Friday, September 4, 2015

Pattern Change Imminent for North America

Welcome back to The Weather Centre! We are now resuming winter weather posting, which means we're back in business for the next several months! We're kicking off this winter with a pattern change update for North America.

It is expected that a pattern change will impact the United States in the next week or two, leading to substantial air mass shifts across North America.

Over the past week or so, we've seen a persistent, high-amplitude weather pattern over the Northern Hemisphere, particularly in the North Pacific. Deep negative height anomalies have been situated west of the Aleutian Islands, as well as along the coast of western Canada. A strong ridge has maintained position over the Aleutian Islands and across the Bering Sea. This volatile pattern has wedged the United States into a rather stagnant pattern, with warm weather over-spreading the Central United States, making pushes north and east as time has gone on.

This pattern is about to change, as we see two main playmakers rile up the atmosphere.

Tropical Tidbits
The first will be the shifting of that deep trough along the west coast of Canada south into California, and then moving north and east into Montana, as this GFS Ensemble forecast shows on the morning of this coming Sunday. Locally, this means warm weather continues for the Plains, with that ridge still not budging, as well as warmer weather in the Midwest and Northeast. However, as that trough moves eastward, we will start to see the jet stream become more zonal.

Tropical Tidbits
By Wednesday morning, we see significant changes to our atmospheric mid-levels. We now see a dominant zonal flow of the jet stream, as the smoothness of height contours over North America shows. Some slight ridging remains in place along the East Coast in the United States, but this too will be shunted away in due time, as the zonal flow is maintained by the strong storm system in the immediate vicinity of Greenland (some of you more knowledgeable weather enthusiasts may recognize this as a pattern similar to that of a +NAO wintertime pattern).

Tropical Tidbits
By Friday morning, a week from this post's publishing, we see a complete flip in the pattern from where we are currently. Strong ridging has taken over along the western coastline of North America, maximized in the Pacific Northwest into the British Columbia region. In response, similar to a +PNA pattern in the winter, we see cooler, stormier weather working its way into the Central US and East US. A note for any severe weather fanatics still watching the fall tornado season threat, I wouldn't be surprised to see some storms anywhere from Oklahoma to the North Plains under this northwest flow.

There is another factor that we will have to watch in the next couple weeks, and that is tropical storm Ignacio.

Ignacio is currently located in the middle of the Pacific, due south of the westernmost Aleutian Islands. The image above shows forecasted storm tracks from all available GFS Ensemble members. The consensus among these members, as well as other statistical forecasting models, is to have Ignacio curve north and east, making another 'landfall' as a post-tropical system in British Columbia. From there, already-low confidence gets significantly lower, as ensemble tracks take this storm anywhere from the Arctic Circle vicinity (as shown by member AP17), or the Lake Michigan region (as per members AP10, 12, and 14).

As far as impacts, we can split it up into a number of talking points. For one, this will drastically lower forecast model skill over the next two weeks. While it isn't necessarily uncommon for post-tropical systems to make their way into the mid-latitudes, forecast models almost always see a drop in verification due to this occurrence. Therefore, long-range forecasts over the next several weeks are subject to anomalously worse verification rates.
Secondly, as Ignacio was once a Category 4 hurricane, we could see a strong storm system develop over land here in North America. Local impacts may include substantial rainfall, possibly to the point of severe weather potential, but that all depends on where the storm tracks, and if it even makes it to North America to begin with.

To summarize:

- A pattern change will unfold across North America over the next 7-14 days.
- A heat wave will impact the East US, to be replaced by cooler weather about a week from today.
- Post-tropical Ignacio will hinder forecast model accuracy, and may also bring about anomalous weather conditions in the U.S. in the next 10-20 days.
- Cooler weather will prevail in the Central US in the next 7-14 days.