November 17-19 Potential Winter Storm

There is the potential for a winter storm in the November 17-19 timeframe.

COD

Analysis of absolute vorticity values across the Northern Hemisphere shows a pretty active environment, especially with the energy lifting north from the United States into Canada which brought us the strong low pressure system this past week. The main item we want to look at is the piece of energy circled in red in the northeast Pacific, currently moving into the Gulf of Alaska. It is this energy that will provide the chance for this winter storm next week.

COD

The 42 hour forecast from the GFS shows the energy deepening and digging southward into California by Sunday morning. Ahead of the system ejecting into the Plains, we will see a blast of warm air into the Central US, as the trough digs south and forces warm air to advect north over the prairie.

Instant Weather Maps

The real fun starts on the 17th, when model guidance indicates this storm system will eject east into the Plains in the form of a sub-1000mb low pressure system. Here, we see cold air wrapping into the Rockies, and a deformation zone across Colorado and far western Nebraska. It is here where we should see the highest accumulations, should this storm transpire. It is also worth noting the potential for serious flooding in eastern Texas, the Arkansas/Oklahoma border, Missouri, and parts of Illinois.

Instant Weather Maps

Forecasted accumulations range from 6-12" in the western portion of Nebraska, to as high as 48" along the Nebraska/Wyoming border. Colorado could see amounts in excess of four feet, especially if orographic lifting comes into play. The gradient will be sharp in Nebraska and Colorado, and snow ratios should be rather low, given this is an early-season storm.

To summarize:

- A potential winter storm could drop significant snow in Nebraska and Colorado between November 17-19.
- Serious flooding could arise along the Mississippi River due to heavy rains.

Andrew

Long Range Outlook (Published 11/12/15)

Current Analysis

Strong low pressure is currently moving into Canada after igniting tornado-producing thunderstorms on Wednesday. This severe weather event struck over the Midwest, particularly in Iowa, Missouri, and Illinois. As the system moves into Canada it will flush cold air deep into the United States with northerly winds.
With respect to global weather, we do see tropical forcing located over the Central Pacific, exerting some influence on the Pacific jet stream as a jet streak pushes eastward over the Aleutian Islands. This will likely allow for another risk for cyclogenesis, potentially strong, in the United States over the next week or so.

Forecast Discussion

Upper-level divergence will intensify and shift east to the East Pacific over the next five days, eventually branching off into North America. Expect a raised threat for tropical cyclone formation in the East Pacific between 11/14 - 11/22 per latest model guidance, as strong divergence will persist over the area. Expect stronger than normal low pressure systems over the United States from the current time (11/12) to about 11/22. Model guidance favoring what could be one or more strong surface low pressures not unlike the one that is currently pushing into Canada, which could bring about additional chances for severe weather, as well as snowfall. Expect the strongest of these potential system(s) around 11/18 - 11/20.
Pattern then looks to quiet down by Thanksgiving, with upper level convergence advecting over North America from the North Pacific. Expect elevated chances for cooler weather to round out the month.

Very Long Range Outlook
Time period covered: Early to Mid December

As we head into December, a quieter and warmer pattern is expected to take hold over North America, primarily in the central and western parts of the continent, including the North US, South Canada, and the Pacific Northwest. There are early signals for a typical El Nino pattern setting up as tropical forcing shifts around. Those in the Southeast and East should monitor the pattern for an increased risk for winter storms and cold weather.

The next update will be posted on November 19th at 5:00 PM Central Time.

Andrew

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.

ESRL

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.

ESRL

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.

Andrew

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.

ESRL

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.

BoM
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.

PSU

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.

PSU

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.

Andrew

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.

ESRL

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.

Andrew