Saturday, May 8, 2021

New 30-Year Normals

Back in January I started to compare the new standard 30-year climate normal period, 1991-2020, to the previous 1981-2010 benchmark.  This week NOAA released the complete new climate normals, so it's worth returning to the topic, with a focus this time on precipitation.

Most parts of the state saw an increase in annual average precipitation, which is broadly to be expected at high latitudes during a warming trend.  According to NOAA's climate division data, the largest increases were found in the north and west, while the northeastern Gulf coast and south-central Alaska were the only regions with a slight drying trend.


Interestingly, Fairbanks saw a greater increase in precipitation than the regional climate division data would suggest, with a +7.4% annual change that was quite well distributed throughout the year - although August and September stand out as having the largest changes in absolute terms.  July remains the wettest month of the year in Fairbanks; this was true for the first time in the 1981-2010 normals, and prior to that August held the top spot.


Precipitation is quite uncertain in much of Alaska because of the lack of good long-term observing sites across the vast area, so model-derived data can be a helpful complement.  Surprisingly, the ERA5 reanalysis actually suggests that precipitation has decreased slightly in parts of Alaska's interior - see below.  Obviously a more detailed comparison with (the limited) ground-truth data would be worthwhile to see whether Fairbanks is indeed an outlier with its more notable moistening trend.


If we look at seasonal changes around the state, summer and autumn account for most of the change in the west and north, and of course this is the wetter half of the year.  See below for graphics comparing the NCEI and ERA5 data for each of the four standard seasons.  (I attempted to set these up in a side-by-side table for easy viewing, but the blog editor didn't like that plan.)

ERA5 suggests that Arctic Alaska has become wetter throughout the year, but NCEI emphasizes the autumn increase, which is clearly related to reduced sea ice, more open water, and much warmer and moister air.  Climate data from Utqiaġvik does support a more uniform increase in precipitation throughout the year:


The rather notable drying trend in winter across the west-central interior and south-central Alaska is related to the persistent ridge of high pressure that has often dominated the regional circulation over the northeastern Pacific since 2013.  Observations from McGrath do reflect a modest decrease in winter precipitation, although I'm far from confident about the state of winter precipitation measurements at that location.  However, Anchorage shows no sign of drying out in winter.


Since I have the maps handy, here's a similar comparison of NCEI and ERA5 temperature changes, for the annual average and the four seasons.  There's a major discrepancy in Southeast Alaska, where ERA5 shows far too much warming, presumably because of the model's inability to represent the complex terrain, but elsewhere the comparison is fairly good.


Friday, April 30, 2021

Meltout and Breakup

A couple of quick notes this evening to mark the seasonal transitions of meltout and breakup in the middle Tanana River valley: the Fairbanks snowpack melted out on Tuesday, and the tripod went out at Nenana today.

It's typical for meltout to be about a week ahead of breakup at Nenana, but a 3-day difference isn't too unusual.  There have been a handful of years when the ice went first (most recently 2018: May 1st versus May 4th).

The unusual aspect of this year's events is how quickly it all happened, after the severe cold around the 10th of the month, with 37" of snow on the ground on April 11.  This year's meltout is the fastest on record that snow depth of 30-36" has disappeared in Fairbanks - see the chart below. Remarkably, this is very similar to last year, which still holds the record for losing snowpack of around 27".

An interesting factoid - which is probably related - is that today is one of the coldest breakup days on record, with a high so far of only 38°F in Fairbanks.  Nenana breakup has never been observed with a high under 40°F, so this might be a new record.  It seems very likely that the ease with which the ice moved today is related to the huge meltout flows that are putting a lot of pressure on the ice; without the big snow melt, the ice probably would have held on a few more days.

[Update: the high was 39°F - and a trace of new snow was observed.  Not your typical breakup weather.]

Monday, April 26, 2021

Thaw Proceeds Apace

After the coldest start to April in more than a century, Fairbanks has seen unusual warmth persist for nearly two weeks now, and consequently the seasonal thaw is advancing very quickly.  As of the last daily report, Fairbanks snow depth was down to 3" in the official climate measurement, which is now taken on UAF West Ridge.  Remarkably, the accumulation of thaw degree days (excess of daily mean temperatures above freezing) is very close to the normal of the last 30 years for this date - and well above the longer-term normal.

Looking back at Fairbanks climate history, there's never been such a rapid seasonal thaw immediately after such extreme cold: over 100 thaw degree days within 15 days of a -29°F temperature.  Nothing even comes close, as illustrated in the chart below.  The coldest that previously occurred immediately prior to 100+ TDDs (in 15 days) was -13°F on April 10, 2004.


The cold start to spring, and the magnitude and sudden onset of the warm-up, keep reminding me of 2013, but in fact that spring saw unusual cold linger more than a month longer, so the similarity is not particularly close in the end (compare the two charts below).  Breakup was of course extremely late in 2013, but it may not be much different from normal this year.


Friday, April 23, 2021

850mb Temperature Records

A noteworthy aspect of the recent cold snap - as highlighted in previous posts (e.g. here) - is that Fairbanks saw its coldest air mass on record for April, with the 850mb temperature dropping well below -30°C in the heart of the event.  Last week I noted that 3 daily low temperature records were broken at 850mb, which equals the number of daily cold records in the preceding 7 years.

Remarkably, however, the extreme warm-up that followed has now broken 2 daily high temperature records at 850mb in Fairbanks, on April 19 and 20; so the chart I produced last week is already out of date.  Here's the updated version, showing numbers of daily 850mb records by year.

I promised to follow up with a look at the seasonal breakdown of records, so here are the results for each of the standard climatological seasons:


There are a few interesting aspects here.  First, I think it's very intriguing how the cold records are more strongly clustered in a few highly unusual years: in particular, 1999 (early February) for winter, 1964 (March and May) for spring, 1969 (late July into August) for summer, and 1992 (September) for autumn.  In contrast, the most anomalous warm events have produced fewer warm records; and so it seems that the most extreme cold episodes have a staying power that warm episodes don't.

Second, the complete absence of any cold records in winter in the last 15 years is amazing - although there have been a few in November.  Of the four seasons, winter has the strongest trends for both warm and cold records.

Third, the lack of increasing warm records in summer is rather striking; the trend is actually marginally down.  However, cold records have also disappeared in summer, so 850mb temperatures have simply been rather bland in comparison to earlier decades.

On an annual basis, the relatively small increase in warm records is also quite interesting, as I noted in the earlier post.  The same is not true on a hemispheric basis, at least if ERA5 reanalysis data is to be believed - see below.  This chart shows an area-average (land+ocean) of annual counts of daily records, again at 850mb; the trend is much more striking than at Fairbanks, and it's also much more symmetric between warm and cold.  It would be worth a bit more investigation to see how consistent the reanalysis trends are with the radiosonde data for Alaska; there are known biases with historical radiosonde data, but mainly in the stratosphere and at tropical latitudes.

Monday, April 19, 2021

Sun and Snow

Remarkable warmth has continued to build over eastern and southern Alaska, under the influence of a tremendous ridge of high pressure aloft.  Yesterday the southern Panhandle community of Klawock reached 75°F, the earliest on record for such a high temperature anywhere in the state.

Fairbanks saw 64°F yesterday, close to a record for the date, and this was with plenty of snow still on the ground.  Brian Brettschneider spotted that it was actually the warmest on record for such a substantial snow pack; the figure below illustrates this point.  There have been warmer spring days with a lesser snow cover, but none so warm with more than a foot remaining.

Here's the early evening scene on the UAF campus today:

And here is yesterday afternoon's 500mb analysis, courtesy of Environment Canada.  We've seen a lot of this in recent years, and it's amazing how the pattern flipped right back to its multi-year default after the excessive cold of 10 days ago.  Notice too the northerly flow rushing down the other side of the ridge into western Canada and the Lower 48; folks down south are wondering where their spring disappeared to.

Thursday, April 15, 2021

Extreme Warm-Up

Following hard on the heels of last week's cold snap, spring has arrived almost immediately in southern, central, and eastern Alaska, with an extremely impressive warm-up.  After a frigid -29°F on Saturday morning in Fairbanks, the temperature reached 50°F on Monday; this 79°F temperature rise over just 2 days is tied for largest on record since 1912 - at any time of year.  For April it's a new record, beating the 75°F warm-up from April 13-15, 1992.

Not only did the thaw arrive with extreme haste, it is also sticking around, with 3 of the last 4 days reaching 50°F, and more warm days are ahead.  This too is very unusual: Fairbanks has never seen more than 4 days reach 45°F within a week of a +3°F (or lower) high temperature, like last Friday.  This time we'll likely see 5 days at 49°F or above within that 7 day window, depending on tomorrow.

And all this with still more than 2 feet of snow on the ground.  It's quite remarkable.

Of course the explanation is that the flow pattern has completely switched around, with deep southerly flow now importing very warm air from far to the south.  Compare this morning's 500mb situation (below) to that of last week.

An extreme of one sign in the atmosphere typically begets an extreme of the other sign elsewhere, and sometimes the flow shifts in such a way that both occur in the same place in quick succession.  Of course this week's warmth is nowhere near as unusual as last week's cold - it would have to be 70+°F for that to be true.

Here are a couple of very nice photos of the Tanana River at the mouth of the Chena River this morning, courtesy of NWS Fairbanks on Twitter (click to enlarge).

Monday, April 12, 2021

Cold Snap Follow-Up

Following up on the dramatic events of last week... First, Rick Thoman put together a nice summary graphic of the lowest temperatures reported at many locations around the state - see below (click to enlarge).  That's some deep cold for the second week in April (but interesting that Chicken didn't get colder).


Second, I looked back at the Fairbanks history of 850mb temperature measurements to see how rare it's becoming to set or tie daily cold records.  Last week, three daily records were broken for 850mb temperature: April 6, 9, and 10, the latter two by a large margin.

Based on the historical data since 1950, the annual counts of daily records show the unmistakable trend towards more frequent warm records and less frequent cold records in recent years.

An interesting aspect of this is that there's a marked asymmetry between the trends.  The number of warm records per year has increased at about 3.5 per decade, but the number of cold records has diminished more than twice as quickly: the trend on the cold side is about -9.5 per decade.

Put another way, only 56% of the warm records occurred in the second half of the history, but 67% of the cold records occurred in the first half.  And so we see that cold records are becoming rare; only 3 occurred from 2014-2020, and all were marginal:

12Z November 18, 2015 (broken by 0.4°C)

00Z June 12, 2018 (tie)

00Z October 16, 2020 (broken by 0.4°C).

The 2013 cold records were of course in May, and like last week, those were substantial new records that probably won't be broken for a long time.

I'll look at the seasonal distribution of records over the years in a subsequent post.