December 8-15 Thoughts

[donsutherland1]

As I continue to look at whether the NAO/PNA forecast (GFS ensembles) can add some value for the distant medium-range (generally 10-17 days out), I will continue to post my thoughts for that timeline. Needless to say, other factors are also considered.

The December 8-15 period looks to begin on a cool note in the East, but warm up over time, especially as one gets closer to mid-month. The west looks to feature generally moderate conditions with near normal to somewhat above normal readings. A possible invasion of much colder air into the Northern Plains near or just after mid-month can’t be ruled out.

At this point, there are no strong signals on the GFS ensemble forecast for either the NAO or PNA e.g., no Archambault event signals: the NAO is forecast to remain generally positive, while the PNA is expected to peak at positive levels and then slowly decline.

My highest confidence in terms of temperature anomalies, with support from the NCEP ensemble mean anomalies, typical patterns associated with the major teleconnection indices as described above, the objective European and GFS analogs, and the 11/25 run of the GSM is for the Northern Plains, Southern Plains, and Southeast.

My thinking for the average temperature anomalies during the 12/8-15 period is as follows:

Average Regional Temperature Anomalies (12/8-15):
Northeast: Somewhat above normal
Mid-Atlantic: Near normal to somewhat above normal
Southeast: Below normal
Great Lakes: Near normal to somewhat above normal
Northern Plains: Above to much above normal
Central Plains: Above normal
Southern Plains: Somewhat below normal to near normal
Pacific Northwest: Near normal to somewhat above normal
Southwest/Rest of West Coast: Near normal to somewhat above normal

For the major cities from Washington, DC to Boston, the 12/8-15 period is likely to feature:

- Nights generally in the 30s throughout the region with some 20s. Boston should see about an equal number of minimum temperatures in the 20s and 30s.
- Days mainly in the 40s but also 50s, especially later in the period in Washington, DC
- One or more days with a high temperature of 60° or above in Washington, DC
- One or more days with lows in the 20s throughout the region.

In the Southeast, Atlanta will likely see highs generally in the 50s with some 60s. Lows should generally be in the 30s and 40s, but at least one day with a sub-30° low is possible given the pattern.

Although one cannot rule out the idea of some snowfall, especially across the northern Mid-Atlantic and southern New England regions, the forthcoming pattern has not been conducive for a big snowstorm in the past. Nonetheless, one of the 10-14-day GFS analogs is December 10, 1960, so perhaps there is a long-shot prospect for such an event. A significant qpf event (0.50” to 1.50”) had occurred in the Southeast and East during periods when the teleconnection indices have been similar to those currently forecast, but most precipitation events were smaller. Atlanta saw frequent precipitation events with precipitation falling almost once every two days. Overall, I don't expect a big East Coast snowstorm (6" or more) during the December 8-15 timeframe.