Cold and Dry

Wintry cold continues to hold on tenaciously across interior Alaska, at least during the nighttime hours.  Here we are after the spring equinox, and temperatures in the -30s Fahrenheit are still occurring overnight in the usual cold spots.  This morning the Kanuti Lake SCAN site reported -34°F, and the Salcha RAWS hit -31°F.

In Fairbanks-land, Goldstream Creek reported -27°F this morning, and the airport thermometer registered -18°F.  Remarkably, every night but one so far this month has seen a low of -13°F (-25°C) or colder at Fairbanks airport.  The 22 nights at -15°F or colder is tied with 2007 for most on record in the month of March, and the NWS expects another one tonight.

Here's a chart showing the last two weeks of 2m temperature measurements from UAF's Smith Lake site: sub-minus 20 every night, and daily temperature traces indicative of generally clear skies.  So far this month more than two-thirds of the sky condition measurements at Fairbanks airport have indicated either clear skies or "few" clouds (25% coverage or less).

Total snowfall this month in Fairbanks has amounted to only 1.7", but this in itself is not very unusual: the 1981-2010 median is only 3.2" for the month.  April is usually even drier, with a normal liquid-equivalent precipitation of only 0.1".  March and April are the driest months of the year, climatologically speaking, in Fairbanks, and a few days ago Brian showed that this is also the driest time of year across most of interior northwestern North America:

https://www.facebook.com/AlaskaClimateFacts/photos/pcb.825178884300842/825177814300949

Why is late winter generally a sunny and very dry time in interior Alaska?  Part of the reason is that the air is very dry, and that's because evaporation from the earth's surface is much reduced in the Northern Hemisphere at this time of year: ocean surface temperatures are near their coolest, ice cover is near its peak on both oceans and fresh water bodies, and forests are still mostly dormant.  The lifetime of water vapor in the atmosphere is only about 9 days on average, so precipitation must decrease in tandem with evaporation rates on a hemispheric scale.

The graphic below shows an estimate of how the total atmospheric water vapor content (precipitable water) varies through the year in the Northern Hemisphere: the values shown in the chart are the west-east averages for each month and each line of latitude.  It's interesting to see the earlier summer peak of vapor content in the Arctic compared to the tropics, as solar heating drops off more rapidly in late summer at high latitudes.

Here's a similar chart, but for a single line of longitude running approximately through Fairbanks (148°W).  Over Alaska and the oceans to the south, the lowest normal value of precipitable water occurs in March.

A second reason for dry weather in Fairbanks in March and April is that there's simply not much storminess to generate clouds and precipitation.  The absence of fronts and storms is a more localized factor than hemispheric dryness, because other parts of the hemisphere (indeed of the USA) have plenty of storminess and wet weather in spring.

In the vicinity of Alaska, storminess is reduced in the spring because the North Pacific Ocean is cool, leading to reduced north-south temperature gradients and lower potential energy for storms.  Another reason is that high pressure tends to build over the Arctic basin in late winter and spring.  The chart below show the time-latitude cross-section of sea-level pressure along 148°W; notice the prominent high pressure zone around 75-80°N from February through April.  At the latitude of Fairbanks (65°N) the pressure is lowest in October and then increases into late winter; and while there's much more to the weather pattern than just MSLP, this does illustrate the overall tendency towards generally more clear and dry weather as the winter advances.