Brendan Heberton

An early season snow this September
[https://www.weather5280.com/2020/09/09/first-snow-of-the-season-in-the-books-for-denver-ties-second-earliest-on-record/] 
gave the illusion that winter may come early this year, much like it did in
2019. This was clearly not to be, with exceptional warmth and drought continuing
to be the dominant weather pattern across the greater region. 

This, of course, should come as no surprise if you've been following our
long-range outlooks over the last year or so. As we've tracked the development
of La Niña
[https://www.weather5280.com/2020/09/18/la-nina-status-and-an-updated-look-at-analog-years-for-the-upcoming-winter/] 
over recent months we've been rather consistent in our messaging of a drier and
warmer than average weather pattern dominating the region to date. 

The question is, where do we go from here? As is always the case with our
October "winter" outlooks, we will cover the period between now and April. It's
tricky to issue a winter outlook that's focused on the Dec - Feb timeframe as
our cold season is really much longer than that of the official winter months.
This is clearly a long period with plenty of variation expected at smaller
timescales. Here we will try to capture the overall themes for the coming
months, and deal with shorter long-range outlooks as the cold season progresses.
Even in a warm and dry six month outlook there can and will be cold and snow. 

While "high confidence" and "long-range forecasts" are not exactly synonymous,
you'll see throughout the article we do feel like we have a good idea of where
we think this winter is headed given our analog set, and with support in much of
the modeling this does perhaps give us as greater-than-average confidence in
this outlook than some other years. 

The setup
By a number of measurements, we head into the winter season with a much
different landscape than at this same time last year. This is certainly true
when looking at the progression of drought across the West from October, 2019 to
October, 2020. 

A year ago the Four Corners region and central Texas were experiencing
abnormally dry and drought conditions, but the extent of D3 and D4 level drought
classifications were at some of their lowest levels in years across the CONUS.
By comparison, at this time in October 2019 just 10.5% of the CONUS was
classified as experiencing Severe Drought or worse, today that number is up to
48.8%! For Colorado, 97.23% is experiencing Severe Drought or worse, up from
11.23% at this time last year. 

We also see some notable differences in the sea surface temperature patterns as
well. While the Northern Pacific continues to roast (no real change from last
year) we see clearly the development La Niña across the Pacific equatorial
region (outlined), and a warming of waters in the Indian Ocean. 

These two factors combined –– extensive drought conditions across the West
(drought begets drought!) and the formation of La Niña –– will be two of the
primary drivers of our weather over the coming months, as well as other
teleconnections we'll touch on below. 

Teleconnections
The Pacific Decadal Oscillation (PDO)
[https://www.ncdc.noaa.gov/teleconnections/pdo/] has averaged -0.98 since last
fall. During the same time, the AMO
[https://climatedataguide.ucar.edu/climate-data/atlantic-multi-decadal-oscillation-amo] 
has been quite positive. This in and of itself should help answer why we have
seen drought advance over recent months and why we continue to stress our
concern for drought through the 2020 - 2021 cold season. 

The graphic below shows drought frequency for various configurations of the PDO
and AMO, with 25% equaling normal drought frequency. Two takeaways here: hard to
avoid drought across Eastern Colorado(!) and the -PDO/+AMO configuration we are
in now shows a stout signal for increased drought frequency across the region: 

The outlook for the coming months doesn't show much change in the PDO/AMO, and
coupled with what precipitation patterns you would "typically" expect during a
La Niña winter – little reason to believe areas being hit hardest by drought
right now will have much of a chance at recovery in the short and mid-term. 

The analogs
When we look through our analog sets we see a clear patter emerge for the coming
months. A continued tendency toward warmer and dryer than average from I-70 (or
so) south, and perhaps as better storm track and cooler temperature anomalies
for points north. 

While there are many instances of years with La Niña, -PDO, and +AMO
configurations, a year where the Northern Pacific is as warm as it is this year
with a weak to moderate La Niña present just doesn't seem to exist, at least not
since 1950. 

A year that comes close is 2017. However, the Niña wasn't as strong, and the
warmth in the Northern Pacific was not as warm as this year. See the comparison
below: 

The unfortunate ramification of having both a La Niña present AND NPAC
temperatures so far above average could be the presence of an even more
persistent ridge across the West/Southwest this winter. 

This was true in 2017, with the mean ridge from Nov to April over the Southeast
United states, leading to a warmer and dryer than average period for the Four
Corners states and Western Texas, with chilly but dry conditions along the
Northeast edge of the dominant ridge across Kansas and Nebraska. 

So, even in the absence of any perfect analogs, here's our list and the outcomes
for Nov - Apr of those years: 

> Temperature analogs Nov - April
> Precipitation analogs Nov - April
For our region the most impactful element of these analogs is that of the
continued drier than average conditions persisting across the Southwest,
Southern Plains, and East-Central Colorado into Kansas and Nebraska. This, in
combination with the wetter signal showing up for the Pacific Northwest also
happens to line up with AMO/PDO teleconnections graphic above, and fits pretty
well with what you'd expect from a "normal" La Niña cold season. 

The models
When we look at what the models are showing for the next several months we see
1) good agreement between the various models, and 2) pretty good agreement with
the analogs we've shown above. 

The largest take aways: a wetter Pacific Northwest, some potential for the
Northern Plains, and again wet across the greater Ohio River Valley. Dry
anomalies are present across the southern tier in all the climate models, with
Colorado being split between near average/slightly above average cross the
northern half of the state, and below average for Southern Colorado. 

In each of the breakdowns below the left image is Nov/Dec/Jan, and the right
image is Jan/Feb/Mar for precipitation anomalies. 

> European model
> CFS model
> NMME model
As for temperatures, all models agree in above average temperatures for most of
us. Areas with the greatest potential for below average temperatures would
likely be across the northern tier and Pacific Northwest. 

Our Nov - April outlook
Based on the above, our winter (or cold season) outlook won't come as much of a
surprise. Here is what we are thinking for the Nov - Apr time period: 

> Temperature outlook
> Precipitation outlook
Denver snowfall?
The takeaway you should have so far is that chance for an above average winter
for snowfall across the Denver area looks like a long shot from where we sit
today. 

This should be couched a little bit, however, as La Niña winters can deliver for
Northeast Colorado even if the likelihood is lowered. You'll recall from our in 
depth look at El Niño and La Niño a few years back
[https://www.weather5280.com/blog/2015/08/13/a-closer-look-at-what-el-nino-means-for-rain-and-snow-in-colorado/] 
that La Niña events (while general promoting dry conditions across the state)
can deliver from time to time, and the core winter months are the most likely
time for that to occur should it happen. 

So, while the season the the whole may end up below average along the Front
Range, IF we are to see a period of snowier weather, perhaps it'll come during
the heart of the winter. If we look at climate region 4 (containing the northern
urban corridor) we see that the majority of years with La Niña events had above
average precipitation from December to February, which has been especially true
the stronger the event: 

If we look at a breakdown of all our analog years we were looking at for this
winter (the list is longer than that used above) we see that really no matter
how we slice it Denver's most likely to see below average snowfall this season.
Of the 26 analog years we looked at, here is a look at the various seasonal
snowfall breakdowns. Denver's average seasonal snowfall is 56.5 inches. (These
averages have been weighted in favor of our top analogs in the list of 35). 

 * Average all analog years: 46.26" 
 * Average La Niña years only: 43.17" 
 * Average of moderate La Niña years in list: 52.20" 
 * Average La Niña years with -PDO: 38.82" 
 * Average La Niña years with -PDO & +AMO: 33.75" 
 * Avg. La Niña years w/ -PDO & +AMO (no 1950s, no extreme PDO): 30.58" 
 * Average: 40.25" 

As you can see, a pretty grim look for those hoping for a big snow winter. The
moderate La Niña winters served the northern Front Range best (which we could
see this winter), but the -PDO/+AMO configuration really hurts those odds. The
average of averages is a season some 16" below the long-term average.

Our snowfall probabilities are the following (for DIA): 

 * Less than 35 inches: 15% 
 * 35 - 50 inches: 55%
 * 50 - 60 inches: 20% 
 * Greater than 60 inches: 10%

Considerations and conclusions
As is always the case we will continue to track all these considerations as we
head into November and the winter. Should we see any large scale changes to the
oceans or other atmospheric drivers we will adjust this outlook accordingly. 

Given the good agreement in modeling, and the lack of an active Pacific tropical
storm season which can act to upend SST configurations, we don't see much change
to this outlook at least in the near term. November to April is a LONG period,
so of course things can change and we will adjust accordingly. 

Perhaps even if the overall period is is a dry one, we can sneak out a more
active/stormy period as we head toward December, at least for folks along and
north of the I-70 line. Time will tell.

Winter outlook 2020 - 2021