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