Thursday, September 18, 2014

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.



Wednesday, September 17, 2014

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:

Sunday, September 14, 2014

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.

Saturday, September 13, 2014

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.

Friday, September 12, 2014

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).


Wednesday, September 10, 2014

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.


Monday, September 8, 2014

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/