Tuesday, February 28, 2017

Snow Piling Up

Fairbanks has seen something of a snow onslaught in the past week, with a remarkable 19" of snow over six days, taking the seasonal snowfall total to 78".  This is the highest in over 20 years.

The entire month of February normally only brings about 6" of snow, as the weather usually becomes increasingly dry in late winter.  On average, the heaviest 1-day snowfall in February is 2.5", but Fairbanks just saw 4 consecutive days with more than 2".  Interestingly this has only happened 8 times before since 1930, and never after mid-January.

Another interesting factoid: the months of December through February brought 15 calendar days with at least 2" of snow in Fairbanks this winter, and that's the highest on record; the previous record was 13 such days in December 1936 through February 1937.

The map below shows reported snow depths today.  A remarkable aspect to me is how high the valley-level snow depths are from the Fairbanks area down to Nenana; the snow depths are no greater at some of the elevated Snotel sites northeast of Fairbanks.  Keystone Ridge is reporting 35" on the ground - barely more than Fairbanks airport (32").

The percent of normal snow water equivalent at the long-term Snotel sites ranges from 112% at Mt Ryan to 198% at Monument Creek.

Here's a chart showing the seasonal snowfall totals and maximum snow depth in Fairbanks for each winter since 1930-31; the long-term median values are indicated with dotted lines.  For as unusual as the snow has seemed this winter, the season as a whole is nowhere near some of the extraordinary winters of the past; but what has been very unusual is the concentration of snowfall in the past three months.

Saturday, February 25, 2017

Yukon River at Dawson

Temperatures are near or above freezing across most of the western half of Alaska south of the Brooks Range this evening, as a very warm and moist airmass has been imported by strong southwesterly flow aloft.  In Fairbanks the temperature has been close to the melting point since around noon yesterday, and today's minimum temperature is so far sitting at 29°F; this is close to record territory for the time of year.  The westerly component of the flow means that it's not a chinook event; in contrast, heavy snow is occurring in many areas.

Interestingly Anchorage has a high temperature of 27°F so far today.  It's very rare for the low temperature in Fairbanks to be above the high temperature in Anchorage during winter; it's less rare in June and July, but it hasn't happened in the winter months since 1974.

On another topic, regular readers may recall that a couple of months ago I drew attention to the lack of complete freeze-up on the Yukon River at Dawson.  Remarkably, the same situation is still in evidence, with a persistent lead of open water in the middle of the mighty river.  Here's a webcam image from earlier today.

Can the presence of open water be explained in terms of unusually warm weather?  No; the chart below shows the accumulation of freezing degree days this winter (black line) compared to the last 5 winters.  This winter has been colder, on balance, than any of the last 3 winters, and the river did not fail to freeze over in those years.

The explanation appears to lie in the early formation of an ice jam a short distance upstream, cutting off the formation of ice floes that would normally provide the building blocks for a complete ice cover.  As the images below demonstrate, the open lead has instead been freezing over from the edges, which is a very much slower process.  It will be interesting to see if the lead manages to disappear before the end of winter; my guess is that next week's much colder weather (indeed perhaps extremely cold) may finally do the job.

February 6:

January 16:

December 31:

Tuesday, February 21, 2017

Daily Minimum Temperatures

Here's a chart showing the lowest temperature reported statewide in Alaska each day so far this winter, along with the same analysis for last winter.  This is constructed with data from NOAA's ACIS tool, but I've excluded Snotel sites because of quality concerns.  Many, though not all, remote automated observing sites are included in the analysis.

The contrast with last winter has become pronounced since the beginning of the year: from January 1 to present, only one day had a higher statewide minimum than the same date in 2016.  The Kanuti Lake SCAN site dropped below -50°F again this morning, so that makes the 4th separate episode with sub-minus 50°F observed in Alaska; and that's a lot more like normal than recent winters.

Update Feb 22: here's the same chart for the two prior winters, 2013-2014 and 2014-2015.  A similar peak occurred at the turn of the year in 2014-2015, so this is the third winter in a row with this feature.  Based on the current forecast, it looks like this will also be the third year in a row with a notable late February warm spell.

Saturday, February 18, 2017

Duration of Cold Spells - Part 2

A few weeks ago I showed some evidence that severely cold spells have become shorter in recent decades in Fairbanks - in other words, the coldest extremes of the modern climate (which are warmer than earlier cold extremes) have become less persistent as warmer conditions have tended to return more quickly.

This follow-up provides a quick look at the average 500mb height patterns associated with the most severe cold spells in two contrasting periods, 1931-1960 and 1981-2015.  To examine this I used data from the 20th Century Reanalysis, courtesy of NOAA/ESRL/PSD.  The 20th Century Reanalysis uses historical sea-level pressure and sea surface temperature observations to produce an evolving 3-D estimate of atmospheric conditions back to 1851.  Owing to the strong correlation between sea-level pressure and upper-level heights, the historical 500mb height estimates should be broadly correct back to the early 20th century or before; by 1925 there were already well over 1000 daily pressure observations north of 20°N (although not at all evenly distributed around the Northern Hemisphere).

The two sets of maps below show the mean height anomaly (departure from normal) for 1931-1960 cold spells on the left (as defined in the earlier post) and 1981-2015 on the right.  The first pair of maps is for 10 days prior to the first day exceeding the cold threshold, and then pairs of maps are shown at 5-day intervals until 15 days after the cold spell onset.  Click to enlarge the images.

-10 Days

-5 Days

0 Days

+5 Days

+10 Days

+15 Days

With respect to the main question of interest here, it's immediately clear why cold spells in the earlier period tended to last longer than more recent cold spells.  In 1931-1960 the average flow pattern remained quite similar from the date of onset through 10 days later, with high pressure over the Bering Sea and low pressure over western Canada; this pattern funnels cold air southward across the interior.  In contrast, the 1981-2015 pattern is completely different by +10 days, with low pressure over the Bering Sea and high pressure over western Canada; this pattern brings warmth to Fairbanks.

It's also interesting to note that the precursor flow patterns were quite similar between the two periods at a lead time of 10 days, with high pressure over and south of the Aleutians and low pressure over the Arctic Ocean.  However, at 5 days in advance, the modern pattern is much more amplified, with a very strong northerly flow anomaly over the Bering Sea (rather than northwesterly in 1931-1960).  It seems as if modern severe cold spells tend to arrive with greater ferocity, but the high-amplitude pattern continues to evolve rapidly so that the cold blast is followed quickly by warmth.  In the "old days" the cold-advection pattern gives the impression of being more stable and long-lived, at least as far as the Alaska-centric flow anomalies are concerned.

A possible next step would be to investigate historical sea surface temperature anomalies to see if we can identify a reason for the preferential patterns shown above.

Tuesday, February 14, 2017

Extreme Warm-Up

I was going to write a few words on the brief but notable cold spell over the weekend, but the cold has already been eclipsed by an incredibly rapid warm-up over the eastern interior.  The map below show 24-hour high temperatures through 5pm today in the wider Fairbanks area: most places above freezing, and +40's in the hills to the east.

A broader view shows clearly where the southerly chinook flow reached the surface today and, conversely, where it did not; the Yukon Flats have remained cool, and locations west of Fairbanks have been less warm; even Nenana was far enough west to stay 10°F cooler than Fairbanks.  Interestingly the upper Tanana Valley was cooler as well.

The rate of warming at certain spots has been really remarkable: the Salcha RAWS, for example, warmed from -45°F at 9am on Sunday to +45°F at 1pm today.  This is extreme even by interior Alaska standards.

At Fairbanks the low on Sunday morning was -41°F, and today's high so far is +35°F; this is the warmest it has ever been within two calendar days of a -40°F or colder reading.  The warmest it's ever been within a single day of -40°F is +21°F (in 2012), and the warmest within 3 days is +44°F (2009).  So this is one for the history books.

Here's a sequence of 500mb charts showing the flow adjustment that has brought such a dramatic change of conditions.  The maps are from 3am on Sunday, Monday, and today, respectively; notice the strong upper-level low dropping south over western Alaska, allowing the the flow to pivot quickly to the south across the Alaska Range.

The readiness of the low to drop south appears to have been aided by the deep trough that was already in place over the North Pacific at around 160-180°W (evident in the first map above).  This in turn was favored by widespread below-normal sea surface temperatures in a west-east band across the North Pacific, reflecting the negative NPM phase that has become entrenched this winter.  Here's a recent SST analysis:

And here's the typical 500mb height pattern when the NPM is strongly negative in February; notice the similarity of the trough position to where the low ended up this morning.  This is a nice example of a single weather event reflecting the favored mode based on long-term (seasonal-scale) forcing mechanisms.

Thursday, February 9, 2017

Arctic Update

It's time for another update on temperatures at selected surface observing sites adjacent to the Arctic Ocean.  For background on the choice of sites, see here.  January was another very warm month, relative to the average conditions of earlier decades, with a mean January temperature more than 4°C above the 1981-2010 normal.  This is down a little from the incredible warmth of last January, but it was still the 5th warmest anomaly of any calendar month in this data set (1971-present).  The top 3 months were November, January, and October of 2016.

Daily mean temperatures remained well above normal throughout January, and the 19-station mean has pushed back up to nearly +10°C in recent days (see the chart below).  The chart also shows a recent spike up to nearly +30°C in the red line, which shows the warmest of the individual stations on any given day.  This spike reflects extraordinary warmth a week ago at Cape Billings on the northern coast of Chukotka (176°E); the temperature reached +4.6°C or 40°F, at a time of year when the 1981-2010 normal mean daily temperature is about -17°F.  The departure from normal of the daily mean temperature last Friday was +27.7°C or +50°F, and this sets a new record for the most extreme daily warm anomaly at any of the 19 stations since 1971 (narrowly beating the extreme daily warm anomaly from two months ago - noted here).

The continued drumbeat of extreme warmth in the Arctic is of course reflected in sluggish growth of sea ice and record low sea ice extent for the time of year, according to the NSIDC's recent update.  January's update from the University of Washington's PIOMAS model indicates that Arctic sea ice volume is likely also at record lows and well below anything observed on this date even in recent warm winters.

Saturday, February 4, 2017

Satellite Temperature Estimates

Back in December I mentioned that I had found a way to begin working with land surface temperature estimates from the Suomi NPP satellite (launched in 2011); the modern radiometer on the satellite measures infrared radiation at a spatial resolution of 750m, which allows for nice high-resolution estimates of surface temperatures when clouds are absent.  I've recently also decoded similar data from the MODIS instrument on the TERRA and AQUA satellites, launched in 1999 and 2002 respectively.  The MODIS resolution is about 1km.

Here are a couple of images showing estimates of the minimum surface temperature observed by these satellites on January 18 over the Fairbanks area and points eastward into the hills; this is the day that temperatures bottomed out during the cold snap.  I've interpolated the data to a 750m grid in both cases for consistency (click to enlarge the images).  The similarities are striking, but greater spatial detail is evident in the S-NPP (lower) image, as expected.

It's gratifying to see that both maps show the cold conditions on the Salcha River in the bottom right; the Salcha River RAWS (SLRA2) reported -59°F, and the satellite data support this very cold reading.  On the other hand, the -65°F reported by the Upper Chena River HADS site (UCHA2) is not supported and was almost certainly in error.

Below is a comparison of the minimum temperatures (in °F) reported at each of the sites marked on the maps.  The agreement is, in my view, quite impressive, with only the Stuart Creek RAWS seeing a substantial difference if we exclude the Upper Chena HADS site.  At both Stuart Creek and at the Salcha RAWS, the estimates are better for the S-NPP, which probably reflects the superior resolution of the S-NPP instrument.