Updated Dec Outlook:
Temperatures:
- We have warmed our December forecast a good bit vs. our initial idea for several reasons, but we first want to note that the evolution of the Polar Vortex and high-latitude blocking is fluid and needs to be watched carefully.
- Our top analogs right now are 2017, 2011, 1999 and 1998. These years were generally warm for December, but 2017 and 2011 did have a few colder blasts. The evolution of the MJO into phases 3/4/5 the first half of December should allow the month to start off very warm and we feel like this period will likely win out regardless of what happens later in the month.
- The strong La Niña base state favors a strong Pacific jet and a flood of warm air and our analogs generally support a lack of blocking (perhaps some intermittent –NAO which keeps the NE cooler).
- There is the risk that especially the Western half of the US could trend cooler if the Polar Vortex continues to rotate down into W. NA in Early December. The other risk we need to watch is if the MJO were to move into 6/7/8 late in the month, we could have some colder blasts as these phases favor more high-latitude blocking. Confidence is low on this potential but it is worth watching.
Precipitation:
- Precipitation is a little less tricky as the big driver will be the strong North Pacific jet which should flood moisture into the northern half of the country.
- With the tendency for an Eastern/Central US ridge at times, we think that areas in the SE US and South Central US can remain drier than normal.
- At times there can be more notable dry stretches between systems related to a Central US ridge.
- The risk here may be those drier stretches, but given the amount of moisture likely pumped into the northern tier of the US, we think the Northern Plains and Upper-Midwest probably average out above normal.
- This will lend to some snow chances north with a a few decent chances of winter weather possible in the N. Plains and Rockies. However, wintry risks likely remain below normal for the Midwest and Ohio Valley especially the first half of the month. If we can get the MJO to move late in the month it may present more chances for these areas.