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