September Precipitation Days

After the tremendous deluge on September 1st and 2nd, no measurable precipitation has been observed at the Fairbanks International Airport. Is this unusual? Since 1915, the average amount of precipitation between September 3rd and September 17th is 0.49". This is only the 4th year (out of 99) with no precipitation during those dates. The other years were 1963, 1974, and 2006. Also, there are normally 4.3 days with measurable precipitation during these 15 days; but of course none this year.

For the entire month of September, the long-term average number of days with measurable precipitation at the Fairbanks International Airport is 9.8. When days with Trace values are included, the number jumps up to 16.0 days. The 9.8 days with measurable precipitation is actually on the low end of other readings around Fairbanks. Figures 1 shows the normal (average) number of days with measurable precipitation in the Fairbanks area for all stations with at least 10 complete years of precipitation data since 1950. Figure 2 shows all days with at least a Trace of precipitation.

Figure 1. Long-term average number of days with measurable precipitation in the greater Fairbanks area for all stations with at least 10 complete years of summer precipitation data since 1950.

Figure 2. Long-term average number of days with any precipitation (Trace or measurable) in the greater Fairbanks area for all stations with at least 10 complete years of summer precipitation data since 1950.

Readers of this blog probably know that September is past the peak of the wet season for Fairbanks. July is typically the month that observes the most rainfall and the most days with measurable precipitation. Figure 3 show which portions of Alaska are wettest during which month. Most areas south of the Alaska Range are wettest in September through December.

Figure 3. The wettest month of the year across Alaska based on NCDC normals.

At the statewide level, we see that a significant gradient exists with regard to the number of days with measurable precipitation during the month of September from northeast to southwest (see Figure 4). Approximately 1/3 of days see measurable precipitation northeast of a line from Glenallen to Wainwright. Southwest of that line, significantly more days with measurable precipitation are observed.

Figure 4. Long-term average number of days with measurable precipitation in the state of Alaska area for all stations with at least 10 complete years of summer precipitation data since 1950.

What is really interesting is to see the number of days with any precipitation (Trace values included) at the statewide level (see Figure 5). Note the very large number of Trace+ days along the North Slope and the west coast. The portion of the state that sees half or fewer days with a Trace or more of precipitation is now much smaller – with a minimum around Tok.

Figure 5. Long-term average number of days with any precipitation (Trace or measurable) in the state of Alaska for all stations with at least 10 complete years of summer precipitation data since 1950.

El Nino and Positive PDO in Winter

Back at the end of July, reader Tracy asked if I could look at the relationship between Alaska winter climate patterns and El Niño or the PDO phase.  I've been very remiss in not getting to this earlier, but now with the change of seasons the topic is more immediately relevant.  Earlier in the year I posted some maps illustrating the El Niño connection to summer and spring conditions, so we can do the same thing for winter.  For the purposes of this post, I've defined winter as November through February: the season that is both cold and dark.

First, the maps below show the distribution of temperature and precipitation anomalies in the top 10 El Niño winters from the period 1951-2013.  The colored columns represent the fraction of years falling in the three climatological terciles, i.e. below-normal, near-normal, and above-normal.  We see that strong El Niño winters most often bring unusual warmth to southeastern and south-central Alaska, but temperatures are most often close to normal overall in the interior and west.  Below-normal precipitation is a common occurrence from Barrow and Bettles down to Cold Bay in these winters.

What if El Niño conditions are combined with a positive PDO phase?  This is the situation that we are currently looking at, and it seems fairly likely to persist into winter, as a weak El Niño episode finally seems to be taking hold in the tropical Pacific, and the PDO phase is moderately positive.  As the maps below indicate, a warm winter becomes more likely in most of Alaska - except for the west and at Barrow - when the PDO phase "lines up" with El Niño.  Also, the precipitation signal is more mixed and shows no clear large-scale signal when the PDO is decidedly positive along with El Niño.

If the PDO were not positive, then the odds would favor near-normal temperatures in many locations (see below) - and the Niño-related warmth in the southeast would be gone.  Recall that we saw the same thing with the earlier maps for spring and summer: El Niño years look vastly different in Alaska depending on the PDO phase.

To confirm the strong connection with the PDO by itself, the maps for the top 10 positive PDO winters show an overwhelming signal for warmth in the interior, south-central, and southeast.

Finally, below are the mean anomaly maps for winter sea-level pressure and 850 mb temperature in the top El Niño winters and top positive PDO winters.  These maps suggest, interestingly, that the patterns are really quite similar between El Niño and the positive PDO, but the surface warmth is much more dominant in the case of the positive PDO.  Cloud cover differences may be an important part of this, but that will have to be a topic for another day.

MSLP, El Niño:

MSLP, Positive PDO:

850 mb Temperature, El Niño:

850 mb Temperature, Positive PDO:

Summer in September

It doesn't get any better than this in mid-September in Fairbanks: mid-70s F, abundant sunshine, and light winds.  Fairbanks International Airport is reporting 76 °F, which is higher than the normal high temperature at the climatological peak in early July.  It's also just two degrees shy of the warmest ever observed so late in the season.

The scene at the Golden Heart Plaza is simply glorious:

Chinook and Record Warmth

Southerly flow ahead of the Bering Sea storm has brought an intense chinook and very warm conditions to interior Alaska in the past two days, and if you thought records were being broken, you were correct.

At Delta Junction, yesterday's midnight-to-midnight low temperature was in the 60s °F, probably 63 or 64 °F (the official value is not reported yet), and this is a record for September - the previous record highest low temperature for the month was 60 °F in the great chinook of late September 1995.  The latest in the year that a low temperature as high as 63 °F has been reported before was August 28, 1989 (with mostly complete data back to 1943).

The lack of nighttime cooling in the Delta Junction area was of course associated with high winds, which were sustained near 30 knots and gusting over 40 knots for many hours yesterday.  The National Weather Service reported many high wind observations, including a gust of 76 mph from a station 11 miles SSW of Fort Greely.  Blowing dust was reported yesterday and Friday on the METAR observations from PABI; this is confirmed by some FAA webcam photos that I saved yesterday afternoon:

At Fairbanks the high temperature of 74 °F yesterday tied the record for the date, but the warmth was not as extreme as in the 1995 chinook, which took temperatures to 78 °F in Fairbanks on September 21.

Yesterday's low temperature in Kotzebue was 52 °F, which ties the record from 1933 for warmest low temperature ever observed so late in the season.  The high temperature of 65 °F on Friday set a new record for the date, but in terms of absolute anomaly this event again falls short of the 1995 event, which brought 66 °F to Kotzebue on September 21.

Upper Air This Summer: Part I - Temperatures

Summer 2014 will long be remembered in the Interior for the amount of rain that fell. Most stations around Fairbanks had their wettest summer on record. Bettles had their second wettest summer and their greatest number of days with measurable precipitation. In addition to the moisture, the temperatures were generally a degree or two below normal. Those are all surface based statistics. How did the upper levels look?

Figure 1 provides a point-specific summary of conditions for the Fairbanks upper level balloon soundings. For all categories, the 2014 temperatures were slightly below the normal value. Somewhat surprisingly, the precipitable water (PW) value was also slightly below normal. This phenomenon was discussed earlier in the summer. A summary can be found here.

Figure 1. Upper level summary at Fairbanks during June-August time period.

If we move beyond Fairbanks, a more complete picture of upper level conditions can be assessed. In regards to temperature, the upper levels were slightly below the 1981-2010 normals. Figures 2 and 3 show the normal 850 mb temperatures and the 2014 850 mb temperatures and Figure 4 shows the 2014 value as compared to normal (units are Celsius). Only Nome, Annette Island and St. Paul had appreciable warmer than normal 850 mb temperatures in 2014. As Figure 5 indicates, with the exception of Cold Bay, the 850 mb temperature anomalies mirrored the surface anomalies. Surprisingly, Cold Bay had near normal 850 mb temperatures even though they experienced their warmest summer on record. Kotzebue's surface departures were also somewhat disconnected from their 850 mb temperatures. In the case of Cold Bay the significantly warmer than normal sea surface temperatures kept temperatures in the lowest levels significantly above normal.

Figure 2. Normal (1981-2010) 850 mb temperature for the June-August time period.

 Figure 3. 850 mb temperature in 2014 for the June-August time period.

Figure 4. 850 mb temperature difference from 1981-2010 normal for the June-August time period.

Figure 5. Surface temperature departure from normal for the June-August time period.

At the 500 mb level, we see a similar pattern; i.e., most of Alaska was cooler at 500 mb except for Nome, Annette Island and St. Paul (see Figures 6 and 7). Again, Cold Bay and Kotzebue were extremely warm this summer but at 500 mb they were below normal. This would seem to indicate that their atmosphere was more unstable (steeper lapse rates) but both stations were below normal for the summer.

Figure 6. Normal (1981-2010) 500 mb temperature for the June-August time period.

Figure 7. 500 mb temperature in 2014 for the June-August time period.

Part II of this post will look at the precipitable water values statewide and how well it correlated to observed precipitation.

Tropical Cyclone Energy

After a chilly start to September, the temperature trend has gone into reverse and brought unseasonably late warmth to interior and northern Alaska.  The temperature exceeded 70°F in Fairbanks today, which happens this late in the season in only about 15% of years.  A strong low pressure system in the Bering Sea is largely responsible for the deep southerly flow that has brought warmth northward; heavy rain and strong winds were experienced in southwestern Alaska today.  The visible satellite image below shows the enormous comma-cloud appearance of the storm this afternoon as it was getting wound up.

The rapid intensification of the storm is evident in two sea-level pressure analyses from Environment Canada - see below.  The first is from 4 pm AKDT yesterday and the second is from 10 am today; in the 18 hours between these maps, two distinct low pressure centers merged and a rapid deepening was observed.

The more southerly of the two low pressure centers at 4 pm yesterday was the remnants of Tropical Storm Fengshen, which passed to the south of Japan on Monday as a 60-knot storm.  I'm no expert on the extratropical transition of tropical cyclones, but it's likely that the deep tropical moisture associated with the southern storm helped feed the intensification of the combined system.  Ironic as it may seem, the climate of Alaska is closely tied to events in the tropics.

Update: here's a wonderful infrared satellite image of the storm on Saturday morning, as linked by Gary in a comment.  The image is from the water vapor band, so light areas show regions of cloud and/or high humidity in the middle to upper atmosphere, while darker areas represent low humidity aloft (not close to the surface).

Heating Degree Days and Elevation

Residents of Fairbanks are well aware of the profound elevation-dependence of temperatures in the cold season, and it's presumably also well-known that heating costs are generally higher in low-lying areas where cold air pools in the winter months.  Hillside residents tend to enjoy lower annual heating costs, and I've been told the differences can be substantial.  As I thought about this recently, I wondered if it would be possible to identify an "optimal" elevation at which heating costs might be minimized.  Obviously, if one moves too high up in the hills, then increased heating costs in the warm season might offset the gain from avoiding the valley inversion in winter.  Note that we ignore the effects of wind speed in the following analysis; higher-elevation locations will tend to have higher wind speeds, which would increase heating costs, perhaps significantly in some locations.

To take an "idealized" look at this, I estimated annual heating degree days (HDDs) from the twice-daily balloon sounding data from Fairbanks airport.  I took the mean daily temperature to be the average of the 3am and 3pm sounding temperatures, and then calculated the daily HDDs (base 65 °F) in the usual manner.  The following chart shows the 1981-2010 mean of the estimated annual degree days, calculated at 10 m elevation increments up to 600 m above sea level.  I also performed the calculation for McGrath, the only other sounding location in interior Alaska.

The data confirm that, as expected, there is a climatological minimum in annual HDDs not too far above the ground.  At Fairbanks this minimum is near 350m above sea level, or about 700 feet above the elevation of the airport.  At McGrath the minimum is only about 500 feet above ground, and the temperature difference from the surface to the minimum aloft is much smaller than at Fairbanks.

The second chart, below, shows the HDD profiles for the warm season (May-September) and for deep winter (December-February).  We see that heating needs increase almost from the surface up in the warm season, as there is little inversion in the daily mean temperatures, but in winter the inversion is very deep on average - the warmest temperature in the sounding is above 4000 feet on average, as we showed here.

Having looked at the estimated HDDs from the balloon soundings, it's interesting also to examine the observed dependence of HDDs on elevation at Fairbanks-area stations; the chart below shows the NCDC 1981-2010 normal annual HDDs for all available stations within 20 miles of the Fairbanks airport.  There is tremendous variability among the low-elevation stations owing to local patterns of cold air drainage and urban-versus-rural temperature differences; but the basic result shown above is confirmed in the station data: the lowest HDD values are found at moderate elevations near 1000-1500 feet above sea level.  However, we also see that being at a higher elevation is no guarantee of a more benign temperature regime, as sheltered valley locations at higher elevation (e.g. Gilmore Creek) can be just as cold as lower elevation sites.

Follow-Up Odds and Ends

A few items of follow-up in relation to recent posts may be of interest to some readers.  First, the year-to-date temperature time series plot from Cold Bay is quite breathtaking in terms of the persistence and magnitude of the unusual warmth.  Nearly 60 percent of days in 2014 have been at least 1 standard deviation warmer than the 1981-2010 normal.

Second, Brian reminded me that global reanalysis data allows for a large-scale view of the remarkable October warming that has taken place over Alaska since 2002.  The three maps below show the differences of the 850 mb temperature, 850 mb height, and 500 mb height, between the two periods 1981-2001 and 2002-2013.  The large-scale warming and higher pressure over the Arctic are notable, but we also see the low pressure in the Bering Sea that has caused warm air to be transported north across Alaska in the autumn.

Finally, reader Gary helpfully linked to the Ivotuk website showing webcam images from this remote location.  Mostly for my own interest, I put together a simple page showing thumbnails of the most recent images from several North Slope webcam locations; the images may be enlarged by clicking on them (and a second click shrinks them again).  The page may take a while to load because it pre-loads all the images.

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

Note that several other North Slope webcams are available at the FAA Aviation Camera page:  http://avcams.faa.gov/

Summer Contrast - Cold Bay and Barrow

In future years when we look back at the meteorological summer of 2014 (June through August), we might conclude from the 850 mb temperature anomaly map that average temperatures were not greatly different from normal in most parts of Alaska (see below).  Mean 850 mb temperatures were slightly below normal over the northern Gulf of Alaska, and the far southwest, southeast, and north of Alaska were somewhat warmer than normal.

However, this conclusion would be incorrect, as in fact surface temperatures were significantly below normal at several interior stations and also at Barrow, while the southwest - and especially Cold Bay - were very warm.

The temperature anomalies at Cold Bay and Barrow are particularly notable - at Cold Bay because of the sheer magnitude of the warmth, and at Barrow because of the change from recent years.  Here are some statistics on the summer temperatures at these two locations:


Cold Bay

• The June-August mean temperature was 3.8 standard deviations above the 1981-2010 normal (54.1 °F vs 49.8 °F).  An anomaly of this magnitude (either warm or cold) has a return period of about 1700 years if we assume that non-overlapping 3-month seasonal anomalies are independent of each other, the climate is unchanging, and the distribution is Gaussian.
• The June-August mean daily maximum temperature was 4.5 standard deviations above normal (59.7 °F vs 54.4 °F).
• Only a single day (August 29) had a mean temperature below normal.
• 54 of 92 days reached 60 °F, compared to a 1981-2010 normal of 11 such days and a previous record of 24 such days in 1997 (period of record 1950-present).
• 16 days reached a relatively balmy 65 °F, compared to a normal of 1 such day per summer and a previous record of 10 such days in 1953.  30% of summers fail to reach 65 °F in Cold Bay.
• 48 days had a low temperature of 50 °F or above, compared to a normal of 10 such days and a previous record (set last year) of 42 such days.

Barrow

• Overall it was the coolest summer since 2001 (mean temperature 1.1 SD below the 1981-2010 normal).
• The average daily high temperature was the lowest since 1983 (1.4 SD below normal).
• The temperature exceeded 50 °F on only 9 days, the least since 1983.
• The summer's highest temperature was only 58 °F, which is a tie with 1945 for coldest on record in the Weather Bureau/NWS era (1930-present).
• The longest period of time at or above freezing was 15 days, the least since 2002.  The chart below shows the annual maximum number of days without dipping below freezing.

The charts below show the time-height cross-sections of temperature (departure from normal) at Cold Bay and Barrow for the summer months.  At both locations it's clear that the temperature anomaly was strongly focused near the surface, i.e. mainly below 2000 feet above sea level.  At Cold Bay the warmth was directly related to the very warm water temperatures in the North Pacific, and at Barrow the anomaly was caused by persistent low-level northerly flow and a cold low-level Arctic air mass.  Higher up in the troposphere the temperatures fluctuated but remained close to normal on average at both locations, as we saw in the 850 mb temperature map.

For the sake of interest I'll throw in the Fairbanks chart, where we see that there was little variation of temperature anomaly with height; cooler than normal temperatures were the rule in June and July, but August was a good deal warmer.

Chilly at New Ivotuk CRN

I couldn't resist a brief post about the chilly temperatures being reported today from the new high-quality CRN station at Ivotuk on the north side of the Brooks Range.  The new station has been in operation since mid-June and promises to supply a wealth of interesting data in years to come.  This morning the temperature dropped to 6 °F there, and the chill in the surrounding area is evident from the VIIRS I05 band thermal infrared image provided by the Suomi NPP satellite at 4:20 am AKST today (click on the image for a larger version).

Kotzebue Sound is visible in the lower left and the Yukon River in the lower right; darker shades are colder, so unfrozen ocean, lakes, and rivers appear white.  Interestingly, the Brooks Range river valleys were warmer than the high terrain this morning, which I assume must be because the valley bottoms are snow-free, while most of the higher terrain is snow-covered.  However, an exception to this is seen near Ivotuk, as some of the high-elevation valleys just to the southeast of Ivotuk were very cold.

The table below shows the five-minute temperature observations from the Ivotuk CRN station since midnight local time.

[Update: it appears there is no credible historical report of a temperature this low so early in the season in Alaska.  More investigation might be warranted to confirm this.  I expect the Ivotuk station will set many new records of this sort in coming years.]

Early Hard Freeze

The temperature dropped to 28 °F this morning at the Fairbanks airport, marking the first freeze of the season.  As I mentioned last week, the first freeze at the airport typically waits until the 850 mb temperature is a few degrees below freezing, especially when it's early in the autumn.  Last week the 850 mb temperature dropped to -2.7 °C, but it was still August and no freeze occurred; but yesterday the 850 mb temperature was measured at -4.7 °C above Fairbanks.

Today's frost was one week earlier than normal and the earliest at the airport since 2005; but 28 °F or lower is not typically observed until September 23 (1981-2010 median), so in that respect we're nearly three weeks ahead of schedule.

In more rural areas of the interior much cooler temperatures were observed this morning, including:

Norutak Lake RAWS in Kobuk National Preserve  14 F
Circle Hot Springs  15 F
Chicken  17 F
Eagle airport  18 F
Goldstream Creek 19 F
Bettles  21 F
Tanana  22 F

Summer Rain Totals

There are seven stations in the Fairbanks area with complete summer precipitation data for 2014 and at least 14 other years. Only College Observatory did not break the record for the wettest summer on record among those stations. North Pole had the highest amount relative to the station average (277% of average).

Figure 1 below shows the location of the seven stations with at least 15 years of June-August precipitation. The ranking, period of record, actual precipitation, and average precipitation are shown. Figure 2 shows the stations in the entire state that set their 1st, 2nd, or 3rd wettest summer on record. There are three pockets of record precipitation. 1) the greater Fairbanks area, 2) the region from Juneau to Haines, and 3) parts of the Kenai Peninsula.

Figure 1. Precipitation ranking for all stations near Fairbanks with at least 15 years of June-August precipitation data.

Figure 2. All stations in Alaska (63) with at least 15 years of June-August precipitation data. Those stations with a ranking of 1, 2, or 3 are displayed in unique colors.

More Rain for Fairbanks

After completing the wettest summer on record, Fairbanks is getting started on some new records. A staggering 2.24" of rain has fallen since noon on Monday (September 1st). This already puts Fairbanks into the Top 10 for wettest September's on record and makes for the wettest 24 hours ever in the month of September.

There have now been 4 days with at least 1" of rain from midnight to midnight this year. No other year has seen more than 3 days with 1"+ of precipitation on a calendar day. Since 1915, a total of 48 days have exceeded 1" of precipitation.

If we drop the threshold down to 0.75", there have been 9 days in 2014. The next highest year is 1962 with 6 days at or above 0.75".

If you take the ten wettest days of 2014, it adds up (10.59") to just a shade under the normal annual precipitation total. No other year has seen as much as 9" when the 10 wettest days are added together.

Figure 1. Precipitation from Mesowest of the 24-hour period ending at 9:45 a.m. on September 2nd.