North Slope Meltout

Spring has arrived in earnest on the North Slope in the past week, and the NWS noted today that breakup is underway:

209 PM AKDT WED MAY 27 2020

...BREAK-UP UNDER WAY ACROSS THE NORTH SLOPE...

WARM TEMPERATURES IN THE BROOKS RANGE ARE CAUSING RAPID SNOW MELT
THAT WILL CONTINUE THROUGH THIS WEEKEND. RIVERS DRAINING TO THE NORTH
SUCH AS THE COLVILLE, KUPARUK AND SAGAVANIRKTOK RIVERS ALL HAVE 
ICE REMAINING NEAR THE COAST, AND COULD HAVE ICE JAM ISSUES THIS 
WEEK AND WEEKEND AS RISING WATER LEVELS MOVE DOWNSTREAM. 

RIVERS NEAR THE ARCTIC COAST SHOULD BE MONITORED CLOSELY FOR ICE 
JAMS AND POTENTIAL FLOODING THROUGH THIS WEEKEND.

Here's the latest NWS breakup map (click to enlarge).

The average date for breakup at the Colville River delta is June 3, based on reports since 1996, so this year will not be far from normal.  The historical chart - see below - suggests a trend towards later breakup, but this is entirely because of two early years at the beginning of the series.  The long-term trend in temperature during May is most decidedly up.

Here are a couple of satellite images that illustrate the extent of snow cover remaining; the first image below is from Monday afternoon, and the second image is from today (click to enlarge).  Patchy cloud cover obscures the situation slightly, but there's no doubt the snow is disappearing.

[Update Friday pm: here's a great view from today, courtesy of and annotated by Rick Thoman.]

The Umiat webcams confirm that snow is gone on the middle Colville River (289' elevation), and even up at Toolik Lake (2460') there's a lot of bare ground now.  Both of these sites have exceeded 60°F already; summer's here.

Tripod Out

The Nenana tripod went out yesterday, marking the break-up of the Tanana River at its confluence with the Nenana River.  As I noted last week, it's highly likely that the break-up was earlier than it would have been without such abundant run-off from excessive late winter snowfall across the region.

This claim is supported by the fact that this year's break-up occurred with the least number of thawing degree days (TDDs) on record in nearby Fairbanks: the total TDDs through yesterday in Fairbanks was 74.0, which narrowly beats out 2002 (74.5) and 2006 (75.0).  We might say this year's thaw season leading up to break-up was the coolest on record.

Here's an updated chart of the relationship between TDDs and precipitation amount from March 1 to break-up.

An interesting nuance here is that most of the years with high precipitation and low TDDs also saw relatively late break-up, in contrast to this year.  The chart below classifies the years as "early" or "late" simply based on whether break-up was before or after the long-term median of May 3 (and yes, there's a significant trend).

It seems that it's typical for wet years to also be cool in April, and this delays break-up in comparison to dry years; i.e. dry and warm tend to occur together in late winter and early spring.  This complicates the picture a bit, because the stronger sunshine at later dates allows late break-ups to occur with lower TDDs independently of precipitation effects.  So the precipitation/TDD relationship isn't quite as simple as "more run-off means earlier break-up"; the relationships are complex.

A final bit of chart analysis (see below) illustrates that the very latest break-ups reliably occur after low TDD totals; but this year was unusually early for such low TDDs.  I suggest this is almost certainly because of the increased run-off related to the wet conditions of the past 6 weeks.

Lots of Water

Interior Alaska is finally escaping the grip of winter, with snow disappearing quickly and break-up beginning to advance on some rivers.  Remarkably abundant snowfall in late winter combined with recent rains has led to an excess of run-off and the potential for flooding if temperatures rise too high too quickly.  Here's the NWS flood potential map from last week:

Fairbanks is already down to a snow depth of only 4 inches, but the snow pack remaining in the hills is quite monumental: the Munson Ridge SNOTEL site at 3100' elevation near Fairbanks is reporting 15" of liquid equivalent water.  That's a lot of water to come down the creeks and rivers in the next month.

According to the SNOTEL instrument, 6.8" of liquid-equivalent precipitation has fallen on the hill since March 1 - this is quite remarkable for the (usually dry) time of year.  It's believable too; here's a chart of recent precipitation at the top-quality CRN site (1140' elevation).

The massive influx of melt water will almost certainly bring forward the date of break-up at the Nenana tripod, owing to the increased stress caused by high water.  There's some evidence of this effect in the history of the Nenana break-up; the chart below shows that when precipitation is very high after March 1, the heat input (thawing degree days) required to reach break-up tends to be lower than normal.

This year Fairbanks has seen 2.86" of precipitation since March 1, which is the highest on record for March 1 - April 21.  The previous record for this period was in 1967, which happens to be the year of the great August flood in Fairbanks (hmm...)

Here's the latest break-up summary from the NWS.

HYDROLOGIC OUTLOOK
NWS ALASKA PACIFIC RIVER FORECAST CENTER
ANCHORAGE AK
1 PM AKDT WED APR 22 2020

..SPRING BREAKUP OUTLOOK FOR ALASKA...

Breakup Update: The Forty Mile river upstream of the Taylor Highway
has broken up as well as the Kuskokwim River near Nikolai. The Tozitna
River has broken up with an ice jam forming and releasing. The Aniak
River is reported to have broken up dynamicaly and is mostly open at
the confluence with the Kuskokwim. Ice is beginning to lift on the
Kuskokwim, Yukon and Koyukuk Rivers.

The 2020 Alaska Spring Breakup flood potential is forecast to be
generally above average south of the Brooks Range and average flood
potential for the North Slope. The Flood Potential forecast is based on
observed snowpack, ice thickness reports, and long-range temperature
forecasts and focuses on the Kuskokwim, Tanana , Yukon, Koyukuk,
Copper, and North Slope rivers. Recent observed and forecast
temperatures suggest a generally earlier than normal, more dynamic
type breakup for locations statewide.

Timing of breakup statewide is expected to be several days earlier
than normal. Breakup along the Yukon River upstream of Fairbanks
is expected to be 1 to 3 days earlier than median dates; generally 3 days
earlier at locations downstream from Fairbanks to Anvik; and 3 to 5 days
earlier at locations downstream from Anvik to the mouth. Breakup along the
Kuskokwim River at Nikolia downstream to Tuluksak are expected to be 1
to 2 days earlier than median dates and 2 to 5 days earlier downstream
from Tuluksak to the mouth.

Temperatures - Statewide, temperatures over the past few weeks generally
have been normal to below normal. However, temperatures over the next
several weeks are expected to be normal in areas north of the Brooks
Range and above normal for the remaining parts of the state. Statewide,
temperatures for April and May are expected to be above normal.

Ice - The April 1st ice thickness data indicate that ice thickness is near
normal across the state. A few measurements across interior Alaska
ranged from 32 inches at Galena, which is 82% of average; 33 inches
at Nenana, which is 80% of average; 48 inches at Eagle, which is
117% of average.

Snow - April 1st snowpack by the Natural Resources Conservation Service
(NRCS) indicates greater than normal snowpack across interior Alaska
ranging from 117% in the central Yukon Basin to 197% of average in
the Kuskokwim Basin. The Yukon government is also reporting greater
than normal snowpack across the Upper Yukon River Basin in Canada.
The snowpack is near normal along the North Slope and below normal along
the Gulf Coast and throughout Southeast Alaska.

Climate Outlook:

April and May weather is the most important factor determining the severity
of river ice breakup. Dynamic breakups have a high potential for ice jam
flooding and typically require cooler than average temperatures during March
and into the first few weeks of April followed by an abrupt transition to
warmer than normal temperatures in late April to early May. Thermal breakups
have a low potential for ice jam flooding as river ice generally rots in place.

The Climate Outlook for Spring 2020 suggests a more dynamic type breakup this
year. Observed temperatures in March thru the first week of April were generally
below normal to normal. The second and third weeks of April were generally
warmer than normal.  Temperature forecasts for the last week in April
indicate near normal temperatures statewide with equal chances for above and
below normal for early may. The mid-March 3-month Climate outlook
indicates increased chances of above average temperatures throughout Alaska.

Further information can be found at the following web sites:
Weather-
https://www.weather.gov/aprfc/AGAK78PACR

Snow-
https://www.weather.gov/aprfc/nrcs_ak_swe
https://www.weather.gov/aprfc/snow_depth
https://www.wcc.nrcs.usda.gov/snow/

Ice thickness-
https://www.weather.gov/aprfc/icethickness

Climate prediction-
http://www.cpc.ncep.noaa.gov/

River Ice Breakup Flood Threat:

The two generalized types of river ice breakup are dynamic (mechanical)
and thermal. A dynamic breakup moves from the headwaters of a river
downstream in a somewhat linear fashion. Ice jam flooding occurs more often
during a dynamic type breakup than a thermal type. A thermal type breakup
results in the river ice rotting in place primarily due to gradual to rapid
warm ups and little snowmelt runoff. Thermal type breakups usually result
in fewer ice jams and less chance of flooding.

Statewide, breakup this year is expected to lean generally to the dynamic
type. Temperatures statewide over the past few weeks generally have been
normal to below normal. However, temperatures over the next several weeks
are expected to be normal in areas north of the Brooks Range with above
normal temperatures for the remaining parts of the state. Temperatures
statewide for April and May are expected to be above normal. South of
the Brooks Range, the expected more dynamic type breakup, above normal
temperatures, and above average snowpack has increased the potential for
flooding this year to above average, while flooding potential north of the
Brooks Range is expected to be average.

THE FOLLOWING TABLE GIVES AN ESTIMATION OF FLOOD POTENTIAL FOR
VARIOUS LOCATIONS AROUND THE STATE AND BASIN RUNOFF VOLUMES.
---------------------------------------------------------------------
SNOWMELT RUNOFF VOLUME...EXPECTED WATER VOLUME FROM SNOWMELT DURING
                         THE MELT SEASON.

FLOOD POTENTIAL...THE LIKELIHOOD OF FLOODING FROM SNOWMELT AND/OR
                  ICE JAMS.  THIS IS INITIALLY CALCULATED BASED ON THE FLOOD
                  FREQUENCY FOR THE 2000 TO 2019 HISTORICAL RECORD AND ADJUSTED
                  TO REFLECT CURRENT CONDITIONS.

*  MEDIAN BREAKUP DATES ARE FOR THE PERIOD 1980 THROUGH 2019 AND ARE
      CALCULATED FOR LOCATIONS WITH AT LEAST 5 YEARS OF DATA.
** ACTUAL BREAKUP DATE FOR THIS YEAR
---------------------------------------------------------------------

                      SNOWMELT    FLOOD     MEDIAN  NO. OF  FORECAST
RIVER - REACH          RUNOFF    POTENTIAL  BREAKUP  YEARS   BREAKUP
                       VOLUME                DATE*    USED    DATE
-------------         ---------- ---------  -------  ------  --------
SOUTHEAST PANHANDLE     ABOVE

KENAI RIVER            AVERAGE    LOW       OPEN TO SKILAK LAKE

ANCHOR RIVER            BELOW     LOW       MOSTLY OPEN

MATANUSKA RIVER       ABOVE AVERAGE  LOW

SUSITNA RIVER         ABOVE AVERAGE
  GOLD CREEK                      MOD
  SUNSHINE                      LOW-MOD     05/02     31  04/26-05/02

YENTNA RIVER          ABOVE AVERAGE
  LAKE CREEK                    LOW-MOD     04/30     29  04/27-05/03

SKWENTNA RIVER        ABOVE AVERAGE
  SKWENTNA                      LOW-MOD     04/29     25  04/23-04/29

COPPER RIVER BASIN     AVERAGE
  GAKONA RVR @ HWY                LOW       04/29     32  04/23-04/29
  GULKANA RVR @ HWY               LOW       04/29     30  04/23-04/29

CHENA RIVER           ABOVE AVERAGE
  CHENA LAKES PROJECT             MOD
  FAIRBANKS                       LOW       04/25     27    OPEN

TANANA RIVER          ABOVE AVERAGE
  CHISANA @ NORTHWAY             LOW-MOD    04/26     27  04/17-04/23
  SALCHA                         LOW-MOD
  FAIRBANKS                      LOW-MOD    04/28     19  04/26-04/02
  NENANA                         LOW-MOD    04/29     40  04/24-04/30
  MANLEY                         LOW-MOD    05/02     29  04/26-05/02

KUSKOKWIM RIVER       ABOVE AVERAGE
  NIKOLAI                        LOW-MOD    04/22     34   **04/21
  MCGRATH                          MOD      05/04     40  04/30-05/06
  STONY RIVER                    LOW-MOD    05/01     32  04/27-05/03
  SLEETMUTE                        MOD      05/01     31  04/27-05/03
  RED DEVIL                        MOD      05/03     34  04/28-05/04
  CROOKED CREEK                    MOD      05/03     34  04/29-05/05
  ANIAK                            MOD      05/04     37  04/29-05/05
  KALSKAG                        LOW-MOD    05/04     31  04/30-05/06
  TULUKSAK                       LOW-MOD    05/06     28  05/01-05/07
  AKIAK                          LOW-MOD    05/08     34  05/01-05/07
  KWETHLUK                         MOD      05/03      8  04/30-05/06
  BETHEL                         LOW-MOD    05/09     40  05/01-05/07
  NAPAKIAK                       LOW-MOD    05/10     25  05/03-05/09

YUKON RIVER (UPPER)   ABOVE AVERAGE
  DAWSON, YT                     LOW-MOD    05/03     40  04/29-05/05
  EAGLE                            MOD      05/03     40  04/28-05/04
  CIRCLE                         MOD-HIGH   05/07     36  05/02-05/08
  FORT YUKON                       MOD      05/10     36  05/04-05/10
  BEAVER                         LOW-MOD    05/09     24  05/04-05/10
  STEVENS VILLAGE                LOW-MOD    05/10     23  05/05-05/11
  RAMPART                        LOW-MOD    05/10     25  05/04-05/10

YUKON RIVER (MIDDLE)   ABOVE AVERAGE
  TANANA                         LOW-MOD    05/07     35  05/01-05/07
  RUBY                           LOW-MOD    05/08     34  05/02-05/08
  GALENA                           MOD      05/10     39  05/04-05/10
  KOYUKUK                          MOD      05/08     14  05/04-05/10
  NULATO                           MOD      05/10     23  05/04-05/10
  KALTAG                         LOW-MOD    05/11     34  05/05-05/11
  ANVIK                          LOW-MOD    05/13     32  05/07-05/13

YUKON RIVER (LOWER)    ABOVE AVERAGE
  HOLY CROSS                     LOW-MOD    05/14     33  05/06-05/12
  RUSSIAN MISSION                LOW-MOD    05/14     34  05/06-05/12
  MARSHALL                         MOD      05/14     28  05/06-05/12
  PILOT STATION                  LOW-MOD    05/13     23  05/07-05/13
  MOUNTAIN VILLAGE               LOW-MOD    05/15     33  05/08-05/14
  ALAKANUK/EMMONAK                 MOD      05/20     32  05/12-05-18

KOYUKUK RIVER          ABOVE AVERAGE
  BETTLES                        LOW-MOD    05/09     38  05/01-05/07
  ALLAKAKET                      LOW-MOD    05/09     33  05/03-05/09
  HUGHES                           MOD      05/10     33  05/03-05/09

SEWARD PENINSULA       ABOVE AVERAGE
  BUCKLAND                         MOD      05/15     30  05/09-05/15

KOBUK RIVER            ABOVE AVERAGE
  KOBUK                            MOD      05/13     36  05/07-05/13
  SHUNGNAK                       LOW-MOD    05/16     29  05/10-05/16
  AMBLER                         LOW-MOD    05/16     35  05/10-05/16

NOATAK RIVER           ABOVE AVERAGE
  NOATAK                         LOW-MOD    05/13     23  05/17-05/19

BROOKS RANGE - NORTH     AVERAGE
  COLVILLE @ UMIAT               LOW-MOD    05/24     19  05/16-05/22
  COLVILLE @ COLVILLE            LOW-MOD    06/02     22  06/26-06/01

SAGAVANIRKTOK RIVER      AVERAGE
  DALTON HWY                     LOW-MOD


For more detail and to see a Flood Potential Map, refer to our Website
at https://www.weather.gov/aprfc/FLOODPOTENTIAL

The next Spring Breakup Outlook is scheduled at 2PM AKDT Friday April 24th, 2020

River Ice Update

Break-up is essentially complete now on the Tanana and Kuskokwim Rivers and is under way on the Yukon River; the Yukon ice moved out on May 3 at Dawson, May 4 at Eagle, and today at Circle.  These dates are very close to long-term normals.  The tripod at Nenana fell on May 1, which is also about normal, but Bethel's break-up guessing game ended on May 6, a few days ahead of normal.

As we've noted before, the long-term trend towards earlier break-up is less statistically significant at Bethel than at Nenana.  There are two reasons for this: first, the (assumed linear) rate of change itself is smaller at Bethel, and also the inter-annual variance is greater at Bethel.  However, the early break-ups of recent years (2014-2017) have been much more unusual at Bethel compared to long-term norms.  The reason for this is probably the proximity of Bethel to the persistent and very unusual warmth in the northern Pacific Ocean.

While we're on the topic of variance and spring warming, here's a very interesting chart that Rick Thoman sent over, showing the long-term history of the first date of 65°F or higher in Fairbanks.  The three trendlines represent linear regression applied to the 90th percentile (blue), the median (green), and the 10th percentile (orange) of dates.  While the trend lines for median and early (10th percentile) warmth have advanced over time as expected, the 90th percentile has barely changed.  This implies that the variance has increased quite substantially.  While this could perhaps be a statistical quirk of this particular temperature threshold, it's an interesting result that deserves further investigation.

Spring Warming Trends (continued)

In my last post I suggested that annual dates of river break-up in Alaska may be able to provide quantitative information about long-term temperature changes at this time of year; this is possible because of the high degree of correlation between break-up dates and spring temperatures.  The annual date of "green-up" in Fairbanks can function in the same way as a climate marker.

After pondering the topic for a couple more days, it now seems clear that the method I used in the first post (read it here) was far from robust.  There's surely no significant difference between a correlation of (say) -0.86 and -0.87, so it's probably not reasonable to pick out the precisely optimal correlation and then claim to have identified a "true" temperature trend.  Accordingly, I think it's probably just coincidence that the results for Fairbanks and Nenana lined up so closely.

Not being one to give up, however, I moved on to a different approach by considering what range of temperature trends could be consistent with the observed correlation between temperature and break-up (or green-up) dates.  This is something that can be addressed with statistical simulation; so I produced 1000 simulated histories of southeast interior April-May temperatures by generating values based on the observed correlation with break-up date.  Each of the simulated histories has approximately the same correlation with break-up date as the actual reported temperature history, but the trends differ widely owing to the random component of the simulations.  Note that I'm assuming there are no other long-term changes that have systematically affected the break-up dates one way or the other.

Here are a couple of examples: the top chart shows the break-up dates and a simulated history that happens to have a very low (near zero) temperature trend, and the second chart shows an example with a very high temperature trend.

For both of these examples, the correlation of the simulated temperatures with break-up dates is very close to the correlation observed in reality, so these are outcomes that "could have" happened based on the physical connection between break-up and temperature.  However, both of these are unlikely outliers; the chart below shows the full distribution of trends from the 1000 histories, plotted as a cumulative distribution function.

It's nice to see that the 50th percentile of simulated trends lies almost exactly on the reported trend, so we can say that the changes in break-up at Nenana are entirely consistent with the temperature trend reported in the southeast interior climate division.  Bear in mind that the simulation process has no knowledge of the actual temperature trend; we have backed it out from the break-up dates.

Here's the same chart derived from green-up dates and April-May temperatures in Fairbanks.

In this case the simulated trends do not line up perfectly with the reported trend, as about 60% of the simulations have more warming than reported by the thermometers.  This is consistent with the simplistic result in my first post, but now we can quantify the probability that the trend is higher than reported.  Based on these results, and if my assumptions are correct, it is about 60% likely that Fairbanks has warmed more than the official climate record indicates (based on a 1974-2016 linear trend line).

Finally, here's an interesting result based on break-up dates of the Koyukuk River in Bettles.

Here we find that nearly all of the simulated histories have less warming than the reported April-May temperatures from the Bettles observing site; it appears to be 90% likely that Bettles has over-reported the warming trend.  In fact, more than 50% of the simulated trends are negative, and when we look at the break-up dates (see chart below) it becomes immediately obvious why this is the case: the linear trend-line for break-up dates shows a very slight increase over this period (although several years are missing near the beginning).  The lack of change in break-up date appears to be inconsistent with the reported warming at Bettles; more investigation is required.