Wednesday, April 30, 2014

Decadal Temperature Trends

The second half of April 2014 in Fairbanks will end as one of the warmest on record, although the full month will be only a little above normal because of the cold spell earlier in the month.  Unusual warmth in spring is very consistent with the trend in the past decade or so in Fairbanks, at least compared to decades prior to the strong 1987 El Niño.  For example, the April-May mean temperature in Fairbanks has been above the 1951-1980 average in 22 of 27 years since 1987; and ten straight years from 2003 to 2012 were warmer than that older normal.  Last year was of course a dramatic exception to the rule.

The charts below show a comparison between long-term mean temperatures by month, by season, and for the annual means; the green columns show the differences between the 1981-2010 mean and the 1951-1980 mean, while the blue columns show the difference between 2004-2013 and the 1951-1980 mean.  Looking at Fairbanks first, the most recent 10 years were warmer than the 1951-1980 mean in all four seasons, with the largest warming in spring (April-May).  The winter season warmed the most in the 1981-2010 period, but the negative PDO phase has brought cooling more recently in winter.  Curiously, Novembers have been cold of late, but Decembers have been very warm.

The situation at Barrow is remarkable, of course, with every month and season except March being warmest for the 2004-2013 period.  The recent warming has been most dramatic in October and November.

Charts for Anchorage, Nome, and Cold Bay are included below.  I added Cold Bay because of the extraordinary warmth there in the past several months, as highlighted by Brian yesterday.  Each of these locations has seen slight cooling in winter and spring (and for the annual mean) in the most recent 10 years compared to 1981-2010, but each location has warmed in autumn.

Tuesday, April 29, 2014

Temperatures Since October 1st

As Rick noted the other day, there has been a wide divide in the daily temperature anomalies across the State. Since October 1st, almost all of Alaska has ranged between above normal to way above normal. Figure 1 shows the average daily temperature anomaly between October 1st, 2013 and March 31st, 2013. Western Alaska in particular has been quite warm. This is in stark contrast to the Lower 48. Figure 2 shows the temperature anomaly for the same time period for the entire U.S. (Note: the color scale for Alaska is different).

Figure 1. October through April temperature anomaly for Alaska.

Figure 2. October through April temperature anomaly for the entire U.S.

As noted above, Alaska was quite warm compared to the rest of the country. Figure 3 shows the list of largest temperature anomalies in the U.S. from October through March. Only one non-Alaska station made the list.

Figure 3. Table showing the stations with the largest positive temperature anomaly in the U.S. Note: Cooperative stations were evaluated for Alaska but not for the Lower 48.

Another way to look at the distribution of temperatures is to break them down by climatological thirds. By definition, a tercile is a grouping where the expected value is 1/3. The upper tercile for temperatures is the group that you expect the anomalously warmest 1/3 of your days to fall into. The lower tercile is the 1/3 of days you expect the anomalously coldest 1/3 of your days to fall into. The middle tercile is the 1/3 of days that hover around the daily normal. If temperatures are normally distributed, over long time periods you would expect that 33% of days would fall within each tercile. Figure 4 shows the tercile breakdown for 10 geographically diverse stations across Alaska from October 1st to April 18th (200 days). Amazingly, Cold Bay has been in the upper tercile over 70% of the time!

Figure 4. Tercile breakdown for 10 stations in Alaska.

Finally, if we just look at the percentage of days either above or below normal, we can get a sense of the temperature trend regularity without the magnitude. Figure 5 shows the percentage of days that were above normal between October 1st and March 31st in Alaska. Figure 6 shows the corollary data (below normal percentage) for the entire U.S.

Figure 6. Percentage of days from October through April in Alaska that were above normal. 

Figure 7. Percentage of days from October through April in the entire U.S. that were below normal. 

***** Update Section ***

Figure 8. ESRL reanalysis of October through March temperature anomalies in the Northern Hemisphere. Units are in degrees Celsius.

Monday, April 28, 2014

Fairbanks Cloud Cover vs Temperature

Reader Eric suggested that the relationship between cloud cover and temperature in Fairbanks would be nicely revealed using scatter plots; he is right, of course, and the results are below.  The winter positive correlation between cloud cover and temperature is significant from November to March (and strongest in November), while the summer inverse correlation shows up quite strongly in June through August (and is strongest in June).

Note the different horizontal axis scale in the plot for summer; monthly mean cloud cover is nearly always considerably more than 50 percent in summer.

Sunday, April 27, 2014

Around Alaska in 2014: Temperature Departures

For fun, here's a scatter plot of daily standardized temperature departures (1981-2010 normals) so far for 2014 for Fairbanks, Nome, Anchorage and Juneau. The color scheme here is by terciles, so ±0.43 standard deviations is the cutoff between for the "near normal" category. If you look at this full screen, you can see that many of the lower tercile (blue) departures were at Juneau, while Nome has had a lot of upper tercile (red) departures.

Thursday, April 24, 2014

Sunny Spring

Fairbanks has enjoyed predominantly dry and relatively sunny weather since late winter, and temperatures have been mostly above normal except for the brief cold snap earlier in April.  Measurable precipitation since March 1 has occurred on only 5 days, compared to a long-term median of 8 days in this typically dry time of year.

I recently downloaded the history of hourly cloud cover observations from Fairbanks, and these reveal that more than half of the days since March 1 have been mostly clear; I defined this based on a daily mean cloud cover amount of less than 50%.  The long-term normal (1950-2013) is for about 35% of days to be mostly clear in this period, so in 2014 there has been a 50% excess of clear days; but this is not close to a record.  Between March 20 and April 2 there were 14 straight days with mostly clear skies, but this too is less than the springtime record of 19 such consecutive days in 1983.

The chart below shows mean daily cloud cover compared to normal so far in 2014.  The normal values are derived from the NCDC 1981-2010 normals for each category of cloudiness (clear, few, scattered, etc).  The updated temperature chart is also included below, and the seasonal change in correlation between cloudiness and temperature is evident.  In winter, clear skies are strongly associated with cold conditions, but as the sun strengthens and days lengthen, clear skies increasingly bring warmth while clouds begin to suppress temperatures.

Wednesday, April 23, 2014

Interior Alaska and 2013 Daily Anomalies

As we have discussed several time in the last few months, 2013 was a very anomalous year for much of Alaska. Even though the annual temperature deviations were not too extreme in most cases, there were many, many days with large temperature deviations. One common method of measuring dispersion is to look at the distribution from an expected mean. Assuming temperatures are normally distributed, which they pretty much are, we would expect that 95% (95.4% to be precise) of days would be within 2 standard deviations of the daily mean. In other words, we would expect that 4.6% of days would be more than +2 standard deviations or less than -2 standard deviations from the daily normal.

In 2013, this was true for the vast majority of the U.S. To analyze this, I paired stations in the GHCN v.3 database with 1981-2010 daily normal values published by NCDC. For the Lower 48, I only used primary stations (n=866) and for Alaska I used both primary and Cooperative stations (n=103). In each case there had to be no more than 25% of days with missing or flagged data.

On the maps, anything shaded in blue indicates that less than 4.6% of days were more than 2 standard deviations from the daily mean. Yellow, orange, and red colors show areas that exceeded the expected rate of days that were more than +2 standard deviations or less than -2 standard deviations from the daily normal.Only a few areas in the Intermountain West and much of Alaska had significantly more than 5% of days greater than 2 standard deviations from the daily mean.

The 'Winner' for primary (USW) stations in the entire U.S. was Anchorage's Merrill Field with a value of 10.1%. The Delta 6N Cooperative station in Alaska actually had a value of 11.9%. Figure 3 shows the stations with the largest percentage of +/- 2 standard deviation days.

Figure 1. Percentage of days in Alaska more than +2 standard deviations or less than -2 standard deviations from the daily normal.

Figure 2. Percentage of days in the U.S. more than +2 standard deviations or less than -2 standard deviations from the daily normal.

Figure 3. List of stations ranked my the highest number of percentage of days more than +2 standard deviation or less than -2 standard deviations from the daily normal.

Sunday, April 20, 2014

The May 1964 -1°F Low Temperature

The most difficult daily temperature record to beat in Fairbanks is the -1°F on May 9th, 1964. Using the 1981-2010 normals, a value of -1°F on May 9th is 6.33 standard deviations below the daily normal low temperature. This is also the latest sub-zero reading on record for Fairbanks. Figure 1 shows the 5 most difficult daily low temperature records to beat for Fairbanks.

Figure 1. The five most difficult daily low temperature records to break in Fairbanks using 1981-2010 normals.

In looking at the hourly observations for Fairbanks for the time period of May 6th through May 11th, a sharp cold front clearly passes through Fairbanks around 6 p.m. on May 7th, 1964. The temperature dropped precipitously and the air pressure rose over 20 mb in 12 hours. Figure 2 shows the air and pressure readings during this time period. On 5 of the 6 days, the official low temperature deviated from the hourly observations by no more than 1°F. However, on May 9th, the lowest hourly temperature reading was 5°F at both the 5 a.m. and 6 a.m. observations. Readings of 6°F were measured at 2 a.m., 3 a.m., 4 a.m., and 7 a.m. To achieve a low temperature of -1°F, the temperature must have dropped and rebounded by at least 6°F between hourly observations. This situation occurs occasionally during the dark winter months but a mechanism to explain this phenomenon in the month of May in lacking. Not shown on the chart is a consistent wind from the north between 7 and 12 mph during the time period with temperatures between 5°F and 6°F.

Figure 2. Hourly air temperature and pressure measured at the Fairbanks International Airport between May 6th and May 11th, 1964.

Looking at the temperatures regionally, I pulled all the station data for Alaska during the May 8th to 10th, 1964, time period to find the lowest temperature. A three-day period is used to negate the effect of observation time effects shifting an observation to the following day. Figure 3 shows the lowest temperature during this time period.

Figure 3. Lowest temperature between May 8th and 10th, 1964.

Other that the Fairbanks International Airport reading of -1°F, no other station around town was colder than +2°F. The only other station with hourly observations was Eielson Field. Their low temperature on May 9, 1964 was 4°F and their lowest hourly observation was also 4°F. Given the correlation between the hourly readings and the official daily minimum on all days except May 9th at Fairbanks International Airport, it seems unlikely that any sort of sloshing of airmass occurred which could account for intra-hourly reading anomalies.

It is worth emphasizing how uncommonly cold this airmass was for May. The values shown in Figure 3 would be uncommon for early April, much less early May.  The 850 mb temperature at Fairbanks was -25.6°C on 5/9/64 at 12Z. No other May has seen an 850 mb temp below -20°C.

So, is the -1°F a valid reading? In my opinion, it is a questionable reading that requires further investigation.

Figure 4. Excerpt from the Fairbanks Daily Newsminer from May 9, 1964.

Friday, April 18, 2014

Sudden Onset of Thawing

After a slightly delayed start, the thawing season has suddenly gotten under way in the interior, and in only a few days the snow depth at Fairbanks airport has dropped in half (from 20" on Sunday to 10" today).  The year-to-date accumulation of thawing degree days, defined as the excess of the daily mean temperature above 32 °F, will be above normal in Fairbanks after today.  The chart below shows the annual thawing degree day accumulation through April 17; I think there is a strong connection here between earlier onset of thawing and El Nino conditions in the preceding months.

It is of some interest to note that this year looks like being one of the unusual ones in which freezing conditions in Fairbanks (mean daily temperature below 32 °F) give way to thawing conditions (mean daily temperature above 32 °F), with no period of back and forth.  This occurs in about 10 percent of years and last happened in 2007.  For comparison, a sudden transition in the opposite direction in the autumn happens in about 15 percent of years - a little more often, because the autumn cooling is more rapid than the spring warming.

Wednesday, April 16, 2014

Winter Low Temperatures Around Fairbanks

Once or twice a year I make a map of the temperatures around Anchorage showing the number of 70°F days in the summer and the number of 0°F days in the winter. This time I decided to see how Fairbanks did this winter for achieving certain temperature thresholds.

I downloaded all observation data for the ASOS, RAWS, and CWOP stations around Fairbanks from the Mesowest site since November 1st. For those stations that are also part of the GHCN network, I downloaded the official daily summaries as well. The Mesowest site only receives temperatures every 15-60 minutes so many instances of minimum temperature thresholds may have been missed between observations. For example, if we only looked at the hourly readings for the Fairbanks International Airport, we would think that 11 days were -30°F or colder. However, the daily summary for Fairbanks indicates that 14 days were -30°F or colder. For the Ft. Wainwright RAWS station, the Mesowest data and the daily summary data matched very nicely. That is due to the much more frequent rate of observations sent to the Mesowest site. The same is true of the CRN site (Fairbanks 11 NE).

For the Anchorage map, I added a shading to indicate likely values between points. However, I decided not to do the same for Fairbanks due to the fact that very small elevation changes have a dramatic effect on the total numbers displayed on the maps. Those elevation differences are too subtle to represent effectively at this time.

Here are the maps for the number of days where a temperature of -10°F, -20°F, -30°F, and -40°F, were recorded. Once again, for the GHCN sites, I used the official daily summary. For the other sites, I used the lowest daily observation that was sent to the Mesowest site. The last image is the map of sub 0°F days in Anchorage.

Goldstream Valley (D1454), Ester 5 NE, Fort Wainwright, North Pole, and Eielson seem to be the most consistently cold.

Monday, April 14, 2014

Warmest Six Months Compared to Normal in Barrow

The month of March was another very warm month compared to normal in Barrow, with a monthly mean temperature of -4.8 °F, or 7.9 °F above normal (and the third warmest March on record).  Last month we highlighted the remarkable persistence of warmth this winter in Barrow, and March took this to a new level: this is the first time that six straight calendar months have been observed with mean temperature more than 5 °F above the 1981-2010 normals. The previous record was five consecutive calendar months, from August to December 2007.

The persistent warmth also broke another record: the October through March period was the warmest six (calendar) months relative to the 1981-2010 normal in Barrow's history. This was a marginal break of the previous record from 2007:

October 2013 - March 2014: +6.93 °F
July - December 2007: +6.87 °F
July - December 1998: +6.70 °F

The chart below shows each month's temperature anomaly since 1997, relative to the 1981-2010 normals.  The ultra-warm spells in 1998 and 2007 are clearly seen.  Interestingly, the warmth in late 1998 followed the intense El Niño and Atlantic warming of the previous year, while the 2007 warmth occurred in conjunction with the then-record Arctic ice melt-out; this winter's record warmth seems mostly connected to the North Pacific ocean temperatures and the persistent North American circulation pattern.

The recent persistence of warmth is also seen in the daily anomaly chart since October 1 (see below).  From October to March, 71 of 182 days were 1 standard deviation or more above normal, while only 44 days were below normal.

Saturday, April 12, 2014

Cold and Warm Spots in U.S. With and Without Alaska

A few months ago we blogged about the stations in Alaska that recorded the lowest temperature in the state most frequently in 2013 (see post HERE). Since we now have data for primary stations across the Lower 48, I thought it would be interesting to see how often Alaska had the daily high or low when looking at the entire U.S. during 2013.

This analysis is slightly different that the January analysis in that only primary stations are evaluated; i.e., no Cooperative, SNOTEL, RAWS, or Mesonet stations. The volume of data would overwhelm my computer to assess all of that data.

Figures 1 and 2 show the count of the number of days a primary station (n=1025) recorded the lowest high temperature in the U.S. with and without Alaska respectively. If a tie occurs, each station receives a tally. Each small gray dot represents a station used in the analysis. When Alaska is included, here is the top 5 list of stations:

1) Barrow 4 ENE, AK (116)
2) Nuiqsut AP, AK (99)
3) Deadhorse AP, AK (46)
4) Mt. Washington, NH (17)
5) Barrow Post Rogers AP, AK (15)

When Alaska is excluded, here is the top 5 list of stations:

1) Mt. Washington, NH (147)
2) Beaver 15 E, UT (27)
3) Boulder 14 W, CO (27)
4) Northgate 5 ESE, ND (22)
5) Mauna LOA 5 NNE, HI (20)

Figure 1. Count of times a station recorded (or tied) for the lowest high temperature in the U.S. (Alaska included).

Figure 2. Count of times a station recorded (or tied) for the lowest high temperature in the U.S. (Alaska excluded).

Of course Alaska frequently records the lowest minimum temperature in the country on most days of the year. The next two maps show how the count of daily low temperatures across the country look when Alaska is included (Figure 3) and excluded (Figure 4).

Figure 3. Count of times a station recorded (or tied) for the lowest low temperature in the U.S. (Alaska included).

Figure 4. Count of times a station recorded (or tied) for the lowest low temperature in the U.S. (Alaska excluded).

When Alaska is included, here is the top 5 list of stations:

1) Nuiqsut AP, AK (60)
2) Barrow 4 ENE, AK (42)
3) Deadhorse AP, AK (37)
4) Eagle AP, AK (33)
5) Stanley RS, ID (32)

When Alaska is excluded, here is the top 5 list of stations:

1) Stanley RS, ID (60)
1) Mt. Washington, NH (42)
3) Provo 22 E, UT (35)
4) Yellowstone Lake, WY (28)
5) International Falls, MN (19)

Not surprisingly, Alaska did not record a single occurrence of having the warmest high temperature or the warmest low temperature in the U.S. in 2013. Figure 5 shows the number of days that a station recorded the highest maximum temperature in the nation and Figure 6 shows the number of days that a station recorded the highest minimum temperature in the nation.

Figure 5. Count of times a station recorded (or tied) for the highest maximum temperature in the U.S. (Alaska included).

Figure 6. Count of times a station recorded (or tied) for the highest minimum temperature in the U.S. (Alaska included).

Thursday, April 10, 2014

El Nino and Spring

One of our readers posted a comment today asking if the maps from last month's post on "El Niño and Summer" could be reproduced for April and May instead of June and July.  This is easy enough to do with the computer code I used before, so here are the corresponding maps for spring:

Top 10 El Niño years in April and May:

 Years falling in the El Niño tercile and the positive PDO tercile:

Years falling in the El Niño tercile and negative or neutral PDO terciles:


Top 10 positive PDO years in April and May:

Compared to the earlier maps for June and July, the El Niño effect by itself is apparently much stronger for temperature in spring, as above-normal temperatures are very likely indeed when strong El Niño conditions prevail.  However, the PDO phase is still very important, as we see that colder than normal conditions are most likely when the PDO is neutral or negative, even when the Niño regions are warm.  Presumably this means that strong El Niño episodes in spring are almost always accompanied by a significantly positive PDO phase (I verified this, 8 of 10 strong El Niño years in April-May had a substantially positive PDO phase at the same time).

With regard to current conditions, the PDO is still quite strongly positive (though not quite in the top 10), but El Niño conditions have not yet developed.

Wednesday, April 9, 2014

April Cold Snap

For the second year in a row, interior Alaska is experiencing winter-like temperatures with April already well under way.  Fairbanks airport reached -12 °F this morning, which ties the record for the date from 1986, and Bettles hit -22 °F.  The automated computer forecast for Bettles calls for -28 °F tonight, which would be the coldest ever observed there so late in the season... we'll see.  This is what a cold air mass, 28" snow pack, and clear skies can do, even with 15 hours of sunshine daily at this time of year; and remarkably, it wasn't even flat calm at Bettles last night.

Some other notably cold temperatures from last night:

Goldstream Creek: -16 °F
Galena -17 °F
Tanana -15 °F

And farther north:

Lake Galbraith -31 °F
Nuiqsut -31 °F
Umiat -31 °F
Barrow USCRN site: -33 °F

Tuesday, April 8, 2014

Wind Chill Climatology

We have devoted a lot of time discussing wind chills over the last few months, so what harm is there in another post, right? Yesterday a comment showed up on a post regarding the cold temperatures in the Lower 48 as compared to Alaska this winter. The comment noted that while temperatures are usually colder in Fairbanks, the windier conditions in the northern Great Plains might make them colder as measured by wind chill. Since there are not very many unsolicited analysis requests, I thought it would be interesting to see if the commenter was correct.

I first took a look at the winter (December, January, and February) wind chills in Fairbanks, Fargo, Bismarck, and International Falls, since 1984. The wind chills for each station were generated by calculating a daily wind chill (from the average daily temperature and the average daily wind speed) and then taking those 90 days (or 91 days in a Leap Year) and averaging them together. That seems like an awful lot of averaging. To corroborate, I looked at four years of hourly observations for each station and generated a seasonal average and compared those to the daily average. The numbers were nearly identical. Therefore, my confidence in the daily average value is high. Figure 1 shows the results of the average daily winter wind chill for the four stations listed above. Indeed, Fairbanks has a lower daily windchill than the other three stations nearly every winter. This past winter is the exception to the 30-year period. In no other winter was Fairbanks behind the other three stations.

Figure 1. Time series of wind chills in Fairbanks, Fargo, Bismarck, and International Falls.

Winter 2013-2014 Wind Chill:

When looking at the entire U.S., there is actually quite a large area whose winter-long average wind chill was colder than Fairbanks. Stations in six states had winter wind chills that were lower than Fairbanks. Figure 2 shows the this winter's average daily wind chill for Alaska and Figure 3 shows this winter's average daily wind chill for the Lower 48.

Figure 2. Average daily wind chill in Alaska during the winter months (DJF) in 2013-2014. This map was generated from only those stations that reported daily average wind and daily temperature for a minimum of 85 out of 90 days (41 stations).

Figure 3. Average daily wind chill in the U.S. during the winter months (DJF) in 2013-2014. This map was generated from only those stations that reported daily average wind and daily temperature for a minimum of 85 out of 90 days (889 stations).

Winter Wind Chill Climatology:

Making a 30-year wind chill climatology is a little more challenging. Due to the number of records in the database that would require processing over that time period (~18,000,000) and the limited power of my computer, a shortcut was needed. I was able to use the 30-year climatology of wind that I processed a week or two ago for all primary stations with complete records for 28, 29, or 30 years (which involved several computer crashes). That data set was then combined with the NCDC normals data set and a raster calculation of the two could commence using the 2001 wind chill formula. As a check of the results, I manualy processed several stations using an hourly method, a daily method, and a seasonal method.

For example, the Fargo, ND, 30-year normal DJF temperature is 12.6°F and their 30-year average DJF wind speed is 12.4 mph. That produces an average wind chill of -2°F for the entire three month period over 30 years as a single calculation. If you take each individual daily temperature and wind speed over a 30-year period, the average of those 10,950 daily wind chills for Fargo is -1°F. I also looked at hourly observations for several years and they were within a degree too. So, it is not an exact match but in my opinion it is certainly good enough.

Figures 4 and 5 show the 30-year climatology of winter wind chills for Alaska and the Lower 48 respectively. On average, Fairbanks' wind chill is far lower than anywhere in the Lower 48. Even Anchorage is lower than 90%+ of the Lower 48. So, while the Lower 48 may have bested much of Alaska for wind chill coldness this winter, on average, it is not even close.

Figure 4. Normal daily wind chill in Alaska during the winter months (DJF). This map was generated from those stations that have a 30-year climatology of daily wind speeds (41 stations) and a published NCDC normal daily temperature (189 stations).

Figure 5. Normal daily wind chill in the Lower 48 during the winter months (DJF). This map was generated from those stations that have a 30-year climatology of daily wind speeds (889 stations) and a published NCDC normal daily temperature (7,454 stations).