Wednesday, April 19, 2023

Cold Days in Context

The cold spell is relaxing its grip for many areas today, with temperatures rising widely above freezing; but thermometers in the interior will reverse course this weekend as another very cold air mass drops in from the northeast.

It's useful to view the recent spate of significant cold days in the context of the last decade.  For example, here's a chart showing the monthly number of days with a mean temperature at least 2 standard deviations above or below normal in Fairbanks.  I've used the 1981-2010 normal as the baseline, because it predates the period shown here.

The dramatic flip from cold to warm in spring 2013 is the standout feature; it really was a most remarkable sequence of events.  Following that reversal, warm days were favored quite heavily through 2019, but since then there's been a more even split in the frequency of very warm and very cold days (relative to the time of year).  April 2021 and now April 2023 stand out as the most unusual calendar months of recent years; but the current cold spell has a long way to go to equal the April-May 2013 cold.

How about Nome, which tied its April record for cold earlier this month?  In this case the prevalence of warmth was overwhelming from 2014 through 2020, with not a single 2SD cold day for more than six years.  Ordinarily, of course, +/- 2SD days would occur about 5% of the time (combined) for a Gaussian distribution (but in fact temperatures are quite significantly non-Gaussian).  But now April 2023 has the most cold days of any month since the notorious September of 1992.

The chart for Bethel looks quite similar, although there was more significant cold there in 2011 through May 2013.  Early summer of last year saw extraordinary heat, leading to very bad fire activity for the Y-K Delta region.

As for Anchorage, warmth in 2018 and 2019 was extreme, and even though this month has been quite wintry, the cold hasn't been significant enough to show up on this chart.

Juneau saw more frequent cold days in 2020 and 2021, and that can be attributed to La Niña and the negative PDO phase; but there was a lot of warmth last year despite those bigger climate anomalies persisting.

Finally, we see the unending warmth of Utqiaġvik, with a mere 3 days of 2SD cold in the past 12 years.  This month's cold isn't registering on the chart at all, and just last month there were 7 days of 2SD warmth relative to the 1981-2010 baseline.

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