Tuesday, January 9, 2018

Long Range Outlook: January 9, 2018

This is the long-range outlook, published on January 9th, 2018.

I'll begin with a topic that I've discussed a few times on this blog since November 2017: that of the stratospheric warming event that unfolded in December.

Shown above is a line graph showing the 30-millibar level temperature from September 2017 to the present day. I had highlighted how a stratospheric warming event appeared imminent in late November, though as mentioned this was not expected to be a sudden stratospheric warming (SSW). This is what transpired, though this warming event was prolonged and came in a number of waves after the initial failure of the first wave of warmth to penetrate the stratospheric polar vortex in the Arctic Circle. As the above graph shows, however, the warmth did eventually make its way into the stratosphere, weakening the stratospheric polar vortex and delivering the colder than normal temperatures to the eastern 2/3rds of the country to kick off the new year, as pictured below.
Following the stratospheric warming event, the warmth in the stratospheric levels of the Arctic Circle has dissipated, leading to the sharp drop in the 30-millibar temperature that has reversed only recently, though still remains well below normal. Similar to how a warming event in the stratosphere can lead to cooler than normal weather in the eastern 2/3rds of the country roughly 2-3 weeks later, a rapid cooling of the stratospheric Arctic Circle region can encourage warmer than normal temperatures in the United States, as the colder than normal air becomes "locked up" in the upper latitudes. We'll get to the forecasts for the medium-range shortly to see if this does end up being the case.

For the time being, though, the Pacific is showing some very interesting features that I'd like to review.
Above is a four-panel graphic showing the global orientation of 200-millibar wind speeds (color fill) and geopotential heights (contours) in five-day periods, from December 18th to the present day. From the five-day period including Christmas Day to today, the Pacific jet stream has strengthened and become extended to the waters due south of the Aleutian Islands. A rule of thumb when observing the Pacific is that an extended jet stream can portray the threat of a storm system in the following days, and this does appear to be the case for this week, but will require a post of its own. For now, it is worth noting the extended nature of the Pacific jet stream, as this will more than likely lead to an opportunity for unsettled weather in the U.S. in the next two weeks.

Let's now take a look at one of my favorite longer-range graphics to watch, the 8-10 day geopotential height anomaly forecast from the three primary global weather models.
The ECMWF model's forecast for 8-10 day geopotential height anomalies is shown on the left-most panel, with the GFS model's guidance in the middle panel. The CMC (Canada's global weather model) provides its outlook in the right-most panel.

The three models certainly have some key differences in the projected pattern for the eight to ten day period, but there are a few similarities to be gleaned from the guidance. For one, all three models indicate the presence of a Rex Block in western Canada into Alaska and the Gulf of Alaska, albeit to varying magnitudes. The GFS is most pronounced with this pattern, producing a 'textbook' Rex Block in the form of the strong, cut-off ridge over a portion of the Arctic Circle and Alaska, as well as the negative anomalies in the Gulf of Alaska, offshore the Pacific Northwest. One could argue the actual Rex Block is formed with that ridge and the apparent upper level low over north-central Canada in the GFS panel, but such an orientation (slanting to the southeast, from the ridge to the upper level low) is rather unorthodox.
The ECMWF and CMC generally agree with the idea of the Rex Block, though neither model cuts off the ridge as the GFS does, instead setting up a positive-PNA-looking ridge from the American Rockies into a portion of the Arctic Circle, as one continuous ridge. Both the ECMWF and CMC still appear to support this as a Rex Block, however, with the zonal downstream flow in the United States, save for the waters just offshore of New England, where a suppressed ridge holds in place as a consequence of strong negative geopotential height anomalies in Europe and a mixed North Atlantic Oscillation (NAO) pattern. Those two features will combine to discourage colder than normal air buckling south into the eastern two-thirds of the country, instead supporting seasonal temperatures.

It's not enough to simply eyeball weather models, however. We now turn our attention to the Madden-Julian Oscillation.
Above is the ECMWF model forecast for the Madden-Julian Oscillation (MJO), with the ECMWF ensembles displayed individually via the thin yellow lines. Although this forecast period began on January 1st and ends January 15th, more-recently issued forecasts still support the outlook of the MJO transitioning from Phase 2 to 3, and perhaps then to Phase 4, albeit at a weaker grade than the previous phases. As the screenshot below and on the right shows, from the Climate Prediction Center,
temperatures in the United States tend to be cooler than normal for much of the Central U.S. and the majority of the East U.S. during Phase 2 in the winter months. This follows the rule of thumb (portrayed by these same composites) that Phases 8, 1 and 2 of the MJO are most favorable for colder than normal and snowier than normal conditions in the eastern two-thirds of the country during the winter months. Similarly, Phases 4, 5 and 6 are most encouraging for warmer than normal temperatures in the United States for the winter months.

The expected movement into Phases 3 and 4 of the MJO by the middle of the month suggests warmer than normal temperatures for much of the United States around the same time period. This seems to go hand in hand with the three primary models' projections of the 8-10 day period, and indicates that the middle of the month (likely a little bit beyond the middle of the month) should be warmer than normal for much of the country. I expect the warmer than normal tendency to continue through the later part of January in part due to that rapid cooling in the stratosphere that was observed at the end of December, and will translate to the warmer than normal weather for the middle and end of the month.

But what should we be on the lookout for at the end of the month? Will the warmth continue, or will the cold return?

Just as this post began with the stratosphere, it will close out with the stratosphere. Above is a multi-panel forecast from the ECMWF model for different variables in the stratosphere. We will focus in on the top three panels to make some prognostications for the end of January and into early February.

The top panel shows observed zonal wind speeds at the 1-millibar level of the stratosphere - in other words, the strength of the far-upper stratospheric polar vortex. The 1-millibar level isn't as important as the 30-millibar or 50-millibar levels. As an example of this, it has not been uncommon for a warming event to occur at the 1-millibar level but not the lower levels of the stratosphere, resulting in the effects of that 1-millibar level not showing up in the troposphere. Regardless, it is useful as an early indicator of a weakening of the stratospheric polar vortex, which can then show up in weakening at other levels of the stratosphere. The black line shows the ECMWF forecast of the 1-millibar wind speed, and as the wind speeds strengthened with the rapid cooling of the stratosphere (indicating a strengthening of the stratospheric polar vortex, as is to be expected), they look to again weaken over the next couple of weeks, albeit still remaining well above levels that would imply a "weak" stratospheric polar vortex. If you're a 'glass half-full' person and enjoy cold weather, then this is a good sign. Personally, the weakening of the wind speeds from a projected 90 m/s to a projected 50 m/s at the end of the forecast period, while a notable change, isn't something to really pay attention to just yet. If the wind speeds continue to weaken, then it could be something to take note of, but for now the upper-stratospheric polar vortex looks set to remain in firm control.

A similar forecast is shown in the second panel for the 10-millibar zonal wind speed (blue line) and the 30-millibar zonal wind speed (red line), where a strengthening of the wind speeds is then followed by a weakening, though still to rather-elevated speeds. To make matters worse for those who are seeking some colder weather, both the geopotential flux and heat flux indicators in the third panel are set to significantly increase during the forecast period, usually a positive sign for those who want the polar vortex to weaken. However, if you've analyzed these graphs before, you'll note how the EP-flux on the bottom panel is pointing up and to the left, when looking at it two-dimensionally. Stratospheric warming events are most probable when the geopotential and heat fluxes are high (which is forecasted) and the EP-flux arrows are pointed directly up (which is not forecasted). As such, the stratospheric polar vortex looks set to keep the colder air locked up in the Arctic Circle through at least the second-to-last week of January, helping to severely hurt any chances of a sustained colder-than-normal pattern for the eastern two-thirds of the country through at least early February.

To Summarize:

- The recently-discussed stratospheric warming event has ended and has been followed by significant cooling.
- Atmospheric oscillations, the stratosphere, and medium-term weather model guidance agree on warmer than normal temperatures more likely than colder than normal temperatures in the eastern two-thirds of the country through at least mid-January.
- The stratosphere supports this likelihood of a warmer than normal weather pattern persisting into at least early February.
- A separate post on the potential for a strong storm system this week is forthcoming.


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