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.