October Climate Data

Looking back at October, there was a lot of active weather in Alaska, especially in the second half of the month, and so it's not surprising that temperatures were highly variable.  However, the statewide average temperature (as calculated by UAF) didn't see any extremes of either sign; neither the warm nor the cold anomalies were all that unusual - see the right-hand portion of the figure below.

The "near-normal" characterization of the monthly average temperature extended to most parts of the state, with the exception of somewhat significant cool in the north-central interior and the northern Panhandle, and warmth on the North Slope.  The southern Yukon Territory was pretty chilly, however.

Warmth on the North Slope is a given these days in October, because of the lack of sea ice.  Every October since 2002 has been significantly warmer than the earlier multi-decadal normal, as illustrated so clearly in Rick Thoman's signature chart for Utqiaġvik - check out his latest post:

https://alaskaclimate.substack.com/p/alaska-october-2024-climate-summary

October precipitation was significantly above normal for the West Coast north of the Y-K Delta and across to the central and northern interior, as well as the Brooks Range.  But I'm not sure the ERA5 data (see below) fully captures the magnitude of the precipitation anomaly in the Fairbanks area, where it was the wettest October in many decades.

Not surprisingly, it was a windier month than normal for much of the state, and especially so for the West Coast.  Every month this year except February and September has been notably windier than normal for a majority of the West Coast.

The mid-atmosphere circulation pattern involved a strong ridge over and to the south of the central Aleutians, and a trough from northwestern Canada to the Gulf of Alaska.

This is very much a negative PDO pattern, with the ridge corresponding to above-normal sea surface temperatures to the east of Japan, and the Gulf of Alaska trough corresponding to cool ocean temperatures.  The warm anomaly is much more extensive and far more intense than the cool anomaly - see below.  It's interesting to see that the dividing line between warm and cold lies right along the western Aleutian chain.

Speaking of the PDO, the monthly PDO index for November was the most negative of any month since 1955.

The negative phase is being driven mostly by the extreme warmth in the northwestern Pacific, and the latest seasonal forecast models show no reprieve at all in the coming months.  Here's the NMME multi-model forecast all the way out in late spring:

And here's an ensemble of model forecasts of the PDO index:

If this forecast is correct, and I have high confidence that it is, then Alaska will continue to be strongly affected by this PDO anomaly for the foreseeable future.  We can expect an enhanced probability of unusual cold in southern Alaska this winter, and perhaps farther north as well.

More Snow in Anchorage

Anchorage is off to a remarkably snowy start to winter, with 22" reported since the beginning of the week, and a snow depth of 19" on yesterday's climate observation.  Only a handful of years have seen greater snow accumulation by this date, and none has reached a snow depth of 19" this early.  With consistent climate data back to 1953 (at the international airport), the previous earliest date with 19" or more on the ground was... last year, November 10.  Prior to that, the earliest was November 21, 1994.

This marks the third year in a row with significantly unusual heavy snow in early winter.  Last year it was the snow onslaught of November 5-13, and in 2022 the city was buried in two major events in the first half of December.

https://ak-wx.blogspot.com/2023/11/south-coast-snowstorm.html

https://ak-wx.blogspot.com/2023/11/a-few-notes.html

https://ak-wx.blogspot.com/2022/12/anchorage-snow.html

With a snow pack of 10" in Fairbanks, this also marks the second earliest that both cities have recorded at least 10" on the ground at the same time.  The earliest was in the notorious autumn of 1992, but even then Anchorage didn't have sustained snow cover until October 27.

Here's a chart of snowfall through November 15 in Anchorage, with this year's total included through November 1.

The variability is enormous, and the change in 2020 really stands out compared to 2012-2019.  The only other period with 5 consecutive years of at least 10" in this early winter period was 1979-1983.

It seems unlikely to be coincidental that 2020 is when the PDO phase switched to negative, after 6 years of a mostly positive phase.

Here's a comparison of early winter snowfall with the PDO index in November:

There are some notable PDO extremes that coincide with snowfall extremes, e.g. high snow and negative PDO in 1955-56 and 2011, and low snow with positive PDO in 1957, 1986, 2002, 2014, and 2016.  However, the overall correlation seems to have been modest at best before the turn of the century - see below.  Since 2000, there's been a much better correlation.

One hypothesis to explain this might be that in earlier decades the positive PDO didn't prevent early winter snow, because it was still cold enough, but in recent years a positive PDO has brought greater warmth that has led to rain rather than snow.  More investigation would be required to explore this; but without doubt, the negative PDO regime is a snow-lover's friend these days in Anchorage.