Saturday, November 9, 2013

November 17-20 Potential Winter Storm

There is a chance of a winter storm to impact the Plains and Midwest over the November 17-20 timeframe.

(From the post on November 6):

Shown above is the 12z GFS projection of 500 millibar height anomalies across the North Pacific, valid for November 10. If you look over by Japan, you will see a swath of negative height anomalies over that area. This is suggestive of a storm system in the area. As some of you may have seen on this blog and elsewhere, it has been discovered that troughing in East Asia tends to lead to a storm system and associated cold weather event in the East US roughly 6-10 days later, per Joe Renken. The image above is valid for November 10th, meaning we are looking at the November 16-20 timeframe for a potential cold weather event and winter storm.

On November 6, I discussed above how we should be watching the November 16-20 period for a winter storm due to a storm system in Japan, as is shown above. Well, the storm system is currently dropping down into Japan, and should go through the area in the next couple of days. This means the November 16-20 timeframe remains on track. And as of this morning, model guidance is beginning to catch on to this idea of a winter storm for someone in the East or, more likely, the Central US.

The ECMWF model shows three major players in its 240 hour 500mb vorticity forecast: a strong piece of energy in the Southwest, a piece of energy in south central Canada, and ridging along the West Coast of the United States. These three factors will influence where this potential storm system ends up, and how strong it may eventually become.

The prognosis is that a piece of energy will enter the Western US at around the same time another piece of energy drops down into the center of Canada. Now, the energy that will be hitting the US will ride up along the West Coast ridge and then drop down into the Rockies as it crawls along the Southwest to eventually push east. As it drops down from going over that ridge, it appears some of its energy will be torn off and push nearly due east, as the main bulk of the system drops down into New Mexico and Arizona. This torn off piece of energy will strengthen and interact with the energy in the middle of Canada to create the blue and green vorticity swirls you see on the Canadian/US border on the forecast image above.

The reason we want to watch this area of vorticity on the border is because it may do one of two things. It may: A) Pull the main body of energy north towards Canada, giving a snowstorm to the Plains and possibly the Upper Midwest. or, B) Have no interaction with the main body of energy, in which case the main body may prefer to move east-northeast and bring a snowstorm to the Midwest and Ohio Valley. The ECMWF does not go out past 240 hours, so we cannot see which solution it prefers (to me, it looks like the ECMWF may be leaning towards option A at this time, but I can't confirm that). However, we can see what the rival GFS model says.

Right off the bat, let's take note of how the GFS and ECMWF are very similar in their set-ups with this event for Hour 240. Both have ridging along the West Coast, both have the two pieces of energy I was discussing in the previous paragraphs, and both models have high pressure in the Southeast, something that will act to try and push the potential winter storm north into the Midwest or Plains rather than east into the Northeast.

The GFS model has the US/Canada border piece of energy further south than the ECMWF, and for that reason, the Southwest piece of energy (shown above in Texas and New Mexico) is weaker and more likely to push north into the Plains. I do believe that it is the US/Canadian border energy that will end up having the most say in where this storm goes, because if that energy is further south like the GFS shows, the storm system in question should push north and hit the Plains. But, if the Canadian system is weaker and more detached from the potential winter storm, we could see the storm affect the Midwest and Plains. And yes, if you're wondering, the Plains is currently in the best spot for this potential storm, but it's all still up in the air for another ~10 days.

An interesting piece to this puzzle is how the North Atlantic Oscillation is projected to evolve over this timeframe. The ECMWF model has the NAO changing phases twice between November 16 and November 21st. As you weather junkies may know, storm systems can be favored in the Northeast and along the East Coast when the NAO is changing phases. The model prognosis from both global guidance systems above would tell me that the storm system is unlikely to head east towards the Mid-Atlantic and such, but the changing NAO makes me want to keep an eye on that region around the November 16-21 timeframe in case something does develop. I'm not saying something will develop, but for all intents and purposes, I'd like to monitor projections over the East Coast in coming days if the projections for NAO changing phases continue.

Here's my current diagnosis on the situation. Bear in mind this is all preliminary, as we are still 240 hours away.