Breakup Flooding Outlook

Despite the wintry chill in the air this morning (-6°F in Tanana, -16°F near Huslia), the sun is winning the seasonal battle, and major river breakup is only a few weeks away.  The Alaska-Pacific River Forecast Center in Anchorage produces a breakup outlook once a week, and yesterday's update discusses the wide variation in snowpack and therefore flood potential across the state.

https://www.weather.gov/aprfc/breakupProducts

Snowpack is greater than normal from the central interior up to the Brooks Range, and so it's no surprise to see above-average expected meltwater runoff for these areas, and a "moderate" flood potential for several communities:

In the breakup outlook's table of flood potential, the Chena River basin is listed as "above normal", and that's consistent with the hefty snowpack in the hills above Fairbanks.  The NRCS April 1 snow survey report describes it thus:

The report lists Cleary Summit (2250' on the Steese Highway) as having a snow depth of 48", with water content of 11" (177% of normal).  This is one of the most impressive snowpack numbers in the Alaskan interior, and is quite remarkable for the elevation.

Hopefully the process of meltout and breakup will be gradual and not all at once.  The short-term forecast looks encouraging: near normal temperatures in the next 10 days, allowing some melting to get under way.

Here's a link to get updates for this graphic (enter the ICAO code for any airport location, e.g. PAFA for Fairbanks, PAEG for Eagle, etc):

https://www.weather.gov/aprfc/breakupAirTemps

And for bonus content, here's an animation of the Yukon River view at Dawson today, courtesy of dawson.meteomac.com.  It looks like people are still crossing the ice bridge, although it officially closed on Monday:

https://www.yukon-news.com/news/dawson-city-ice-bridge-closes-for-season-7931513

March Climate Data

March was another warmer-than-average month for most of Alaska, although not excessively so; it was the 15th warmest March in the NOAA/NCEI history since 1925.  The only region that wasn't warmer than the 30-year normal was the west, and the Bering Strait region was actually significantly colder than normal.  Here are my usual "percentile rank" maps, showing how the month compared to the same month in the past 30 years. 

The monthly average mid-atmosphere circulation pattern isn't what we would usually think of as producing a warm month for the state as a whole: a ridge to the north and a trough over the Gulf of Alaska tends to be a colder pattern in the cold season.

However, there were tremendous changes in the flow orientation through the month.  The first 10 days were very warm owing to a Bering/Aleutian trough (a typical warm pattern), but this reversed to an Aleutian ridge by late in the month.  Here are 500mb height anomaly maps for one-third portions of the month:

It seems that the northern Bering Sea and Bering Strait region managed to remain north of a frontal zone for much of the month, and so Arctic air dominated that region and kept temperatures relatively low.  The persistent frontal zone can be seen on a map of solar radiation, which was below normal (i.e. above-normal cloudiness) from the central Bering Sea to Bristol Bay:

March precipitation was below normal for more of Alaska than it was above normal, and according to NCEI both the Northeast Interior and Northeast Gulf divisions were significantly drier than normal for the second month in a row.

Given that March is typically a dry month, the precipitation anomalies made relatively little difference to the snowpack, which remains quite similar to a month earlier - although the positive anomalies have generally diminished in the western and northern interior.

Snowpack remains seriously lacking in the southwest, parts of South-Central, and the Seward Peninsula, according to ERA data:

Winds were lighter than normal for large parts of the state in March, which makes sense in view of the overall circulation anomaly (ridge to the north, trough to the south).

The extended winter period of November through March ended up as the sixth warmest on record - not as anomalous as December through February (third warmest).  November was the coolest month relative to normal (although still not cooler than normal), and January was by far the warmest and wettest.

The North Slope climate division had its third warmest November-March, trailing only 2017-18 and 2018-19, and the Northwest Gulf was fourth warmest on record.  In the case of southern Alaska, it's worth considering again how remarkable it is that this kind of warmth can prevail with a significantly negative PDO phase, although admittedly the PDO index did rise to neutral by the end of March.  Here's a chart of the PDO index for the last decade or so (click to enlarge).

Precipitation for the extended winter season was above normal for most of the state except the Panhandle, largely because of January; but as noted above, this generally only produced a good snowpack to the north of the Alaska Range, because of all the warmth (again, especially in January).

More on Persistence

I suspect that not everyone finds this as interesting as I do, but nevertheless here's a follow-up on seasonal temperature persistence in and around Alaska, this time from a map perspective.  Using ERA5 reanalysis data, I calculated the correlation of consecutive monthly temperature anomalies from 1950 through 2020, with the linear trend (specific to each month) removed.  A positive correlation means that the sign of the anomaly (i.e. above or below trend) tends to persist from one month to the next, but a negative correlation indicates that it tends to reverse.

It is usually a safe rule of thumb that weather and climate anomalies tend to be "persistent" even over land - there is a positive autocorrelation - so it's a surprise to see that the overall correlation is slightly negative for a portion of east-central Alaska and an adjacent zone in northwestern Canada.

This is actually the only place on the planet that has a negative month-to-month correlation, according to ERA5 data for this particular historical period.  There are plenty of regions with very low correlations, but this small region just to the north of Eagle is the only place with inverse persistence.  Here's a map for the Northern Hemisphere extratropics.

The tendency for temperature anomalies to reverse sign is mostly found in the winter for interior and eastern Alaska.  Here's the December-January correlation:

The rest of the year is added below.  The maps confirm the observation I made in the previous post: for the state as a whole, persistence is very much heightened in April-May and in July-August.  One might say those pairs of months are temperature twins: they tend to resemble each other in terms of departure from normal.

The other striking point is how much higher persistence is in summer than in winter for the maritime southern regions, and for late summer and early autumn near the Arctic Ocean.  Clearly the warm season temperature anomalies are dominated by slowly-varying ocean temperature regimes in locations close to open water, whereas relatively chaotic atmospheric flow patterns tend to control the month-to-month temperature changes during winter or over ice-covered ocean.

Seasonal Persistence

As spring gets under way in Alaska, temperature anomalies tend to become more persistent from week to week and from month to month.  What I mean is that colder than average - or warmer than average - weather tends to stick around more in spring than in winter; but winter temperatures are more variable over the course of weeks and months.

The chart below shows some evidence to back up this claim.  I've taken the NOAA/NCEI monthly temperatures for Alaska as a whole, removed the 1950-2024 trend, and calculated the month-to-month similarity of departures from normal.  The orange columns show the traditional correlation coefficient of adjacent monthly anomalies, and the blue columns show a "persistence index" that I defined here: the index takes a value of 1 if the adjacent monthly anomalies always have the same sign (perfectly persistent), and a value of 0 if the anomalies always reverse sign from month to month (perfectly anti-persistent).

The only pairs of months with fairly strong month-to-month persistence of statewide temperature anomalies are April-May and July-August, although there's a secondary peak of modest correlation in October-November.  Month-to-month persistence from November though March is remarkably low.

Looking at weekly data from Fairbanks shows a more prominent peak of persistence in the autumn (early October at a one-week lag), and there's also a clear peak in mid-April.  These two peaks are undoubtedly related to the persistent impact of snow cover anomalies: if there's more snow than usual at those transition times, it tends to remain cold, but if snow is lacking, it tends to remain warm.

It's interesting to note a pronounced dip in persistence in early June for Fairbanks, but I can't immediately think of an explanation for that.

In Anchorage we see much more of the late summer persistence that characterizes the statewide temperatures.  It's tempting to attribute this to the persistent effect of sea surface temperature anomalies around the western and southern parts of the state, but I'm not sure why the effect would show up more prominently at just that time of year (late July, early August).

The NCEI data for the Cook Inlet climate division confirms July-August as peak season for temperature persistence in the South-Central region, but the correlations are also significantly positive throughout spring and early summer.

Hopefully readers agree that "secondary" climate statistics like this provide interesting nuance and subtle insight into seasonal climate: there's a lot more to climatology than just the progression of normals and averages.  The next step in this analysis will be to calculate persistence from the gridded data so that we can examine the spatial distribution for each time of the year.

Late Winter Chill

Clear skies and a cold air mass have produced very chilly overnight temperatures in the past several days across interior and northern Alaska.  Some of the coldest readings have come in from the remote (but high quality) CRN sites, including the Selawik NWR (-38°F this morning), the Koyukuk NWR (-35°F), and the Toolik Lake site (-31°F).  It was -36°F this morning at the Beaver RAWS on the Yukon Flats, and Anaktuvuk Pass dropped to -33°F at the other end of the elevation spectrum.

Fairbanks airport dropped to -7°F, but this isn't at all noteworthy from a historical standpoint; it is typical to see -20°F at some point in the second half of March.

Here are this morning's lows across the central and northern interior (click to enlarge):

Interestingly, the change of weather pattern has brought significant snow to Anchorage for the first time since autumn.  Anchorage saw its least snowy winter (December-February) on record, with a measly 4.6" of snow in those three months, but now they've managed to accumulate 14.6" in the past week.  Adding this month's total to October's heavy snow gives a total of 36.1", well over 3 times the amount that fell in November through February.

The following chart illustrates the contribution of November-February to the total seasonal snowfall in the modern Anchorage climate history.  Typically about 30% of the seasonal snowfall occurs outside of Nov-Feb, and this ratio has been as low as 11% (1986-87) and as high as 62% (2001-02) in past years.  This year the ratio is 78% and the snow season isn't over yet.  Note that there's zero correlation between the two sub-totals over the years.

It's a topic for another day, but there is actually a statistically significant downward trend in the contribution of autumn and spring snow to the seasonal total in Anchorage.  This is what we would expect if the climate warms, because the autumn and spring temperatures are more marginal for snow.  So this winter very much counters the trend, because of the extremely dry winter (from the standpoint of snow - there was rain instead).  Last winter was the polar opposite, with only 12% of snowfall outside of Nov-Feb.

Fairbanks sees only slightly more of its seasonal total snowfall in months other than November-February; the average is about 33%.  The ratio has ranged from only 7% (2016-17) to 63% (1947-48).  Unlike in Anchorage, there is a slight positive correlation between the two sub-totals; but there is no significant trend in the contribution of autumn and spring snowfall in Fairbanks.

February Climate Data

NCEI and ECMWF have updated their climate data for February, so we can have a look back at last month and also the traditional climatological winter season of December-February.

February was the "least warm" month of winter for Alaska as a whole, but it was still warmer than the 1991-2020 average across most of the state's land area.  It was especially warm in the west and southwest, and quite warm on the North Slope too:

The obvious difference from December and January was in the Southeast, where February was colder than normal, and quite significantly so in the southern Panhandle.  This change occurred because instead of a ridge over western Canada, February saw a trough from the north-central Pacific to southern Canada, and the flow orientation brought cold interior air to southeastern Alaska.  November was somewhat similar, with Southeast being colder (relative to normal) than the rest of the state.

For Alaska as a whole, December-February was the 3rd warmest such period in the NCEI climate data history (1925-present); only 2000-01 and 2015-16 were warmer.  While unusual warmth was universal across Alaska, it was most pronounced for the Alaska Peninsula and the northwestern Gulf coast.  Both Cold Bay and Kodiak had their second warmest Dec-Feb on record (2013-14 and 2014-15 were the warmest at these two sites respectively).

While February didn't mark a dramatic break with earlier warmth in most of the state, the precipitation contrast between January and February could hardly have been greater, as nearly all of the state was drier than normal in February.  In southwest and south-central areas the deficit was really unusual; Anchorage saw its driest February on record, and according to Rick Thoman several locations in the Palmer area reported no measurable snow at all.  This is really remarkable, following January's onslaught of moisture.

January was so wet that despite February's dryness, large parts of the state were significantly wetter than normal for Dec-Feb overall.  According to ERA5 model data, only the North Slope was notably dry, but the NCEI February estimate strongly disagrees on this.  Given the paucity of reliable ground-truth winter precipitation data, I would tend to trust ERA5 more, but Utqiaġvik did report over an inch of liquid equivalent, almost enough to be in the top 10 for Dec-Feb precipitation.

As of the end of February, snowpack was very healthy for monitored areas in the western and northern interior, but major deficits are evident in South-Central.  The Iditarod start was moved to Fairbanks for the 4th time (other years 2003, 2015, 2017).

ERA5 snow data shows a similar north-south contrast in snowpack fortunes:

The lack of snow in southern areas increases the risk of early season wildfires, and that has been reflected in the fire potential outlook:

https://akfireinfo.com/2025/02/28/low-snow-high-risk-what-alaskans-need-to-know/

Burn permits will be required beginning just one week from now across southern Alaska:

https://akfireinfo.com/2025/03/06/early-wildland-fire-season-expected-in-alaska/

Finally, I'll close with ERA5's estimate that the Dec-Feb season was much windier than normal across the northern half of Alaska (see below).  This is broadly consistent with the warmth, as winds tend to mix out the surface-based inversion that is associated with valley-level cold; but the relationship is quite weak and is very elevation-dependent.  See here for some previous results on the temperature-wind correlation in the model data:

https://ak-wx.blogspot.com/2020/12/wind-and-cold.html