Tuesday, December 27, 2016

Surprising Snowfall

In a change of scene that is no doubt refreshing for some, Fairbanks has seen a significant amount of snow in the past several weeks.  The total so far this month is 16.9", compared to a normal accumulation of just under 10" for the full month of December.  With more snow in the forecast, it looks like this month will beat the 18.6" of December 2012 and become the snowiest December since 1992 (28.5").

Here's a chart of the season-to-date snow accumulation (green line) compared to normal (brown line), recent years, and the historical extremes.  I created the chart on the xmACIS2 interface, which provides a wide variety of handy analysis options.  Fairbanks is still in the lower tercile for season-to-date snowfall owing to the very dry start to winter.


A couple of weeks ago I mentioned that the PDO was still moderately positive, which was interesting in view of the notable cold spell that managed to envelop most of the state earlier this month.  The positive PDO phase also tends to be associated with below-normal snowfall in interior Alaska, as we saw in the past couple of winters; and so the current snowy spell does not reflect the PDO influence.  Furthermore, the NPM (North Pacific Mode) is strongly negative at present, and this too is normally associated with dry weather in winter.

The chart below shows the rank correlation coefficient by month between snowfall and the PDO and NPM index values.  For the November-March seasonal average, the NPM actually has a slightly stronger correlation with Fairbanks snowfall than the PDO.  Given that the current NPM index is among the most negative on record, and the PDO is also positive, it's quite surprising to see a snowy weather pattern emerge in Fairbanks.


Here's a recent SST analysis, showing North Pacific temperature anomaly patterns that are characteristic of the positive PDO and especially the negative NPM:



See these earlier posts for more perspective on the PDO and NPM and their impacts:

http://ak-wx.blogspot.com/2015/11/summary-of-sst-impacts.html

http://ak-wx.blogspot.com/2016/11/north-pacific-temperature-update.html

10 comments:

  1. What in the PDO or NPM exclude the creation and occurrence of a typically seasonal forecast occlusion bringing snow and warmth to Interior Alaska?

    Gary

    ReplyDelete
  2. Nothing is excluded, but the likelihood of snow is reduced when the PDO and NPM are similar to the current phases. The +PDO/-NPM pattern is related to a persistent anomalous trough over and to the south of the eastern Aleutians, and a strong ridge over western Canada. Translation: southerly flow and downsloping over Alaska's interior; dry and generally warm weather (but the warmth is related to the PDO, not the NPM).

    ReplyDelete
  3. looks like one model's forecast jetstream loop is going to trump persistence and create SW flow near the surface of Alaska:

    http://virga.sfsu.edu/scripts/jetstream_model_fcst.html

    Gary

    ReplyDelete
    Replies
    1. If the jet stream is weak and wavy enough to allow near freezing temps at the North Pole, it shouldn't be too surprising that wetter conditions would weave themselves into Interior Alaska.

      I wonder if there is a index that measures the strength/waviness of the jet stream and what that currently would be?

      Delete
    2. Good point Eric on the North Pole warmth. Yea I wonder too if there's a wavy index for the Jets? That Arctic Amplification/Polar Vortex deal discusses the potential for waves but it's over my head as to why.

      Gary

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    3. The Arctic Oscillation is closely related, although it's not directly a measure of jet stream properties. Follow it here:

      http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/teleconnections.shtml

      The AO was strongly negative in October (i.e. high pressure in the Arctic, weak jet stream), somewhat negative last month, and is currently positive. The stratospheric polar vortex is also currently stronger than normal.

      Delete
  4. https://earth.nullschool.net/#current/wind/isobaric/850hPa/orthographic=-140.71,69.22,616

    This will be fun to watch during the storminess...if we still have power in Fairbanks. Click on "Earth" to access menu and change displayed parameters.

    Gary

    ReplyDelete
  5. HAPs, SOMs, and the jet stream forced by Arctic Amplification: A proposal.

    https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4455715/

    Gary

    ReplyDelete
    Replies
    1. A more current interpretation of the proposal cited above re: wavy jets:

      http://iopscience.iop.org/article/10.1088/1748-9326/10/1/014005

      Gary

      Delete
  6. Snowfall reports as of Friday morning 12/30/16. More on the way this eve plus a SW breeze.

    https://nwschat.weather.gov/p.php?pid=201612301722-PAFG-NOAK49-PNSAFG

    Gary

    ReplyDelete