November, 2012

Maybe SOMEthing to Hang a Winter Outlook Hat On, Finally.

November 23rd, 2012 at 3:27 pm by under 4 Warn Weather

Thanks to Tom Niziol, who forwarded to me an interview with winter outlook researcher and forecaster, Judah Cohen. The interview was conducted by a private sector meteorologist, who works for a group called The Capital Weather Gang, in Washington. They are regular contributors to the Washington Post. Surprisingly, Dr. Cohen has partially “spilled the beans” in a public way early in the season. While part of the interview’s focus centers on D.C., much of what he discusses is relevant to the NE as well.

A couple of important notes before you hop on the link, please. Cohen and his group have developed a new index on how to look at October Siberian snowfall; the Snowfall Advance Index, or SAI. He now correlates a faster rate of snow accumulation  with a -AO more than with just a monthly anomaly. It is clear that this professional (me) and some of you weather hobbyists have not been interpreting the Rutgers Global Snow Lab data (which he mentions) the way Cohen does. As for this year, despite what we saw on the Rutgers site, the SAI is favorable for a colder than average winter in the east and NE, and a more -AO/NAO. I think many of my fellow fans of wintry weather will be greatly encouraged by much of what he says here.

Another note: Cohen does not generally go into detailed precipitation outlooks. I would remind you all again that the “Snowmaggedon” winter of 2009-2010 left Buffalo with only 74 inches of snow. That record negative AO brought us cold, but not that much snow. So in MY opinion, we still can’t extrapolate much on snowfall from this hopeful outlook, except to say that both statistical odds and expected pattern dominance would favor more snow than last winter. The statistical side of that is an easy call. The pattern dominance is a predictive call.

And 2 puzzles: Cohen and a few others seem to enjoy treating the NAO and AO as if they were virtually identical, and virtually a single index. I can’t say I agree with that. There are parallels, but I have seen periods in which one was highly negative and the other wasn’t. Clearly, the majority of forecasters still go with 2 indices. Cohen may be favoring his approach to simplify matters for clients and the public…I don’t know.

The other puzzle is why he never mentions the MJO, nor does The Capital Weather Gang. The MJO is recognized by CPC as playing a role in temperature anomalies during the winter season. There is a highly suggestive body of evidence that its anomalous behavior last year was a very significant forcing mechanism in “defeating” la nina climatology over much of the northern latitudes of the U.S. And, when I asked Dr. Kerry Emanuel of MIT about this in August (one of the most accomplished meteorological scholars we have), he agreed it was a good hypothesis.

Now that I’ve said my piece, here’s the interview:

Please use the link in my first comment for ease.

STILL Not Much to Hang a Hat On for the Winter

November 15th, 2012 at 8:36 pm by under 4 Warn Weather

The Climate Prediction Ctr has updated its Winter Outlook and its Monthly (December) Outlook today. If you were looking to the CPC–or me–or anyone else to reduce the uncertainty about a winter outlook, you’ll be disappointed. CPC is maintaining its EC/Equal Chances status for much of the east. They’re reasoning is partially tied to the lack of predictability of the phase of the AO/Arctic Oscillation or the NAO/North Atlantic Oscillation beyond a couple of weeks. The MJO/Madden-Julian Oscillation is briefly mentioned but, not much was said about its current active phase due to that lack of predictability. On the matter of time scales CPC put a little more emphasis on the role of the PDO/Pacific Decadal Oscillation, which is in a Cool/Negative phase. Before anyone gets excited about what that might mean, PDO phases tend to run on the order of 20-40 years, as opposed to the typical 30-60 days for the MJO. They seemed to imply that the negative PDO, which weakened in October but is expected to strengthen again by summer next year, may have been enough to kill off what would have only been a weak el nino and keep ENSO neutral.

I might have expected them to give more weight to the current active phase of the MJO, which favors above average temperatures in the central and eastern U.S. Since this active phase has run only 2-4 weeks, that would still leave it with at least several weeks more time in this phase, which could weight the start of winter with above average temps in the central & ern U.S. In my mind, that might have tilted the odds toward a positive temperature anomaly for December, with the emphasis on the first couple of weeks.

Now that I’ve said all that, I do not have the tools and experience of the long range specialists of CPC, and I have very little experience balancing the PDO with shorter term oscillations. That is not false humility. That is a statement of fact. If the MJO returns to the normal longevity for its phases, we could well see a notable flip in the overall pattern  later next month. The length of active phase time the MJO took on last winter was highly anomalous, and didn’t follow the rules. Could that happen again? No one knows. If it does not happen again, we may see a return to more variability in 4-6 weeks.