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:

Saturday, June 3, 2017

Extending the Tanana River at Nenana Spring Break-up Date Record

Hi, Rick T. here. One of the omnipresent issues in doing climate-scale analysis for Alaska is the comparatively short record of most variables at most places. So when a chance comes along to extend a record, we jump at it. To that end, yesterday I stumbled across several lists of break-up dates on the Chena River at Fairbanks. The version below was published in the Fairbanks Daily News-Miner on March 19, 1951. You will immediately notice this record starts in 1903 and is unbroken through 1950. In the same issue, the News-Miner also published break-up dates and times for the Tanana River at Nenana, which are the same as we use today and are available from many sources.
Now the Chena River today is a much different watercourse than it was when E.T. Barnette was unceremoniously booted off the Lavelle Young on the south bank of the Chena River on August 26, 1901. For our purposes here, the major event was the 1940s construction of the dike from Moose Creek Bluff across Piledriver Slough, which over time effectively ended the flow of Tanana River water into what today we call the Chena River at Fairbanks (Chena Slough in the early days). More details on this history can be found in this 2000 USGS report on Noyes Slough Restoration Project. So while this list may be primarily of historical interest for Fairbanks, can we use it to learn something about the timing of break-up of the Tanana River at Nenana prior to the start of observations in 1917?

The first thing I did was look at the correlation of the Chena River at Fairbanks dates against the Tanana at Nenana dates for the 34 years of overlapping data, 1917-1950 (plotted below). The correlation is considerably higher than I expected (+0.94), with 1940 (and to a lesser extent 1919) the only significant outlier. In general, break-up on the Chena River occurred a few days before the ice went out at Nenana, but there are several years (e.g. 1919 and 1920) when Chena dates are later than the Tanana at Nenana. So how can we use this to estimate break-up dates 1903-1916, when we have dates for the Chena at Fairbanks but not the Tanana at Nenana?
The simplest way is to just apply the regression equation (plotted above) using the 1903-1916 Chena at Fairbanks dates as the predictor. This will yield a single value, but there are of course uncertainties. There are many methods we could use to estimate the uncertainty of the estimated Tanana at Nenana dates, but here I'll simply use the standard errors of the regression coefficients to generate some idea of the uncertainly of our estimated dates. For the 10th to 90th percentile estimate, this turns out to be a bit under three days either side of the mean estimate. Below is a plot with the estimated dates and uncertainty estimates prepended to the observed data.
The estimated dates look reasonable when compared with the climate data that we have for the central Interior for this period: there are some cool springs (e.g. 1914) and a couple of warm springs (most notably 1912). All in all, it seems pretty clear that break-up dates on the Tanana River at Nenana in the 14 years prior to the start what we now call the Ice Classic were typical of the pre-World War Two era dates and not like the modern, early break-ups.

Friday, June 2, 2017

Rapid Chukchi Melt

The seasonal melt of sea ice is off to a fast start this year in the Chukchi Sea, according to data from the National Snow and Ice Data Center.  The daily-updating multi-sensor ice analysis, which is available from 2006 onward, shows that the decline is several weeks ahead of the recent normal.

The latest NWS analysis shows a remarkably wide zone of open water off the northwest coast of Alaska.

Farther to the south, the ice is just about gone from the Bering Sea, which is not unprecedented for this date but definitely unusual.  It's interesting to see the advance of seasonal ice retreat since 2012: nearly every year has seen a faster melt-out than the year before in the Bering Sea.

In contrast, Beaufort Sea ice is in much better shape than last year, and the Arctic Ocean as a whole currently has a greater ice extent than last year.

For reference, here's a map showing the boundaries of the ocean regions as defined by NSIDC.

1 - Beaufort Sea
2 - Chukchi Sea
3 - East Siberian Sea
4 - Laptev Sea
5 - Kara Sea
6 - Barents Sea
7 - Greenland Sea
8 - Baffin Bay / Gulf of St Lawrence
9 - Canadian Archipelago
10 - Hudson Bay
11 - Central Arctic
12 - Bering Sea
13 - Baltic Sea
14 - Sea of Okhotsk
15 - Yellow Sea
16 - Cook Inlet

[Update June 4] The first map below shows that wind speeds were lower than normal in May over the southern Chukchi Sea area, according to reanalysis data; so we probably can't blame the early melt on increased storminess.  It was, however, warmer than normal throughout the region in May; and winter as a whole was extraordinarily warm (see the third map below).  There's little doubt that Chukchi Sea ice was much thinner and weaker than normal as melting commenced this spring.

The PIOMAS model's estimated Arctic ice volume is at record lows (see below), so I wouldn't be surprised to see the remarkable Chukchi Sea ice loss extend to other areas in the coming months.

Saturday, May 27, 2017

Snow in the Hills

Spring has suffered a setback in the past few days across interior Alaska, with snow falling in the hills.  Light mixed rain and snow was reported at Fairbanks airport yesterday morning, but in the nearby hills the scene was distinctly white.  Here are some webcam photos from Cleary Summit (2233') on the Steese Highway at about 5am Friday morning.

The photos below were taken a few hours earlier, at approximately solar midnight.  It's an odd combination: 24-hour daylight with snow on the ground.

The graphic below shows the vertical profile of temperature (red line) above the airport at 4am yesterday morning; the freezing level is right at 925mb or 2450' elevation.

The history of observations from Fairbanks only includes 925mb data back to 1992, but in those years there has only been one other occasion when the 925mb temperature was so cold this late in the spring: the early June cold snap of 2006.  Back then the surface temperature fell to 29°F at the airport on June 4, which was the coldest on record for so late in the spring (excluding pre-1930 data).

The most impressive late season wintry weather on record in Fairbanks was the record late snow of 1931, when 1.2 inches accumulated on June 2.  Nothing comparable has been observed in the years since; the latest significant valley-level snowfall of recent years was in mid-May of 1992.

Wednesday, May 24, 2017

Medium-Range Model Performance

There's been some chatter among meteorologists in recent days about unusually poor forecast performance by the U.S. Global Forecast System, a model that predicts the global circulation for weather forecasts out to 15 days.  Summer is a more challenging time for weather forecasts in general, because relatively unpredictable small-scale features are more important than in winter.  But even compared to normal for the time of year, the recent performance has been poor.

The pair of maps below shows a 7-day forecast of 500mb height anomaly (departure from normal) on the left and the ensuing verification on the right; the forecast was issued on May 14.  While some of the features were correctly anticipated, the forecast was badly wrong from easternmost Siberia to Alaska and also in western Europe.  The spatial correlation of anomalies ("anomaly correlation") was only 0.26.

Here's the same pair of maps but for the 7-day forecast from ECMWF.  It's widely known than ECMWF's data assimilation and modeling methods produce generally superior weather forecasts compared to GFS, but in this case the difference was dramatic.

To put this event in context, I calculated the daily anomaly correlations over the Northern Hemisphere north of 20°N for 7-day forecasts back to early 2016, and I also looked at Environment Canada's global model forecasts (labeled as "CMC" in the charts below).  The first chart shows the dramatic drop-off in skill in recent days for both GFS and CMC; both models have suffered the same fate, but ECMWF has remained relatively unscathed.  Note, however, that ECMWF is not without fault - it had a remarkable failure last summer.

If we plot the differences between the models, we see that the ECMWF forecasts have outperformed more dramatically in recent days than at any other time in the past year or so.

Zooming in on the North Pacific region (see below), there is of course more noise in the statistics, but similar patterns are evident.  The CMC model has not fared quite as badly as GFS in recent days, but again the ECMWF has performed much better than the other two models.

In conclusion - let's hope the current episode proves to be instructive for the model developers so that the science can continue to advance.  NOAA scientists are paying attention to these forecast skill "dropouts" - see for example the article beginning on page 5 of the following newsletter:

The bigger picture from the results I've shown here is that modern global circulation models now have the ability to predict weather patterns out to 7 days or more with fairly good skill on average, and I find that to be a remarkable achievement.