Cold Outbreak

The first serious cold spell of the winter is now under way for Alaska, as strong northerly flow has brought cold air plunging down from the Arctic in the past day or so.  Temperatures have dropped below -40° at the usual cold spots, and Fairbanks airport is sitting around -30°F this morning for the first time since last January.

The immediate cause of the cold outbreak is a powerful high pressure ridge that ballooned up over the Bering Sea and far eastern Russia in the past couple of days, driving a sharp change to northerly flow over Alaska.  The shift in the flow pattern has been very dramatic: check out the 500mb flow change over Alaska from Thursday morning to this morning:

The north-south pressure gradient across Alaska is extreme today, with MSLP near 1040mb on the Beaufort Sea coast and near 980mb on the south coast of the Kenai Peninsula.  As a result, strong winds are producing ground blizzard conditions and nasty wind chill in exposed locations.

Remarkably, today's near-60mb north-south pressure difference is close to (or may be) an all-time record; here's a chart showing the annual maximum and minimum MSLP difference between Deadhorse and Seward, based on daily mean data.

The ERA5 record for the daily mean pressure difference is 55.8 mb on December 8, 1955.  I need to rerun the analysis using hourly data and compare it to today's conditions.  Note that very strong pressure differences like this are found in winter, when the jet stream is strongest and flow anomalies are greatest.

Looking ahead, there are strong signs that the Bering Ridge setup could have some serious staying power, judging from model forecasts.  For example, here's a diagram of 500mb heights averaged over the 50-80°N latitude band, with the time progression from top to bottom.  Look at the yellow/orange colors at the left and right sides in the forecast section (below the dashed line): the ECMWF model expects a ridge to persist near the Date Line all the way into mid-January.

This would mean persistent, significantly below-normal temperatures for southeastern Alaska and western Canada.  The CPC forecast is locked into this signal for their 6-10 day, 8-14 day, and week 3-4 forecasts:

Declining Alaska Observations

Yesterday UAF's ACCAP announced the results of a study evaluating the availability of weather data from FAA and NWS observing sites in Alaska:

https://uaf-accap.org/projects/alaska-weather-station-data-availability/

Unfortunately the results are not pretty: there has been a serious decline in availability of hourly weather data in recent years.  The number of sites reporting consistently on an hourly basis has dropped by about 40% in the last decade, and only 19 reliable FAA/NWS sites remain in off-road-network locations.  Check out the linked article for some striking graphics.

It's not difficult to observe the trend in the data.  Here's a chart for Utqiaġvik showing the percentage of days each year with temperature and wind data from at least 18 of 24 hours (not necessarily at the top of the hour):

Last year the observation reliability fell of a cliff, with virtually the entire summer missing data in the overnight period.  This year the same thing happened in March and April, but not in summer.

From a scientific standpoint it's a shame to see the lack of commitment to maintaining weather and climate monitoring with the FAA/NWS instrumentation, and from a public service standpoint it's bad news, as reliable real-time ground-level weather data is important for forecasting and decision making.

A significant counterpoint, however, is that NOAA's Climate Reference Network has expanded its coverage across Alaska in the last 20 years.  There are now 25 sites installed, and while several aren't reporting currently, the overall volume of data has become substantial in recent years.  The chart below shows the annual number of daily observations with maximum and minimum temperature across all the sites:

With the CRN sites being located preferentially in stable and often remote locations, and with high-quality instrumentation, the scientific value for long-term climate monitoring is potentially much higher than with airport instruments; but for real-time weather monitoring and forecasting, the CRN network can't replace the loss of FAA/NWS data in recent years.

Stratospheric Disruption

Meteorologists around the Northern Hemisphere have been excitedly watching events high up in the stratosphere recently, as the circulation aloft has become very unusual for the time of year.

Normally at this point on the calendar there is a robust vortex of cold air circulating high above the Arctic, but in recent days this vortex has been pushed well off its axis by a big ridge over the Canadian side of the Arctic.  The anomaly is ongoing; here's a forecast pressure map for Friday at the 10mb level, or about 100,000 feet above sea level.

The average 10mb westerly wind at 60°N is hovering around zero, which is approximately the weakest it's ever been at this time of year; more typical values are 20-40 m/s (45-90 mph).  The green line in the chart below shows the recent evolution of this closely-watched "U10/60" index, and the red line shows a forecast.

What does this have to do with Alaska, you may ask?  Well, the high pressure anomaly aloft also extends down through the lower stratosphere and the troposphere, so it's connected to the surface-level weather pattern over Alaska.  Here are maps for Friday at 100mb (about 50,000 feet) and 500mb (18,000 feet):

As of this morning, the 500mb ridge is centered over the Chukchi Sea, bringing relatively clear skies and cool northeasterly flow to northern and western Alaska:

Forecasters like to watch the stratospheric winds because they tend to foreshadow circulation patterns at lower levels, sometimes giving valuable information weeks in advance of important pattern changes.  In other words, what happens in the stratosphere doesn't stay in the stratosphere.

For instance, here's an analysis of 500mb heights 10-40 days following previous dates this century when the 10mb 60°N westerly wind dropped to zero in winter.  Above-normal heights near Greenland in the map below signify a negative Arctic Oscillation, a well-known effect of such "sudden stratospheric warming" events.

Closer to home in Alaska, there tends to be a trough over northern Alaska and a ridge over the Gulf of Alaska, and that is usually a mild setup for southern Alaska:

More strikingly, there is a strong wet signal for western Alaska: liquid-equivalent precipitation is commonly above normal in the month or so after these major stratospheric disruptions.

It will be interesting to see if the historical pattern plays out this time around.  CPC does show a strong wet signal in the 6-10 day period, but focused on the eastern interior at this time:

Autumn Clouds

Following up briefly on last week's post, I took a closer look at cloud cover and solar radiation data from the Fairbanks area, seeking to document the remarkable lack of sunshine in the last few months.  All lines of evidence indicate that it has, indeed, been persistently and unusually cloudy in Fairbanks since mid-August.  Here's a chart showing the daily solar radiation measured since summer at the high-quality CRN site just to the northeast of town:

Of course it's typical for rain and cloud to increase across much of Alaska in August (see this post), but the change this year was more pronounced than usual for the interior, and the weather remained generally cloudier than normal all the way through autumn.

For the 90 day period ending November 10, the CRN instrument measured less radiation than in any other year, with data going back to 2002.  Interestingly, the last 3 years have seen the lowest solar energy in this short history, so it's a persistent anomaly that I strongly suspect is related to the remarkable North Pacific warmth of the last few years.

ERA5 reanalysis data provides a much longer history of estimated solar radiation for the same window, and it sends a similar message:

Of course most of the solar radiation in this autumn window occurs in the first month or so, while the sun still has some strength, so it's worth looking at cloud observations from the airport ASOS for a more evenly distributed perspective on cloudiness.  Here's the result: this was the cloudiest autumn since at least the mid-1950s, if the cloud cover observations are to be believed (and there are certainly inconsistencies in the observing method over time).

To get a sense of the spatial extent of the recurring anomaly, here are solar radiation anomaly maps for the August-October window back to 2020, the last relatively sunny autumn in Fairbanks.

Again, the analysis for the whole seasonal period is focused on the earlier part of the window (mainly August) because of the annual cycle.  However, the last 5 years have also been cloudier than normal in each month separately, so it's not just a late summer change.  The maps below also show a similar trend in the Arctic - at least according to the ERA5 model data.  It's not good news for autumn aurora watchers, or for those who enjoy clear and crisp weather at that time of year.

Autumn Climate Data

Temperatures have been dropping off sharply in the past 10 days in the interior, and the change has felt more acute because of the unseasonable warmth before that.  Fairbanks didn't see anything more than a mild freeze until three weeks ago, when the temperature first dropped into the low 20s Fahrenheit (about 3 weeks later than usual).

The last week or so has felt very different.  Starting on November 4, high temperatures have been below 20°F; the first sub-zero low temperature occurred on the 9th; and then yesterday the high was -1°F.  Winter has most definitely arrived.

Last month I missed my usual monthly climate summary owing to the focus on ex-Typhoon Halong; so let's look back at September and October.  Both months brought unusually low pressure to northern Alaska and stronger than normal westerly flow across the state.  There was a particularly strong trough over the western Bering Sea in October, and this bears part of the blame for pulling Halong up from the south.  Both months saw above-normal high pressure to the south of the Aleutians.

The southwesterly orientation of the flow in October explains the unusual warmth in Fairbanks, and the anomaly was widespread; but September was somewhat cool in western Alaska, as the air had a more northwesterly origin.  Both months were very unusually warm in the southeastern interior and along the Gulf coast, according to ERA5 data:

Recall that early September saw a very unusual heatwave in the southeastern interior, and there was another rare warm extreme in early October: Northway reached 62°F on the 8th, with a low temperature of 40°F (easily the latest on record for such a high daily mean temperature).  This was followed by unrelenting warm anomalies for several weeks:

Another theme of the two months was the unusual excess of cloud cover across large parts of Alaska.  October was especially cloudy for the interior, according to ERA5 data.

Looking at cloud cover data from Fairbanks airport, the situation has been dreadful since the middle of August.  Until this week only one day (September 17) reported mostly clear skies for more than 20% of the day, and the fraction of hourly reports with mostly clear skies is easily the lowest on record for the September-October window.  This is so anomalous that I think it's worth a separate post after looking at data from the CRN solar radiation instruments.

Not surprisingly, precipitation was very abundant across much of the state in October, and northern Alaska was substantially wetter than normal for three months in a row (August through October).  Bettles saw 10.6 inches of precipitation in the three month period, the second highest on record for that period.

The active westerly pattern produced above-normal wind speeds in October (in part due to Halong of course), but September was relatively calm for the central and eastern interior.

Finally, it's worth noting that the enhanced wind speeds across the northern North Pacific produced a dramatic drop-off in sea surface temperatures from just north of Japan to the Gulf of Alaska.  Compare the two figures below (August versus October):

The North Pacific Mode (NPM) SST index, which I revisited recently here, has dropped off precipitously:

Given that La Niña will prevail during at least the first part of this winter, the disappearance of the positive NPM probably means a greater chance of cold in Alaska; but much will depend on the high-latitude flow, which is about to become very perturbed in connection with a rare upper-atmosphere disturbance over Arctic Canada.  More on that later.

Ocean Temperatures and Halong

In an earlier comparison of ex-typhoon Halong versus other historic autumn Bering Sea storms, I noted an intriguing similarity: Halong, Merbok, and the major storms of 2011 and 1974 all occurred during prolonged La Niña episodes in the tropical Pacific.  It's worth digging into this a bit more to see what might be going on.

Looking back at historical data since 1950, there have been 11 years - including this year - when La Niña occurred in late autumn for the second or third consecutive year.  (It's not uncommon, by the way, for major La Niña episodes to extend over multiple years, in contrast to El Niño, which tends to be "one and done".)  Here's the average sea surface temperature (SST) anomaly pattern in September through November of those years (relative to the long-term trend):

For comparison, here's the September-October SST anomaly this year:

There's a lot more warmth in the northeastern Pacific this year compared to the historical pattern, but there is a similar contrast between a cool tropical Pacific and warmth extending eastward from Japan.  The warmth from Japan eastward is very characteristic of a negative PDO phase, and indeed the PDO has been strongly negative in recent months.

Zooming in on the North Pacific, it's interesting to see that Halong and Merbok both passed over very warm ocean water (relative to normal) to the south of the Aleutians before moving up into the Bering Sea.  Based on the analysis above, that region of warm water is typical of persistent La Niña and negative PDO regimes.

The storm of 2011 also originated over a region of warm water, although the North Pacific as a whole was much cooler back then.

It's tempting to speculate that the extra warmth and moisture available from the unusually warm ocean to the south of the Aleutians provided more fuel for these storms than would be derived from cooler oceans.  Rick Thoman mentioned to me that modeling research is already under way to quantify the role of the unusually warm SSTs for Halong, with preliminary results showing a significant impact.

Another aspect of the persistent La Niña/negative PDO SST pattern is that the north-south ocean temperature gradient is greater than normal near the Aleutians, and this gradient tends to enhance the North Pacific jet stream, providing more upper-atmosphere support for strong extratropical cyclones.  Here's the average 500mb height anomaly in the same "persistent La Niña" years; notice the strong trough from the East Siberian Sea to Alaska.

Again, this is not too dissimilar to what we've seen this autumn so far:

It's worth noting that autumns in which La Niña is just developing do not have the same magnitude of warmth to the east of Japan, and they lack an upper-level trough near the Bering Sea that would support (and reflect) strong storm activity.  The PDO tends to be less negative.

Part of the reason for the difference is that La Niña favors high pressure ridging over the central North Pacific that gradually produces warmer ocean temperatures over time, and so the warm anomaly east of Japan becomes more amplified by the second or third consecutive La Niña winter.  In contrast, an initial La Niña is often developing after El Niño, and so the mid-latitude North Pacific SSTs tend to be cooler owing to the lingering El Niño influence (lower pressure with more wind and cooler conditions to the south of the Aleutians).

An interesting corollary to this discussion is that the long-term trend seems to be favoring more frequent and persistent La Niña and negative PDO conditions in the Pacific; and this therefore seems to raise the risk of more frequent severe Bering Sea storms in autumn.  Last year's UAF ACCAP report "Alaska's Changing Environment" indeed documented a recent increase in Bering Sea storms, but also indicated that no clear long-term trend has yet emerged.  It will be interesting to see how this assessment evolves in the coming years.

https://uaf-accap.org/alaskas-changing-environment/