Warm and Humid on the North Slope

After a slow start to the warm season, much higher temperatures have reached the North Slope in the past couple of weeks.  On Tuesday the airport at Deadhorse saw a high temperature of 80°F, making this the fourth consecutive summer that 80°F has been reached or exceeded there.  Of course the all-time heat record of 85°F was set in 2016, as discussed here.  But prior to 2015, the site went 13 years without getting out of the 70s.

It's also worth noting that the humidity has been very high, relative to normal, in recent days.  According to the ASOS instrument at the Deadhorse airport, the dewpoint reached 61°F on Sunday evening, and the daily mean dewpoint was almost the highest on record - only a couple of days in 2004 were higher.  The top-quality 5-minute data from the nearby CRN site tell the same story, with a peak dewpoint of 62°F on Sunday evening, and Sunday was the most humid (highest dewpoint) of any day in the short history of the CRN site at Deadhorse.

For context, the highest dewpoint so far in Fairbanks this summer is 58°F, and only two of Alaska's 21 CRN sites have seen a higher dewpoint than Deadhorse this summer (i.e. Nowitna NWR and Selawik Refuge).

Peak Summer Heat

The first and perhaps only sustained period of really hot weather this summer developed over the interior at the weekend.  The National Weather Service noted several readings in excess of 90°F in the Fairbanks area on Sunday, but this did not include the airport climate site (88°F); it has been more than 5 years since 90°F was exceeded at the airport.

With mid-80s again today in Fairbanks, the last 7 days are already the warmest week of the summer despite the fact that it was distinctly chilly a week ago.  This is a little later than normal to be observing the peak warmth of the season, although the absence of heat was itself a bit unusual until now.

The chart below shows a histogram of the dates of peak weekly mean temperatures in Fairbanks, based on daily highs (red), daily lows (blue), and daily mean temperatures (gray).  Daily high temperatures tend to peak from late June through mid-July, but daily low temperatures often rise into mid-July as humidity and cloud cover increase.  But by the latter third of July it's distinctly less common to see the summer's warmest weather.

Hourly Rainfall History

Recently I have spent a good bit of time digging into the history of hourly precipitation data from Fairbanks, with a focus on the warm season months of May through September.  NOAA's hourly precipitation data set ostensibly covers 1949-2011, although there is inexplicably a long gap from 1952-1962 in Fairbanks; so I have filled this in by transcribing numbers from the historical LCD forms (a somewhat painstaking effort).  I also brought the data up to the present using hourly ASOS observations.

Now that I'm in possession of a reasonably complete data set, there are a number of questions that can be answered.  For instance, is there evidence that short-duration heavy rain events have become more common in Fairbanks?  We might expect this in a warming climate with increasing moisture content.  The chart below shows annual counts of days (May through September) with at least 0.5" of rain within a two-hour interval.  I used a two-hour period so that the analysis captures events that crossed the top of the hour, and the chart starts in 1956 because we have essentially complete data since then.  (But see a note at the bottom on the 1997 event.)

A striking result is the remarkable cluster of heavy rain events in 2005-2010: there were 8 separate events in 6 years, but the previous 50 years had seen only 10 such days in total.  If we assume a Poisson distribution based on the first 50 years, it is extremely unlikely (p<0.0001) that 8 events would occur in 2005-2010 by random chance, so "something" in the climate was different (perhaps a remote climate influence).  Interestingly, however, the last 7 years have seen only one of these heavy rain events.

The overall 62-year mean frequency of these events is 0.31/year, or a return period of 3.3 years, and this is nicely consistent with the NOAA precipitation atlas (https://hdsc.nws.noaa.gov/hdsc/pfds/):

For more analysis on heavy precipitation events in Fairbanks, but at a daily time scale, see this post from last October:

http://ak-wx.blogspot.com/2017/10/heavy-rain-frequency.html

And regarding the 1997 event that is included in the chart above: 0.75" of rain was reported for the daily total on June 8, 1997, but the hourly totals were not reported owing to a rain gauge malfunction.  There's no way of knowing for sure, then, whether 0.5" fell in two hours at the airport, but it turns out this event was a well-documented severe storm and there's little doubt that it qualifies for the analysis here.

Here's a write-up on the storm by one of the NWS staff in Fairbanks:

http://worldagweather.com/richmond2007_fairbanks_storm_june_1997.pdf

Looking at the hourly METAR reports, a key observation was made at 8:58pm on the 8th, stating that hail and rain had begun at 8:13 along with a peak wind of 50 knots (perhaps the rain gauge was blown over!).  Apparently hail fell for 18 minutes and rain for another 22 minutes.  None of the other hourly observations from that day reported more than light rain, so it appears that most of the 0.75" fell in less than one hour.

It just so happens that the Pedro Dome radar was offline for most of the day, but a scan from 8:55pm showed a strong storm southwest of Fairbanks - see below (image courtesy of weather.us).  As the storm was reported to be moving south in the METAR data, this is almost certainly the cell that brought chaos to the city.

Still Frozen on the North Slope

It has been a relatively chilly summer so far in the northernmost parts of Alaska, with Utqiaġvik (Barrow) recording the coolest June since 1994; despite a high temperature of 57°F on the 25th, the monthly mean temperature was only 33.5°F.  Only one June since 1980 has been cooler (1994); but prior to 1980 this was a typical mean temperature for June.

The cool conditions have been quite persistent for nearly two months now.

As a result of the chilly weather, some of the fresh water lakes on the North Slope are still frozen, as seen in the land-cover imagery from the Suomi polar orbiting satellite.  The image below was taken at 4pm today; ice is indicated by light blue colors in the Arctic Ocean and the larger lakes of the North Slope.

Today's webcam images from the observatory at Teshekpuk Lake (the large lake in the image above) confirm the presence of ice cover:

The same images from yesterday showed more ice cover near the shore; meltout appears to be getting into full swing now.  Compare the situation to what was observed on June 17 of last year: https://ak-wx.blogspot.com/2017/06/north-slope-thaw.html

The blame for the persistent chill lies with a trough that has transported Arctic air south into northern and interior Alaska; Fairbanks felt the effects of this about a month ago.  Here's a map of the 500mb height anomaly for the month of June.

However, the 925mb temperature map for June puts Alaska's cool anomaly in perspective - see below.  In contrast to the localized and rather mundane region of chill over northern Alaska, the central part of Arctic Siberia saw a very large and pronounced warm anomaly, and indeed the Siberian warmth was very extreme by historical standards.  At the town of Saskylakh at 72°N (nearly the same as Utqiaġvik), the June mean temperature of 60.0°F was more than 5°F above any other June, with data back to 1936, and the month was a remarkable 17.5°F above the 1981-2010 normal.

Possible El Niño Modoki

There has been a lot of talk in climate circles about the possibility of a new El Niño episode developing in the Pacific Ocean over the coming months, and this could become an influence on Alaska's weather patterns during the autumn and winter.  Of course it was only two years ago that the very strong El Niño of 2015-2016 ended, so I think it would be surprising to see another major episode so soon; but there is a strong consensus among the seasonal forecast models that a significant El Niño is on the way.  Here's the sea surface temperature (SST) forecast for early winter from the NMME models:

The models expect the warmest SSTs, relative to normal, in the central equatorial Pacific, and this has also given rise to a bit of speculation that the coming El Niño (if it happens) may be more of a "Modoki" variety than a classic episode.  El Niño Modoki is sometimes considered to be a distinct climate phenomenon in which warming occurs in the central portions of the equatorial Pacific, as opposed to the classic El Niño pattern that involves the most pronounced warming in the east.  However, others emphasize that the Modoki versus classic distinction is really a continuum and is mostly related to the strength of the warming episode.

Regardless of the semantics, it's interesting to compare the winter climate patterns between central-Pacific and eastern-Pacific El Niño's.  The Aleutian Low is usually stronger than normal during strong classic El Niño winters, and there is a pronounced low pressure anomaly centered to the south of the Alaska Peninsula, as shown in the map below.  The years listed here were obtained by taking the top El Niño winters (based on the Multivariate ENSO Index) that were not also top-10 Modoki episodes (based on the El Niño Modoki Index).

Enhanced southerly flow associated with the strong Aleutian trough tends to bring warmer than normal conditions to much of Alaska.

El Niño Modoki winters look very different; rather than showing a strong Aleutian Low, there's a notable tendency for high pressure from the Bering Sea to southern Alaska.  It also tends to be warmer than normal over the Bering Sea and surrounding areas (see maps below).  Note that these years are the top Modoki events that did not also have top-10 Niño3 (eastern equatorial Pacific) SST anomalies; so by definition they are also not strong El Niño episodes.  We might think of them as weak-to-moderate warming episodes that were clearly focused on the central equatorial Pacific.

As we consider which "flavor" of El Niño might be more likely this year, we can ask how the current global SST pattern compares to the typical precursor patterns for the two varieties.  First, the map below shows the SST pattern in June and July prior to east-Pacific El Niño's.  The central and eastern tropical Pacific tend to be warmer than normal already by this time of year, and there's also a strong warm signal in the Indian Ocean.

Does this resemble the current setup?  Not at all; June SSTs were near-normal in the Indian Ocean and near or even slightly below normal in the eastern Pacific.

The map below shows the pattern for summers preceding Modoki episodes; there tends to be warmth in the central Pacific and also - interestingly - in the northern North Pacific, and especially the Gulf of Alaska.  Overall this is a lot more consistent with the current pattern, and so this fits with the idea that El Niño, if it does emerge and persist into winter, is more likely to be a central-Pacific episode.  It will be interesting to see how it plays out.