Implications of a Warm Dry Summer

The very warm, dry summer that occurred this year in interior Alaska raises the question of whether similar tendencies are likely to persist in the months ahead - in other words, is warmer and drier than normal weather likely to continue on average?

One way to explore this question is to look at previous years in which the summer was warm and dry, and then examine what happened in the subsequent months.  This is a kind of analog technique in which we explore the historical data to see if there might be any predictability based on the observed anomalies.  There are at least two possible reasons why we might hope for success: it's possible that whatever caused the summer warmth might continue to act as a consistent forcing through the next few months; or perhaps the sea surface temperature patterns that have emerged over the summer near Alaska might help nudge the autumn climate in one direction or another.

I chose to base the investigation (at least initially) on Fairbanks temperature and precipitation between June 1 and August 15, which this year were the 2nd highest and 4th lowest on record, respectively (since 1930).  I picked out all years since 1930 which had June 1 - August 15 temperature in the top 25% of all years AND June 1 - August 15 precipitation in the bottom 25% of all years; this provides 11 "analog years".  The summer was significantly warmer and drier than normal in all these years.

I then pulled out the temperature and precipitation data from the subsequent months in these years and classified each month as "below normal", "near normal", or "above normal", based on an equal division of the data in the 1951-2010 period.  The chart below shows the results for September, for Barrow, Fairbanks, and Anchorage; the colors indicate the number of years out of 11 that fell into each category.  So for example, Fairbanks September temperature was above normal in 3 of the 11 years, near normal in 3 of 11, and below normal in the remaining 5 years.  More significantly, September precipitation was below normal in 8 of the 11 years, suggesting that there is a good chance that September will be dry overall in Fairbanks (but see a cautionary note at the bottom).

The other major signal that emerges for September is at Barrow, where 7 of the 11 years were wetter than normal, and none of the 11 were drier than normal; thus Barrow seems quite likely to be wet.

The corresponding charts for October and November are shown below.  Interestingly, all three locations show a preference for above normal temperatures in October (especially Barrow), and in November the warm signal becomes relatively strong for Anchorage.

Another way of looking at the patterns in the analog years is to examine some maps.  The first map below shows the frequency of above normal 500 mb height in September; note that here I am simply comparing to the 1951-2010 mean rather than the three-way terciles (i.e. the map has no near-normal category).  There seems to be a fairly clear-cut signal for low heights in the Chukchi Sea, and the second map below shows a tendency for low pressure along the North Slope; this would explain the high chance of a wet September in Barrow.  It's not entirely clear why Fairbanks would be dry, unless perhaps the favored higher pressure over eastern Siberia means that the flow is more likely to be northwesterly (drier) rather than southwesterly (wetter) over interior Alaska.

The next three maps show the analog temperature pattern for September through November.  These are based on the NCEP global reanalysis, and so the quality is probably "questionable" over Alaska; but the general trends should be approximately correct.  In agreement with the charts above, we see a slight warm signal for the North Slope in September, more so for October, and then a rather notable warm signal for most of southern Alaska in November.

So to answer the original question: based solely on the history of past warm/dry summers, it seems likely that unusually dry weather will re-emerge in September in Fairbanks, but there's no indication that September will be unusually warm.  In fact, there is a slight signal that early autumn will be cooler than average.  For example, in Fairbanks 8 of the 11 analog years first reached 25 °F earlier than usual, and 7 of the 11 years saw the first half-inch or greater snowfall earlier than usual.  In October and November, the summer analog suggests warmer than average temperatures as the most likely outcome, especially for southern Alaska in November.  One caveat here is that a negative PDO (Pacific Decadal Oscillation) phase, as we're currently seeing, would favor a cold November in the south, so that raises the level of uncertainty for the November outcome.

** Note: forecasts for many weeks or months in advance are rife with uncertainty, and analog indications like these usually give only a hint of where the probabilities may favor one side of the climate distribution or the other.  We have to bear in mind that random chance alone can produce patterns and apparent signals in the data, so some of what we see here may be a statistical fluke.  To be precise, when we randomly select any 11 years and categorize the data into terciles, there is about a 37% chance that 6 or more of the years will fall into one of the categories, leaving 5 or fewer years in the other two categories.  The chance of 7 or more years in one category is about 12%, and the chance of 8 or more is 2.6%.  So when we see 6 of 11 years in the warm category for Fairbanks in October, there is only a 63% chance that this is a "real" signal, not random chance.

Light Frost

It's a frosty morning at northern interior locations; the new FAA webcam at Allakaket (see below) shows mist and what looks like frost.

Bettles and Eagle both dipped below freezing for the first time this autumn; this is very close to the median date for Eagle, and just a few days earlier than normal for Bettles.

Autumnal Air

Snow fell in the Brooks Range yesterday as cooler air worked its way down from the Arctic, and temperatures this morning are close to freezing at a number of interior locations farther north than Fairbanks.

At Barrow, the mean temperature yesterday was 32 °F, the first time this has happened since late June; 55 consecutive days had a mean temperature above freezing.  The chart below shows the annual (maximum) number of consecutive days with mean temperature above freezing in Barrow for each year since 1930.  This year's maximum of 55 days is lower than in some recent years, when the mean temperature did not dip to freezing until mid or even late September, but is higher than the long-term median.

Frequent Rainfall in Anchorage

The summer rainfall pattern is firmly entrenched in Southcentral Alaska. Of particular note is the number of days with substantial precipitation. So far in 2013 there have been 15 days with at least 1/3" of rain or melted snow. That is that most ever through this point in the season. The normal tally is only 8.7 days through August 19th. When looking at all precipitation events, the numbers are much closer to normal. 70 days have had at least 0.01" of precipitation while the normal number of days is 62. With all of the heavy precipitation events, it would not be surprising if 2013 had the most rainfall season-to-date of any year, but that is not the case. 2013 is in 5th place through August 19th on the list of wettest years so far. As the second chart below shows, the typical amount of precipitation on days where at least 1/3" is actually only 0.45". This tells us that there have been many days with 1/3" but very few with substantially more than that.

Twilight Lengthens

It's starting to get just about completely dark each night now in Fairbanks.  Astronomical twilight was reached yesterday for the first time, and starting tomorrow each night will have more than 6 hours below the civil twilight limit.

In Barrow, nautical twilight begins today, i.e. a period each night when it's dark enough to need artificial lighting for normal outdoor activities.

Here's this morning's north-northeast view from the FAA webcam at Point Hope, at the western end of the Lisburne Peninsula.  The sun is rising well around to the east now.

Summer 2013 - Arctic Temperatures

Two brief items of interest.  First, here's a nice summary graphic for "summer 2013" that Rick showed on Thursday at a NOAA "state of the climate" national briefing.

Second, after some discussion prompted by reader Mike, I created an animation of recent Arctic temperatures and temperature anomalies.  It's nice to be able to track the development of cold air masses as winter begins in the high Arctic.  Here's the link; the data should update once a day.  It may take a few seconds for all the images to load.

http://www.worldagweather.com/arctic/t2m.html

Here's last night's temperature analysis.  The air mass north of Alaska and Canada's Arctic Archipelago is unusually cold for the season, but temperatures are well above normal north of Scandinavia.

Season-to-Date 60, 70, and 80 Degree Days

With all of the talk about the number of 70 and 80 degree days so far this summer, it seems like a good time to take a preliminary look at season total up to this point in the year. The following three maps show the number of 60, 70, and 80 degree days across Alaska in 2013. There are 172 stations that have a reported temperatures through mid-August (61 Coop, 54 RAWS, and 57 NOAA). The analysis utilized an inverse distance weighting (IDW) algorithm to estimate values for all location. It is not an "exact fit" technique so some stations may be shown in a different category than their data would otherwise indicate. In other instances, the best guess by the program is clearly erroneous (e.g., Alaska Range, St. Lawrence Island, etc.) since it uses the values of the 6 nearest stations. Sometime toward the end of September and end-of-season analysis will be done. Note: Some of the RAWS data in particular may have a warm bias.

Climatology of Warm Summer Nights

Back in July we had some extended discussion about high daily minimum temperatures in Alaska, so I've been meaning to look at the climatology of "warm nights" in the state.  I use the phrase loosely as the highest daily minima tend to occur when "night" is fleeting or non-existent.  For the purpose of this project, I looked at daily minimum temperatures of 60 °F or above; this is well above the peak summer average minimum temperature for any Alaska location, and so these events are unusual and notable wherever they occur.  In my mind, a 60 °F overnight minimum also marks something of a threshold in terms of comfort level - anything warmer, and summer heat tends to accumulate in buildings.

The first map below shows the average percentage of days in June through August when the daily minimum is at or above 60 °F, based on the 1981-2010 data.  I'm showing only stations with nearly complete data.  Blue "0.0" markers indicate locations where no such events occurred between 1981 and 2010, while green "0.0" markers indicate at least one event but less then 0.1 % of all days.

Some notable aspects of the data include:

- the high frequency at Fairbanks airport, 3.4 %, the highest of any location
- the low frequency at the Eagle COOP station, 0.2 %.
- the relatively high frequency at Kotzebue (1.4 %), which is not only above the Arctic Circle but also adjacent to the cold northern ocean.

Looking more closely at the situation in the Tanana valley, we see that Fairbanks airport reports "warm nights" much more frequently than other Fairbanks locations:

Fairbanks airport   3.4 %
College 5NW (a few hundred feet up from the valley floor)   2.0 %
College Observatory (UAF West Ridge)   1.2 %
University Experiment Station   1.2 %

The differences in frequency mirror the differences in average minimum temperature in summer, with the airport being warmer than the other Fairbanks locations by 1.5-2 °F on average.

The long-term warming trend across Alaska is evident if we compare the above results to the same calculation for 1951-1980, see below.  Nearly all stations with data from both periods show a higher frequency of warm nights in the more recent decades, with notable increases at Fairbanks and Kotzebue.  A major exception is Kodiak, which showed a large decrease; this is interesting because the average minimum temperatures are slightly higher in the more recent period.  It seems Kodiak had an exceptional number of warm summer nights in the 1950's and again in 1979.

It's also interesting to look at the seasonality of warm nights; the map below shows the median date of occurrence, for locations with at least 10 events between 1981 and 2010.  Early July is the most likely time of year in the northern interior, but the median date is pushed back as you move west or south, with more maritime influence and a later seasonal peak in temperature.

The seasonal distribution at Fairbanks airport is shown below; the frequency ramps up quickly in late June, drops suddenly after July 10, and then tapers off gradually into August.  Over 60 percent of the events are between June 21 and July 10.

Finally, the analysis would be incomplete without a comparison of 2013 to previous years in Fairbanks, see below.  The first thing to note is that prior to 1965, 60°-plus nights were almost unheard-of in Fairbanks.  Secondly, although 2013 beats out 2004 for number of warm nights, surprisingly it is still well behind the record of 21 days in 1975.  Nearly all of the 60°-plus nights in 1975 occurred in July, which remains - by some margin - the warmest month on record in Fairbanks.  Remarkably, the monthly average high temperature in July 1975 was only 7th highest on record, but the monthly average low temperature was an extraordinary 59 °F, by far the highest on record.

Change Approaches

Computer model forecasts are indicating that the end is near for the upper-level ridge and attendant warmth over eastern Alaska and western Canada; the pattern should soon be replaced with westerly flow and much cooler temperatures.  Of course, this is overdue for the time of year.  The images below show two of the leading computer model forecasts for the middle of next week (Wednesday afternoon Alaska time); the shading indicates temperature anomaly (departure from 1981-2010 normal) at 850 mb, and the dotted contours show the 500 mb heights, which run approximately parallel to the upper-level flow.

Although the outcome could differ quite a bit from these model predictions (a week is a long time in Alaska weather), both forecasts indicate major change.  Fast westerly flow is expected along with below-average temperatures across northern Alaska.  Translation: much cooler in the interior and north, with rain likely for many places.

Will 2013 Be Fairbanks' Warmest Summer?

As of August 13th, Fairbanks is on pace to have the warmest summer on record. The average temperature from June 1 through August 13 has been 65.6 degrees. The warmest summer on record, 2004, was 65.5 degrees through August 12th. Will 2013 break the record? If the temperature in Fairbanks were normal for the rest of August, 2013 will end up in second place. To break the record, the average daily temperature must average 60.05 degrees between August 14th and August 31st (5.66 above normal). A tall order to say the least. Only 3 seasons (2007, 2004, and 1977) have had and average temperature greater than 60 degrees between August 14th and August 31st.

Warmest Week of Record in Inuvik

This isn't quite Alaska weather and climate, but isn't far off and seems notable enough to write about.  The Canadian town of Inuvik, close to the Mackenzie River and not far from the Arctic Ocean, recorded their warmest week on record for the week ending Friday, August 9.  Below are the top 6 warmest weeks (non-overlapping; see a note at the bottom about the station locations):

August 3-9, 2013      71.9 °F
June 30-July 6, 1998      71.4 °F
July 10-16, 1989      70.6 °F
July 17-23, 2001      70.6 °F
July 28-August 3, 1994      70.5 °F
June 26-July 2, 1982      69.7 °F

As we've seen for Alaska locations this summer (notably Fairbanks), it was the high overnight minimum temperatures that really set this event apart from others in the past.  The highest week-average minimum temperature broke the previous record by a large margin:

August 4-10, 2013      62.0 °F
June 30-July 6, 1998      59.9 °F
July 10-16, 1989      59.3 °F
July 30-August 5, 1994      59.3 °F
July 17-23, 2001      59.3 °F
July 2-8, 2012      59.1 °F

The weekly maximum temperature was the third highest on record.

Besides the event itself, it's interesting how late in the year it occurred, with average temperatures about 3 °F lower than at the climatological peak around July 10.

The chart below shows the minimum temperature observations since May 1;  most of the summer was not outstandingly warm until August arrived.

**Note: I've combined data from Inuvik Airport (1957-2006) and the Inuvik climate/upper air site (2004-present).  The two sites are just a couple of km apart, with the climate site being 35 m higher.  I verified that during the brief 2004-2006 period of overlap, the June-August daily minimum temperatures had almost no systematic difference (climate site less than 0.1 °F warmer), whereas the maximum temperatures were warmer by 1.5 °F on average at the airport.  This makes the record all the more significant, given that observations are now taken at the climate site.

Canadian Ridge

**Update 6 pm AKDT Aug 12** Eagle airport is reporting 90 °F at this hour - the hottest ever recorded in Eagle so late in the season.  So we have a significant record after all.

**Update 6 am AKDT Aug 12**  Eagle dropped to 54 °F by midnight, so the 24-hour minimum was 54, not 60.  No records broken.  Also, there appears to be little doubt that the September 1995 temperatures were correct.  The Eagle RAWS data show hourly temperatures of 60 or above, with a southeasterly breeze, from 6 am on the 20th to 7 pm on the 22nd.  The Eagle COOP observations were taken at 7 am, so the numbers match very well; in fact the midnight-to-midnight lowest hourly reading at the RAWS was 67 for the 21st.  Also, numerous other interior Alaska sites reported minimum temperatures of 60 or above on the 20th and 21st, including 63 at North Pole on both days, and 65 at Fairbanks airport and Eielson AFB on the 20th.  Clearly the 1995 event was an extreme outlier for the time of year; it probably deserves its own write-up. **End of Update**

Extremely warm air continues to reside over northwestern Canada under a persistent upper-level ridge.  Close to the border, Eagle airport recorded an overnight minimum temperature of 60 °F last night, which is the highest recorded so late in the season except for a remarkable pair of readings in late September 1995.  The all-time highest minimum temperature at any of the Eagle stations (as far as I can tell) is 64 °F in that 1995 event, which admittedly looks somewhat suspect and deserves further investigation.  Minimum temperatures of 60 °F or above are quite rare in Eagle, as I'll show in a subsequent post that I'm working on about the climatology of high minimum temperatures.

Elsewhere in northern Alaska, there was a hint of autumn in Bettles this morning, with fog and 37 °F.  The median date for Bettles to first reach 37 °F or lower in the late summer is August 10, so this is right on schedule.

Summer Smoke

The Fairbanks International Airport has observed smoke the last several days from the Mississippi Fire near Delta Junction. So where does the summer of 2013 rank in terms of the number of smoky days in Fairbanks? Through August 10th, 9 days have recorded smoke during the hourly observations. Since 1997, smoke was observed an average of 10.6 days during the months of June, July, and August. It appears that Fairbanks is on track for an average number of smoky days.

Hottest Fair EVER

Seven days down and three to go for the Tanana Valley State Fair, and with high temperature again exceeding 80ºF on Thursday, this is already the hottest TVF ever, — at least if you're counting days with a high of 80ºF or higher: five days have had highs of 80ºF or higher.

Here's the plot the number days with high temperatures of 80ºF or higher since 1952:

As reader Gary pointed out, the dates of the Tanana Valley Fair have varied over the years, as has the length. Fairs were generally 3 or 4 days long until the early 1970s, when they bumped up to 5 to 7 days in length. Since the early 1990s Fairs have been mostly nine or ten days long. However, it's clear on the above plot that the frequency of hot days corresponds with the change to in start dates of the Fair from sometime in the middle of the month (usually the second week) to the first Friday of August.

Notable Temperatures

Here's a brief round-up of temperature records and near-records in the past couple of days:

- Fairbanks max temp of 85 F on Wednesday was a daily record; Fairbanks reaches 85 this late in the season in less than 10 % of all years.

- Bettles max temp of 83 F on Wednesday was a daily record.  Also, the preliminary minimum temperature of 60 F this morning would be only the 3rd time that a low in the 60's has been recorded so late in the season.  The August record high minimum is 61 F.

- As mentioned earlier, Fort Yukon apparently reached 88 F on Wednesday, which would tie the record highest temperature observed there in August and also tie the highest so late in the season.  However, the Fort Yukon GHCN data is highly incomplete in recent decades.

- Deadhorse reached 72 F and Nuiqsut reached 77 F on Wednesday, both notable but not records.

- Delta Junction reported an overnight minimum of 66 F last night, but I'm not sure which historical station series to compare to for a possible record.

Over the border in Canada this morning:

- At Inuvik airport, the lowest hourly temperature overnight was 64 F; the record highest minimum so late in the season is 63 F (data from 1957-2005).

- Komakuk Beach, on the Arctic coast just across the border, reported a minimum hourly temperature overnight of 61 F, which compares to an August record highest minimum of only 56 F (data from 1959-1992).  The all-time highest minimum is only 62.

Finally, on the other temperature extreme, and on the southern coast (oddly enough!):

- Kodiak airport reported a temperature of 40 F this morning, a new daily record minimum temperature.  The temperature has been lower in the first half of August on only 4 occasions.

Most Extreme Year Ever for Fairbanks?

** Updated at 1:00 p.m. on 8/8/13 to correct a rather embarrassing math error **

As a follow-up to Richard's excellent post from last week, I decided to re-calculate the daily anomalies based upon the "new" NCDC formula that he and I discussed via email for generating daily standard deviations. The following charts for Fairbanks and Anchorage show the results of that reanalysis.

Here is how to interpret the charts - from bottom to top.

1) The red line shows the annual percentage of days that the temperature was more than three standard deviations either above or below the mean; e.g., +3.2, -3.5, etc. Since it uses percentages, 2013 can be compared to other years on equal footing. Also note that the categories are non-overlapping.

2) The dark blue line shows the percentage of days where the daily temperature anomaly was between 2 and 3 standard deviations either above or below the mean; e.g., +2.1, +2.7, -2.4, etc. The dashed line is the expected value based upon the normal distribution.

3) The solid burgundy line shows the percentage of days where the daily temperature anomaly was between 1 and 2 standard deviations either above or below the mean; e.g., +1.5, +1.2, -1.6, etc. The dashed line is the expected value based upon the normal distribution.

4) The solid green line shows the percentage of days where the daily temperature anomaly was between 0 and 1 standard deviations either above or below the mean; e.g., +0.2, -0.9, -0.7, etc. The dashed line is the expected value based upon the normal distribution.

5) The solid orange line at the top shows the Chi-Squared goodness of fit value. It is a squared, weighted measure of the difference between the actual and expected values and is the gold-standard for categorized (grouped) data. A value of zero indicates that the distribution of anomalies exactly fit a normal distribution. Any Chi-Squared value less than 6.0 (with 2 degrees of freedom) indicates that that year's temperatures approximated a normal distribution at he 95th percent significance level. The larger the value, the more anomalous (less normal) the distribution is. It can also be thought of as a measure of distribution extremes. By this metric, Fairbanks in 2013 (through August 7th) has had the most extreme temperatures of any year on record (post-1930) by a large margin. In fact, if the rest of 2013 is exactly normally distributed, it will still be the most extreme year on record. For Anchorage, 2013 is also in 1st place by a wide margin.

The graphic below shows the calculation for Fairbanks in 2013.

It is worth noting that if additional categories are used that to distinguish positive and negative anomalies, 2013 is still easily in first place overall for Fairbanks but not by as much. If the rest of the year was "normal," 2013 would only rand in 10th place. 1993 is a good year to look at. That year 11% of days were between -1 and -2 SD and 17% of days were between +1 and +2 SD. When you look at those as separate categories, they each differ fairly significantly from the expected value of 13.6. However, when they are combined, the number matches almost perfectly with the expected value. Of course, that knife cuts both ways.

Here is a good explanation of Chi-Squared:  http://www.stat.yale.edu/Courses/1997-98/101/chigf.htm

Here is a good site to manually enter numbers and check the Chi-Squared value: http://www.quantpsy.org/chisq/chisq.htm

Fairbanks Precipitation Update

Reader Mike asked about Fairbanks precipitation since May 1 and whether the dry conditions are within normal climate variability.  The chart below, showing annual values for May 1 - Aug 6 precipitation, indicates that the dryness is very unusual, but not unprecedented - the precipitation for this period is the 3rd lowest in the Weather Bureau/NWS era, exceeding only 1957 and 1966.  The number of days with measurable precipitation (17 out of 98 possible days) is also the 3rd lowest.

On another note, it appears Fort Yukon reached about 88 °F today, which ties the highest recorded temperature for so late in the season according to GHCN data - although the GHCN historical record is highly incomplete in recent decades.  Rick may be able to comment on whether more recent data are available.

Hot Weather and the Tanana Valley Fair

Tuesday was the fifth day of the Tanana Valley Fair, and the third day thus far with a high temperature in the 80s.  So this raises the question; how unusual are 80 plus temperatures this late in the summer in Fairbanks?

Here's a plot of the calendar day frequency (1930-2012) of days with maximum temperature ≥80F (disclaimer: the plotted data is a three day running mean frequency, just to smooth out the curve):

As you can see, the frequency of hot days in early August is about half that in late June and early July. So while 80ºF temperatures are uncommon this time of year, this is by no means unprecedented.

So, today's trivia questions are: what's the most number of days during the Tanana Valley Fair with a high temperature of 80F or higher?  And will the 2013  Fair break that record?

Warm Surge

Here's a nice visible satellite picture of the low pressure system currently spinning over the ocean south of the Alaska Peninsula.  The low was analyzed at 983 mb this morning, making it the deepest low to the south of Alaska since April.  Far to the northeast, over Canada's Northwest Territories and Arctic Archipelago, there is an enormous blocking ridge, as evidenced by the mostly clear skies in the interior.

As the low weakens and pulls back to Bristol Bay in the next couple of days, a powerful surge of warm air will be drawn northwestward across interior and northern Alaska.  The most unusual conditions relative to normal may be along the eastern North Slope, where the computer models are indicating 850 mb temperatures above 15 °C by late tomorrow.  The forecast graphic below shows a time-height cross-section of temperature at Barter Island; the horizontal axis shows time (UTC) increasing to the right, beginning at 12 UTC (4 am AKDT) today, while the vertical axis indicates pressure in mb.

It seems rather likely that some high temperature records may be broken across northern Alaska in the next day or two.

Uniform Temperature Days

Last November I noticed that Juneau had a day where the high and low temperatures were identical. It got me to thinking about how common of an occurrence this is. Well, here is the answer for the "Big 3" cities in Alaska. Surprisingly, Fairbanks has had more of these days than either Anchorage or Juneau. Note: I will check surrounding stations for the June 1905 occurrence in Fairbanks.

Anchorage - 1 occurrence
1/8/1917  Max = 11, Min = 11

Juneau - 2 occurrences
10/26/1942    Max = 57, Min = 57
11/13/2012    Max = 32, Min = 32

Fairbanks - 3 occurrences
11/12/1904    Max = 10, Min = 10
6/25/1905    Max = 54, Min = 54
12/24/1909    Max = 8, Min = 8

Unusual Temperatures

As a follow-up to Brian's and Rick's illuminating posts on Friday and Saturday, I thought it would be interesting to examine more closely how "unusual" the "unusual temperatures" of 2013 have been.  Brian showed that, in Fairbanks and Anchorage, temperatures more than 1, 2, or 3 standard deviations from the mean have occurred much more often in 2013 than would be expected based on the normal distribution.  The question is, how often does this occur, or might it be unpredecented in the modern historical record?

The charts below show the number of days each year since 1931 when the daily mean temperature was more than 1, 2, or 3 standard deviations from the preceding 30-year average, for dates between January 1 and August 2.  Note that I compared the temperatures to the preceding 30-year average rather than the 1981-2010 average to help reduce artifacts associated with the long-term warming trend.  For example, temperatures in 1961-1970 are compared to the 1931-1960 mean and standard deviation.  (If we used the modern mean, many of the older temperatures would look artificially cold and would boost the counts in earlier years.)

We can see that 2013 really stands out in the "2 SD or more" category, with this year's count exceeding any previously observed for both Fairbanks and Anchorage.  The number of days at least 1 SD from the mean is very high but not unprecedented, while the 3 SD count is a record in Fairbanks but not Anchorage.

Note that my numbers differ slightly from Brian's for 2013, which is probably because the calculation method was slightly different for the 1981-2010 daily means and standard deviations (a smoothing method is required).

It is interesting that the total 83-year count for the "3 SD or more" category is much higher for Anchorage than Fairbanks (87 versus 39).  If the daily temperature anomalies truly followed a normal distribution, the counts would be around 48.  This suggests that the Jan 1 - Aug 2 Anchorage temperature distribution might have "fat tails" compared to the normal distribution, i.e. a higher chance of high-sigma events.  But strong decadal variability or long-term trends could also be playing a role - more investigation is required.

Warm Spells and Cold Spells in Fairbanks

I have thought a lot about what constitutes a cold spell or a warm spell over the years. We have all heard countless times something close to, "what a nice stretch of weather we have had" or "it's been really cold for the last two weeks." Based on a cursory review of the literature, there is nothing even approaching a definition of where to put the bounds on a homogeneous grouping of days. So how do we quantify something that is inherently ambiguous. We can all agree that if the temperature is 2 standard deviations above the mean for 15 straight days that that would constitute a warm spell. But what if day 8 (of the 15 days) was cloudy and wet and was 0.1 standard deviations below the mean; leaving 14 of the 15 days 2 standard deviations above the mean. Is the 15-day period a single warm spell with one outlier or is it two distinct warm periods? What I have temporarily proposed is that if 6 out of a group of 7 days are in the upper third (tercile) of the expected temperature range, each of those days is a member of a warm spell. Conversely, if 6 out of a group of 7 days are in the lower third (tercile) of the expected temperature range, each of those days is a member of a cold spell. This is an entirely subjective criteria and I am very interested in suggestions from the readers for an alternative criteria. I discussed this with Rick a few months ago and he chimed in with a few thoughts so he is off the hook (for the time being). The first graphic below shows standard anomalies for Fairbanks during May 2013. The cold spell at the beginning of the month and the warm spell at the end of the month are identified. Make note of the text on the graphic that explains why certain observations are included in the respective groupings. Also, there is usually a transition period between one spell and the next but the example below does not have such a transition period.

The next chart shows the length of cold spells in Fairbanks since 1920 using the methodology described above. The typical length of a cold spell is 13.5 days and has been nearly steady over the last 90 years. However, the time interval between cold spells has not always been steady.

Here is the companion chart for warm spells in Fairbanks. The length of warm spells has increased from 13 days to 14 days over the last 90 years. However, as with cold spells, the interval between warm spells has not been uniform.

The final chart shows the number of days per year that Fairbanks has been in a cold spell (blue), a warm spell (red), and the combined number of days (black dashed).

As stated earlier, I am interested in hearing the thoughts of others on the criteria described in the introductory paragraph.

Fairbanks Temperature Updates

Here's some updated Fairbanks temperature and anomaly plots to compliment Brian's post Friday. First, the plot of the daily mean temperature reflected around the 1981-2010 normal:

The sustained, though not monolithic warm weather since late May is evident. Next is the raw daily departure from normal (and for good measure the year to date running departure):

As we all know, absolute temperature departures from normal are much smaller in the summer than in the snow cover season. So, of course I have to include a standardized departure plot, which accounts for the much smaller temperature variability in the summer.

This is the five-day running standardized departure, which smooths the day to day variation and clearly illustrates that we've had a lot of unusual weather since late March.

Year to Date Temperature Anomalies

So far the temperatures in 2013 have run hot and cold both literally and metaphorically. Through August 2nd there have been 214 calendar days for which daily temperature anomalies can be evaluated. The chart below categorizes the daily temperature anomalies for the three main population centers in Alaska by standard deviations from the mean. An expected value, based on the normal distribution, is also shown for each category. What really stands out is how many days both Anchorage and Fairbanks were more than on standard deviation from the mean and how many days all three cities were more than two standard deviations from the mean.

Note: So far this year Fairbanks is 1.4 degrees below normal, Anchorage is 0.6 above normal, and Juneau is 0.4 degrees above normal.

July anomalies

Below are a couple of maps showing the July temperature and precipitation anomalies at observing locations across Alaska.  The format was inspired by Rick's practice of comparing anomalies to the width of the local historical distribution; this allows a more meaningful comparison between locations with widely differing climates.

In the first map (temperature), the area of each circle shows the magnitude of the standardized anomaly, which is calculated as (July mean - 1981-2010 mean)/(1981-2010 standard deviation).  The second map (precipitation) depicts the significance of the precipitation anomaly by using the rank of the July total compared to the 30 July totals in the 1981-2010 period; so the largest possible circle is obtained when the July total is either smaller or larger than any in the 30-year period.  If the July total falls in the middle of the historical distribution, the circle has zero radius (i.e. precipitation was close to average).

The numbers at the observing locations show the actual values for the mean temperature and total precipitation.

Sunshine Duration Through the Year

The sun sets at Barrow on Friday morning, August 2nd at 157am AKDT for the first time since early May. So here's a plot of possible sunshine (duration the sun is above the horizon) through the year for some representative Alaska location:

I've include 18 months to get the full effect of month the winter minimum and summer maximums.

Why So Warm?

Reader Gary asked about the cause of the warm summer in much of Alaska and pointed to the upper-atmosphere flow pattern as an explanation.  In a certain sense this is undoubtedly correct, because observed weather patterns at the earth's surface are always closely related to the configuration of the air flow at various heights in the atmosphere.  This summer has seen unusually strong upper-level high pressure across Alaska, especially in the south.

Looking at a few maps, the first chart below shows the average (1981-2010) 700 mb height field in June and July, which can be thought of as the pressure pattern at about 3 km or 10,000 feet above sea level; there is a mean trough in the Bering Sea and extending into the Gulf of Alaska, and a ridge extending up from British Columbia to eastern Alaska.

Now let's look at the average 700 mb height this summer so far (since June 1), see below.  Heights have been much higher than normal over most of Alaska, as the southeastern ridge has been very strong and the Bering Sea trough has been weak.

Taking the difference between this year's average and the long-term mean shows where the height has been most unusual, see below.

What does this all mean?  Two things, as I see it: first, the source region of the air over southern and interior Alaska has been predominantly far to the south, so that the prevailing air mass has been very warm.  Chilly Arctic air has stayed out of the picture, because the strong ridge has kept cold fronts well to the north and west.  Second, the ridge has caused subsidence and low humidity in the lower atmosphere, leading to plentiful sunshine and dry conditions, which in turn favors higher temperatures.  Thunderstorms have been scarce because the atmosphere has been stable.

Having said all this, in my view the upper-air flow pattern is ultimately unsatisfying as an explanation for the warm weather, because the question immediately turns to "what caused the ridge"?  Why did the weather pattern evolve this way in the summer of 2013?  This is an enormously difficult question, and I only have one small piece of possible insight to offer: this summer's pattern may (perhaps) be (partially) related to the phase of the Quasi-Biennial Oscillation (QBO).  The QBO is a fairly regular cycle in east-west winds in the tropical stratosphere, i.e. about 15-50 km above sea level at the equator.  Over a period of about 2-3 years, the wind direction reverses from east to west and back again, and it is known that the phase of the oscillation affects weather and climate variations around the globe.  This summer there happens to be a strongly positive phase of the QBO, so I looked up previous years with similar phase and found the following pattern of 700 mb heights near Alaska:

The map shows that higher heights are more common from the Bering Sea to southeast Alaska when the QBO phase is significantly positive in summer.  So it's possible that the QBO phase this summer raised the likelihood of a strong ridge developing over Alaska, although it certainly cannot be cited as the only reason.  Note that several of the top QBO years listed on the map brought cool summers in Fairbanks (e.g. 2008, 2002, 2006), so it is far from a one-to-one correlation.  Also, I can't comment on the exact mechanism by which the QBO might influence the flow pattern near Alaska - it's purely a historical observation and may be a coincidence.

Rick and Brian may have more to add on causes of the warm summer - and any other insight or suggestions from readers would be most welcome.