Friday, September 25, 2015

Long Range Outlook: Warm Pattern Expected to Persist

This is the first long range outlook of the winter weather season. In these outlooks, conditions over the next 7-14 days, and possibly beyond, will be discussed.

We begin with an analysis of what's happening at the upper level of the troposphere, with the chart above showing wind speeds at the 150-millibar level as of this past Tuesday. We're going to focus in here on the conditions evolving over the North Pacific. Note how we see a number of disturbances in this area- mainly, two low pressure 'cells' and two high pressure 'cells', each demarcated as needed. One of these high pressure cells rests in the Bering Sea, while the other is found just southeast of Japan. A low pressure anomaly is noted directly south of that high pressure cell in the Bering Sea, while the other is found along the West Coast of North America. 

This is a good example of a Rossby Wave Train, where we see these upper-atmospheric features positioning themselves and becoming nearly stationary for weeks at a time. As a result, the atmospheric pattern will lock in for a prolonged period of time, and that's what we're going to see over the next couple weeks, like we have seen lately. 

That low pressure cell along the coast of California will maintain surface low pressure along the West US, and should provide for warmer than normal conditions in the Central and East US over the next 7-14 days. More on this a little later in the post, but the pattern is pretty clear-cut with respect to these Rossby Waves, in that we're looking at a much better chance for warm weather than cool weather in the eastern 2/3rds of the country.

This image represents forecasted temperature anomalies over the next 8-14 days, based on a number of analog dates as produced by the Climate Prediction Center. Notice the abundance of warm weather across the Midwest, Plains and East US, with well-below-normal temperatures across the West US. This scenario seems rather plausible, given the Rossby Wave pattern supporting surface low development in the West, thus forcing a ridge in the Central and East US.

The precipitation forecast based on the same 8-14 day analogs as those in the temperature graphic highlight a heavy rain event over the South Plains in the next week or two, while mainly dry conditions prevail in the East US. A heavy rain event may also occur in the North Plains, as well as in the Pacific Northwest. I tested this method out this past winter on an irregular basis and found the temperature graphics to verify better than precipitation graphics, so don't take this at face value.

The above covers the 7-14 day period, possibly into the 16 day period. Beyond then, there are some indications of a cool-down for the country, but it's too far out to get a good handle on what could unfold. Latest analog projections as of this morning (I created the majority of this post over the last couple days) show the warmth shifting back west in the 14-ish day range, but whether that happens remains to be seen.

To Summarize:

- Warmer than normal weather is expected to persist across the Central and East US.
- There are signals for a heavy rain event in the South-central US, but confidence is very low.
- A return to more neutral conditions is possible beyond the two-week timeframe.
- A look at the 2-6 week outlook reveals the potential for a cooler than normal pattern setting up, but confidence is very low.


Friday, September 18, 2015

Special Post: 2015-2016 Winter Pattern Beginning to Form?

In today's special post, we're looking at how the upcoming winter pattern may be giving a sneak peak at what it will do this cold season.

The above two-panel image shows daily sea surface temperature anomalies for September 16th on the left, with the 500-millibar height anomalies for September 17th shown on the right. I've discussed the importance of realizing an ocean-atmosphere relationship for some time, and this latest evidence is only adding credibility to that relationship.

I've highlighted two areas of interest on the right-most panel. From Alaska, south into the British Columbia and Pacific Northwest regions, negative height anomalies were observed on September 17th. In the other highlighted region, positive height anomalies were extending from the Bering Sea into the waters north of Hawaii. Although not shown, we also saw a closed upper level low placed due south of that ridge in the Bering Sea, forming a Rex Block pattern.

It is no coincidence that sea surface temperature anomalies reflect this atmospheric pattern. We see a swath of above-normal to well-above-normal water temperatures across the North Pacific, with a body of below-normal SST anomalies just south of the Aleutian Islands, extending back towards Japan. In addition, we also see a cooling of water temperatures over the last month in the Gulf of Alaska / Northeast Pacific, as the graphic below shows.

Change in water temperatures over the last calendar month
In addition to this pattern showing up in the past, model guidance insists it is here to stay, at least for the next 10 days.

The GFS ensemble mean 500-millibar height anomaly forecast for 132 hours out is shown above on the left panel. We see a set-up very similar to what we have been seeing, with a Rex Block orientation in the North Pacific, a deep trough in the Gulf of Alaska, and a stagnant ridge in the Central and East US.

Even in the very long range, over 10 days out, the GFS ensembles still show a signal for ridging in the Bering Sea, a trough in the Pacific Northwest, and a ridge in the Central US. While accuracy at this long range timeframe is quite low, the pattern has a decent chance of locking up like this if it verifies 5 days down the road.

While we may see sea surface temperatures change drastically over the next few months, I'm getting the feeling that we're closing in on our winter pattern, and it could resemble something like the pattern shown above.
This current pattern, and the one forecasted over the next two weeks, is not unlike a modified strong El Nino, as shown in this link (click here), which I've been discussing as a favored set-up for the winter (a modified textbook El Nino pattern). Whether it sticks around and/or returns during the winter remains to be seen, but I find it plausible we enter into this pattern for the winter. This would bring about a warm winter for much of the Central and East US.

To Summarize:

- The current weather pattern is forecasted to continue over at least the next two weeks.
- This pattern is following sea surface temperature anomalies over the Pacific, making it more likely to maintain.
- This pattern resembles a modified Strong El Nino pattern, and could stick around for the winter months.


Thursday, September 17, 2015

Winter Coastal Storm Threat Enhanced From Warm Atlantic Waters

It does appear that the threat for significant East Coast winter storms is on the rise, as we have seen rapid and significant warming of the Atlantic waters along the coastline in the last few weeks.

The above image shows daily sea surface temperature anomalies for September 15th. While there are a number of variables we could analyze in depth, I want to focus on the situation unfolding in the central Atlantic today.

Take note of strong positive SST anomalies from the East Coast through Nova Scotia, partially reaching into the open waters of the Atlantic before being stopped by a swath of below-normal SST anomalies west of Europe. These well-above-normal water temperatures could be a focal point for the coming winter.

Coastal storms that develop along the East Coast can turn into Nor'easters, infamous for dropping massive amounts of snow on inland Northeast communities, and sometimes large rain amounts along the coast. If we are to see this warmer than normal body of water maintain itself along the East Coast, both in position and strength, it's quite likely we will see an enhancement of any coastal storms that do develop along the East US. This would occur as the warm waters feed additional moisture and energy into the storm system, increasing precipitation and strengthening the storm system.

To Summarize:

- Warmer than normal water temperatures off the East Coast may increase the threat for stronger coastal winter storms in the coming cold season.


Tuesday, September 8, 2015

Global SST Anomalies Exhibiting Significant Changes

There are a number of changes underway across the globe with respect to sea surface temperature anomalies, a number of which may impact our coming winter, should these changes persist into the cold season.

The above animation shows sea surface temperatures across the globe over the last several weeks on the top panel, with anomalies for the same time period on the bottom panel. We're going to focus our attention today on the bottom animation, and I will refer frequently back to that bottom panel. There are four important items we need to analyze in this post.

1. Warming Waters Southwest of California
Over the last few weeks, an arm of increasingly-warming waters has been protruding from the Baja California region towards the central Equatorial Pacific. As of the last update on this animation, there is a small sliver of 3ยบ C above normal water anomalies in this swath. Over the last two winters, we saw a warm pool in the Gulf of Alaska provide a semi-permanent ridge, delivering those intense Arctic air outbreaks. A positioning of warm waters to the south could have a similar effect, but if it maintains into the winter, my gut tells me we could see something of a suppressed ridge along the Southwest coastline, which could be bad news for those in California hoping for a wet winter.

2. Sudden Warming in the North Atlantic
This event has only commenced in the last couple of weeks, and it remains to be seen how long it can persist. It appears the previously-cool waters in the north Atlantic are now experiencing rapid warming, stemming from well above normal waters off the East Coast of the United States. This development proves intriguing, as the cold waters stationed near Greenland would have encouraged a positive NAO pattern to develop in the winter, which would be favorable for warm, calm weather in the East US. However, if this warming can sustain and flourish in the north Atlantic, the situation could turn more interesting, particularly for winter weather fans in the East US.

3. Warm Waters in the Indian Ocean
There has been some gradual warming of surface temperatures in the Indian Ocean over the past few weeks. You might be asking, what does that have to do with the United States? Well, those warmer waters could very well encourage increased tropical convection in that area. That region is where the Madden-Julian Oscillation phases 2-5 tend to be, come winter-time. If you didn't get what that means, it can be summed up like this: increased storms in the Indian Ocean could prove favorable for those wishing for a warm winter in the eastern 2/3rds of the country.

4. Gulf of Alaska Warmth Migrating West
This may be the most pivotal development in this post, narrowly beating out the Central Pacific warming. The infamous Gulf of Alaska warm pool, which has been stagnant over the past two winters, allowing frequent and brutal intrusions of Arctic air, appears to be making a move to the west. Over the past few weeks we have seen the strongest positive SST anomalies make a shift from the Gulf of Alaska to the waters due south of the western Aleutian Islands. If this warm pool stops in that part of the Pacific, the risk of a warm winter in the Central US spikes. If it keeps pushing west, then we have a whole new ballgame. It could also migrate back east into the Gulf of Alaska. The point is, there's a lot of uncertainty, which isn't good, since this could be a critical part of the coming winter.

To summarize:
- There are a number of global sea surface temperature anomalies undergoing changes in recent weeks.
- These changes are proving to be favorable for both another harsh winter, but also a warm winter.
- We remain a few months away from winter, so there is a lot of time for these factors to change yet again.


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.