Wednesday, March 13, 2024

Sustained Wind Chill

Long-time blog readers will recall many previous comments about wind chill at Howard Pass in the Brooks Range.  It's a notorious location with the unusual climate characteristic that the windier it gets, the colder it gets, as low-level cold from the North Slope gets funneled through the low pass (only 2062' elevation) from north to south.  We know this because of instruments maintained by the National Park Service.

The past few days have seen a prolonged episode of hefty wind chill at Howard Pass: at or below -60°F since Friday evening, or nearly 120 hours.


Temperatures have hovered in the range -22°F to -32°F, with sustained winds of about 30-45 mph.  Howard Pass has seen much worse in the past (search the blog archives), but this episode is actually the longest on record with wind chill remaining continuously at or below -60°F.  The previous record was 105 hours in February 2013, the first winter we have data for.  Also, late November 2021 saw 10 straight days with an average wind chill below -60°F, but with higher daily maximum values.  [But note that the Howard Pass instrumentation was knocked out by severe conditions in a couple of winters, so there may have been more prolonged episodes that weren't recorded.]

[Update March 14: the wind chill finally rose above -60°F at 1pm today, so the new record is 136 hours.]

As in every other case of severe wind chill at Howard Pass, the wind has been continuously out of the north-northeast, i.e. perpendicular to the mountain range.  The second graphic below shows the location:



Actual temperatures on the North Slope have been very cold: -40s for overnight minimum temperatures in many locations since Sunday, and even below -50°F at the Umiat HADS site (the Umiat RAWS isn't reporting).  The Deadhorse ASOS reported -46°F on the 11th, which ties the coldest so late in the season since 2012 (when it was -49°F on March 16th).

The MSLP analysis from Sunday morning shows a very typical setup for North Slope cold and Howard Pass wind chill: high pressure draped across the Arctic coast, and a significant (but not extreme) north-south pressure gradient across state.  The bunched isobars across the Brooks Range (to the north of Bettles, circled) highlight the potential for strong winds in the mountains.  Click to enlarge:




Friday, March 8, 2024

February Climate Data

Climate data for February has arrived, showing that it was a warmer and wetter than normal month overall for Alaska.  Both December and January were slightly colder than the 1991-2020 normal, so February was the only month of climatological winter that was on the warm side; but actually all three months were quite close to normal statewide.

As is often the case, of course, "near normal" masks a lot of spatial and - in this case - temporal variability, as Alaska started and ended February with well below normal temperatures.  Here's the UAF statewide temperature index for December through February:

For February as a whole, colder than normal conditions were confined to some parts of the eastern interior, whereas western Alaska and the North Slope were significantly - but not dramatically - warmer than the baseline of the last 30 years.


The temperature rank map for Dec-Feb shows a moderately significant cold anomaly in the eastern interior, but more significant warmth occurred across  the North Slope and Southeast Alaska.


Fairbanks was colder than 8 of the 10 past winters with a Dec-Feb mean temperature of -5.5°F, but that's only slightly below the 1991-2020 average of -4.4°F.  On the other hand, Utqiaġvik had its 3rd warmest winter on record (only 2016-17 and 2017-18 were warmer).

February precipitation was very high in southwestern Alaska, locally over 300% of normal according to ERA5 data, and this was caused by a persistently strong Bering Sea trough:





December and February weren't particularly wet over the southwestern mainland, but February made for a much wetter than normal Dec-Feb overall.  The northeastern Gulf Coast also had a very wet winter, but that was mostly December.


As for wind, February was a stormy month for southern (but not Southeast) Alaska, with a big north-south pressure gradient across the Aleutians driving strong winds:




The Dec-Feb wind anomaly pattern is remarkably similar to the February pattern: it was an unusually windy winter from the Aleutians to southwestern and south-central Alaska.


All three aspects of the winter's climate outcome (temperature, precipitation, wind) were quite atypical for a strong El Niño winter.  The typical influence of El Niño is to produce unusual warmth in eastern (not western) Alaska, generally below-normal precipitation in most areas, and more often than not reduced winds from the Bering Sea to the northern Gulf.

It seems that the main reason for the difference is that El Niño's typical "Aleutian low" was shifted northward into the Bering Sea, and that's likely because there was a strong mid-latitude ridge associated with very unusual oceanic warmth extending east from Japan.  The persistence of such widespread warmth in the North Pacific was really unusual for El Niño, which historically has been associated with below-normal wintertime SSTs in the central North Pacific.

 

Wednesday, March 6, 2024

Yukon at Dawson Still Open

This has been a recurring theme in recent years, and a problem for residents of Dawson, YT: the Yukon River is reluctant to freeze over next to town.  As in some other recent winters, it hasn't been possible to build the usual ice bridge across to West Dawson.  Here's today's webcam view, suggesting that a complete freeze-up won't occur at all this winter:


And a video confirming the flow of water in the open channel:


November and December were significantly warmer than normal in Dawson, but January and February were both slightly colder than normal, so it seems unlikely that the lack of ice can be blamed on the weather.

Monday, March 4, 2024

Anchorage Wind

A reader inquired about the persistent north wind event of last week in the Anchorage area, so let's take a look at this.

First, the facts on the ground: there was indeed a very lengthy period of stiff northerly winds, lasting from early morning on Wednesday (Feb 28) through Saturday morning (March 2).  Here's a graphical look at observations from Anchorage airport, with wind speeds in mph:


Much the same sort of thing was observed up in Palmer, and with higher wind speeds, although there was a break on Thursday morning:


Remarkably, the peak wind gusts were 60mph or higher for 4 consecutive days in Palmer.  The maps below (click to enlarge) show the daily peak gusts from Feb 28 (top) to March 2 (bottom).  This was a remarkably persistent event:





Taking a quick look at historical hourly data from Anchorage airport, I see only one other event since 2000 with comparable sustained winds: it looks like a very similar event occurred at the same point on the calendar in 2017 (Feb 28 - March 3).  Here's the average MSLP for the 3 windiest days in both cases:  2017 (top) and 2024 (bottom):



Clearly the driving factor in both cases was the strong pressure gradient between low pressure in the eastern Gulf of Alaska and high pressure to the west and north.

The long duration of the event reflects the unusually persistent MSLP setup, with low pressure slow to depart or weaken in the Gulf of Alaska, and with building high pressure to the north maintaining the gradient as the low pressure eventually decayed.  Here's a sequence of MSLP analyses at 3am AKST for 5 consecutive days: February 27 through March 2.






Farther aloft, the situation was unusual, with a sharp ridge over the Bering Sea progressing slowly eastward behind a broad and deep trough over the Gulf of Alaska.  The 500mb map from Thursday afternoon shows powerful northwesterly flow in the very tight pressure gradient over southwestern Alaska, but yet the 500mb wind was out of the south at Anchorage (highlighted with the red circle below):


To me, this highlights the large degree of spin (technically "vorticity") in the atmosphere aloft, which helps explain the longevity of the low pressure in the northeastern Gulf of Alaska.  This upper trough and associated jet stream, by the way, is what brought extreme mountain snows to California over the weekend.  It was windy down there too: there's a lot of energy in this jet stream, and this is probably attributable to El Niño.

Friday, March 1, 2024

ECMWF AI Forecasts

The weather industry has been abuzz with excitement in the past year about the new AI (Artificial Intelligence) forecast models; I penned a few comments back in November:


The latest news is that ECMWF is now providing the realtime forecast data from its AIFS model, and it's open and free for all to use.  You can see the 4 forecasts per day on Levi Cowan's website:


The skill of the model is comparable to the leading physics-based models, so the new data will provide a useful tool for forecasters.  I'll be keeping an eye on it for Alaska.

We should bear in mind, however, that ECMWF currently runs only a single AIFS forecast each time, rather than an ensemble of forecasts like the ECMWF, NOAA, and Canadian ensemble systems.  Ensemble forecasts provide valuable information on confidence ("how similar are the ensemble members?"), and the ensemble-average forecasts tend to be more stable from run to run.  The AIFS forecasts will have a tendency to jump around from run to run, so take each iteration with a pinch of salt.

For example, here are the 4 latest AIFS forecasts for the morning of March 11, i.e. 10 days ahead, and probably at or beyond the limit of deterministic predictability for the Alaska region.  From oldest to newest forecasts:





The general theme is the same - cold in northern Alaska - but the individual forecasts disagree on the extent of cold farther south.

For comparison, here's the NOAA GEFS ensemble mean for the same time.  In this case the overall agreement is pretty good; and these forecasts (GEFS vs AIFS) are produced by completely different methods.  Impressive technology for sure!



Wednesday, February 28, 2024

Calendar of Cold

It's back to cold(er than normal) in Alaska again, with a brisk -31°F observed at Fairbanks airport this morning, and -40°F in colder spots to the north, including Bettles.  Wind chills are unpleasant on the North Slope too, but as noted by reader Mike, Arctic Alaska is very much still locked in winter.  The seasonal minimum in temperature occurs a lot later for the North Slope - and indeed for many of Alaska's coastal locations - than for the interior.

For a visual perspective on the timing of the seasonal minimum, here's a map based on ERA5 1991-2020 data.

According to this analysis, the seasonal temperature cycle bottoms out before the turn of the year for a few areas in western and interior Alaska, but most of the interior sees its coldest "normals" in the first half of January.  Cold peaks later for the North Slope - the first half of February for the eastern North Slope - and the highly maritime climate of the North Pacific sees a seasonal minimum even later, in late February or even early March.  The pronounced lag over the ice-free Pacific is related to the very large heat capacity of the ocean's upper layers, as well as the ample cloud cover that prevents a strengthening sun from bringing warmth to the ocean surface.

I'm a little surprised by the early seasonal minimum for coastal western Alaska, where I would have expected sea ice to keep temperatures suppressed to a later date.  The ERA5 results are confirmed by the NCEI 1991-2020 daily normals: Nome has its lowest normal temperature around January 12, and Bethel around January 9.  For comparison, Fairbanks also bottoms out around January 12.  I don't know why the west coast doesn't have more of a maritime influence in this aspect of the climate.

But a couple of caveats are worth noting in this analysis.  First, I'm using harmonic functions to smooth the seasonal temperature cycle through the year, and the details of the method (e.g. the number of functions) can affect the timing of the estimated seasonal minimum.  Second, the sample size of 30 years also implies some uncertainty in the date of the minimum, because the random timing of major historical cold and warm episodes will affect the details of the calculated cycle.  It would be interesting to quantify the uncertainty with some statistical experiments.

On a broader scale, the map below shows that the European side of the Arctic Ocean has a later seasonal minimum than the Pacific side: it even extends past March 15 for a small area near Iceland.


And expanding the domain into the mid-latitudes, we see that the eastern North Pacific has the latest seasonal minimum of anywhere in the hemisphere: as late as March 29 according to this analysis.


Another question I can't answer is why the eastern North Pacific has so much more lag than the western North Pacific.  Perhaps it's as simple as the western ocean being influenced by continental air from the vast land mass of Asia, which warms up quickly in late winter.


Wednesday, February 21, 2024

More Context for Cold

Another round of downslope/chinook warming brought another widespread thaw to the Tanana River valley and Fairbanks-land yesterday.  Temperatures above 40°F were widespread, including 45°F at Fairbanks airport, the highest February temperature in 20 years.  The year-to-date average temperature in Fairbanks is now only 2.2°F below normal.

Looking back again at the late January and early February cold snap, I was curious to see how the lower atmosphere (not surface) temperatures compared to past cold spells on various time scales.  However, with NWS weather balloons no longer going up during severe cold, there's some missing data for the recent event; so I extracted data from the ERA5 reanalysis.  I looked at Fairbanks 850mb temperatures and 1000-500mb thickness, with the latter being an excellent measure of the average temperature of the lower half of the atmosphere.

Here's the annual (winter) minimum for 3-day average 1000-500mb thickness:


On this time scale, the recent event was one of the coldest since 2000, but it was nowhere near as cold as the more extreme events of earlier decades.  The complete absence of seriously cold events after 2000 is quite remarkable, and suggests that the cold tail of the distribution has been dramatically curtailed since then.

Below is another perspective, showing annual minimum values of 3-day 850mb temperature (y-axis) versus 1000-500mb thickness (x-axis).  The values are quite highly correlated, of course, but there's some variability in the extent to which cold is concentrated at lower atmosphere levels like 850mb.  On a 3-day basis, the recent event was relatively less unusual in terms of 850mb temperature, with several other events being colder in recent years.


The 7-day and 14-day charts below show quite similar findings on these time scales, with the recent event being a bit more anomalous overall on a 14-day basis, as I noted before; but it was still not in the least unusual compared to earlier decades.



The two "granddaddy" cold spells of 1989 and 1999 stand out clearly here; those two events were much more extreme than anything in recent years, and also worse than anything that happened in the otherwise colder 1950s, 60s, and 70s.


Finally, it's interesting to observe that two quite extreme cold spells occurred in the last 20 years when we look at a 30-day average: late February and early/mid March of 2007, and January of 2012.



I hadn't previously realized how anomalous that 2007 event was, but in fact ERA5 shows the event having the lowest 30-day average temperature since 1950, by both metrics (thickness and 850mb temp above Fairbanks).  Given that it was so late in the winter, the surface temperatures were nowhere near as low as they would have been a few weeks earlier; but nevertheless March 2007 was the second coldest on record in Fairbanks, and 27 of 31 days had a low of -10°F or lower (the highest such number for the month).

Here's a look at the 500mb height pattern for the 30-day period: what a classic!


The 500mb height anomaly:


And the 850mb temperature anomaly, according to the older (but still basically reliable) NCEP reanalysis: