Matt Makens

December is upon us and the first week looks anything but wintry for much of the
Front Range and Plains.

The snowpack certainly isn't looking very good at the moment but the season is
early and we shall see some progress in parts of the West this month. The
snowpack to close out November is shown here, lots of red and orange on that map
for well-below normal snowpack.

In order to recover, a negative-phased Arctic Oscillation can help give snow to
the Central and Northern Rockies. Although we may have moments in that phase,
the month overall will be near neutral to a positive-phased Arctic Oscillation.

A positive-phased PNA Oscillation can help spread moisture east of the Rockies
but brings less snow to the West versus a negative-phased PNA. Like the AO,
we'll see some variation, but data shows a bias toward neutral or more of a
positive phase this month.

How about any blocking to the east to help slow the storm flow? Well, a
negative-phased NAO could help with this but we are likely more neutral to
positively phased this month.

The MJO will shift through phases that will support moisture for the West early
in the month, combined with the next Atmospheric River event will be good for
some snowpack growth. However, the MJO likely shifts toward drier phases for us
from next week into the mid-month and perhaps beyond. 

So, we have a lot to overcome this month in terms of a favorably snowy/wet
weather pattern for the West and for Colorado. If you look at how the
oscillations combined together have treated us in the past, you have the
following for moisture frequency:

And temperatures:

If we look at the latest longe range guidance from the European weeklies we see
a bit more optimism for precipitation across our region than analogs might
suggest. This is particularly true the further south you are as the southern jet
tries to deliver. 

Temperature guidance meanwhile matches pretty well! Favored cooler than average
across Western North America with anomalous warmth east, near average for
December along the Front Range and adjacent plains: 

I think it will take a shift in the Polar Vortex structure to make much of a
change to the outlook for the month, but let's say that the PV is disrupted at
some point and we could see a shift in conditions to more of:

For Western Colorado through the mountains to the Divide, that doesn't offer a
lot of incentive for betting on a snowy month.

So, unless we get an unexpected change to the overall pattern, the snowpack
situation for Colorado and surroundings will remain struggling. The West Coast
ranges may see a boost due to the Atmospheric River events, however those are
most coastal in nature so here too may see basin data remain below normal
throughout the month.

For Denver and the Front Range, we have a cool start to the month with a little
bit of moisture - mostly in the mountains though - before a very warm week next
week, at least through Thursday or so. That is the result of a strong ridge that
will boost our temperatures ahead of the next trough out of the west next
weekend. 

Even with a cool down as that ridge weakens toward the 9th and 10th, we are only
cooling back down to near averages, at least in the ensemble means. Some
operational model guidance has had a bit more of an impactful storm for our area
by the 9th or so, but this idea is far from certain, and given historical
context may not be all that favored. Guidance has at times been dropping the Low 
well south of Colorado which would fit our analogs better than a more northerly
track (and more impactful one for Colorado) but importantly no scenario is off
the table. 

In fact, as one should expect with large cutoff lows 7+ days out each run has
been wildly different, with the same model pulling the track much further north
and more favorable for moisture across Northeast Colorado for the same time: 

Wobble, wobble. Nothing to get too attached to just yet, but it looks like we'll
certainly be tracking this one as we head through the weekend and into next week
as we already have been this last week
[https://www.weather5280.com/2023/11/29/slow-start-to-the-season-for-western-u-s-snowpack-will-that-change-in-december]
.

The odds for increased frequency of troughing may increase for our area later in
the month but until then ridging is the main driver at this time. Should it
indeed weaken and we get some troughing we will see colder air for the West and
Southeast U.S. with moisture orientated as shown in those "Low Probability"
cases above.

Weather5280 Insider: Weather outlook for December