September Climate Data

With September climate data now available, let's look back at the predominant weather patterns for the month.  The monthly-mean circulation anomaly was quite similar to August, with a high-pressure ridge retaining a strong influence across southern Alaska.  In contrast, low pressure was dominant over the Arctic Ocean, and the Arctic Oscillation was virtually tied with 1975 as the most positive on record in the month of September.  The mid-atmosphere height (pressure) anomaly shows the strong north-south gradient:

The corollary of a strong pressure gradient is, of course, strong wind, and the enhanced westerly flow brought abundant moisture and frequent rainfall to central and northern Alaska.  Again the pattern showed similarities to August, and much drier than normal conditions continued from the Kenai Peninsula to Southeast Alaska.

The monthly precipitation total in Fairbanks was the highest for September since 1925, and as noted in Rick's comments, some rivers saw their highest measured flow on record for so late in the season.

Check out the contrast in sunshine fortunes across the state: western Alaska and the northern interior were extremely cloudy, but southeastern regions were much sunnier than usual.

Despite the broad similarity in the pattern, temperatures were not as overwhelmingly warm in September as in August; the flow direction across central and northern Alaska had more of a southerly component in August.  The figures below highlight a strong north-south gradient that fits with the pressure gradient and the enhanced westerly flow.

Warmth in southern Alaska was amplified by ongoing high sea surface temperatures across the North Pacific to the south of Alaska, and especially to the south of the Aleutians.  Although the core region of unusual warmth is quite a long way from mainland Alaska, southwesterly winds commonly track from the Aleutian region up into southern Alaska, making the oceanic warm anomaly a "source region" for unseasonably warm air.

As for Arctic sea ice, the minimum ice extent on September 12 was the 10th lowest in the satellite-era period of record.  Seasonal meltout was less than might have been anticipated at the start of summer, owing to a relatively cool summer with persistent low pressure across the high Arctic Ocean.  For this reason, the NSIDC describes the melt season as "unusual in the extreme".  My impression, however, is that much of the ice that qualified for "ice extent" (based on the 15% area threshold) on the eastern side of the pole was very tenuous, and a different weather pattern could easily have produced a large drop in extent.  Here's a map of estimated ice concentration just a few days before the date of the seasonal minimum:

Compare the eastern hemisphere coverage to the NSIDC "ice extent" map from the same date:

Weather Data from Central

I recently noticed that weather data from the town of Central started showing up on the surface observations map, and it seems that 24-hour data is indeed now flowing from instruments there for the first time since 1989.  Central's location tucked up against the east side of the White Mountains makes it an interesting spot for weather.  Here's the location (click to enlarge):

So far this month, Central has seen persistent snowfall on four days, and judging from webcam photos today, there is a modest snow cover:

Interestingly, Central saw snow for much of Sunday and Monday earlier this week, while Fairbanks saw none, as cold air flowing from the northeast was lifted over the north side of the terrain but subsided on the Fairbanks side.  The temperature in Central was as much as 18°F colder on Sunday afternoon: 27°F and snowing in Central, 45°F and dry in Fairbanks.  We normally think of chinook warming reaching Fairbanks under southerly flow across the Alaska Range, but the White Mountains can also produce a similar effect.

The sheltered interior location of Central creates a surprisingly cold winter climate compared to some other northern and eastern interior valley sites.  To illustrate this, I calculated the monthly average temperature difference of several locations compared to Fairbanks, using all available overlapping dates in the joint periods of record back to 1930.  (The climate history is a lot more complete in some locations than in others, but the temperature differences between two locations tend to be quite stable over time, so I'm not worried about issues of trend in this analysis.)  Click to enlarge:

Central, shown with a green line, is about 8°F colder than Fairbanks in late winter, and is also significantly colder than Eagle to the southeast.  Even though Central is above the Yukon River valley (its elevation of 940' is higher than Eagle's elevation), Central's winter cold is more akin to that of Fort Yukon (430'), although it's not quite as severe as the lower elevations of the Yukon Flats.

Central is also quite a bit colder than Bettles in mid-winter, even though Central is well to the south; but it's certainly not as cold as Allakaket (not far from Bettles).  As an aside, Allakaket is a real contender with Chicken for coldest winter climate in inhabited interior Alaska, based on the respective comparisons with Fairbanks (but Allakaket data ended in 1982).

Central's ICAO code is PACE, and the latest observations can be viewed here:

https://www.weather.gov/wrh/timeseries?site=pace

[Update October 9]

A case in point this morning: low temperatures around 20°F in the Fairbanks area, but 6°F in Central.  Eagle (not shown on the map) got to 13°F.

Much Colder

The first deep freeze of autumn has enveloped northern Alaska in recent days, and some notably chilly conditions have been registered for this early in October.  Here are some of the lowest readings in the past few days:

October 1:  12°F Ambler

October 2:  8°F Bettles

October 3:  5°F Wiseman, 2°F Kanuti NWR RAWS

Temperatures of 14°F (-10°C) or lower were particularly widespread yesterday morning, and a few sub-zero readings showed up on the North Slope:

Snow cover is widespread on the hills and - judging from webcam photos - much of the southeastern interior, but the western interior is mostly snow-free to the south of the Yukon River.  Here's a satellite image from yesterday afternoon, with blue color enhancement showing where the algorithm picked up snow cover:

How about this for late autumn views in the northeastern interior settlement of Arctic Village this morning:

Light snow fell in Fairbanks for about 10 hours on Thursday, but the measured accumulation was only 0.1", and that melted out rather quickly with a high temperature of 36°F later in the day.  The following day - October 2 - was chilly, with a high temperature of only 33°F; but it's not unheard-of to get daily high temperatures below freezing by this time of year.

It's interesting to look at how the calendar affects the probability that snow on the ground will melt out before further snow arrives in Fairbanks - consider the chart below (click to enlarge).

At the beginning of October there is still about an 80% chance that a 1-inch snow cover will disappear at some point in the subsequent days or weeks, but this drops to 50% on October 10 and only 20% on October 20.

The probability of a 3-inch snow cover melting out in Fairbanks drops even more rapidly in early October: from over 60% in the first few days of the month to 25% by October 10.  So even a relatively heavy snowfall at the very beginning of October is not safe from subsequent melting; but you only have to wait a week or so for much better odds of the snowpack remaining through winter.

As for a 6-inch snow depth, there have only been 3 years in which such a substantial snow depth melted out in late autumn: 1936, 1972, and 2015.  In 1936, 6 inches fell on the 17th of October but disappeared by the 26th, and in 1972, temperatures above 50°F in mid-October melted an apparently robust early October snowpack.  The 2015 meltout occurred following record snow in late September, and snow didn't return until October 29th.  The blog archives make for interesting reading, e.g.

https://ak-wx.blogspot.com/2015/09/historic-snow.html

https://ak-wx.blogspot.com/2015/10/fairbanks-melt-out.html

Of course no mention of early winter snowpack loss would be complete without mentioning the "brown Christmas" of 1934, following the great early-December chinook - unquestionably one of the most extreme weather events in the recorded climate history of Fairbanks.

https://ak-wx.blogspot.com/2011/12/its-white-christmas.html