December 25-27 Significant Winter Storm: First Call

**This post is dedicated to Dylan Hockley, age 6, a victim of the Connecticut shootings.**

"The Northeast's First Major Winter Storm Is On The Way."

I have upgraded this storm system's title from 'Potential Winter Storm' to 'Significant Winter Storm'.

Above, we see Hour 87 of the SREF ensembles. These are several forecast members in the small images, with the average of all the members on the bottom right. The warm colors indicate areas of low pressure, and blues show high pressure areas. At Hour 87, we see our system is digging into New Mexico, as shown by the oranges present in that region.

This is where the models have issues. Because we have a low pressure system pushing down on the jet stream, physics dictates that high pressure must form to the west and east of said low pressure system. This is evident with a strong high pressure system in the West Coast (positive PNA, will be discussed later), and another, weaker high pressure system along the Gulf Coast. Now, low pressure systems, contrary to the 'opposites attract' theory, do NOT want to go towards high pressure systems. In the way that the atmosphere is a river, a fish does not want to head towards a rock, because it doesn't want to get held up, stuck, etc.

Extrapolating (using conditions to forecast ahead) this Hour 87 forecast tells me that the system will probably dig a bit further southeast just because the positive PNA regime supports such a pattern, and there is not high pressure in western Texas. Now, if we do see high pressure build in in that area, the storm track would be further to the northwest.

This new 12z GFS forecast, valid the morning of December 26, is by far the one that makes the most sense. See that deep depression in the West US? That is a negative PNA, the opposite of a positive PNA. In the positive PNA, storm systems can go south and towards the Ohio Valley, Midwest, etc. The negative PNA forces high pressure to form in those same areas, pushing the storm track north. In this forecast, we clearly see the negative PNA influence in the Northeast, with high pressure reigning supreme, evidenced by arcing lines (depressed lines signify low pressure). Adding to the push for a more northwest track is a high pressure system in the Rockies, which would try to act like a slingshot effect- if you pull the slingshot back, you have two pieces of plastic holding the elastic up (the high pressure systems). The slingshot itself is being pulled back (low pressure system), but if the pieces of plastic are too strong, you will have to reduce the pull on the slingshot, meaning the low pressure system tries to go north.

More interesting is that lobe of vorticity spinning off of the main storm system, shown as a closed low in the Ohio Valley. That lobe is still digging as a negatively tilted system, and thus has reason to want to go off into the Southeast. Based on the presence of high pressure in the Northeast, the lobe will want to move north, and eventually the two pieces of energy will want to merge, probably off the coast as evidenced in previous model runs.

To cover all the bases, a baroclinic zone (where a gradient of temperatures is found) will be present in the Midwest and Ohio Valley, thanks to recent snowfall and snow cover. This snow cover will act as a stationary cold front and will amplify the first storm system as the cold snowpack's air goes up against the warm Gulf moisture. Cyclogenesis (forming of extratropical cyclones) commonly occurs in baroclinic zones.

For the evening of December 26th, we see our two storm systems are still in play. System #1 (in southeast Ohio) continues to produce precipitation (likely snow) across north Ohio and west Pennsylvania, but is now weakening. Why? Because System #2 (offshore the Mid-Atlantic) is now strengthening in response to Atlantic Ocean influence. The two systems then begin to share energy, and System #1 is enveloped into System #2. This combination results in a major coastal snowstorm for the entire New England area. Again, thanks to that high pressure system in Maine, the merged system will now want to ride the coast.

Here's the GFS Snowfall Forecast:

Based on all the above, including the PNA, baroclinic zone, atmospheric river analogy, and just basic laws of physics, here is my first call for the December 25-27 Significant Winter Storm.

Andrew

Long Range Lookout, Apocalypse Edition: Stormy Pattern Continues, Cold Outbreaks Likely

**This post is dedicated to Ana Marquez-Greene, age 6, a victim of the Connecticut shootings.**

This edition of Long Range Lookout was published on the Apocalypse, December 21, 2012.

The stormy pattern that many have recently experienced should continue, and increasing cold patterns should emerge as a response.

The above image shows a side-by-side forecast from the ECMWF model (left) and the GFS model (right). The forecasts are for the 500mb height anomalies from the Day 8 to Day 10 forecast period. We'll start with the stormy potential. See that low pressure anomaly over the West US? That is a negative Pacific-North American index (PNA). In the negative PNA, you'll see storm systems forming over the Southwest and propagate through the Plains before going somewhere in the Central and East US. This negative PNA has been present in the last few weeks, creating several stormy situations over parts of the nation. As long as this keeps going, more storms will continue to appear, some of them significant.

The second thing is the rampant reds over the Arctic. This signifies a negative Arctic Oscillation. In a negative AO, high pressure forms over the North Pole. This high pressure weakens the polar vortex, a permanent low pressure system that regulates the flow of cold air that stays and leaves the Arctic. In a negative AO, cold air is more prone to escaping the polar vortex and fleeing south into lower latitudes, including North America. The PV may be weakened so much it could crash, and more on that can be found in this link.

Andrew

Polar Vortex Collapse, Arctic Freeze Imminent

I have concluded that a polar vortex collapse and subsequent Arctic Freeze is imminent across the nation in January. I'll explain why.

Here, we see observed stratospheric temperatures at the 70mb level, commonly known as one of the lowest levels in the stratosphere. Looking at the 2012 section, we see that recent stratospheric temperatures have taken an uptick to normal temperatures, expressed by the dashed green line. This uptick tells us that the stratosphere is now at average temperatures, and the polar vortex is now at average strength. That's a good starting point to tell us what the stratosphere is currently like.

Let's continue our analysis of the observations of the stratosphere. This is an animation of observed temperatures at the 50mb level (slightly higher up in the air than the 70mb level) over the past month. In late November, we saw a large blob of red appear and propagate into the Arctic Circle. This was a significant stratospheric warming (SSW), meaning warm air from the lower atmosphere was pushed into the stratosphere. This was also shown in the 70mb chart by that spike in observed temperatures a month ago.

I want to dig deeper into the time just before that warm air shot into the Arctic Circle. Try and watch for the observation on November 25-27. If you can see it, you can see how the warm air entered east Asia before blowing up and shooting into the Arctic. Looking at the most recent December image, we can see that another small blob of warm air has emerged in central Asia. Judging by the last two or three images, this blob also looks to be entering East Asia, meaning we could see another stratospheric warming if this warm air shoots into the Arctic Circle.

This is the 8-10 day height anomalies at the 500mb level. The 500mb level is the best area to identify areas of low pressure (blues) and high pressure (reds). Stormy patterns and quiet patterns correspond to these colors, respectively. The image is of two model forecasts- the one on the left is the ECMWF model, and the one on the right is the GFS model. For those unfamiliar with the polar vortex, it is a permanent low pressure system stationed over the Arctic. When enhanced low pressure is over the Arctic, the polar vortex (PV) is stronger than normal, and high pressure weakens the polar vortex.

The model forecasts above both show big high pressure centers interrupting the polar vortex and splitting it into two pieces. Such a split exemplifies the traditional weakening of the polar vortex. When that polar vortex is weakened, the cold air locked up in said polar vortex flows south, possibly into North America, maybe into the US. This is a very good sign coming from the models at this point in time to go along with the stratospheric observations.

Continuing with watching the models, we turn to a charted forecast of the ECMWF model. I want you to look at the bottom image, called the EP Flux. In short, the EP flux shows the direction and strength (shown by length of arrow) that air moves into the stratosphere. In events where warm air enters the stratosphere, long, extended arrows are commonly seen. The forecast into the days before New Year's Day show a batch of extended arrow motions, suggesting a large motion of warming may be incoming into the stratosphere. This enhanced EP Flux would theoretically enhance warming potentials in the Arctic, thus further weakening the Polar Vortex. I find it likely that the aforementioned 50mb wave over East Asia would be to blame for this rise in EP Flux values.

Take a look at the two ECMWF Forecasts below:

These two images are for different levels of the stratosphere. The top level is the 100mb level, essentially the lowest part of the stratosphere. The bottom image is the 1mb level, the highest part of the atmosphere. Look closely and see if you can spot the 'L' letter in each image. This is the predicted center of the polar vortex at the Hour 240 for both images.

The top image of the 100mb level has the polar vortex centered in East Asia. Note the warming occurring in North America, another sign of cold potential at the surface (more on that later). Now take a glance at the 1mb image. The polar vortex is now shown to be to the east of Greenland. They are pretty far apart! Why does it matter? Take a look at the analogy below.

Think of the game Jenga. You must try to take out pieces of the tower to get as many pieces out, but still have it standing. The more pieces you take out on one side, the more unstable that side becomes, and eventually the tower falls down because of the difference in stability of the tower (a.k.a. where more blocks are placed). In a similar situation, when you have the polar vortex far apart in different levels, like what is forecasted above, the polar vortex becomes more unstable and weakens, possibly to the point of collapse.

One more analogy for you: Think of the polar vortex like a cylinder full of cold air. If you cut the cylinder in half and move one half away from the other, the cold air will sink. That is exactly how the stratosphere. If you have displaced parts of the stratosphere, the cold air that is held in the polar vortex will be released and flow down into lower latitudes. If the PV is displaced enough, it may collapse altogether, leading to an icebox solution over parts of the world.

The GFS model also shows the displacement of the polar vortex at Hour 240:

Pressure at hour 240 at the 1mb level

Pressure at Hour 240 at 100mb level

Here is what I have gathered:

-Current stratospheric temperatures are at average values.
-A strong warming event looks to occur at the end of December.
-Very elevated EP Flux forecasts enhance such a warming potential.
-The polar vortex will become displaced as a response.
-High pressure forms over the Arctic, further messing up the polar vortex.
-AAM QBO values are already turning positive, increasing the potential for the permanent warming event to switch winter to spring later on in 2013.

All of the factors above combined, severe damage looks to be done to the polar vortex. I can envision a partial collapse of the vortex occurring, which would only lead to enhanced cold outbreaks in the United States. I am not confident on this collapse occurring, but if there is a best time for such a collapse to happen, it could be coming up at the end of this year.

Keep a sharp eye out for prolonged cold weather if these forecasts by the ECMWF and GFS verify.

Andrew

December 25-27 Potential Significant Snowstorm

**This post is dedicated to Ana Marquez-Greene, age 6, a victim of the Connecticut shootings.**

We are looking at the possibility of a snowstorm along the central and eastern portions of the nation in the days immediately after Christmas, something that could no doubt cause a travel nightmare. This is a forecast from the National Weather Service's GFS model, also known as the American model. It shows 500mb height anomalies, basically where areas of low pressure are (cool colors), and where higher pressure is located (warm colors).  We see our storm system in the southern Ohio Valley as a deep depression of purples on the morning of December 26th.

Before we dive into the concept of where this system will go, let's analyze the atmospheric structure first. The first thing I want to point out is two low pressure systems to the north of New England. This phenomenon is known as a 50-50 Low, as discovered by wxrisk.com. The 50-50 low acts as an attractant for the storm track to head towards the Northeast. In response to the 50-50 Low, high pressure develops close to or in the Great Lakes. This pushes the jet stream south into the US, pressing into the storm track and forcing storm systems to go into the Northeast, because storm systems do not head into high pressure systems.

Some of you more avid weather enthusiasts may recognize a high pressure system to the west of Greenland as a west-based negative NAO. Not so fast. See that system of low pressure right up against Greenland? That's the Achilles heel of the NAO. Typically, when you have high pressure over Greenland, the jet stream wants to push south and encourages cold and snow to hit New England. However, such an encouragement of cold and snow can be enhanced by the high pressure system being WEST of Greenland, a.k.a. a West-Based negative NAO. I find this potential negative NAO to be too far west of Greenland to be completely effective. The presence of a low pressure anomaly right against the east coast of Greenland then disbands any effects the ridge may have on the NAO. I expect this index to be neutral- neither supportive or unsupportive of snow in the Northeast.

Here's another 500mb height anomaly forecast for the morning of December 26th, but this forecast is from the European's ECMWF model. The ECMWF is commonly known as the most accurate model in the world, and forecasters are frequently watching the ECMWF and GFS to see which one is showing which forecast.

The ECMWF has the system stronger than the GFS, that's the first and foremost issue. A common thing to remember for storm systems is that they will turn north the stronger they are, as is observed with tropical systems. Additionally, the ECMWF is lacking a good 50-50 Low. It has half of the phenomenon, but not the whole package. This, combined with that dicey negative NAO situation, is not all that encouraging as far as snow prospects for the Northeast go. However, that big high pressure over Canada suppresses the storm from the Midwest and thus induces a higher likelihood for a Nor'easter track.

This is an observation map from November 4th of this year, just about 52 days from the forecast date of December 26th. You should see a storm system located in the Southeast. This is another weather phenomenon known as the Lezak Recurring Cycle (LRC). It is associated with a repeating pattern developing every fall, with each repetition falling between 40-60 days. This year's LRC repetition period, known as a 'cycle', is 52-53 days. This is why I went back to find this chart.

I went forward a couple more days with this system and found that it went out to sea instead of riding up the coast, a solution that would be very possible if a negative NAO and/or 50-50 Low failed to formulate. However, I have a feeling that if the huge high pressure system is present in Canada where the ECMWF/GFS are both forecasting it to be, the storm system should rightfully move into the Northeast, bringing significant snows to that region, as well as the Ohio Valley.

Early estimate on best snowfall areas are the Ohio Valley and Northeast. No specific amounts as of yet.

Andrew

December 24-25 Clipper System

**This post is dedicated to Ana Marquez-Greene, age 6, a victim of the Connecticut shootings.**

It is becoming increasingly likely that a small clipper system will deliver precipitation and disturbed weather to the Plains and Midwest during Christmas Eve and the day of Christmas.

Above is the NAM model forecast, valid for the afternoon of Christmas Eve. We see a small, negatively tilted area of vorticity in the upper left hand part of the collage of images above. The system looks to be a clipper, pushing to the southeast from Canada. It's lack of sustained moisture and unimpressive isobar gradient tells me this will not be a winter storm event at all. 6 hour precipitation values in the upper right tell of light snow ongoing in the Upper Midwest in response to this system. High pressure stationed in the Plains will provide a base for cold air in the region.

Two forecasts, both based off of the NAM model but known as parallels (slightly changed from the NAM model itself), show 3 hour snow in north Wisconsin and Minnesota during this system as light, nothing extremely impressive. Based on the low prospect for any big accumulating snows, and the NAM's lack of moisture in the top image collage, this does not look like anything to get excited about. If you're into the traditional snowing on Christmas Morning, those in the West Great Lakes may have a shot at such a prospect as the clipper continues pushing to the southeast.

Andrew