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.

Saturday, April 10, 2021

One More Cold Night

Last night was the climax of the great April cold snap for the eastern interior and south central Alaska, and -40s were observed in a number of the usual cold spots, including:

-47°F  Salcha RAWS

-45°F  Marguerite Creek HADS near Healy 

-44°F  Nome Creek HADS (40 miles NNE of Fairbanks)

-42°F  Granite Creek SNOTEL near Delta Junction

-41°F  Tok 70SE CRN

-40°F  Livengood RAWS

The thermometer on the Salcha River spent more than 5 hours below -40°F:

There was also a -48°F reported by a USGS river gauge on the Denali Highway, but it looks a bit suspicious to me, with a rather large temperature drop just for a single hour:

And so it seems that the April state temperature record of -50°F (Umiat, April 5, 1986) remains intact.  It's also not the coldest so late in the season: a Dalton Highway site on the North Slope reported -49°F on April 12, 1974, and 3 of the 4 previous days were also -45 or colder at that spot.

As for Fairbanks, the -29°F this morning didn't quite break the April record (-32°F in 1911), but yesterday's high temperature of only +3°F was easily the coldest on record so late in the season.  The previous coldest day after April 7 was +10°F in 1959 (April 17) - the same cold snap that I mentioned yesterday.

And not to overlook Anchorage, yesterday the airport climate site there broke daily records for lowest max temp (20°F), lowest min temp (6°F), and highest snow depth (24"), all on the same day (thanks to Brian Brettschneider for noticing this).  The snow depth aspect is remarkable, as total snowfall has actually been marginally below normal this winter in Anchorage; this reflects the persistent chill and absence of melting episodes.

It will be cold again tonight in many spots - although not like last night - and then the temperature will keep rising towards more seasonable levels and quickly beyond, as a major warm-up ensues.  It's going to feel seriously tropical by early next week.

Friday, April 9, 2021

In The Freezer

Another day, another update on the remarkable cold snap that is now gripping much of Alaska.  First, to follow up on yesterday: Kotzebue reached -9°F, so not quite the coldest day this late in the season; but the high temperature in Bettles was confirmed at -2°F, the latest sub-zero day in the history there (1944-present).  And today is even colder: so far no higher than -4°F.  [Update: the high was -3°F.  Back-to-back sub-zero days have never been observed in April in Bettles before.]

As for an overnight low, Bettles dropped down to at least -38°F this morning, but the final number won't be known until midnight tonight, because the ASOS inexplicably failed to report the 6-hour minimum at 18 UTC.  They may have nicked -40°.  [Update: -39°F]

Broadly speaking, temperatures weren't as low as expected last night, as some wind and/or cloud, combined with the short night, prevented most thermometers from really bottoming out.  -30s were observed quite widely on the North Slope, but south of the Brooks Range relatively few spots dropped below -30°F; Bettles was one of the coldest.  Click to enlarge the following:

The lowest temperatures in this event so far - courtesy of Rick Thoman - appear to be the following:

-45°F  Ivotuk CRN

-43°F  Atigun Pass SNOTEL

-43°F  Atigun River USGS

-43°F  Jim River DOT NWS Mesonet

As for upper-air temperatures, Fairbanks sent up a special 18 UTC sounding (thanks, NWS!) to sample the cold air mass at approximately its nadir.  The results were spectacular: the instrument reported easily the coldest air on record for April, with data back to 1948:

1000-500mb thickness 4814 gpm, old record 4871 April 7, 1986

1000-850mb thickness 1164 gpm, old record 1182 April 18, 1959

850mb temperature  -32.9°C, old record -29.6°C April 18, 1959

Here it is, in all its glory:

In the chart below I've plotted all Fairbanks 850mb temperature measurements from the entire history, and the red marker shows this morning's temperature.  The only comparable cold outliers in spring - in terms of departure from normal - were those of April 18, 1959, March 27-28, 1963, and of course the granddaddy of them all, May 9, 1964 (when the surface temperature reached -1°F in Fairbanks on May 9).  1964 was the year of record latest breakup at Nenana until the record was broken (by just 3 hours) in 2013.

Thursday, April 8, 2021

Arctic Blast

Regular readers are probably surprised to see such a blizzard of posts from me this week, but the cold snap now unfolding across Alaska is an historic episode that's worth documenting.  In the depths of winter this would be a hefty cold outbreak; but here we are, nearly 3 weeks after the equinox; the North Slope is already seeing 15 hours of daylight.  And of course the chill is all the more noteworthy in view of the general absence of really significant cold in recent years.

To illustrate the character of the air mass moving down across northern Alaska today, here are hourly conditions from atop the Dalton Highway at Atigun Pass.  A temperature of -42°F with a 19mph breeze amounts to a wind chill of -77°F in the latest observation; and that was in the middle of the afternoon.

Lower elevation Brooks Range sites, and most locations on the interior North Slope, have been in the -20s this afternoon, although Toolik Lake is sitting just below -30°F.  Anaktuvuk Pass dropped to -32°F with a gusty breeze earlier today.  Here are a couple of webcam photos showing the hazy sunshine there this morning.

In terms of records for the time of year, they will be too numerous to list by the time we get to the weekend.  Bettles failed to reach 0°F today, even with brilliant sunshine; this is the latest sub-zero day of record at Bettles (1944-present).  As of this hour, Kotzebue has only reached -10°F; this would be the coldest on record so late (although the day isn't over and -9° wouldn't quite do it).

Even on the North Slope, we're close to record levels: Umiat's high of -18°F (as of 5pm) is very nearly the coldest this late in the season.  Up at Deadhorse, the combined Deadhorse/Prudhoe Bay history since 1968 has only one day with a -20°F high temperature at a later date: April 12, 1985.

The real "fun" will begin tonight when the sun goes down; many locations will fall into the -30s, some will drop below -40°, and we may very well see a -50° either tonight or tomorrow night.  The short duration of night will of course limit the additional cooling; but the sheer magnitude of the cold argues for a severe extreme.  Consider this: by tomorrow morning, the air over Fairbanks is likely to be colder (based on 850mb temperature) than anything seen - at any time of year - since January 2012.

Wednesday, April 7, 2021

Extreme Cold Approaches

Unprecedented cold for the time of year will expand across Alaska in the next 48 hours; the forecast remains on track.  Here is today's GEFS ensemble update for 850mb temperatures in Fairbanks:

The cold has already reached the North Slope, with coastal temperatures around -15°F to -20°F on a westerly breeze.  Utqiaġvik dropped to -19°F this morning and remained at -18°F all day; it was the coldest afternoon so late in the season there since 1991.

Confirming the extreme nature of the air mass, the 4pm balloon sounding from Utqiaġvik measured an 850mb temperature of -34.5°C.  This is the coldest on record for April, and colder than anything previously measured outside of December-March.

What's responsible for this?  It's an intensely cold Arctic low pressure system that has dropped down from the central Arctic Ocean in the past few days and is now located almost directly above Utqiaġvik.  Here's an animation of 500mb heights over the past 3 days and the next 3 days:


As for explaining why the Arctic low is dropping down so dramatically, I think a key factor is the strong ridge that has prevailed since Monday over the Aleutians.  Here's a static image from yesterday morning, when the ridge was at its peak:


As the jet stream rounds the peak of this ridge over the Bering Sea, it naturally then drops south into a trough on the east side over the Gulf of Alaska.  The jet then spins up a strong surface cyclone in the Gulf of Alaska tonight and tomorrow, intensifying the trough and dragging the Arctic low farther south.  Here's a 250mb wind map, again from yesterday morning.

As it happens, the ridge to the south of the Aleutians was a very prominent aspect of last month's North Pacific pattern, and there's little doubt it was favored and reinforced by very unusual warmth in the North Pacific waters to the east of Japan.  So I would argue that this extreme cold outbreak is actually directly linked to the remarkable and persistent North Pacific warmth.

Tuesday, April 6, 2021

Even Colder On The Way

In the wake of the big snow, clearing skies and much reduced winds allowed temperatures to plummet in Fairbanks last night, as well as more widely across the interior (click to enlarge the map below).  This morning's low temperature of -24°F in Fairbanks is very nearly the lowest on record for so late in the season; only April 7, 1986 (-24°F) and April 10-11, 1911 (-32°F) saw this degree of cold at a later date.

Even colder readings were logged as follows:

-36°F  Jim River DOT (Mile 138 Dalton Higway) 

-34°F  Norutak Lake RAWS

-33°F  Bettles

-31°F  Salcha RAWS

-30°F  Tanana

The Fairbanks balloon sounding measured a temperature of -25°C at 850mb yesterday afternoon, which is the coldest in April since 1992.  But as hard as it may be to believe, an even colder air mass is on the way by Friday.  How much colder?  The chart below shows the 850mb temperature forecast from the GEFS ensemble: there's a strong probability of the 850mb temperature dropping below -30°C.

Fairbanks has never measured an 850mb temperature of -30°C in April, with data since 1948; the lowest was -29.6°C on April 18, 1959.  Perhaps surprisingly, the surface temperature never dropped below -8°F in that event, apparently because the wind didn't die down.

The latest date with -30°C at 850mb was March 28, 1963, and that episode brought -31°F to Fairbanks and -40° to Chena Hot Springs.

So buckle up, hardy residents of Fairbanks: it's looking quite likely that the Golden Heart City will see its lowest April temperatures in recorded history (the monthly record is -33°F).  The state record for April is -50°F, and incredibly that is very much in play as well.

Monday, April 5, 2021

Rare Snow Onslaught

Winter is putting on a remarkable final act in Fairbanks-land, with extraordinarily heavy snowfall in the past several days - especially for the time of year - and very unusual cold.

First, the cold: the high of only 11°F on the first of the month in Fairbanks was the lowest daily maximum temperature in April since the remarkable events of 2013.  Today has seen similar temperatures, but it felt even colder with a very stiff breeze.  And still colder conditions are on the way; the National Weather Service is going for just 4°F on Friday, which would be the coldest daytime high on record for so late in the season (1930-present).

As for snow, Fairbanks is up to 16.7" in the first 4 days of the month, which already makes this the snowiest April since 1948.  The two-day total of 13.2" marks the first time on record that more than a foot has fallen in a two-day period in April.


Late winter and spring are typically the driest time of year in Fairbanks, so it's really noteworthy that 1.2" of liquid equivalent precipitation fell in the two-day period.  There have only been 4 other events in 90 years when more than 1" fell in two days between late January and late May, i.e. during a period spanning one-third of the year.  Three of these involved a rare snow onslaught like the current event, bucking the seasonal normal:

- March 23-25, 1963: 20" of snow in 3 days, snow depth went from 18" to 35"

- Feb 11-13, 1966: 35" of snow in 3 days, snow depth went from 20" to 52"

- March 23-25, 1991: 21" of snow in 3 days, snow depth went from 39" to 54"

1966 and 1991 were the only winters since the 1930s that saw a snow depth above 50".  Last night's snow depth measurement came in at 40", which is the greatest since 1993.

It's also worth noting that this event produced the most two-day precipitation to fall as snow since the March 1991 event; so it's been 30 years since a snowstorm of this character occurred in Fairbanks - at any time of year.  (Late September 2015 saw more than a foot of heavy wet snow, but about half the total precipitation fell as rain.)

Here's the webcam view from the University of Fairbanks this afternoon: not very spring-like.