Matt Makens

We've been teased before, but this time the support for a PNA flip grows and that'll mean parts of Colorado and the region may catch a bit of winter this winter.

The PNA oscillation may change phase soon, snowier for the mountains

We've been teased before, but this time the support for a PNA flip grows and
that'll mean parts of Colorado and the region may catch a bit of winter this
winter.

Thus far, PNA forecasts have been challenging beyond 7 days.

Shown above, forecast verification for 10 and 14-day forecasts of the PNA have
been poor. This led to multiple forecasts calling for colder/wetter changes that
did not always pan out. We've stayed quite cautious with forecasts to date
calling for a big change due to the PNA forecast weakness.

Now, however, there's growing support for a change. Both the Euro and GFS
ensembles call for a negative PNA with some confidence within each product.

This change will take over the second half of January, and be impactful to the
weather pattern across Colorado and the region. 

With that said, we remain somewhat cautious about calling for a big pattern
change for Denver. We believe this change will bring the greatest benefit of
precipitation to the mountain, with those on the leeside perhaps not favored as
much as we'd hope. Still, any time we see a shift in the PNA it gives good
reason to keep an eye on the forecast along the urban corridor, as it at least
opens a window for a storm to move through. 

Still, even with the PNA set to go negative, it's far from certain it'll equate
to much of an uptick in snow across Eastern Colorado. This is partly due to the
time of year, when it is not climatologically all that wet for Colorado east of
the mountains, and partly due to the northwesterly flow that is expected to
remain over the state leading to downslope issues for moisture on the leeside,
even as the western ridge breaks down. 

What we would like to see is the ridge over the western/southwestern United
States weaken more than what is being shown in the Euro ensemble modeling. If
this scenario pans out to be most accurate, leeside dryness will certainly be a
factor.

However, the GFS ensembles show a more pronounced -PNA signal in the middle
atmosphere. Troughing over the western US certainly would help the moisture
outlook east of the mountains for latter parts of January.

Let's turn to the precipitation forecasts through the end of the month. Despite
the differing 500H patterns, we do see a reasonable agreement between models. 

First the Euro: 

And the GEFS: 

That general agreement shows lower potential precipitation leeward of the
Rockies for Colorado and New Mexico. However, considering the time of year, even
a tenth or two of an inch is average for some.

As it stands now, the greatest benefit from the potential pattern change will be
for moisture in our mountains. Modeling suggests quite of bit of water, well
over an inch, for the latter parts of the month.

Considering all of Colorado's basins are drier than average, and average
snowpack continues to drop through mid-January, let's take what we can.

To briefly look at temperatures, which can be stubbornly warm in La Niña
winters, we see the greatest potential for cold remains to our north.

Let's wait and see if we can't sneak a stronger western trough to support wetter
and colder conditions... too hopeful, perhaps, based on data to date.

PNA may finally flip, what that means for the weather pattern across Colorado and the West for the second half of January