Tuesday, January 31, 2017

Winter So Far

January has been another relatively snowy month in Fairbanks, with a total accumulation of 17" - the highest January total since 2005.  The combined December-January total was almost exactly 50", which is the 5th highest on record for the two-month period.  Temperatures have averaged slightly below the 1981-2010 normal and close to the long-term (1930-present) normal, but with alternating cold and warm spells.  Snowy winters are typically colder than dry winters in Fairbanks.



As noted in earlier posts (e.g. here), the cool and snowy conditions are surprising in view of this winter's North Pacific sea surface temperature patterns.  The positive PDO phase and negative NPM phase, which continue to be evident in the latest SST data, would normally promote warm and dry conditions in the interior; we discussed this back in November.  Here's another map showing the average 500mb height anomaly (departure from normal) in November through January for 6 winters with a significantly positive PDO and significantly negative NPM:

Compare this pattern to what has actually transpired in the last 3 months:

The patterns are fairly close to the inverse of each other, and this of course explains the weather in Fairbanks: the flow pattern has produced enhanced westerly flow across western Alaska, thereby transporting moisture to the interior, and the tendency for higher heights in the Bering Sea has allowed cold air to drop south across Alaska at times.

The reason for the discrepancy appears to be that the weak La Niña in the tropical Pacific has had a much more dominant influence on the circulation pattern than expected (at least by me).  The map below shows the height pattern for the 10 strongest La Niña winters since 1950-51; this is a better match to what has happened this winter, although admittedly the La Niña flow is a colder pattern for Alaska than we've seen this winter.


The most intriguing part of the situation, I think, is how the North Pacific SST patterns have remained much more reminiscent of El Niño than of La Niña, despite the evidently profound influence of the La Niña episode on the atmospheric circulation.  A positive PDO and negative NPM are generally observed in association with El Niño, like last winter - but not this time.  But regardless of the cause, this winter's outcome will serve as a cautionary tale about the danger of relying too heavily on the North Pacific SST patterns in isolation for making a forecast.

Update Feb 2: This nice graphic from NWS Fairbanks shows the complete contrast between last winter and this winter in terms of early versus mid-winter snowfall accumulation; the totals before and after December 1 are almost perfectly reversed.


14 comments:

  1. With respects to accumulated snowfall, winter 2016-2017 has been a tale of two winters for Fairbanks. On the first day of December, only 5.5 inches of snow had fallen and folks were wondering when the ski season was ever going to start. Something certainly shifted in mid-December to awaken Ullr, the God of Winter. It will just be an interesting and stark contrast once you can compare early winter snowfall (Sept-Oct-Nov) to late winter snowfall (Dec-Jan-Feb) data.

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  2. Good point, Tracy - thanks - and it wasn't just Fairbanks that started dry. Bettles and McGrath both had only 5" of snow on the ground as late as December 15.

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    1. NWS Fairbanks had a nice graphic today showing the contrast, and it's the opposite of what happened last year. I updated the post.

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    2. That graphic is fascinating. How bizarre that the two consecutive winters were almost exactly inverse of one another. Thanks for the update.

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  3. Some required reading:

    http://ak-wx.blogspot.com/2014/09/el-nino-and-positive-pdo-in-winter.html

    http://ak-wx.blogspot.com/2016/01/pdo-enso-and-temperature-extremes.html

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  4. The snow doesn't seem to be settling much; may be normal condition for early February. Lots of multiple layers due to the frequency of snow events, and of course wind driven relatively solidified intermediates to support and hold the current level. Frozen above ground limnology is interesting.

    Gary

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    1. Yes, interesting. What sort of snow depths are you seeing around town? The airport reports 20", the same as January 1, so that suggests a fair bit of settling. Since November 1 the snow depth has increased by 37% of the total accumulation, which is just a little less than normal.

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    2. Depends upon location and the prior exposure of snow to wind events and likely other prior processes such as sublimation and compression due to gravity.

      Obviously if it's snowed 55" and the airport reports a physical 20" it's settled or sublimated at their observation site. In other locations near their site (?) wind disturbed or driven snow depth is greater, but 20-30" would seem a reasonable average.

      The current snow water equivalent values are above median:

      https://wcc.sc.egov.usda.gov/reports/UpdateReport.html?report=Alaska

      This paper offers a refined method of estimating SWE from depth data and classes of snow climate:

      http://journals.ametsoc.org/doi/full/10.1175/2010JHM1202.1

      Gary

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    3. I might also note that in September 2015 we had an early snow event that later pretty much disappeared prior to October. The total for the season remained, but the SWE and depth restarted later in the season.

      Gary

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    4. Snow info for Alaska's reported obs sites:

      https://www.wcc.nrcs.usda.gov/webmap_beta/index.html#version=76&elements=W,D&networks=!&states=!&counties=!&hucs=&minElevation=&maxElevation=&elementSelectType=all&activeOnly=true&activeForecastPointsOnly=false&hucLabels=false&hucParameterLabels=false&stationLabels=&overlays=&hucOverlays=&mode=stations&openSections=dataElement,parameter,date,basin,elements,location,networks&controlsOpen=true&popup=&base=esriNgwm&displayType=station&basinType=6&dataElement=WTEQ&parameter=PCTMED&frequency=DAILY&duration=custom&customDuration=&dayPart=E&year=2017&month=2&day=1&monthPart=E&forecastPubMonth=2&forecastPubDay=1&forecastExceedance=50&seqColor=1&divColor=3&scaleType=D&scaleMin=&scaleMax=&referencePeriodType=POR&referenceBegin=1981&referenceEnd=2010&minimumYears=20&hucAssociations=true&lat=63.45&lon=-148.10&zoom=4

      Gary

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  5. I heard on the radio that the Iditarod organizers are contemplating on moving the race to Fairbanks again. Apparently the high amounts of snow didn't make it throughout all the state. A decision will be made by February 11th. How much snow did the area south of the Alaska range actually get?

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    1. Plenty to the south, I believe, but low snow in the usual problem areas:

      https://www.adn.com/outdoors-adventure/iditarod/2017/02/01/low-snow-in-rainy-pass-and-dalzell-gorge-could-mean-an-iditarod-restart-in-fairbanks/

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  6. This is a better source than that huge Beta deal I linked above:

    https://www.weather.gov/aprfc/Snow_Depth

    Gary

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    1. Thanks Gary, nice link - I think that's new this season.

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