Tuesday, January 31, 2017

Winter So Far

January has been another relatively snowy month in Fairbanks, with a total accumulation of 17" - the highest January total since 2005.  The combined December-January total was almost exactly 50", which is the 5th highest on record for the two-month period.  Temperatures have averaged slightly below the 1981-2010 normal and close to the long-term (1930-present) normal, but with alternating cold and warm spells.  Snowy winters are typically colder than dry winters in Fairbanks.

As noted in earlier posts (e.g. here), the cool and snowy conditions are surprising in view of this winter's North Pacific sea surface temperature patterns.  The positive PDO phase and negative NPM phase, which continue to be evident in the latest SST data, would normally promote warm and dry conditions in the interior; we discussed this back in November.  Here's another map showing the average 500mb height anomaly (departure from normal) in November through January for 6 winters with a significantly positive PDO and significantly negative NPM:

Compare this pattern to what has actually transpired in the last 3 months:

The patterns are fairly close to the inverse of each other, and this of course explains the weather in Fairbanks: the flow pattern has produced enhanced westerly flow across western Alaska, thereby transporting moisture to the interior, and the tendency for higher heights in the Bering Sea has allowed cold air to drop south across Alaska at times.

The reason for the discrepancy appears to be that the weak La Niña in the tropical Pacific has had a much more dominant influence on the circulation pattern than expected (at least by me).  The map below shows the height pattern for the 10 strongest La Niña winters since 1950-51; this is a better match to what has happened this winter, although admittedly the La Niña flow is a colder pattern for Alaska than we've seen this winter.

The most intriguing part of the situation, I think, is how the North Pacific SST patterns have remained much more reminiscent of El Niño than of La Niña, despite the evidently profound influence of the La Niña episode on the atmospheric circulation.  A positive PDO and negative NPM are generally observed in association with El Niño, like last winter - but not this time.  But regardless of the cause, this winter's outcome will serve as a cautionary tale about the danger of relying too heavily on the North Pacific SST patterns in isolation for making a forecast.

Update Feb 2: This nice graphic from NWS Fairbanks shows the complete contrast between last winter and this winter in terms of early versus mid-winter snowfall accumulation; the totals before and after December 1 are almost perfectly reversed.

Friday, January 27, 2017

Duration of Cold Spells

Last week's cold snap turned out to be rather short-lived in Fairbanks, as there were only 3 days with a midnight-to-midnight minimum temperature below -40°F.  The cold persisted considerably longer in the western interior, and a relatively small distance made a big difference: Tanana saw daily minimum temperatures below -40°F for 9 consecutive days, including 5 days below -50°F.  And not far to the north, the SCAN site in the Kanuti NWR reported minimum temperatures between -56°F and -61°F for 6 consecutive days.

Prompted by a question from reader Gary, I looked into the duration of cold snaps in Fairbanks to see if there's evidence that periods of intensely cold weather have become shorter over time.  This is not a question of how much warming has occurred in the mean - we know Fairbanks now doesn't see the extreme cold that used to occur - but rather we are asking how long the coldest conditions stick around during periods of relatively cold weather.  Here's a chart of each winter's lowest daily mean temperature since 1930.

I began by looking at the evolution of daily mean temperatures during the most severe cold spells relative to the climate of two periods: 1931-1960 and 1981-2015.  It turns out that in the first of these two periods, 20% of the winters produced at least one day with a mean temperature below -50°F, and in the more recent period, 20% produced at least one day below -45°F.  We'll consider these the thresholds for a severe cold spell in the two periods respectively.

The two charts below show the evolution of daily mean temperatures for each of the cold spells within these two periods, as defined by having at least one day at or below the threshold temperature.  The events are labeled with dates indicating the first day that met the criterion (day 0 on the horizontal axis).  Notice that there were two such spells in the winter of 1933-34, but the temperature rose above freezing in between, so I would consider them as separate cold spells rather than a single long one.

The most striking feature of the comparison is that several of the cold spells in the earlier period were lengthy, and half of the events returned to -50°F or lower more than 10 days after the initial onset.  In contrast, only 2 of the 7 cold spells in 1981-2015 returned to a similar degree of cold after a week, and then only for a brief period.

A comparison of the multi-event means for the two periods (see below) shows a divergence after 5-6 days, with the modern climate showing a tendency for continued warming after one week, while in earlier decades the mean remained lower for a few days as cold sometimes returned for an extended period.  Based on this comparison, it certainly appears that cold spells have become shorter on average in recent decades, although admittedly the sample size is rather small.

Another way of looking at the change is to track the number of days spent below the cold threshold for overlapping 30-year periods.  The columns in the chart below show these numbers, and the line and markers above indicate the threshold for cold in each period.  The overall warming trend is reflected by the rising threshold for cold snaps, but the interesting result is the marked decrease in the number of days spent within cold snap territory; in recent decades, cold snaps (as defined here) have lasted only 1 or 2 days on average, whereas 5 or more days was typical in some earlier periods of Fairbanks' history.  So not only has the climate warmed in the mean, but considerably fewer days are spent at the bottom end of the temperature distribution in the modern climate. 

The obvious next question is why has the average length of cold spells become so much smaller over time?  A detailed answer would require analysis of the circulation patterns during the coldest episodes in Fairbanks - a project for another time.  For now I'll simply suggest that the climate of recent decades seems to have precluded the kind of persistent trough over eastern Alaska and western Canada that would be required to maintain very cold conditions in Fairbanks.  Even in last week's cold spell we saw that the ridge over western Canada was reluctant to budge, and as a consequence the trough had a difficult time progressing eastward; and so for much of the time the flow over Fairbanks had a southerly component, and moisture and clouds made their way up from the Gulf of Alaska.  Cloud cover was much more sparse over the western interior, and so of course those areas were much colder.  It would be interesting to look at historical cold spell durations in, say, McGrath, to see how they compare to Fairbanks.

Monday, January 23, 2017

Cold Kusko 300

The Kuskokwim 300 sled dog race occurred over the weekend, with the teams running from Bethel to Aniak and back.  The race is quite notorious for volatile weather conditions, which of course is not surprising as southwestern Alaska is subject to Bering Sea storminess and often experiences highly variable temperatures during winter.

This year the race was a cold one, with temperatures as low as -31°F in Bethel - normally the warmest location on the trail - and as low as -45°F in Aniak.  Looking back at past years, and without accounting for wind chill, this may in fact have been the coldest race in the 38-year history of the Kuskokwim 300.

The chart below shows the average temperature and wind chill in Bethel for each race from 1986-present, based on the 48 hours ending 3pm on the day when the first-place team returned to Bethel.  (I wasn't able to find the dates for the early races from 1980-1985).  Based on conditions in Bethel, this year's race was the coldest, with an average temperature of -25°F.  Mercifully, however, winds were relatively light this year, so the wind chill was not terribly low; the lowest wind chill occurred in 1989, when the race start was postponed 3 days to avoid even worse conditions during the severe cold spell of that January.

Perhaps the most striking aspect of the chart is the change in snow conditions that has occurred over the past few decades; it's been more than 10 years since the race has occurred with more than 10" of snow on the ground in Bethel, but this was common in the 1990s.  However, a quick look at the longer term history suggests that the 1970s and early 1980s also generally saw low snow depths in late January, so the increased snow in the 1990s may have been more of a decadal-scale anomaly than a reflection of a different climate in the past.

Thursday, January 19, 2017

Cold Snap

Clearing skies allowed temperatures to drop to very low levels in most parts of the interior in the past 48 hours, with Fairbanks airport reaching -51°F yesterday morning.  The high temperature for the midnight-to-midnight period was only -41°F.  These are the coldest conditions in Fairbanks since late January 2012.  Many other locations, distributed very widely across the interior, dropped well into the -50s, with the "winner" being the Kanuti Lake SCAN site at -61°F as of 5am today.

The following map, courtesy of NOAA/NWS, shows reported low temperatures below -50°F in the 24 hours ending around 5am today.

As we noted before, the deep trough over Alaska has imported very cold air aloft, so the low surface temperatures are entirely to be expected under clear skies with very little solar insolation and light winds.  The Fairbanks soundings have measured 850mb temperatures around -20°F in the past couple of days, and it's not unusual to see an inversion of 30°F or more from the surface to 850mb at this time of year.  Under the right circumstances the inversion can reach 40°F or more, but that's more common under high pressure.  The sea-level pressure (MSLP) has actually been considerably below normal in the past several days.

The relationship between MSLP and temperature inversions is illustrated by the pair of charts below, which show the 850mb and surface temperatures on high-MSLP days (top chart) and low-MSLP days (bottom chart) according to the winter-season Fairbanks balloon soundings since 1981.  There is considerably more scatter in the relationship when MSLP is high, because the surface conditions are more decoupled from the free atmosphere, and accordingly strong inversions are more common; notice that there are quite a number of days in the top chart with surface temperature more than 40°F colder than 850mb temperature.  In contrast, this almost never happens when MSLP is low, and it's not coincidental that surface temperatures almost never get below -40°F when MSLP is low.  Interestingly, yesterday had one of the lowest MSLP readings on record for a -40°F surface temperature in the Fairbanks sounding, and this attests to the strength of the upper-level trough.

Here's the 500mb analysis from yesterday morning, courtesy of Environment Canada:

Finally, here are some interesting FAA webcam photos of shallow ice fog layers yesterday at Chalkyitsik, Arctic Village, and Livengood.


Tuesday, January 17, 2017

Cold And Snowy

Fairbanks has seen an unusual combination of low temperatures with significant snowfall since early yesterday; temperatures remained well below -10°F during the day yesterday in spite of snowfall rates that brought visibility down to 3/4 mile at times.  The day's liquid-equivalent precipitation was 0.29", which is easily the highest on record in January for a day with a high temperature below -10°F.  The chart below shows the outlier in comparison to the historical distribution of precipitation and daily high temperatures in January.

The 500mb analysis from yesterday afternoon helps reveal the reason for the unusual conditions: see below (click to enlarge).  The cold trough aloft is deeply entrenched over the western half of Alaska, and Fairbanks is under a zone of strong temperature gradient aloft with southerly flow; notice the tightly-packed dashed lines over the southern and eastern interior, denoting a tight gradient between very cold air over western Alaska and much warmer air to the east.  (The dashed lines indicate 1000-500mb thickness, a good measure of overall temperatures in the lower troposphere.)  Cold air is reaching Fairbanks from the base of the trough, but the frontal zone is proving a lifting mechanism to generate clouds and snow.

If and when the frontal zone shifts east and clouds clear out, temperatures will drop sharply in Fairbanks; most of the western interior is cold this morning.

On a related note, there were a few comments the other day about the variability of surface temperatures during cold spells like the current one.  It's an interesting question as to whether surface temperatures are more or less variable when upper-air temperatures are low, so I looked at this by calculating the standardized anomaly of daily minimum temperatures for days in November through March since 1981; the chart below shows the relationship to 850mb temperature.  (The standardized anomaly is the departure from normal, divided by the climatological standard deviation; both the normal temperature and the standard deviation vary greatly by date.)

The chart shows a hint that surface minimum temperatures tend to be less variable when the upper-air temperatures are low; for example, with very low 850mb temperatures below -30°C, the surface minimum temperatures mostly fall in the range of 0.5-2.5 standard deviations below normal.  However, with very warm 850mb temperatures above freezing, daily minimum temperatures can be below normal or as high as 3 SD above normal.

The decreased variability during cold spells is seen in the chart below, which shows the standard deviation of the daily surface temperature anomalies in categories of 850mb temperature.  The relationship is more noticeable for daily low temperatures than for high temperatures, which is why I used low temperatures in the scatter plot above.  The results certainly confirm reader Gary's comment that "with 850 mb temps forecast to be -30 or below there's not a lot of room for variance in cold at the surface"; and I think this is because troughs of very cold air aloft are not generally conducive to very large temperature inversions near the surface.

Thursday, January 12, 2017

Ice Fog and Surface Temperatures

Rick T, here. Just a quick follow-up on the question of ice fog and possible impacts on surface temperatures. Ice fog and its impact on surface temperature is a tough nut to crack to due many confounds: societal changes, e.g. people leaving vehicles running, urbanization changes, airport siting and activity, ASOS vs. human observed visibility, etc.

My sense is that, physically, the long wave radiational depth of the ice fog (analogous to optical depth) is what matters for surface temperatures, which I don't think has much correlation to reported (horizontal) visibility. I think the the temperature manifestation of this can be seen in the RAOBs with very low surface temperatures when lapse rates become neutral or negative (i.e temperatures are steady or following with height). 
Here are two very cold soundings, one from January 1969, the pre-pipeline era, and one from 1989, with a significantly expanded urban environment and many more automobiles operating.  The 1969 sounding looks like a myriad of other winter soundings in Fairbanks: the temperature at the airport was about 15C lower than 400 meters up. The 1989 sounding is much different and quite unusual, with slight cooling in the lowest 400 meters. No two situations are of course the same, but part of the reason for the dramatic difference is, I think, the depth of the ice fog. In spite of very low visiblities in both cases, I'd suggest that in the 1969 case the ice fog was very shallow, while in 1989 it was thick enough that the effective radiative surface was the ice fog top (~600), with weak mixing within the ice fog.
In any case, no hard science here, just my sense of some of the issues involved based on my years of trying to forecast in these conditions.

Arctic Update

Here's a brief update on temperatures at climate observing sites around the Arctic Ocean in the month of December.  According to monthly data from a set of 19 stations, December was nearly but not quite the warmest in the period of record I've used here (1971-present), with an average departure from normal of +3.3°C.  In this data set, December 2005 was slightly warmer than December 2016.

The relatively high temperatures in December continued the pattern of persistent warmth that was observed all year in 2016.  According to data from these particular stations, the average temperature was at least 1°C above the 1981-2010 normal for all 12 months of the year; it's the first time that has happened in this data set.  Perhaps more strikingly, the 19-station mean temperature was above normal for all but 5 days of the year.

If we look at the annual number of days with a mean daily temperature below normal (see below), there was a sudden drop-off in 2005, but for the past decade the number of below-normal days has been relatively stable; the 2005-2015 average was about 75 days or 20% of the year.  However, 2016 was quite different, with below-normal days becoming rare for the Arctic coastline as a whole, and even more so in the second half of the year; the last below-normal day was August 5, 2016 - more than 5 months ago.

The near-complete absence of below-normal temperatures in 2016 is entirely consistent with ECMWF model estimates of high Arctic (80-90°N) air temperatures, as calculated by the Danish Meteorological Institute; their 2016 chart (see below) shows no below-normal days at all outside the summer season.  The charts provided for earlier years on the DMI website show nothing comparable for sustained anomalous warmth in the reanalysis history back to 1958.

Wednesday, January 11, 2017

Historic Fairbanks Cold Snaps

Rick T. here with a short post for future reference. Here's an annotated plot of the most extreme cold snaps during the Weather Bureau/NWS era (since late 1929) in Fairbanks, from downtown, Weeks Field, International Airport, wherever the observations were being made. Here we're looking at all the cold snaps with a 7-day average temperature of 40 below or lower. Note: average temperature, not average low temperature. The dramatic dropoff in frequency and intensity of cold snaps post-1975 is partly PDO shift, partly trend and partly urban growth and the attendant ice fog, which serves as something of a "blanket" for the valley floor.

For those in Fairbanks in 1989: yes, the lowest weekly mean temperature reported from the Airport was -43.0F, and I agree, I think there was a problem with the temperature sensor on at least a few days. But based on the Ft. Wainwright observations, such as there were, it was not greatly lower than that.

Sunday, January 8, 2017

Severe Cold Spell Possible

For those hardy residents of Alaska's interior who enjoy the occasional spell of intensely cold winter weather, and who have felt that the warmth of recent years has been over the top, the latest forecast indications may be welcome.  Others will feel less enthusiastic.  Recent forecasts have been coming into agreement with expectations for a period of very cold weather later this month, and there's a chance the cold may be extreme and prolonged.  Here are the 500mb height anomaly forecasts from 3 leading models for 7-10 days from now (click to enlarge): notice the extremely pronounced trough over Alaska and the good agreement between the models.

Here are the CPC's latest 6-10 and 8-14 day forecasts, emphasizing the likelihood of unusual cold over southwestern portions of the interior in particular:

And here are a few words from Rick Thoman (note this is personal opinion, not official NWS guidance):

"Models having been pointing this way for some days, and now we have increasing agreement on the potential for a period of prolonged deep cold Jan 15-21 or so. The basic forecast pattern has similarities to portions of the 1989 and 1999 cold events. The devil is in the details (of course), especially east of 150W, where there is potential for clouds and snow to wrap around from the GoA, and so keep surface temps higher (just as we saw in the 1999 event).

North of the Alaska Range there is a plausible potential for multiple days with 850mb temps lower than -35C and surface temps lower than -60F, which in the past has resulted in some of the commuter airlines (and Everett's) shutting down service to rural communities."

For reference, here's a chart of the mean 500mb height anomaly during January 20-30, 1989, when extraordinarily cold conditions affected the interior.  It's hard to imagine, but the temperature in Bettles dropped below -60°F on 9 out of the 11 days in this period, including -69°F on January 26.  Fairbanks reached "only" -51°F but had many days that stayed at or below -40°.

Saturday, January 7, 2017

Colder Again

The development of a strong high pressure system (1055mb+) across northern and interior Alaska sent temperatures into a dive in Fairbanks after snow ended on Thursday, with a drop from +22°F to -28°F in 24 hours.   Today the temperature struggled to get above -20°F at the airport.  It seems as if temperatures have been on quite a roller-coaster so far this winter, as seen in the chart below, but in fact the variability is not at all unusual.  Since November 1, the standard deviation of daily temperature anomalies has been 12.6°F, compared to the 1981-2010 normal of 13.3°F for the same period on the calendar.  Perhaps the volatility seems unusual to me because last winter was one of the least variable winters on record as far as temperature in Fairbanks is concerned.

It's interesting to look at how the winter-time temperature variance depends on the phases of ENSO, the PDO, and the NPM.  El Niño winters (like last winter) tend to bring reduced temperature variability to much of Alaska because of the persistent flow patterns associated with low pressure to the southwest of Alaska.  La Niña winters tend to be more variable as high-pressure blocking episodes develop and then dissipate in the vicinity of the Bering Sea.

The chart below shows a box-and-whisker representation of Fairbanks November-March temperature variability for different phases of ENSO.  For reference, last winter's standard deviation of daily temperature anomalies was only 10.8°F, the second lowest on record.

A parallel analysis for the PDO phase shows a similar relationship but with a more notable enhancement of variance on the "cold" side, i.e. negative PDO.

Finally, the NPM chart shows a surprisingly strong relationship, given that the NPM has little relationship with mean winter temperature in Fairbanks; negative NPM winters tend to have lower temperature variance.  Of course the fact that the NPM is uncorrelated with mean temperature does not at all mean that the circulation patterns are similar; the negative NPM flow pattern is more El Niño-like in the vicinity of Alaska, whereas the flow more resembles a La Niña pattern when the NPM is positive.

Given that this winter has so far been characterized by a modestly positive PDO and a strongly negative NPM, we would expect the temperature variance to be lower than normal in Fairbanks.  However, as we discussed with respect to the heavy December snowfall, this winter is not quite conforming to expectations.

Monday, January 2, 2017

Extremely Warm Day in Arctic Alaska

In the wake of the recent Bering Sea storm, a deep and very strong southerly flow has transported extremely warm air into northern Alaska, leading to exceptional warmth across the North Slope.  Remarkably, temperatures rose above freezing everywhere on the North Slope yesterday (or at least everywhere with a thermometer reporting in real-time), and today isn't much cooler.  The map below shows the maximum temperatures observed in the 24-hour period ending 5pm AKST yesterday.

Here is yesterday afternoon's 500mb analysis, showing the torrent of warm air flowing northward over the Bering Sea and far western Alaska.

Looking at the climate observing sites with long term histories, Utqiaġvik (Barrow) and Deadhorse both had a high temperature of 36°F yesterday, and Kotzebue hit 38°F for the midnight-to-midnight period.  This is the first time in the common period of record (December 1968-present) that all three stations have exceeded 35°F on the same day in winter; this has never before been observed between October 25 and April 13.  The chart below attempts to illustrate this in graphical form.