Saturday, January 31, 2015

Will the PDO Influence Return?

Yesterday was the fifth day in a row with a minimum temperature of -40 °F or below in Fairbanks.  This is the longest such sequence since December 2012, which saw 9 straight days of -40 °F temperatures.  The Fairbanks record (1930-present) for longest such sequence is 18 days in January 1965 and January 1971 (the latter month also produced the all-time state cold record at Prospect Creek).

It is of interest to note that the PDO phase is still strongly positive as it has been throughout the winter so far (see below).

Is it therefore a surprise that colder than normal conditions have emerged over Alaska this month?  Wouldn't we expect a positive PDO episode of such magnitude to prevent deep cold from developing over Alaska?  Perhaps - but this is so far only a brief cold spell by historical standards, and this month's mean temperature will still end up above normal in Fairbanks.  Moreover, as we discussed back in December, the PDO influence on Fairbanks temperatures is smaller in January than in early or late winter.  I don't know why this is so, but it's clear in the summary statistics I showed before and in the monthly scatterplots shown below.  Note that for these plots I computed the monthly temperature anomalies with respect to the contemporary 30-year normals.

The historical data reveal that the PDO typically reasserts itself in February, with the scatterplot showing a considerably better correlation than in January.  Given the magnitude of the current PDO anomaly, we would therefore NOT expect February to remain cold overall in Fairbanks.  Of course, there are always exceptions - for example, February 1936 was the 10th coldest February in Fairbanks but also had the second most positive PDO index for the month.  Perhaps this year will be a similar outlier - indeed the first 10 days of the month certainly look cold - but if cold persists throughout the month, then it will be counter to the long-term PDO correlation.


  1. Very nice read Richard. Here's some obs from yesterday 1/30/15.

    A high thin layer of clouds moved over Fairbanks during mid-day from the SW. Temps rose from the upper -30's to the lower -20's by 1800 akst.

    There was no appreciable wind at the valley floor as seen by directed heating exhausts, but the local power plant emissions did move towards the S/SW for some time. Their plumes rose vertically through the inversion and then flat topped and flowed with warmer wind moving above.

    Any mention of "light snow" in the PAFA records related to falling ice fog and power plant derived products. There was no snow was we'd typically call it sourced in clouds.

    Despite a warm PDO it will take more cloud cover and a stronger valley wind to reduce the IR loss and warm/remove the cold air at the surface in low lying areas.


    1. Gary, thanks for the description. There has been steady warming aloft for days - but yes, tough to scour out a cold air mass like this even with some measurable solar input now.


      I often link this view of Fairbanks looking south:

      It's a great visual indicator of local weather, winds, and fog. Today 2/1 it's a clear -20F with a light W/NW wind above the inversion...note the flow of the two power plant plumes. And see the bright planet Venus over the airport PAFA during hours of darkness.


    3. Yesterday 1/31/15 the CPC revised its monthly forecast for mainland Alaska from warmer than normal to EC...equal chances of warm, cold, and whatever normal climatology is for this time of year.

      Gary (Yo PDO! Where'd you go?)

  2. I can't find any posts that describe what percentile of Januarys have 5-6 days in a row with minimum temps below -40. As you said, it's not unusual at all but it is still happening in a very warm winter.

    Is there anyway to apply that blocking high/low analysis from a while back to the conditions of the last few months. Perhaps that will give insight into how stable this cold pattern is.

    1. Eric, the following January's had 5 or more consecutive days with minimum temperatures of -40 or below, with the sequence falling either partly or wholly within the month:

      1932 1933 1934 1947 1951 1952 1953 1959 1962 1965 1966 1968 1969 1971 1973 1975 1983 1989 1990 1993 1997 2000 2009 2012

      So that's 24 of 85 winters or 28%. Of these, 10 were warmer than normal in November-December. However, only 6 of 24 had a positive PDO in January and the most positive was 1959 at +0.69. So it does seem this is an unusual situation.

      I'm not quite following your suggestion on the blocking analysis. But I do hope to look at the recent and forecast upper-air pattern in another post soon.

    2. Thanks, Richard. To sum up: About 1/4 of all Januarys on record have 5+ consecutive days of -40℉. About 1/16 of all Januarys on record have 5+ consecutive days of -40℉ and also a positive PDO. And since we have had one of the warmest winters on record, it's conceivable that 1/85 winters have the cold snap that we've just had. This doesn't seem as unusual when you figure that we went from the coldest Aprils on record in 2013 to one of the warmest May.

      I'm sorry I wasn't clear about the blocking patterns. My bad. I'm just wondering how much of the warm winter and cold snap is related to blocking highs and lows and other persistent patterns.

      Also, did the warm North Pacific waters recede a little? How did the arctic air show up so forcefully? I was watching Alaska Weather the other day and they showed the jet streams more north than you would expect. I assume this is because of the warm Pacific. Did we just get a random southern bulge of the polar jet? I'm sure you upper-air analysis will show all of this.