this pretty cold pattern we look to enter.

To summarize, a chilly pattern looks to envelop the United States in the next several days, persisting for a decent period of time. This cold may bring the risk for snowfall in areas where the cold air and any possible storm systems collide.


1 comment:

Anonymous said...

OK, now..I can deal with some cool temps, but not cool temps & snow! If I had to pick one, cool temps hands down! Just leave the white stuff out! I have to say where I'm at.. I have no one wanting snow here, but I seem to be out numbered on this site!
Here's to cool temps & no snow!