Saturday, January 30, 2016

Exceptionally Dry

Ever since the persistent snows of November ended about 2 months ago, the weather has been very dry indeed in Fairbanks.  Through yesterday with its meager 0.2" of snow, the December-January period has seen only 1.9" of snowfall and 0.08" of liquid-equivalent precipitation.  If this stands, it will be the driest December-January period on record (1930-present); normal snowfall and precipitation are 19.3" and 0.98" respectively for this period.  For comparison, the last 2 winters saw 16.8" and 19.8" of snow in December through January.

However, if we look beyond just calendar months, there have been a couple of occasions when even less snow fell over the same length of time during winter; mid-winter 1952-1953 and early winter 1953-1954 both had 62-day spells with only 1.4" of snow.

I thought it would be interesting to look at the upper-level flow patterns that typically accompany very dry or very wet conditions on a monthly time scale in Fairbanks.  This is a similar analysis to the one I did for extreme daily temperature anomalies, see here.

The maps below show the average 500mb height anomalies that accompanied the 8 driest (left) and wettest (right) instances of each calendar month in Fairbanks since 1950.  In winter the driest conditions in Fairbanks tend to be associated with unusually low pressure to the south of Alaska, which reduces the strength of the westerly flow aloft and prevents weather disturbances (fronts and lows) from reaching the interior.  But it's very interesting to note that the height anomalies accompanying dry weather are less consistent and less pronounced than those accompanying wet weather.  My interpretation of this is that there is more than one way to get dry weather: very warm weather, with chinook flow, is dry, but very cold weather is dry as well.  Higher precipitation amounts tend to occur with near-normal temperatures, as we saw in this post.  Very wet weather, in contrast, tends to occur only one way in winter: when westerly flow is enhanced around an anomalous ridge to the southwest.


























So does the flow pattern in the past 2 months match the historical pattern for dry weather?  The map below shows the 500mb height anomaly since December 1; it closely resembles the dry weather pattern for December.

Unsurprisingly, the dry weather pattern is also quite similar to the typical pattern associated with El Niño.  The map below shows the mean height anomaly during 10 significant El Niño's, indicating that low pressure aloft in the Gulf of Alaska is the preferred circulation state.


8 comments:

  1. Yes Richard, the NWS has confirmed your suspicion about the extreme dry spell in Fbks: http://pafg.arh.noaa.gov/wmofcst.php?wmo=NOAK49PAFG&type=public
    They also mention the similarity to the winter of 1976/77. Since this was my first living in Fbks, I've often wondered why it was so different than the winters before or after. Was it an extreme El Nino year also? Of course it still could get real cold yet this year, but the NWS long term forecast is for more of the same, warm and dry. So what are your thoughts on this? Thanks. Jon

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    1. Jon, 1976 was an auspicious year for Alaska climate, because it marked a sudden transition from negative to positive PDO phase; this had profound effects on climate statewide, most notably in terms of much warmer winters. To this day, 1976-77 remains the second warmest winter (Dec-Feb) on record in Fairbanks; it was marginally warmer in 2000-2001. As noted by the NWS, it was also the only winter on record that never reached -30F in Fairbanks (until this winter??).

      So the main explanation for 76-77 was the positive PDO phase. It was a mild El Niño winter but certainly not extreme.

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  2. Richard,
    What are your thoughts about the upcoming fire season? I suspect that snow water has little to do with fire season onset but what pattern historically follows this pressure pattern? Snow overall in the interior is average or above right now thanks to the generous snows of September. Soil moisture should be high also since last summer was modestly wet.
    Thoughts?

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    1. Brian did a Facebook post awhile back https://m.facebook.com/story.php?story_fbid=497890407029693&id=220671184751618 that showed that there is no statistical correlation between a summer fire season and the amount of winter snow.

      But I think looking at pressure values over time would be an interesting idea since they tell us about the paths of storm tracks and such - which might prove informative.

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    2. Mike, good questions - the topic is worthy of investigation. As I understand it, overall fire acreage is not correlated with winter snowfall (thanks Eric!), but early season fire acreage is - so if it continues dry (as seems likely), the odds may be enhanced for a fast start to fire season.

      Yes, it would be interesting to look at the teleconnection or ocean temperature patterns as potential predictors in addition to current moisture conditions.

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  3. In my opinion it takes two to tango...dry soil duff and a source of ignition to light it off. Soil and atmospheric moisture/humidity help determine how readily the duff ignites and burns. But without unstable air aloft thunderstorms and lightning can't readily develop. Folks tend to start fires but that's another issue.

    So does snow level and water equivalent affect early fire season soil duff moisture? Why not?

    Does snow affect relative humidity and lightning? Probably a weak correlation exists indirectly in the form of how much available moisture is available to get lifted and condensed as clouds. An influx of unstable air aloft has a greater impact I believe.

    Gary

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    1. A big yes to this idea, Gary - ignition is critical. As I recall the extreme heat of early summer 2013 produced relatively few fires because it was so darned dry.

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    2. If we note the process we'll see that after days of hot sunny weather any influx of moisture via a rain event followed by clearing and solar heating can cause lightning if the conditions are right.

      Adequate surface humidity and insolation combined with unstable air/orographic lifting/convergence, etc. will create a swath of lightning depending upon the direction of the flow of unstable cold air aloft.

      An explanation of cloud formation and lightning from an aviator's view:

      http://www.aopa.org/Pilot-Resources/PIC-archive/ASF-Weather/Stability

      Gary

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