Saturday, March 31, 2018

Warm April More Likely

With just a few days left to submit guesses for the Nenana Ice Classic, breakup prognosticators are pondering the extended-range weather forecast for the next few weeks.  The date of the Tanana River's breakup at Nenana is closely tied to local temperatures in April, so an early breakup will be likely if the unusual warmth of recent weeks and months persists.

Looking quickly at some of the forecast data, CPC's most recent outlook for April shows a slight warm tilt to the temperature probabilities over most of the interior. 

NOAA's CFSv2 model has recently signaled an increasingly pronounced warm anomaly centered over far eastern Russia and the Bering Sea.  The maps below show a progression of April forecasts from oldest at bottom to newest at top.  A few weeks ago it appeared possible that the pattern would favor a cold April in the Tanana valley, but this now seems unlikely.

As an aside, I'm in the middle of a very busy period of travel and work, so blog posts will continue to be thin for a while.

Friday, March 23, 2018

Late Winter Snow Showers

My visit to Fairbanks ended on Monday with a rare March instance of strong westerly breezes, adding further to the tally of "strong" westerly wind days in recent weeks (see here and here).  While it's typical for winds to pick up in late winter in Fairbanks, the seasonal breezes are usually from the northeast; in the past 20 years, there have only been 2 other days in March with similar winds (20-30 mph) from the western half of the compass.

In this week's event, the wind was associated with a strong cold front aloft; the 500mb chart below shows the trough axis near Fairbanks at 4pm AKDT on Monday.  Note the temperature of -39°C reported above Fairbanks; by Tuesday morning the 500mb temperature was down to -43.5°C, which was very nearly the coldest air of the entire winter at this level.

However, even as the cold air moved in aloft, temperatures at the surface were up near the freezing point on Monday afternoon.  The chart below shows the full sounding from 4pm on Monday; the (red) temperature trace shows a strikingly steep vertical temperature gradient, indicating an environment with potential for some convective overturning.

In response to the increasingly unstable environment, heavy snow showers emerged on Monday afternoon and moved through the Fairbanks area.  See below for a sequence of radar images at hourly intervals from 2pm to 7pm AKDT.

Interestingly a similar round of showers occurred on Tuesday afternoon as surface temperatures made their way up to the freezing point again, while very cold temperatures persisted aloft.  Tuesday afternoon's temperature difference (42.6°C) between the surface and 500mb was one of the greatest on record for March in Fairbanks and was actually the largest on record for so early in the year.  Here's a Tuesday afternoon radar image:

Reader Bill suggested that it's not uncommon to see heavy snow squalls like this in late winter in Fairbanks, but without going through a long-term archive of radar data, it's difficult to say what the frequency actually is or whether it might be changing over time.

Friday, March 16, 2018

Interior Snow Pack

I'm up in Fairbanks this week, getting a taste of late winter conditions - although warmer and snowier than normal for the time of year.  Today I took a trip up to the Chena River State Recreation Area to get a first-hand look at the snow pack in the hills, and I can state unambiguously that there is a lot of snow on the ground.  The photo below was taken at about 800' elevation.

I wrote about the above-normal Chena Basin snowpack back in January, and the chart below updates that analysis through today.  Remarkably, the average of 5 sites that are reporting snow water equivalent is close to double the 1981-2010 median for this date, and it's close to a record.  The only year with more snow on the ground at this date was 1993 - no other year comes close in the SNOTEL data for these 5 sites (1981-present).  The winter of 1992-1993 brought 139" of snow to Fairbanks (3rd greatest on record), and March 1993 was the last time the measured snow depth reached 3 feet at the airport.  For comparison, Fairbanks has had nearly 80" of snow so far this winter, with a peak snow depth of 32".

It's worth noting too that the snow water equivalent (SWE) is at or very close to record levels for the time of year at 3 of these 5 SNOTEL sites.  The SWE of 9.7" at Mt. Ryan (2800') is not far off the all-time record of 10.5" (April 1993), and the remarkable 14.8" that's currently being reported from Munson Ridge (3100') was exceeded only in 1991 and 1993.

The map below provides a broader look at snow pack in regions where data are available.  Significant shortfalls are evident in south-central and southeast Alaska, and this is related to La Niña, which reduces the number of storms in the Gulf of Alaska.  On the other hand, snowfall has been above normal in the interior and at least as far north as the Brooks Range; the Atigun Pass SNOTEL is currently reporting a snow depth of 57", which is a new record for March and ties the all-time record for the site (1983-present).

Monday, March 12, 2018

Excessive Warmth in Barrow

Much has been written in various outlets about the extremely warm winter that the Arctic has been having this year, and so I haven't felt the need to draw attention to it here, but the persistence of the warmth has been so extraordinary of late that I would be remiss not to make some mention of it.  Here's my usual chart of daily mean temperatures compared to the standard 30-year normal for Utqiaġvik (formerly Barrow); the warmth has been extremely persistent and pronounced, with only a handful of days below normal since autumn.

We've become accustomed to seeing extreme warm anomalies during autumn in Barrow in recent years, owing to the loss of sea ice at that time of year, but the kind of winter-long warmth we are seeing now is really remarkable.  The north coast of Alaska was finally iced in by mid-December 2017 (much later than normal, of course), but that didn't bring an end to the anomalous warmth; it seems that the entire Arctic basin has been unable to cool off this winter, even though there's not that much more open water than normal in percentage terms.

In comparing temperatures this winter to the climate of previous decades, it's remarkable to observe that the respective histograms of Barrow temperatures are non-overlapping to a significant extent - see the chart below.  In comparison to the extreme warmth of this winter, even the notable differences between the 1951-1980 and 1981-2010 climates seem quite minimal; and even the then-record warmth of 2016-2017 seems rather less significant compared to this year.

Another perspective on the anomaly is gained by looking at the annual fraction of December-March days with daily mean temperature at or above 0°F - see below.  This winter's outcome (so far) is an astonishingly large deviation from historical climate normals.

Wednesday, March 7, 2018

Sea Ice and Southerly Winds

Last month's look at ice loss in the Bering Sea provoked a comment from reader Eric about the impact of southerly winds on ice extent, so I thought it would be interesting to make a quick comparison of this winter to previous years in this respect.

The chart below shows some November-February average data from the north-central part of the Bering Sea.  Based on reanalysis data from a grid point near St. Lawrence Island, the black line shows mean temperature at 925 mb (about 2000' above sea level), and the blue line shows the southerly component of the wind at the same height - so negative wind values indicate northerly flow (the usual situation).  The red line shows the southerly wind component observed at the surface at Gambell and Savoonga, based on hourly observations since 1996.

It's interesting to note that this winter so far has produced the record highest mean temperature and southerly wind component at 925mb, according to the reanalysis; the average 925mb wind still had a northerly component, but barely.  The surface winds from the two observing sites on the island are highly correlated with the 925mb winds, and the surface data reveal a positive mean southerly component for the first time on record.  It's no wonder that ice extent has suffered with such an absence of cold northerly flow.

Here's an update of the chart from the last post, showing that the ice extent staged a modest recovery after its collapse in February, but in recent days it has turned down again.

Obviously the ice situation is now well outside the historical range from the satellite era, and this is partly because there was such little ice cover prior to the recent episode of ice loss.  The magnitude of the recent loss is not in itself unprecedented, as 6 previous years since 1979 saw at least as much ice loss over a 15-day period - although presumably it's "easier" to lose a lot of ice when it's being removed from warmer locations in the southern Bering Sea.

The maps below show sea-level pressure anomalies from the aforementioned 15-day periods in which ice loss was greater than the recent episode, and the last map shows this year's anomaly.  The common feature is obviously an enhanced southerly flow between high pressure over the Gulf of Alaska and strong low pressure over the western Bering Sea or far eastern Russia.

The unusual flow pattern appears to be partly related to the modest La Niña anomaly that continues to be evident in the tropical Pacific; the map below shows the average February-March sea-level pressure anomaly for 10 previous years when weak-moderate (but not strong) La Niña conditions were observed in late winter.  Strong La Niña episodes tend not to be as favorable for ice loss, because the PDO is more often negative in those winters and the Bering Sea tends to be more dominated by high pressure (and thus it's colder in Alaska).

Thursday, March 1, 2018

Winter Winds

After suggesting last Saturday that Fairbanks' February snow total probably wouldn't match last year's number, another round of very stormy conditions moved in on Monday, delivering enough snow to beat last February by just 0.1".  The airport's snow depth of 32" is now exactly the same as last year's number on February 28th.

As in my last post, I want to focus on the very unusual winds that have occurred in conjunction with the recent repetitive storms.  Remarkably, the peak 2-minute wind speed at Fairbanks airport reached 20 mph or higher on 9 out of 15 days in the latter part of February, and this is easily a record for any time in the winter, including March (based on data since 1996).  In fact, this many "windy" days has not previously occurred in anything less than a 65-day period in the last ~20 winters in Fairbanks; it's just extremely unusual to have such a focused period of repeated "high winds".

The rarity of recent events raises the question of whether there's a long-term trend towards more wind in winter in Fairbanks.  Based on ASOS data from the past 20 years, the answer seems to be clearly yes.  The two charts below show the average wind speed (10m above ground) for November-December and January-February, along with the maximum values for both 2-minute and 5-second averages.

The sample size is obviously very small for the seasonal maximum wind gust values, so I dug a bit deeper by looking at frequencies of exceeding the 98th percentile of the climatological distribution.  I estimated the 98th percentiles separately for November-December and January-February and then combined the results for the following chart:

The results indicate that both 2013-2014 and this winter had 8 days with peak wind speed above the 98th percentile on both the 2-minute and 5-second time scales; the long-term expected value would be 2-3 days.  The suggestion of an increasing trend is rather striking, although 20 years is a short period, and if we take out the two big outliers the picture would be less clear.

Dividing the results into early and mid-late winter (see below) emphasizes the outliers even more.  The 98th percentile estimates are included on the legends; they're higher for January-February as a result of the normal annual cycle of wind speed in Fairbanks.

Looking farther back in Fairbanks' climate history is not simple, because the pre-ASOS wind measurements are not directly comparable with the modern data.  We can, however, gain a longer-term view from the winds reported by balloon soundings; the charts below show results for mean and maximum 850mb wind speed in November-December (top) and January-February (bottom).

The November-December data show some indication of an uptick in 850mb wind speeds in the past 20 years, but in a longer-term context there's no obvious trend, and recent conditions do not seem to be outside the range of historical norms.

The situation for January-February is a bit more interesting, as the 850mb wind speed seems quite stable, including in the past 2 decades, and there's little evidence of a recent increase as suggested by the surface data.  Could it be that surface winds have increased while winds at around 4000' above ground level have not?  This might be consistent with a change in preferred flow direction, or perhaps it's a reflection of weaker surface-based inversions that allow winds aloft to mix down more often.  It would be worth looking into this more closely, and it would also be interesting to see if the same thing has happened at other locations, including in the Arctic where recent warming has been so pronounced.