Tuesday, December 18, 2012

December 25-28 Potential Significant Winter Storm

I have upgraded the title of this potential winter storm from 'Winter Storm' to 'Significant Winter Storm'.

**This post is dedicated to Rachel Davino, age 29, a victim of the Connecticut shootings.**

We'll start with today's 12z GFS model and its forecast for the 500mb height layer. This is the forecast for the evening of December 27th. We see a deep depression in the 500mb height layer in the Midwest, signifying our storm system. It looks to be centered in central Illinois. If this looks familiar to you from the upcoming Dec. 19-22 blizzard forecasts, then you're correct- but the buck stops there. There's one thing that I think will make this system fairly big: The Rex Block.

This is an image of a Rex Block from the site of theweatherprediction.com. The Rex Block involves a low pressure system in the Southwest US, and a high pressure system in the Pacific Northwest. In response to this highly meridional flow (north to south wind pattern), storm systems in the presence of a Rex Block are strengthened, as shown in this example of a storm system in a Rex Block-turned-Omega Block during May 2005.

While we're on the topic of this storm system in May, let's see if this storm system has any similarities with the GFS forecast above, and the ECMWF forecast below.

Are you surprised? It's almost like a carbon copy of the May 2005 storm is now in this ECMWF forecast for the morning of December 26th. Putting aside the (eventually crucial) time differences, we see the Rex Block has now transitioned to an Omega Block, as was pictured in the May 2005 picture above.

It should be noted that the ECMWF is showing an Omega Block while the GFS is showing a fair Rex Block, with a system in the Southwest and slight ridge in the Pac. NW. The slight difference in these two, as small as it may seem, is actually quite significant, and something we will need to watch later on.

What does this mean? It means that the models are in rare agreement of high pressure setting up on the West Coast, leading to an Omega Block (possibly Rex Block) over the area that then gives the storm system a much more amplified feel. Naturally, as the Omega block/Rex block produces a much more meridional flow, we see the vorticity centers in the low pressure system enhanced by the meridional flow, and suddenly a much stronger storm is born. Another thing interesting is the level of agreement from the ECMWF and GFS on the placement of the storm. Given, there are some time issues that need to be worked out, but it's reassuring to see this storm now showing up on more than one model.

For those wondering what precipitation forecasts are coming out of the models, here's a few images:

Evening of Dec. 27th precipitation forecast. All precip.
above blue line is snow. Multiply value on legend with 10 to get snow.



armando said...

Well good post Andrew, but I don't see this storm to be the highlight of the midwest for a couple reasons. First of all their will blocking. second their is a high pressure right around great lakes so I'm guessing it will make this storm transition toward the coast then let the blocking which is over canada do it's job. Also the sunspot cycle should be near their minimum so all in all it should have an affect on this storm. I just hope that we will be in the line of fire (NJ), and the rest of the east. I bet the models will catch on later. Great post Andrew!

Anonymous said...

i appreciate the post andrew but could next time u MAYBE show the entire picture. not just the precip totals over the north..maybe include kentucky, tennesse ext..cuz i believe their also effected

Anonymous said...

Will nyc get affected by this storm nd if so will it be rain or snow

Anonymous said...

when the storm gets closer can you estamate the snow totals if any in southern ontario and does this storm for the 21/22 look like rain or snow for the toroto area

Storm-Chaser Wx said...

remember that HP systems rotate clockwise, so if there's one in the great lakes, then the HP System will only help the potential for another significant winter storm for the upper mississippi valley, of course though, this is still a good ten days or so away, so a lot can change.
Andrew: remember when you posted the article on a more active december track for the upper plains? I think that this is going to be it because there are storms forecasted for the 20th,
26th, and the 31st-2nd

Anonymous said...

I have a question? When will snow hit the gosh darn mid atlantic because this December so far is a copy of last December