## Friday, October 28, 2022

### Rare Early Ice Jam?

Here's a puzzle for readers, concerning events on the middle Tanana River right now.  In what would be a very unusual event for the time of year, the Tanana River appears to have developed an ice jam yesterday near Fairbanks, causing the river gauge to register a rise of over 6 feet from yesterday morning to yesterday afternoon.  The water has been dropping only very gradually today.

The odd behavior is confirmed by this report via Twitter:

Water "flowing the wrong way" certainly suggests a jam.

My understanding is that it's very rare to see a genuine ice jam on a large river like the Tanana at freeze-up, because the ice is thin and easily dislodged; it's a totally different situation from spring ice jams.  However, with ice developing and increasing during freeze-up, a jam that develops at this time of year can be very persistent and even cause chronic flooding throughout the winter.  This may be a long-lived story.

Having said all that, though, there's a puzzle here: the Salcha River apparently had a very similar rise about 36 hours earlier.  If a single ice jam were the problem, and water were backing up, the rise would have occurred much later at the Salcha gauge, which is about 40 miles upstream from the Fairbanks gauge.  The Salcha-Fairbanks sequence looks more like a big volume of water is making its way downstream, although of course there may be more than one jam.

So which is it, ice jamming or a big new volume of water?  We should find out more sometime tonight, when we would expect a downstream-propagating rise to reach Nenana (another 40 miles downstream).  So far Nenana has seen a drop that aligns rather well with the rise upstream at Fairbanks, so this looks more consistent with the ice jam hypothesis (water held back and volume decreasing at Nenana).

I wonder if reader Gary wants to take a peek from his aircraft?  (Only joking, I imagine the NWS will bear that expense and solve the puzzle soon.)

Here's the evening scene at Nenana:

Update Saturday morning:

No rise overnight at Nenana, and a new observation: the Kuskokwim River at McGrath also saw a sudden and significant rise early yesterday.  It seems that ice jams are occurring widely during freeze-up this year.  But why?  Does anyone recall this happening in any previous year?

Update Sunday evening: another sudden rise, this one on the Koyukuk River at Hughes:

And another on the upper Susitna River.  Clearly this is what freeze-up looks like for interior Alaska this year.

## Tuesday, October 25, 2022

### Wintry Storm, Ice Notes

The weather map is looking a bit wintry this evening, with heavy snow for parts of the southwestern interior and west coast, courtesy of a strong Bering Sea storm.  A cold air mass is settling into northern and eastern Alaska.  In short, autumn is over.

The Bering Sea storm has intensified on approach to the Y-K Delta region, and has a central pressure near 970mb at present.  It will redevelop tomorrow in the northern Gulf of Alaska, reaching about 960mb not far from the northern Gulf coast; that's a strong storm for the time of year.

Ice is running in the Chena and Tanana Rivers, but not a whole lot just yet.  With cold in the forecast, there will be a lot more by the end of the month.

Freeze-up is progressing more substantially on the Koyuk River:

On another note, a question from Rick Thoman today prompted me to update a graphic showing the ice-free season along the Northern Sea Route (Arctic Russian waters) in the past two decades.  As I noted earlier, there was speculation in early August (by the NSIDC, no less) that the NSR might not open this year, but the situation turned around quickly in August in the East Siberian Sea.  In the end, there was a window of about 45 days for ice-free navigation (judging from the NSIDC Sea Ice Index maps).

## Friday, October 21, 2022

### Arctic Temperature Update

Back in June 2021, I wrote about a set of 32 longstanding climate sites that should allow for a reasonably comprehensive ongoing assessment of "ground truth" Arctic air temperatures over land; so it's time for an update.  One reason I think this is worthwhile is because global reanalysis models - a common and generally excellent source of information about climate variability -  tend to struggle with Arctic temperatures because of deficient representations of sea ice, snow, cloud, and radiation physics.  For example:

On the warm bias in atmospheric reanalysis induce by the missing snow over Arctic sea-ice

Evaluation of Six Atmospheric Reanalyses over Arctic Sea Ice from Winter to Early Summer

Of course there are also significant difficulties with long-term temperature trends at surface observing sites - there's no really "pristine" long-term surface data - but it is interesting to take a look at the month-to-month and year-to-year variations for sites with relatively long and stable histories.

With that in mind, here's what the GHCN data has to say about summer 2022 at 30 of my 32 sites (the other 2 had quality control flags this summer):

Kotzebue was the coolest of the lot, with the coolest summer since 2006.  This is actually the third consecutive summer with an average temperature below the 1991-2020 normal in Kotzebue, but this summer's temperature would have been normal about 60 years ago.

More broadly for these Arctic land sites, summer 2022 was a bit warmer than 2021 but cooler than 2020.

Below are the monthly temperature anomalies from May-September this year, expressed in terms of standard deviations.

Two of the monthly site anomalies stand out as being particularly unusual.  Barentsburg on Svalbard had its warmest June on record by some margin, and it was also - easily - the most anomalously warm month on record in terms of standard deviations (compared to the 1991-2020 climate).

The same was true of Hall Beach (locally Sanirajak) in Nunavut in July: warmest July on record, and most anomalously warm month on record.  But we need to put an asterisk by this one, because an alternative site at the same location, which does not report as reliably, was not quite as warm: only 2.4 standard deviations above normal.  The previously more reliable site that I used for the map above seems to have been running warm since May - an example of a problem with surface station data.

Looking at a longer-term view of monthly anomalies since 2010, the 32-station mean seems to have come down slightly in 2021 and 2022 compared to the post-super-El NiÃ±o warmth in 2016-2020, and that may be because of the persistent La NiÃ±a in the last two years.  There's been an absence of very large positive temperature anomalies since winter 2020-21, but that's mostly because the Arctic was relatively cool last winter, at the time of year when anomalies are largest in amplitude.

When the anomalies are standardized by monthly variance, the 32-station maximum jumps up dramatically because of the Barentsburg and Hall Beach anomalies this summer.  Also, notice that while the 32-site average may have avoided jumping to new highs, there has also been an absence of significant cold this year: none of the sites has been below -1.5 standard deviations so far this year.

And check out last March: warmth was remarkably widespread, with the coldest site being only 0.2 standard deviations below normal.  This itself is a record for "lack of cold" at any of the sites.  The history of this data does not yet have a month with all 32 sites above the 1991-2020 normal, but it will probably happen sooner or later.

## Tuesday, October 18, 2022

### September Climate Data

September already seems like a long time ago, but it's worth looking back at how the month stamped an exclamation point on the outstandingly unusual events of summer.  Last month I discussed the extraordinary mid-summer reversal in moisture, such that southern and western Alaska went from record or near-record dryness in early summer to record or near-record high rainfall in July and August.

September continued the trend in abundance, with much above-normal precipitation for large areas.  In particular, the eastern interior turned very wet, after not being as wet as the west and south in mid-late summer.  The two maps below show the September precipitation ranks relative to the prior 30-year history.

Using a longer baseline consisting of the entire NCEI history (1925-present), the following pair of maps highlights the amazing contrast.  For the Cook Inlet division, and for the state as a whole, the driest April-June on record was followed by the wettest July-September on record.

Here's an updated version of a figure I showed last month, showing the unprecedented magnitude of the turn-around.  In the prior 97 years, July-September precipitation was never more than 3 times that of April-June, but this year it was almost 5 times higher.  This year's anomaly is a 7.7 standard deviation departure from the mean of the previous history.

As for September's other aspects, it was a warm month, although not in the top 10 for the state as a whole.  According to ERA5, the most significant warm anomalies were found in the Brooks Range and southeast Alaska.

Unusual warmth also extended east across Arctic Canada, where it was very pronounced and significant in the vicinity of Baffin Bay; this was related to a strong "blocking" pattern near Greenland, with high pressure and a negative Arctic Oscillation.

Back in Alaska, ERA5 data indicates that wind anomalies were variable and not especially pronounced for the monthly mean, but of course we can't forget the extreme west coast storm, ex-typhoon Merbok.  September sunshine was uncharacteristically lacking for nearly the whole state - a very dreary end to summer indeed.  Soil moisture was, of course, far above normal, and I can't help but wonder if that will worsen ice overflow problems this winter.

Speaking of ice, here's a photo from Bettles that should help eliminate dreariness.

## Friday, October 14, 2022

### Changing Fast

A change from warmer than normal to colder than normal in interior Alaska at this time of year is always a rather dramatic affair.  This is partly because the climatological rate of change is at its greatest - for example, in Fairbanks the normal temperature drops by over 6°F per week in mid-October - and so it's easy to get a big drop in short order.  And of course it's partly because a good cold spell at this date means freeze-up for the interior, and sometimes quickly.

Fairbanks airport dropped to 14°F this morning, but other nearby locations were colder: +4°F at the Goldstream Creek COOP and +1°F at Eielson.  It was well below zero in favored parts of the eastern interior: -11°F at Chicken and an impressive -17°F at the Robertson River COOP near Tok.

This is not the coldest observed so early at Chicken, even within their relatively short period of record (1997-present).  In 2000 it was -10°F on the 2nd of October, and in 1997 it was -12°F on the 12th, followed by -25°F on the 18th.

Here are a few webcam views to document the state of freeze-up in select locations.

Toolik Lake has frozen over since Tuesday:

Ice is forming in the shallow water of the Koyuk River (always one of the first places to show ice south of the Brooks Range):

A bit of ice is running in the Tanana River at Nenana:

And in the Kuskokwim at McGrath:

And in the Yukon at Dawson (video courtesy of http://dawson.meteomac.com)

## Tuesday, October 11, 2022

### Snow Arrives; Winter Outlook

The scene is beginning to look and feel more wintry for interior and northern Alaska, with snow making an appearance in many locations as freeze-up begins.  Yesterday the temperature failed to reach freezing in Bettles, and there's a light covering of snow on the ground.

Fairbanks received its first measurable snow of the season on Sunday, and it amounted to just over an inch according to the NWS.  This is a bit later than usual, but more notable is the fact that the temperature hadn't dropped below 30°F until this morning.  This ties with 2017 for latest arrival of the first freeze in the 20s Fahrenheit.  Interestingly, with the forecast looking chilly, it seems possible there may not be another day with a low temperature above the 20s.  This has happened once before: in 1966.

Lakes and rivers will be undergoing freeze-up in the coming weeks.  North of the Brooks Range, it looks like Teshekpuk Lake is partially frozen over, whereas Toolik Lake is open for now.

What sort of winter might we expect for Alaska?  A key consideration is that we still have La NiÃ±a in the tropical Pacific Ocean, and La NiÃ±a winters tend to be colder than normal for nearly all of Alaska, drier than normal along the northern Gulf Coast, and snowier than normal for western, interior, and northern Alaska.  This is all because La NiÃ±a tends to weaken the semi-permanent Aleutian Low and produce more westerly rather than southwesterly flow across Alaska.  Compare the two maps below: the first shows the 1991-2020 average 500mb height, and the second shows the average during 10 strong La NiÃ±a winters.  It's a subtle difference, but a significant one, with fewer chinook-style warm-ups for the interior, less storminess for the Gulf Coast, and a wetter westerly regime to the north and west of the Alaska Range.

Here's the difference between the two: the anomaly produces a northerly perturbation to the mean flow, and hence the cold.

Here are the temperature and precipitation anomaly patterns for a larger set of 20 La NiÃ±a winters.  According to this data, 70-80+% of La NiÃ±a winters see below-trend temperatures across the interior, southern, and most of southeast Alaska.

La NiÃ±a is only one consideration in the forecast, however.  The experts at NOAA's CPC don't foresee a heightened risk of cold, except for the southeast; and there are no strong probability signals anywhere in the state for the December through February mean temperature.

Precipitation, on the other hand, does have a notable tilt in CPC's odds, with wetter being significantly favored for western Alaska:

The absence of cold in the CPC forecast may be a reasonable response to ongoing extreme warmth in the North Pacific to the south of the Aleutians.  The very warm ocean (relative to normal) was probably a factor in last month's damaging Bering Sea storm (ex-Merbok, read more here), and it will certainly boost warmth for Alaska whenever the flow comes from the southwest this winter.  Here's the ERA5 reanalysis SST anomaly map for September:

Above-normal precipitation for western Alaska this winter is supported by the latest NMME forecast, which is a consensus of several long-range computer models:

The NMME temperature forecast also hints at unusual warmth for western Alaska, similar to CPC, but it has a more typical cold La NiÃ±a look for southern and southeastern areas.  It will be interesting to see if CPC holds onto "equal chances" for most of the state when the forecast is updated next week.