Winter 2010-2011

[Mallow]

Here's my winter forecast. The scales are left unlabeled on purpose. Obviously greens, blues, and purples are colder (wetter), and yellows, oranges, and reds are warmer (drier).

No big surprises. Based on ENSO, summer PDO, expected NAO, NPI, and the summer GoA blocking ridge. ENSO analogs are 1949 and 1998. Other analogs are 1948, 1951, 1952, 1980.

============================================================

This one seemed pretty straightforward to me. There are a few big question marks, of course, but less than normal for sure.

Unlike some Nina years, this one developed quick enough that Nina impacts should already be significant by December. Therefore, the southeast ridge will be baring its teeth. However, we should be in a solidly -NAO by December, so New England could do well that month with northern-stream systems. An Ohio Valley/Appalachian storm track, with relatively weak, warm storms will keep areas further south and east from seeing much, though an ice storm can't be ruled out in favorable CAD areas.

The summer -PDO and associated strong GoA blocking ridge is a strong signal for a cold west/warm east pattern, for whatever reason. 1998-1999 would be a great example, except the trough was located too far west, and the EPO was too positive. I expect this positive EPO will be in place for December, but not for January and February. This is a huge question mark, though. If I'm right, December will have a pretty zonal feel to it across the US, with cold anomalies north and warm south. I could see this being a month where Arctic intrusions in the Pac NW make it down to just south of the Canadian border, but no further south. Most of the cold will be bottled up in Western Canada and Alaska.

In January, the -NAO begins to weaken some, and the EPO flips to slightly negative. The southeast ridge begins to build northward somewhat, though New England could still do okay with clippers. The cold begins to infiltrate the west, especially later in the month, as the blocking ridge begins to build in offshore. Heaviest precipitation will be on the southern end of the upper-level troughs, in California.

February is a continuation of the trends of January, with a near-neutral or weakly negative NAO, and a -EPO. The southeast ridge builds in further north and west, and we have a classic cold-west, warm-east pattern. The coldest anomalies will be in California and southern Oregon, with most of the precipitation in central and southern California. The Pac NW will be cold, but somewhat dry this month.

The two biggest assumptions are the winter NAO and EPO. I'm basing my thoughts on those on my analog years, but there's a lot of variability in there, so this is where my greatest uncertainty is.

EDIT: In light of recent discussions, I'd like to correct a misconception in this post. I expect the EPO to average positive either way, but earlier in the winter I think we have to deal with a more "classically positive" EPO. Later on, it could be + or slightly -, but I think there will be significant blocking off the west coast by then.

============================================================

Not that anyone here cares, but since I made a "probability-forecast" deal for Portland and Seattle, I might as well post it here...

SEA (forecast, 12")...

Less than 1": 5%

Less than 3": 10%

Less than 5": 20%

Less than 8": 30%

Less than 12": 50%

Less than 18": 70%

Less than 24": 85%

PDX (forecast, 11")...

Less than 1": 5%

Less than 3": 10%

Less than 5": 20%

Less than 8": 35%

Less than 12": 55%

Less than 18": 75%

Less than 24": 90%

============================================================

FWIW, based on the Sept-Oct pattern (among other things), my latest 'complete' list of analog years would be as follows:

1948

1950

1955*

1962

1964*

1973

1977

1978

1979

1980

1988

1995

1998*

2003

* Most heavily favored.

... the new list definitely doesn't mesh with my winter forecast, especially for January. If I were to remake the maps today, I'd make January generally warmer nationwide, perhaps cooler in the south and southeast (though still near or above normal), and shift the wet anomalies in California up to Oregon and Washington.

[rainstorm]

great work. i hope january is drier though. nothing worse than a rainy winter.