Saturday, June 24, 2017

Hail in Eagle

Eagle was hit with quite a hail storm yesterday afternoon - here's photographic evidence via the NWS Twitter feed.

Here's how the storm was described in a NWS statement:

"The town of Eagle had a thunderstorm from 3 to 4 pm today with hail lasting for over one half hour. By the time the storm was over, an inch or more of hail was covering the ground. Some of the hail was one half inch in diameter. Eagle also had winds gusting to 25 mph, and had over sixty-six hundredths of an inch (0.66) of rain from that storm. A long time weather observer in Eagle reports that they have not had a storm like this since the 1970s."

And here's an animation of webcam images from the airport, showing the dramatic approach of the storm and what appear to be hailstones falling in one of the shots.  Note that the airport only reported 0.25" of precipitation and appears to have had much less hail; but the airport is more than a mile away from the village.

By way of explanation, the main culprit appears to be high humidity.  Surface dewpoints were in the upper 40s yesterday in Eagle as the temperature rose to 78°F, leading to convective instability.  Furthermore, the Fairbanks sounding from yesterday afternoon showed a rather notable moist layer in the mid-troposphere (750-500mb), and this is a favorable environment for storm updrafts to be maintained without dilution by surrounding dry air.

The upper-air map shows a ridge in place over northwest Canada, but high pressure certainly does not always translate into clear and dry weather - notice the correspondence of yesterday's lightning strikes with the upper ridge location.  Note also the lightning on the North Slope; Deadhorse reported a thunderstorm - a rare occurrence up there.

Thursday, June 22, 2017

Arctic Storm On Tap

Last summer I remarked on an intense low pressure system over the high Arctic in August and noted that strong Arctic storms seem to have occurred rather frequently in summer in recent years.  Another instance of this phenomenon appears likely to develop early next week, according to the highly-regarded ECMWF forecast model; it will be interesting to see how strong this one gets.

Here's a map of predicted sea-level pressure and 850mb wind speed for 3am AKST on Tuesday; the minimum pressure is about 968mb, which is comparable to the storm last August.  This is also close to the all-time record for lowest pressure in the Arctic domain during the summer months.

The map above shows the forecast from a single run of the model, but of course it's nice to look at the ensemble data to get an idea of uncertainty.  Here's a chart of the ensemble mean, minimum, and maximum of the 70-90°N minimum pressure; so we see that on average the 51 ensemble members show a minimum pressure of around 967mb on Sunday evening (Alaska time).  The peak intensity is consistent with the single model run shown above, but the ensemble shows a faster-developing storm.

Based on the window of ensemble forecasts, there appears to be a high chance that the storm will deepen below 980mb, and the most extreme member shows a pressure of below 960mb; this outcome is unlikely, but perhaps not impossible.

Saturday, June 17, 2017

North Slope Thaw

It's been a cold month so far along Alaska's Arctic coast, but webcam views are finally showing change in the fresh water environment as winter's ice begins to disappear.  The two images below were taken a little over 72 hours apart at Wadepiper Lake, close to Teshekpuk Lake, with conditions this afternoon obviously much more hospitable for avian visitors.

The much larger Teshekpuk Lake has seen much less change in the last 3 days and appears to still be mostly frozen over:

Here's a nice chart from Rick showing how unusual the cool conditions have been in Barrow lately: the first half of June was the coolest in more than 40 years.

Wednesday, June 14, 2017

Frost Returns

The climate extremes of Alaska's interior have been on display in the past few days, with hard freezes this morning in places that were downright hot a few days ago.  For example, the Salcha RAWS dropped to 23°F this morning after reporting 90°F on Friday.

Perhaps more amazingly, the high-quality Tok 70SE CRN site reached 26°F today after being at 94°F on Friday.  It would be interesting to see how often a feat like this occurs anywhere in the U.S.; I imagine it might not be too uncommon in the desert southwest, but Alaska can compete when it comes to rapid temperature changes even in summer.

The map below shows the widespread extent of the freeze this morning in many of the typically colder parts of the southeast interior.

The two 500mb charts below illustrate the change in upper-air pattern that brought about the dramatic temperature drop; the first shows the 500mb analysis at 4pm on Friday, and the second shows this morning's analysis.  Notice the dashed lines, which depict the mean temperature (thickness) of the lower troposphere, in Friday's map; they show a bullseye of warmth essentially right over the Tok 70SE site.  Farther west over the interior, very warm air was brought up from the south and downsloped off the Alaska Range.

In contrast, by this morning a different upper ridge had developed over northwest Alaska, and much cooler air from the northeast was in place over the interior; and then favored spots with calm winds and clear skies were able to drop below freezing during the hours of minimal solar insolation.

Sunday, June 11, 2017

Heat Update

Here's a list of the highest reliable temperatures observed in the recent heat wave: all are found in the Tanana River valley, from Fairbanks up to the Tok 70SE CRN site (at 2000' elevation, close to the Alaska Highway, and only 10 miles from the border with Canada).

94°F   Tok 70SE
92°F   Whitestone Farms COOP
91°F   Tok #2 COOP (June 9 and June 10)
90°F   North Pole COOP
90°F   Ester 5NE COOP
90°F   Fairbanks Intl AP

The Tok 70SE site experiences a remarkably continental climate with large diurnal temperature ranges in summer; the low temperature on Friday was 43°F, and just 3 days earlier it was 29°F.

Here are the temperatures recorded by the various thermometers at the CRN site, at 5-minute intervals from 5-6pm AKDT on Friday.  The maximum of 34.3°C is 93.7°F; pretty impressive for the 9th of June.

Friday, June 9, 2017

Interior Heat Wave

The heat is on across the interior today, with temperatures pushing up to 90°F in some spots.  The map below shows the high temperatures through about 5pm; note that RAWS thermometers tend to run several degrees too warm on days like this, so most of the low 90s are probably erroneous.  Nevertheless, at 5pm it was a very consistent 89°F at Fairbanks airport, Fort Wainwright, and Eielson AFB.

Assuming the high temperature reaches 90°F in Fairbanks today [update: confirmed by NWS], this is quite a rare event; it happened in 2013 (two days), once in each of 2009 and 2010, but prior to that 90°F was not reached at all from 1995 through 2008.  Today is almost but not quite the earliest in the year that 90°F has been reached; the only earlier occasion was May (!) 28 in 1947.

It's hard to imagine, but temperatures on the eastern North Slope this afternoon are actually below freezing; Barter Island has been hovering around 28°F all day.  It was 23°F in Barrow this morning with clear skies and a strong breeze; it was a beautiful scene, but amazingly cool considering the heat on the other side of the Brooks Range.

The temperature difference we're seeing here between the North Slope and the interior is right at the limit of what has been observed historically at this time of year; for example, the largest difference in daily high temperatures between Fairbanks and Barrow is 58°F in June, and it looks like today's difference will be about 55°F (90°F vs 35°F).

The largest summer-time same-day high temperature difference between Fairbanks and Deadhorse/Prudhoe Bay is 55°F, which occurred in the late June heat wave of 2013 (92°F in Fairbanks, 37°F in Deadhorse).  With the temperature reaching only 32°F so far today in Deadhorse, the record may be broken.

Thursday, June 8, 2017

Arctic Winter Warmth

Last week's post on Chukchi Sea ice (or lack of it) prompted me to look back at the past winter and examine just how warm it was around the Arctic Ocean.  Back in April we saw that the warmth was quite extreme compared to normal (see here and here), but let's look at some alternative data sets to get a sense of the magnitude of the anomaly.

First, here's a map of the October-April mean temperature anomaly, i.e. departure from the standard 1981-2010 normal, based on the NCEP/NCAR reanalysis.  This is the 2m temperature output from the reanalysis model - so it does not represent ground truth observations, but nevertheless it's useful.  The amplitude of the anomaly is extraordinary, greater than +6°C over a broad swath from the Chukchi Sea to the area around Franz Josef Land, and over +10°C in parts of the latter region.

The NCEP/NCAR reanalysis is rather outdated now (the model is over 20 years old), so it's worth looking at some more modern alternatives.  Here are the results from a couple of other reanalysis products:

While there are some modest regional differences between the maps, with MERRA-2 in particular showing less extreme warmth over the Arctic Ocean, the patterns are qualitatively the same.  Moreover, all three models show that the seven months ending April 2017 were the warmest such period in the Arctic region for the entire reanalysis period - warmer even than winter 2015-2016.  The chart below shows the 65-90°N area average of the 2m temperature anomaly according to the three reanalyses.  Despite the models showing some differences in the long-term trends, they agree that the past two winters were much warmer than any others in the modern era.

The obvious next question is - what about actual temperature measurements?  Do thermometer readings at weather stations support the huge warm anomaly produced by the models?  Based on a set of 19 long-term observing sites that I've used previously on this blog, the answer is unequivocally "yes".  The chart below shows monthly mean temperature anomalies from these Arctic sites, and remarkably the 19-station mean temperature was over 3°C above normal for each month from October through April this winter.  (But note that in May the average temperature returned to almost exactly normal.)

If we compare the observed temperature anomalies (from the 19 stations) with the reanalysis model temperatures, it's rather amazing to see that the station data shows even more warming than the models, and winter 2016-2017 was warmer than 2015-2016 by a larger margin.  Of course the stations are not evenly distributed around the Arctic - we could especially use a few more in the Canadian Arctic - but it does seem possible that the models may actually be underestimating the magnitude of warming.

Finally, the area-average reanalysis temperatures from the high Arctic, 80-90°N, show a 7-month average temperature anomaly of approximately 4-6°C above normal, which is greater than for the Arctic basin as a whole.  Of course this represents a very small area of the Earth's surface, but it's a striking anomaly nonetheless.  It's no surprise, really, that estimated Arctic sea ice volume at the end of May was about the same as the normal ice volume for the month of July (see the figure below).

[Update June 20]

A reader pointed me to the reanalysis intercomparison tool at NOAA ESRL, which I've seen before but had forgotten when I put this post together.  Happily, the NCEP/NCAR reanalysis and MERRA-2 results from NOAA's tool appear to be identical to mine: