Saturday, December 22, 2012

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

17 comments:

Anonymous said...

wow...depressing

Anonymous said...

Andrew,

Is Sussex County, NJ, in the 3-6 inch range or 6-12 inch range? Please reply,
Tommy

Anonymous said...

Look at the map

Anonymous said...

i really like this site and the info that is shared here
can anyone tell me is there a site like this that gives the plains states (kansas) the advance notices like this one gives to the upper midwest & northeast

Anonymous said...

Nice site and analysis but I think you're wrong about light snows in the lower Ohio Valley.

Anonymous said...

the weather channel says the places around philly will get all rain with no snow, is this true and it sayes the same for the storm this wednsday

Anonymous said...

Andrew, thank you for writing New Mexico in your post. Hoping for a little snow here even if it's an inch at least where I live anyway. Had a question: Did you remove the Weather Model's section? Just curious, I was out of town this past week and didn't have access until last night. Hope my family in the Northeast gets that storm. I sure miss them.

Storm-Chaser Wx said...

Question: I decided to lake a look at you models page and took a glance at the GFS and the ECMWF and they show a system of different strengths for the upper mississippi valley on the 29th,
12z GFS model weakens the LP System and slows it down, but the ECMWF strengthens it and shoots it up into the upper mississippi valley, which model would you go with? Thanks

Anonymous said...

sure hope this happens, vt needs a few of these storms to get the snowmobile season going. Send any unwanted snow this way....

Anonymous said...

The local meteorologist in Northwest Arkansas is calling for rain Christmas Day Morning and 3 1/2 inches to 5 inches of snow on Christmas Day Afternoon / Night .

John Holleran said...

Andrew, I'm in the Boston metro-north area, what are you thinking for my area? Thanks!

Art Vandelay said...

Not sure about this storm yet and you should not be posting snowfall maps

Anonymous said...

Given that it's his site, he can post pics of half-naked elves doing dances.

Anonymous said...


I LIKE THIS SITE
I LIKE THE COMMENTS
I WOULD LIKE TO SEE ANSWERS
NOT JUST THIS ARTICLE BUT ALL OF THEM
SOME OF THESE ARE GOOD QUESTONS

Andrew said...

Anonymous #1: Add 2 inches to all totals if you want last night's thoughts.

Tommy: The darker the color, the heavier the snow.

Anonymous #4: I don't know of any at the moment.

Anonymous #5: See the first anonymous' response.

Philly Anonymous #6: I cannot give exact locations for areas in than the Northeast- there's still another day of uncertainty.

New Mexico Anonymous: Yes, it had end of the year maintainence, but is back up.

Storm-Chaser WX: Yesterday's 12z GFS looked reasonable.

Arkansas Anonymous: It is definitely possible.

John: I will have more confidence in city-by-city calls later on tonight.

Last Anonymous: I have been tight on time, but I will make more attempts to answer the questions.

Anonymous said...

ANDREW,
LAST ANONYMOUS WAS NOT AIMED IN YOUR DIRECTION YOU DO A GOOD JOB!!
AND MANY OF US ARE THANKFULL FOR IT
I WAS LOOKING FOR THE INPUT FROM OTHER PEOPLES POINT OF VIEW FOR MORE INFORMATION
AGAIN MOST OF US THANK YOU FOR THE GOOD JOB YOU DO

Anonymous said...

The local meteorologist in Northwest Arkansas is now calling for rain Christmas Day Morning and Heavy Snow Christmas Day Afternoon / Night with snow totals being 6 inches to 8 inches , possibly up to One Foot .