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).

Monday, April 7, 2014

Changing Pressure Gradients

A post by Brian the other day led to some discussion about long-term trends in wind speed over Alaska and the lower 48 states - specifically, whether the observed weakening in wind speed is "real" or is at least partially an artifact of changing measurement procedures.  To look a little closer at the changes in Fairbanks, I calculated the mean annual gradient in sea-level pressure from the NCEP/NCAR global reanalysis, which extends back to 1948 at six-hourly time intervals with a horizontal resolution of 2.5 degrees latitude/longitude.  Admittedly the data is spatially coarse, but it does provide a consistent gridded history, and for sea-level pressure the quality of the synoptic-scale variations should be very good.

The chart below shows the results obtained for a reanalysis grid point close to Fairbanks: the black line depicts the annual mean pressure gradient, and the colored lines show the mean gradient in three-month seasons.  It is interesting to note that the mean annual gradient was slightly lower than earlier norms for about ten years beginning in 2000, but it was slightly higher again in 2012 and 2013.  The largest decline from earlier years occurred in winter and fall, whereas spring and summer showed no significant change.


The drop in mean pressure gradient since 2000 is consistent with lower mean wind speeds at Fairbanks, suggesting that the wind speed really has slackened in recent years.  However, note that the annual pressure gradient did not begin to drop off until 2000, whereas the reported wind speed dropped off quite precipitously in 1998, so I think it's likely that there is also an equipment or measurement change involved in the wind speed history.  Of course, we should remember that the surface wind speed is also affected strongly by the inversion and stability characteristics of the air near the ground, so the pressure gradient is not the only determining factor.

The spatial scale of the pressure gradient change is illustrated in the map below, which shows the difference in mean gradient since 2000.  Most of Alaska stands out prominently as having a reduced pressure gradient, while the Arctic Ocean has experienced higher gradients in recent years.  The changes in the lower 48 are rather small, except in New England where weaker gradients are consistent with Brian's map showing weaker wind speeds.