Matt Makens

Looking at the first half of December there are some storm systems, including
today's – however, the next two weeks appear drier than average unless a system
we're watching for late next week wants to overproduce. 

Generally, the what the CFS shows for precipitation over the next two weeks is
hard to argue with. The top image below are the week 1 precipitation anomalies
forecast, the lower image is week 2. 

The CFS week one forecast makes sense. Data doesn't support much total
precipitation from today and tomorrow's system, and this is the only one we have
for the first week. 

The CFS week two forecast will rely on a trough to emerge after the 11th or so
to deliver some water and that will serve as the second system for the first
half of December.

The indication now is that second trough has a lot of work to do, but you can
see the CFS trying to pull some positive precipitation anomalies into northern
Colorado. 

Coming out of the MJO phases 1-3, and just lingering around 4, is a less
favorable phase for precipitation across the region. The mid-month system will
have to have MJO support in phase 5 or revert back to 3 if this signal is to
show much precipitation potential.

For two, the PNA does begin to trend lower during the second week of December,
but the index remains positive through the end of the forecast run. This
indicates to me only a slight increase in trough development potential. For lots
of water we'd want a sharply negative change. The ensemble which takes us
through the 16th does take the PNA close to neutral by the 11th or so – but
without a significant dip, will that be enough? 

For three and four, there's additional model data to support the CFS
projections. 

The first week between GFS and Euro:

The second week:

The first system, moving through now, certain isn't going to start the month off
on a wet note.

The second system may perform a bit better, we shall see, but the overall
atmospheric pattern – at least on basic inspection – is not favorable for a
significant Western US trough in the data thus far.

We'll keep you posted on any changes to projected MJO or PNA phases, and see of
those become more favorable for troughing as the month progresses.

The first half of December doesn't support abundant moisture across Colorado and the region