Brendan Heberton

In the early part of September we released our initial long range outlook
[https://www.weather5280.com/2021/09/13/the-winter-outlook-for-fall-2021-through-spring-2022] 
for the cold season which is now upon us. As promised, today we revisit that
post a bit, will see what's changed and what's stayed the same, and issue our 
annual winter (season) outlook [https://www.weather5280.com/tag/winter-outlook] 
for Denver, Colorado, and the region. 

In most respects, our thoughts from a month ago have changed little. La Niña
looks like it will be a driving factor of our weather over the coming months,
and without the help of an over amplified MJO as we had last year, it'll leave a
lot of folks drier than average from now through March, but could bring areas of
drought relief too, particularly for latitudes further north across the CONUS. 

The setup
The evolution of La Niña as we head deeper into the fall and heart of winter
will be key in determining a major driver in our weather pattern over the coming
months. Recently, we've gone from neutral conditions to full-blown La Niña
conditions, with subsurface cold water anomalies really taking over, as you can
see in the sea surface temperature cross section image below of the equatorial
ENSO region: 

The most recent IRI Model Ensemble forecast for October is as follows: 

> In mid-September, SSTs in the east-central Pacific are -0.4 degree C different
from average. The evolution of key atmospheric variables is consistent with
ENSO-neutral conditions. However, a La Niña Watch remains in effect for Sep
2021. A large majority of the models predict SSTs to cool further through boreal
autumn and winter, and then return to ENSO-neutral levels during late spring
months. Similar to, the new official CPC/IRI outlook issued earlier this month,
this objective outlook calls for La Niña to emerge during Sep-Nov and persist
through winter and early spring, with return to ENSO-neutral in late spring and
early summer of 2022.
Despite a snowy end to last season, and at times active storms across the state
this summer, much of the state remains or is now back under a drought
classification from the Drought Monitor, including Denver and Northeast Colorado
which for a few short months this summer were drought-free. The latest Monitor
places the urban corridor under Moderate Drought: 

Nationally we see drought conditions have expanded notably from a year ago
across the west, Severe and Exceptional drought has expanded across the northern
High Plains, and we've seen some improvement for the Southern Plains where the
storm track has been relatively active since last winter.

A look at current SST anomalies shows La Niña conditions alive and well as we
head into the winter months, denoted by the area of blue (cooler than average
sea surface temperatures) in the checked box. The core of the cold very much
west-based rather than pooled against west coast of South America, which could
give it at least a slightly different flavor than a 'typical' La Niña event, and
differs a bit from what we saw last year (see below). 

If we look at surface temperature anomalies we see similarities in the ENSO
region with the presence of the La Niña, but some differences globally as well –
particularly along the west cost of the United States and in the Gulf of Alaska,
where we see sea surface temperatures that are much cooler than average vs the
very warm water in place last year:

Teleconnections
We share this graphic regularly, but it reiterates the idea that a -PDO (
Pacific
Decadal Oscillation [https://www.ncdc.noaa.gov/teleconnections/pdo/]) and +AMO (
Atlantic Multi-Decadal Oscillation
[https://climatedataguide.ucar.edu/climate-data/atlantic-multi-decadal-oscillation-amo]
) teleconnection configuration is ripe for drought across the Southwest,
Southern Plains, and Colorado. For Eastern Colorado, drought frequency is quite
high for almost any configuration, with a +PDO/-AMO being the exception.

Currently, the PDO is -1.94 and at 0.411, the AMO remains positive – as it has
for most of the last two decades. This configuration is similar in many respects
to last year, although this year the PDO is much cooler given the addition of
the anomalous cold in the northern Pacific. 

AMO Index: 

PDO Index: 

The analogs
We are working with numerous analog years as we kick off the upcoming 6 months.
Our top three vary a bit week-to-week, but are currently 2013, 2008, and 2005.
The year 2017 has also been on the list from time to time, but recent cooling in
the northern Pacific has taking it further down the list as it was quite warm in
the GOA and off the west coast. 

Here we compare SSTs for mid October of 2017, 2013, 2008, and 2005, with a
reference to the same SST anomaly map we shared above for 2021 below. Note the
cool pool present this year in the GOA which is shared in 2013, 2008, 2005 but
not 2017: 

If we take those top three years and look at the temperature pattern in place
across the CONUS for the first half of October in those years, we see a lot of
similarities to this year – cool to the west, warmth focused north and east.
Clearly 2021 is the warmest of the bunch: 

Using the above analogs, weighted in combination with our entire list, the
following November through March periods shake out with the following: 

Nov - Mar temperature departure from average: 

Nov - Mar precipitation departure from average: 

Chilly, but not particularly wet! 

And oh, by the way, the above years had this for temperatures in November...
Quite the opposite of what we've seen so far this October, and quite possibly
where we are headed to end the month and start November as the PNA is forecast
to jump positive, with ridging establishing itself in the west after this wet
period
[https://www.washingtonpost.com/weather/2021/10/22/california-atmospheric-river-bomb-cyclone/]
. Unlike this analog package, we'll see if this pattern holds in November of
this year. Current numerical weather modeling suggests the PNA may dip negative
by mid November, and again promote a cool west/warm east for the latter
two-thirds of the month. 

The models
There's not much in the long range modeling to contradict our thinking for the
upcoming months. It's hard to say if that's a good thing or bad thing! 

The European ensemble seasonal model shows a lot of near average precipitation
across the CONUS for the periods of NOV/DEC/JAN (left) and JAN/FEB/MAR (right)
which isn't particularly helpful. Still, a Niña look, with a wet Pacific
Northwest and drier southern tier: 

The CFS is similar, but generally more bullish in areas that have a chance at
ending up wet, and pulls the drier than average line further south into southern
Colorado. Again, no real surprises here: 

And finally the NMME for the same two periods (NDJ and JFM): 

As for temperatures... well, models won't see any cold at range just in case
there ever is any. Most show a warmer than average period ahead for the CONUS,
with the core of the winter months running warmer than average for basically
everyone. 

It's likely we'll see some cold, even in today's world, and we think this
portion of the outlook aligns better with the analogs than the models.
Nevertheless, here's your winter (Dec-Feb) temperature outlook from the Euro!
With the cold locked up in Western Canada, it's easy to see how some of that
will push south from time to time down the plains and east: 

Weather5280 November through March outlook
Given all of the above, our outlook is likely already given away. We see some
areas come out of the next six months with quite a bit of cold under their
belts... and the best of the storm track being further north vs south as last
year. 

For Colorado, the northwest mountains will stand the best chance at a big snow
year, with probabilities dropping the further south and east you travel. 

Denver snowfall?
For Mile High snow lovers, the above analog set we shared wasn't exactly the one
you wanted to see. There are some grim snow years in the list... with 2017
taking the cake, delivering just 25.7" of snow to the city, which is 30.8" below
the longterm average! Here's to hoping 2017 stays (very) low on the list! Of
course, in contrast, 2020 is somewhere on the long list, and we know that it
certainly bucked the idea of a low-snow winter around here, so never say never. 

As for the top 3 analogs? 

 * 2013: 38.4" (-18.1" below average)
 * 2008: 38.1" (-18.4") 
 * 2005: 30.4" (-26.1") 

If we take the average of our analog years (10 in total) and compare with larger
lists of similar years with comparable ENSO and teleconnections we get the
following: 

 * Average all analog years (10 in total):  38.72"
 * Average La Niña years with -PDO & +AMO: 33.75"
 * Avg. La Niña years w/ -PDO & +AMO (no 1950s, no extreme PDO): 30.58"
 * Average: 34.43"

The average snowfall in Denver from September through May is 56.5". As you can
see, the above is a pretty grim look for those hoping for a big snow winter, and
this includes the BIG snow year of 2020 in that analog list! 

Our snowfall probabilities are the following (for DIA):

 * Less than 30 inches: 15%
 * 30 - 45 inches: 60%
 * 45 - 60 inches: 20%
 * Greater than 60 inches: 5%

Considerations and conclusions
As always, a lot of factors to consider as we head into the winter months! It'll
be interesting to see if this year's Niña event behaves more like a
"traditional" La Niña or if we end up with some anomalous southern jet as we did
at times last year. We think it'll be the former, with a more traditional
pattern in place across the CONUS this cool season. 

How far south across the west coast will we see the moisture remains to be seen.
Northern California is taking it on the chin this weekend with heavy rain across
the drought-stricken Bay Area, Cascades, and Sierra Nevada, but is this telling
of what's to come? Or will the long-term pattern ultimately keep the jet further
north this winter? 

For the plains we see the potential for real cold at times, but even as storm
frequency in a La Niña year can be greater than average, they often aren't in
ample supply of moisture east of the mountains – providing more wind and cold
than anything else. 

There are exceptions to every rule, and each year is of course unique,
especially in a changing climate. We'll keep tracking all the latest here on
Weather5280, and of course update our extended outlook as needed for our
Insiders as we head into the winter months!

Winter outlook 2021 - 2022