December 26-29 Potentially Significant Winter Storm

Model guidance is beginning to pick up on the idea of a strong winter storm impacting the Plains.

WxCaster

The above image shows today's 06z GFS snowfall forecast from the time of posting to midnight on December 29th, 2015. We can see the large swath of snowfall extending from northern Texas through Kansas, Nebraska, and into Minnesota. The heaviest-hit region in the deep south appears to be extreme northern Texas, with amounts in the 12" to 24" range, particularly along the border with Oklahoma, as well as in a spot near New Mexico.
Portions of the Oklahoma Panhandle, as well as southwest Kansas and eastern Colorado, may be in line for up to 36" of snow, according to this forecast model. These are very extreme amounts, and although they have been showing up multiple times on previous forecast runs, extreme caution should be used with these maps, especially given the timeframe.
Amounts in excess of 24" are found in spotty areas to the north and east, particularly eastern Nebraska into southeast South Dakota, with lesser (but still significant) amounts in Minnesota.

WxCaster

The above image shows the 12z GFS interpretation of the coming storm, with total snowfall from posting time to 6 AM December 29th shown. Snowfall amounts are still incredibly high in northern Texas, the Oklahoma Panhandle, and much of Kansas into southeast Nebraska, but trail off from there. Amounts in the 12-16" range can still be found in Iowa and Minnesota, but these amounts are substantially lower than those in the 06z forecast. This variability, both in amounts and track (as you'll notice the heavy snow axis has shifted east) expresses the extreme uncertainty still associated with this system.

Tropical Tidbits

Lastly, we'll analyze today's 12z Canadian model, the GEM model. Amounts on the order of 20-24" are found in north and west Texas into the Oklahoma Panhandle, even in south-central Kansas, but overall snowfall totals are drastically less than the two GFS runs we analyzed earlier. This is a red flag that may indicate one model is either over-doing snowfall (GFS) or one model is under-doing snow (GEM). There's also a plausible chance that they're both incorrect- the point is, there's a lot of uncertainty here.

But just where is this system now?

Tropical Tidbits

Our system is currently traversing the Aleutian Islands, and will drop into the west coast of North America to produce a very strong trough, as per latest model guidance. It remains to be seen if this is model error, as medium and long-range guidance has a tendency to over-amplify storm systems, but for now, it's merely a system to monitor.

To Summarize:

- There is the threat of a winter storm in the Plains over the December 26-29 period.
- Model guidance is expressing high snow totals in the South Plains, but high uncertainty indicates that these models should not be taken at face value right now.

Andrew 

Upper Stratospheric Polar Vortex Warming in Short Term

(Note: This post discusses the stratospheric polar vortex in the short term (out to Day 10). The other stratospheric post discusses expectations next month.)

The upper stratospheric polar vortex is forecasted to experience some intensive warming, in the atmosphere's attempt to get this winter started.

FU-Berlin

The image above shows a graph of the forecasted Wave-1 temperature outlook from the ECMWF model, 10 days from today. While daunting at first glance, it can be explained.
The different levels of the atmosphere are listed on the left-hand legend. We can see the graph covers the atmosphere from the 1000-millibar level up to the 1-millibar level. Note the warmest colors centered right around the 3-millibar line. The bottom legend shows lines of longitude.

Let's depart this forecast for a moment and discuss different modes of stratospheric vortex disruption. There are two primary modes of disruption: Wave-1 and Wave-2. In a Wave-1 stratospheric polar vortex disruption event, a single ridge / body of warm temperatures forms aloft and singlehandedly tries to displace the polar vortex. This singular body of warmth / ridging is why it is called a Wave-1 event- Wave-1 events are also the strongest type of disruption event. Similarly, Wave-2 events involve two bodies of ridging / warmth trying to squeeze into the Arctic Circle, usually to split the polar vortex into two vortexes. This is weaker than a Wave-1 event, but still wields significant power in the atmosphere.

This temperature chart above shows a temperature spike at the 3-millibar level. Remembering this chart specifically identifies Wave-1 events, we can then deduce that the ECMWF model is expecting a Wave-1 disruption attempt at the 3-millibar level of the atmosphere. We can look at this on a 3-millibar forecast map below, valid for the same timeframe as the graph above:

FU-Berlin

Note how we see a single body of warm temperatures on the right-hand side of the hemisphere (if you look closely, the warmth is centered over Eurasia), visually showing that Wave-1 pattern I discussed earlier.

Why is this important? It means that the atmosphere is applying pressure to the stratosphere to try and make the pattern more conducive for wintry weather here in the troposphere, something that's been lacking this December. Now, vortex disruption at the 3-millibar level won't do much down here at the surface, but if it can expand to lower levels of the stratosphere (ideally 30, 50, and/or 100-millibar levels), the influence becomes greater on us here at the surface.

To summarize:

- The upper portion of the stratospheric polar vortex looks to experience warming in the next 10 days and beyond.
- While insignificant at that height of the stratosphere (3-millibar level), it could indicate a pattern more conducive for wintry weather may set up down the road (i.e. into next month).

Andrew

Early Signals For Pattern Change in Early January

There are emerging signals for a change to a colder and snowier pattern in the early days of January 2016.

PSU

The image above shows the GFS Ensemble mean 500mb height anomaly forecast for hour 384- the infamous fantasy land, end of the run panel. The ensembles are expecting ridging to build up along the west coast of North America, with troughing taking over in the Bering Sea and Arctic Circle. This kind of pattern would likely promote an overall-cooler set-up than the one we're currently in, which will feature well above normal temperatures on Christmas Day.

You all know I wouldn't post a 384-hour graphic without good reason, so let's dive in.

CPC

The ensemble forecasts out of the Climate Prediction Center for the Pacific North American (PNA) index show a dip to negative values as we head to Christmas, but then rising to neutral, and even positive values to kick off 2016. The majority of members are in positive territory by the start of the new year. The relative consensus of most ensemble members on a neutral or positive value of the PNA is encouraging for winter weather fans. Positive PNA values typically result in colder and snowier weather for the Central and East US, while negative PNA values result in warmer weather.
Some of you may ask why it's been so warm even though the PNA was positive in December- that can be attributed to the El Nino, producing enhanced convection in a part of the Equatorial Pacific conducive for warm weather here in the U.S., however that's a topic for another post.

ESRL

A grand overview of four key teleconnections reveals this improving pattern (improving, at least in the eyes of winter weather fans). We see a continued positive EPO through the forecast timeframe- if you're more knowledgeable, you'll recall that the positive phase of the East Pacific Oscillation (EPO) encourages colder weather in the eastern 2/3rds of the country, while the negative phase encourages warmth. The WPO (West Pacific Oscillation) closely follows those guidelines as well.
As noted earlier, we see the forecasted PNA rising to at least neutral values in the long range near the start of 2016.

If all of these forecasted indices and teleconnections verify on an as-is basis (which, I may add, is rather unlikely), we could expect a cooler pattern in the Central and East US. This would be mitigated by the positive phases of the Arctic Oscillation (shown in the first image by negative height anomalies across the Arctic Circle) and North Atlantic Oscillation (shown in the first image by weak troughing over Greenland), both of which tend to discourage persistence of colder weather.

To summarize:

- There are indications that a pattern change may be on the way for the start of 2016.
- Model guidance will change drastically, and this is nowhere near set in stone. However, if this does verify to a certain degree, a cooler pattern may be on the horizon.

Andrew

Stratospheric Polar Vortex Expected to Weaken in January

The stratospheric polar vortex is expected to exhibit weakening in the near future.

Garfinkel/Hartmann Publication
(Image obtained from Eric Webb)

The above image, from the Garfinkel / Hartmann publication, 'Tropospheric Precursors of Anomalous Northern Hemisphere Stratospheric Polar Vortices', shows precursors in the tropospheric mid-levels to weakening of the stratospheric polar vortex. In layman's terms, the images above (particularly the left panel) give us a glimpse at what the weather pattern should look like a short period of time before the stratospheric polar vortex weakens.

In that left panel, showing geopotential height anomalies at the 500-millibar level, we see strong negative anomalies (troughs / storm systems) in the Bering Sea extending into the Arctic Circle, even a bit into Greenland, and positive anomalies (ridges / high pressure) over Canada, as well as in the north-central Pacific. We also see ridging over Europe. These are indicators that the stratospheric polar vortex could be weakening down the road.

Let's compare these indicators to the long-range forecast from the GFS Ensembles.

Tropical Tidbits

Attached is the 500-millibar height anomaly prognostication for the 11-15 day forecast period (Dec. 30 to Jan. 4). Here, we see negative anomalies across the Bering Sea into Siberia, all the way across the Arctic Circle into Greenland. We also see weak ridging in Canada and the East US, and ridging in the north-central Pacific. Most notable is a strong ridge over Europe.

If you compare this forecast graphic and that left panel earlier in this post, you'll find that they're incredibly similar, almost identical. In sum, long range model guidance is telling us that we should see the stratospheric polar vortex weaken in January. This is good news for snow and cold fans who have been suffering through this first month of 'winter' (more like an extended fall with these warm temperatures)- even though a weakened polar vortex does not necessarily mean cold and snow, it does raise the chances of more cold air intrusions and snow events for North America.

To summarize:

- Long range model guidance is indicating the stratospheric polar vortex will weaken in January.
- Consequentially, the chances of cold and snow may be on the rise for next month.

Andrew 

Upper Stratosphere Experiencing Minor Warming

The upper stratosphere appears to be undergoing some minor warming, although it is unlikely to set up a polar vortex disruption event.

CPC

The above animation shows temperature anomalies at the 10-hPa level of the atmosphere, in the upper levels of the stratosphere, over the last month. Beginning around November 20th, we started to see increasing temperatures just east of Japan, which really amplified in strength around November 27th. They aren't really moving in a particular direction, although a slight northward push has been noted over the past couple weeks.

CPC

This warming is likely thanks in part to a substantial increase in north poleward eddy heat flux values since about the last two weeks of November, right at the time when we started to see that warming commence in the 10-hPa layer. Typically, increased eddy heat flux values reflect an increased movement of warm air from the lower latitudes to the poles. Higher values can precede or coincide with stratospheric warming events, which is likely why we're seeing warming at the 10-hPa and 30-hPa levels.

CPC

The 30-hPa level, just a bit higher than the middle of the stratosphere, is showing some warming, albeit deflected a bit further east than the 10-hPa warming. In addition, this 30-hPa warming is showing an eastward push, rather than a poleward push. The latter would be more favorable for a polar vortex disruption event / stratospheric warming, so winter weather fans may not be too encouraged to see this.

To summarize:

- Minor warming is being observed at the 10-hPa and 30-hPa levels of the stratosphere.
- This warming is not strengthening quickly and is not making a strong poleward movement, so a stratospheric warming event is not expected over the next several days.

Andrew