Tuesday, December 16, 2014

Atmospheric Trifecta Preparing to Deliver Cold, Snowy January

A trio of atmospheric signals are gearing up for what could be a rather cold, snowy January.

Research I completed last night showed significant (10"+) snowstorms in the Midwest are most favored under the negative phase of the Arctic Oscillation, the negative phase of the East Pacific Oscillation, the negative phase of the North Atlantic Oscillation, and the positive phase of the Pacific-North American index. We look to have at least three of these factors locking down the atmosphere to round out December and kick off 2015.

Tropical Tidbits
The image above shows the GFS ensembles' forecasted 500mb geopotential height anomalies over the Northern Hemisphere, valid on the evening of December 26th. On this graphic, we see a prevailing negative EPO, as highlighted in this graphic with ridging over the Gulf of Alaska. A negative NAO has also emerged just west of Greenland, combining with ridging in Eurasia to provoke a negative AO regime. I'm watching for a positive PNA signal, but as a ridge is passing over the region, I'm content on acknowledging a trifecta for now.

It doesn't end there, however...

Tropical Tidbits
By December 30th, we find more than a few big developments have taken place. We see our negative EPO ridge has blossomed into a massive swath of high pressure, now extending along the West Coast into Alaska... and then some. Ridging over Greenland still exists, paving the way for a continued negative NAO and negative AO tandem. As a result, we continue to observe below-normal anomalies in the East US, signaling a very cold period to end December and start January.

My thinking is we'll see those below-normal height anomalies slowly progress west, as a west-based negative NAO (where the ridge is displaced to the west of Greenland) typically favors cold weather more into the Central US. This would be amplified/supported by the strong -EPO ridge, slowly evolving into a +PNA signal, it appears.

Now, onto the snow...

Tropical Tidbits
The image above shows the GFS ensembles projection on December 28th, showing jet stream wind speed values. There are a few things to note here.
First off, check out the extended Pacific jet stream. We're seeing that occur in the very near future, and it's no coincidence that this extended jet stream into North America is happening at the same time some storm threats are arising. The extended jet allows for cyclogenesis in the Northern Pacific, where energy may be shunted east into the United States, leading to those winter storm chances.
Second, we see a notable subtropical jet stream (STJ), as depicted by the green colors extending from the northeast Pacific into Mexico and the South US. With a negative NAO regime in place, as well as an enhanced subtropical jet stream, it's possible the East Coast could get rocking and rolling with these storm chances.

As for the Central US, I alluded to the below normal height anomalies pushing east. As far as snowfall impacts, clipper systems look to make their return, as the strong ridge pushing into Alaska produces a sustained northwest flow pattern (where the winds are out of the northwest). This could lead to not only episodes of snow in the North US, but also lake effect snow episodes, as all Great Lakes are currently well below last year's ice levels at this same time.

To summarize:

- The atmosphere is preparing to shift from a generally mild December pattern to a rather harsh January pattern.
- Sustained cold weather is likely for most of the nation east of the Rockies.
- Enhanced chances of snowstorms will be seen both along the East Coast, into the Central US (depending on individual storm tracks, of course).
- Buckle up, things are about to get fun.