Wednesday, May 18, 2022

Breakup Progress

River ice breakup has been progressing across the state, and as expected it has been a relatively "dynamic" process with ice jams and flooding in a number of locations (notably McGrath and Manley Hot Springs).  However, it could have been even worse: temperatures have been below normal this month, significantly so at times, and there hasn't been the kind of dramatic warm-up that might have created a more violent collision between rushing meltwater and ice jams.

Here are temperature anomaly plots from (top) Fairbanks and (bottom) the UAF/ACCAP statewide temperature index (click to enlarge).  Every day so far this month has been cooler than normal by both measures.

The map below shows the latest breakup situation, with mostly open water now on the major interior rivers.  A major exception is part of the lower Yukon, where flood watches have been hoisted downstream of an ice jam just upriver from Grayling.  Apparently water is so backed up that flooding is occurring for 70 miles upstream of the jam, and when the jam releases there will be a rush of water and ice into communities downstream.


From the NWS river forecast center:

500 PM AKDT WED MAY 18  2022


Current webcams show the breakup front on the Yukon is right at
Grayling, with ice shifting in the river across from town this
afternoon. Satellite imagery from yesterday showed a very large ice
jam in place 7 miles upstream from Grayling, extending to around 25
miles upstream from the toe of the jam.  This ice jam is causing
water levels to rise overbank and inundate low lying areas nearly 70
miles upstream. Sheet ice is still in place from Grayling down to
Russian Mission. Downriver communities can expect rapid rises in
water levels and heavy runs of ice when this jam releases. Additonal
ice jams could also form as the breakup front pushes into the
stronger sheet ice downstream, thus a flood watch has been issued
for Grayling to Russian Mission. Downstream from Mountain Village
the river was mostly open.

An ice jam released near Old Crow and water levels at the US-Canada
border crested Tuesday.  Rapid rises in water levels along with
heavy runs of ice are expected along the upper Porcupine River.


Statewide, breakup this year has been largely dynamic with ice jams
observed at Manley Hot Springs, McGrath, Sleetmute, Red Devil,
Crooked Creek, Circle, and Galena so far.  While Manley Hot Springs
and McGrath suffered major flooding, the other ice jams created
minor to moderate flooding due to the cooler than normal
temperatures so far this May.

Breakup is complete on the Kuskowkwim, Koyukuk and Middle and Upper
Yukon Rivers.  The lower Yukon River still has some areas of weak in-
place ice and runs of ice with breakup still continuing on the lower
Yukon River.

The Kobuk, Buckland and rivers north of the Brooks Range are still
ice covered.

Thursday, May 12, 2022

April Climate Data

I'm a few days behind on this, but for future reference - here are the NOAA climate division rankings for April in Alaska, expressed in terms of the percentile of each variable within the historical distribution of the prior 30 years.


It was significantly colder than normal in the southeastern interior and southeast Alaska; for example, Northway saw its 3rd coldest April in the past 30 years (coldest since 2013), but April in Fairbanks ended up not being as cold as last year.  Precipitation was very scarce - not an unusual occurrence, given that it's the driest month of the year statewide on average - but this was unusual, with the 4th lowest statewide April precipitation total on record (back to 1925).

Here's Rick Thoman's temperature plot:


At first glance this suggests that western Alaska had more unusual warmth than indicated by the NOAA data, but note that most of the climate sites with +3-5°F temperature anomalies are right on the Bering Sea coast, where sea ice quickly broke up in April.  ERA5 data (below) supports the idea that really unusual warmth occurred near the coast but not so much inland.


April was another windy month in the northern interior, according to ERA5, and this has been a common theme since November, if the model is to be believed.  ERA5 also shows a wet month in the western Alaska Peninsula, in contrast to the NOAA data.  For what it's worth, Cold Bay has mostly complete data and reported less than normal precipitation for the month; but more work would be needed to comment any further on the discrepancy.

Tuesday, May 10, 2022

New Record Snowpack at Munson Ridge

With a deep trough and cold air aloft, light snow continued on and off last night and for much of today in Fairbanks-land, and it has actually been piling up on the higher hills outside of town.  Remarkably, the SNOTEL instrument on Munson Ridge (3100' elevation) has reported a gain of more than an inch of liquid equivalent in the snowpack, and a 7" increase in snow depth, since Saturday.

The current snow water equivalent of 18.6" is a new all-time record for the Munson Ridge site, with data back to October 1980; the previous record was 18.4" at the start of April 1991.  For the month of May, it's a record by a larger margin (previously 17.1" in early May 2018).

Consider this: the 18.6" of water now on the ground at Munson Ridge is equivalent to the annual precipitation in Fairbanks' wettest year of record (last year, 18.74").  It's all going to be coming downstream in a matter of weeks.

Down at valley-level it has been too warm for additional snow accumulation today, but yesterday's official total of 1.2" is the largest snowfall this late in the season in Fairbanks since 1992.

The number of hours of snow falling is very unusual too: 25 hourly observations reported snow falling (sometimes mixing with rain) yesterday and today, and only 3 other years have managed this feat after the first week of May: 1992 (90 hours), 1964 (44 hours), and 1966 (30 hours after accounting for the 3-hourly spacing of obs at that time).

Here's this morning's 4am AKDT balloon sounding from Fairbanks, showing the steep lapse rate that has made it easy to produce recurring snow showers.

Here's the 500mb analysis from the same time, courtesy of Environment Canada.

Monday, May 9, 2022

Spring Snow

An unusual late spring snowfall is today's story in the central interior, with several inches reported at higher elevations around Fairbanks.  As usual for out-of-season snows, elevation dependence was substantial; Rick Thoman reported 1.7" in South Fox (1000' elevation), but 5" was measured on Keystone Ridge (1600').

Rick also notes that the 4.5" that fell in Denali NP is the greatest this late in the season since the remarkable May snows of 1992 (when over 25" fell there, and 14" in Fairbanks).

Here are a couple of webcam shots from Cleary Summit (2200') above Fairbanks early this morning:

The latest accumulating snow on record in Fairbanks appears to be 1.2" on June 2, 1931, and that was two weeks later than the next latest inch-or-more (May 18, 1943).  However, just a few years ago there was a June 1st snowfall around Salcha and Delta Junction:

And just last year Bettles (much farther north of course) saw a couple of inches on May 19:

Spring is something of a fitful and precarious undertaking in interior Alaska.


Tuesday, May 3, 2022

Nenana Tripod Out

Breakup occurred yesterday evening on the Tanana River at Nenana.  This is a couple of days later than the normal of recent decades - no doubt because of the chilly April (5°F below average in Fairbanks) - but it's several days earlier than was typical before about 1990.  Here's Rick Thoman's graphic:

Here's the view at Nenana this morning.

There's little doubt that breakup was brought forward several days by the large volume of water from melting of the deep snowpack upstream from Nenana.  For example, the Little Chena Ridge SNOTEL site (2000' elevation) has lost about half of its deep snowpack in the last 10 days.

Based on April 15 data from 5 SNOTEL sites in the Chena River basin, this year's snowpack water content is the greatest in the past 4 decades.  Looking at Nenana breakup dates since 1982, there is a modest inverse relationship between snowpack water and the amount of warmth needed to cause breakup at Nenana:

Breakup in 2018 and 2020 seemed to be good examples of breakup occurring after relatively little warmth (low thawing degree days) when the snowpack was very deep.  1991 and 1993 don't fit the pattern, but this is almost certainly because April was much warmer in those years - particularly in 1993, which had one of the warmest April's on record, leading to a very early breakup (April 23).  More heat input is required to shift the ice at an early date, because the sun has less time to work on the ice; whereas late breakup tends to occur with lower TDDs because of the long window for increasingly intense solar heat input.  It's a fun multivariate prediction problem.

As for 2002, breakup occurred with both low snowpack and minimal heat input, and that's because it was the wettest April on record, with well over 2" of (mostly) rain in the last week of the month; so the swollen rivers did the job without needing a big snowpack meltout.

Thursday, April 28, 2022

Radiation Normals

Recently I've been taking a look at the (infrared) radiation climate for Fairbanks, based on data from the state-of-the-art ERA5 reanalysis.  I suppose that many people may find it a rather abstract topic, but to me there's nothing more relevant in the world of weather and climate than radiation, and particularly the long-term trends thereof.  After all, solar radiation ("shortwave" radiation) is the fundamental driver of weather, and the imbalance between incoming and outcoming radiation controls weather and climate variations on many time scales.

The chart below shows the monthly normals for the familiar shortwave radiation budget near Fairbanks, according to ERA5.  Solar input (red columns) is close to zero in November through January, it rises quickly in spring under relatively clear skies, and then its decay in autumn is more gradual because increased cloud cover in July and August brings a premature decline (e.g. July has more daylight than May, but much more cloud cover).

The blue columns show the normal upward (reflected) shortwave radiation at ground level, and this peaks in April because most of the month is usually snow covered under relatively strong sunshine.  However, the albedo (i.e. the fraction of incoming solar radiation that's reflected) does drop off markedly in April as snow disappears from trees and eventually from the ground, and it remains near 0.1 until October, when snow cover typically returns.

This much is pretty straightforward to understand.  However, the longwave radiation budget is less intuitive, and in particular it can be a surprise to see that the rate of energy transfer for both incoming and outgoing longwave radiation is much greater than for shortwave - see below.  Probably not many of us would guess that much more radiation is warming us from the sky above than we receive directly in the form of sunshine, even in summer.  Of course this is largely because we're bathed in longwave radiation at all hours of the day and night (emitted by clouds and the atmosphere), with little change from hour to hour, but intense sunshine is confined to only a portion of the day.

But although the incoming longwave is large, the outgoing is even larger at all times of the year, because the ground temperature is higher than the average emitting temperature of the clouds and air above.  The longwave radiation flux is proportional to the fourth power of temperature, so both upward and downward components track very closely with the seasonal temperature cycle.  The only obvious departure from a simple seasonal cycle that I can see is that the downward flux doesn't increase from January to March as quickly as the upward flux, and that's because the surface warms up more quickly than the air aloft (and also because the air stays very dry well into spring - water vapor is very efficient at absorbing and emitting these wavelengths).

So we have a net loss of longwave energy at all seasons, and a net gain of shortwave in all but winter.  What does the overall net look like?

Here we see that there's a net gain of radiative energy from March (barely) through September, and a small loss from October through February; and overall it's a significant gain over the course of a year, which is perhaps a bit surprising at a latitude of nearly 65°N.

Now someone may ask why the temperature in Fairbanks drops so dramatically in the early autumn when there's still a net gain of radiation - even in September, according to ERA5.  The answer is that this radiation budget pertains to the ground surface, not the air above.  The atmosphere doesn't absorb or emit shortwave radiation, so the longwave balance is the only thing at play - and consider that the atmosphere only gains longwave energy from below, while it emits it both downward to the ground and upward to space.  This implies a significant net radiative loss for the atmosphere year-round, and when heat transfer from the surface (by mixing/convection) drops off in the autumn, there's nothing to stop the air from cooling rapidly.

In another post I'll take a look at long-term trends in the different radiation components.

Thursday, April 21, 2022

Breakup Briefing

Here's a link to Tuesday's UAF/ACCAP/NWS river breakup briefing:

The breakup portion starts on Slide 14.  To probably no one's surprise, the key message is, "We expect a dynamic breakup with above average potential for ice jams and flooding."

I thought this was a helpful graphic to explain the difference between dynamic (mechanical) breakup and thermal breakup:

The higher the ice resistance at the point where it is overcome by driving forces, the more "dynamic" the breakup is said to be; at the dynamic extreme, it's a violent process involving what we might call an irresistible force and an immovable object.  At the other end of the spectrum, ice resistance crumbles before the driving force rises a lot, and the whole process is relatively uneventful.  Most breakups occur somewhere in the middle, but this year's abundant snowpack and this month's chilly conditions have tilted the odds significantly towards the dynamic end.

Here's an NWS graphic showing estimated flood potential for settlements across Alaska.  Circle is the only location with a "high" risk rating, with an estimated breakup window of May 9-15.  Note that the risk pertains to both ice jam flooding and snowmelt flooding, with the latter involving the greatest discharge.

Saturday, April 16, 2022

River Ice Wecams

For those interested in breakup progress, here's a nice resource for daily-updated webcam views:

Yesterday's photos are linked below via Twitter.  There's no significant breakup yet; the ice is getting soft under the sun with warm afternoon temperatures, but nights have been cold.  Fairbanks has yet to see a daily mean temperature above freezing, and this is unusual: in the past 30 years, only 2013 and 2002 had zero thaw degree days by April 15.  Of course 2013 was the coldest April on record in Fairbanks, with a record late breakup at Nenana, and 2002 was also very chilly, with Nenana breakup on May 7.

Saturday, April 9, 2022

March Climate Data

With March climate data now available, here's a look back at the major climate anomalies.  For the month as a whole, the broad circulation pattern around Alaska was quite similar to that of February - compare the two maps below - and most of southern Alaska was again wet, cloudy, and mild.  However, there wasn't enough sustained ridging to the west to keep cold locked in over northern Alaska, and it was easily the warmest month of the winter for the state (and the only month that was significantly above normal statewide).

Here are the temperature and precipitation rank maps from NOAA and ERA5 respectively:

And to confirm the widespread warmth, Rick's ground-truth temperature anomaly map:

Wind and solar:

According to the ERA5 data, it was an unusually windy winter overall from the west coast to the northern interior, but it wasn't Bering Sea storms that produced this anomaly: MSLP was generally higher than normal from the Chukchi Sea to the Gulf of Alaska (see below).  Looking at ASOS wind data from Kaltag as an example, virtually all of the windiest days had winds from the northeast; this is the favored interior wind direction, of course, but it appears the strength of these winds was considerably enhanced by repeated episodes of high pressure to the north and northwest.

Winter temperatures overall were not as low as might be expected during a significant La Niña, and south-central Alaska was actually warmer than normal.

Extremely widespread and very unusual warmth in the northwestern North Pacific may help explain the lack of sustained cold in most of the state despite a negative PDO phase and a circulation pattern (Bering Sea ridge) that was relatively favorable for cold northerly flow.  Sea surface temperatures were far above normal to the south of the Aleutians, and that's a source region for Alaskan air during more southerly episodes such as late February and early March.

Extreme precipitation was the biggest story overall, especially for the interior.  As Rick Thoman pointed out, snowpack water content on April 1 in Tok was nearly 50% higher than the previous record from way back in 1967 - see below.

Click to enlarge the April 1 snowpack map below.  According to NRCS, four subbasins have 250% or more of normal snow water equivalent: Nenana River, Tanana Flats - Tanana River, Healy Lake - Tanana River, and Salcha River.  Meltout and breakup continue to be a very pressing concern in terms of flood risk, especially with the forecast continuing to favor below-normal temperatures this month.

Friday, April 1, 2022

Sea Ice Update

First a quick comment again on northern Alaska, where winter is holding on.  Temperatures have dropped into the -20s °F widely across the North Slope the last 3 nights, and not because it's been calm; strong winds have pushed wind chills down into the -40s and even -50s in some spots.

According to airport ASOS data, Point Lay on the west coast has seen nearly continuous blizzard conditions (blowing snow, presumably) since late Tuesday morning.  Winds have been sustained at 30-40mph or greater the whole time, with a temperature between -10°F and -20°F.  Not bad for April.

But of course we're nowhere near record cold for the time of year across the region.  Daily record lows at Umiat are still in the -40s, and just last year it was -38°F on April 3.  The Umiat thermometer notched -50°F on April 5, 1986, and that stands as the latest observed -50°F in the state of Alaska.

And now for an update on sea ice, a follow-up to this post from two months ago.  Bering Sea ice has remained relatively abundant in terms of areal extent, at least compared to the recent history.  Ice extent has mostly been well above last year and it has been far above the extreme lows of a few years ago.

The chart below shows the strong recovery from 2018, with this year's January-March extent reaching 6% above the 1991-2020 median.

The map below shows today's analysis from the NWS.  On the plus side, full ice coverage is found as far south as Saint Paul Island, but on the flip side Norton Sound ice is already in poor shape, with a lot of open water visible on satellite (per Rick Thoman's Twitter comments).

As for other seasonally ice-covered basins of the Northern Hemisphere, the major anomaly has been in the Sea of Okhotsk, where a January-March shortfall of nearly 200,000 km2 (over 20% of normal) has far exceeded the small Bering Sea surplus relative to normal.  The Greenland Sea has also been running a deficit of about the same size as the Bering Sea surplus.

Friday, March 25, 2022

North Slope Cold

For the third year in a row, the North Slope of Alaska has seen a relatively cold winter, i.e colder than in many recent years.  With a week left to go, the November-March period has been the coldest since 2012-2013 in both Utqiaġvik and Umiat.  The deep winter period of December-February was colder two years ago, but that was mostly because of a very cold February 2020.  Interestingly, February has been easily the coldest month of each of the past 3 winters on the North Slope.

This winter the cold was somewhat persistent from mid-November on, and each month from November through February was colder than the 1991-2020 normal (and the 1981-2010 normal) for the North Slope climate division.   This is also true for the Northeast Interior climate division, but nowhere else in Alaska.  However, with significant warmth in early March, it looks a bit unlikely that this month will be the fifth consecutive below-normal month.

In Umiat the number of days (72) reaching -30°F or lower was the highest since 2011-2012, but it only reached -50°F three times (compared to 11 times in Jan-Feb 2020).  Utqiaġvik did not manage to reach -40° after succeeding in the last two winters.

I found myself curious about the correlation between winter temperatures on the North Slope and temperatures elsewhere in Alaska and farther afield, so here's a map.  I've used detrended November-March average temperatures from ERA5, and the map shows the correlation with a grid cell near Umiat.

The interesting aspect of this to me is how the Brooks Range forms such an effective boundary in the temperature anomalies: the correlation between North Slope and interior Alaska winter temperatures is really modest - mostly less than +0.6 (i.e. less than about a third of the variance is joint).  According to ERA5, Umiat winter temperatures are better correlated with temperatures over the central Bering Sea than in Fairbanks or even Fort Yukon.

(Note that I checked the ERA5 data against actual observations from Umiat since 2007, and the performance is surprisingly good: correlation of +0.99.)

Here's the same correlation map but for Fairbanks winter temperatures: this shows a much more expansive area of high correlation, including across the Alaska Range (much higher elevation than the Brooks Range).

To visualize the flow patterns that have the greatest linear relation to winter temperatures in Umiat, the map below shows the correlation with 500mb height.  A ridge anomaly right over Alaska is favorable for North Slope warmth in winter, but a trough tends to brings cold.  This contrasts with the more expansive (PNA) correlation pattern for Fairbanks - second map below.

For completeness, below are the MSLP correlations.  Interestingly, low pressure over the eastern Bering Sea is closely connected to winter warmth in Fairbanks, but it has the opposite correlation (albeit slight) with Umiat temperatures.

I'd be interested to hear any other observations from readers.