Follow-up on Cold

A couple of quick items of follow-up regarding the recent cold snap.  First, here's a look at temperatures in the hills above Fairbanks, using data from several of the SNOTEL sites (not all of them have been reporting data lately).  The two distinct waves of cold are evident - the first culminating in the January 27 cold, and the second reaching its nadir 6 days later, on February 2.

The second cold wave was colder at valley level, with Fairbanks seeing its first daily-mean temperatures below -40° since 2017.  But some of the SNOTEL sites were no colder in the second round, and so the temperature inversion was stronger.  The climatological average inversion at this time of year is about 10-11°F between Fairbanks and most of these sites, although Little Chena Ridge runs a few degrees warmer (as in this event).  So the inversion was roughly normal in the second cold wave, but was certainly weaker than normal on January 27.

We can't confirm the temperature profile for February 2, because the NWS balloon sounding auto-launchers are automatically disabled below -40°, but the 3pm January 27 Fairbanks sounding confirms a total lack of inversion in the lowest 1500 feet or so.

Rick Thoman comments and explains on his blog:

https://alaskaclimate.substack.com/p/late-january-2024-cold-snap-in-alaska

Rick also comments that late January may have been the coldest in the hills since February 1999, based on data from Keystone Ridge and the Fairbanks soundings.  Certainly the -40° on Keystone Ridge on January 27 was the lowest minimum temperature since February 5, 1999, but it's worth mentioning also late January 2012, which saw a high temperature below -30°F on Keystone Ridge (not seen in the recent event).

Looking at the SNOTEL historical data, we get the following comparison for coldest daily mean temperature (°F) in the two years (2024 vs 2012):

Eagle Summit: -30.0 (2024) vs -38.5 (2012)

Munson Ridge  -35.5 vs -33.5

Mt Ryan  -31.5 vs -33.5

Upper Nome Creek  -39.5 vs -39.0

Little Chena Ridge  -32.0 vs -32.0

With the exception of Eagle Summit (which was much colder in 2012 and also colder in other recent years), I'd say it's a toss-up: the recent cold was comparable to 2012 at elevation.

Rick's copious notes from January 2012 (the coldest month statewide since pre-1925) are worth perusing:

https://ak-wx.blogspot.com/2012/01/more-cold_29.html

Airmass Contrasts

[Hat-tip to Brian Brettschneider for pointing out the event discussed below.]

Yesterday I highlighted the strong southerly flow across central and eastern Alaska that raised temperatures aloft up to near the freezing point above Fairbanks.  Elevated locations in the Yukon-Tanana uplands and the Alaska Range experienced this warmth directly: take a look at yesterday's high temperatures between Delta Junction and Glennallen (click to enlarge).

An automated site just above 5000' elevation (Look Eyrie) overlooking Isabel Pass (3600') rose to 36°F around noon:

But below about 3000' on the north side of the Alaska Range, the warmth didn't break through.  Here's the chart for a site at 2430' (Black Rapids Chalet) just to the north of the Richardson Highway pass: notice the high temperature of -17°F yesterday.

In the roughly 10 miles and 1000' elevation between this site and the pass, the back-and-forth "slosh" of the dense surface cold air produced remarkable swings in temperature yesterday evening.  The following chart is from the Black Rapids FAA site less than 2 miles up the road, and just 250' higher:

Check out the alternating wind direction and temperatures between 9pm and midnight (click to enlarge):

Warm air broke through 3 separate times, with the temperature dropping back to -10°F or lower between each episode!  Then finally the cold northerly flow won out by midnight, and the temperature re-stabilized below -20°F, with quite a wind chill (wind speed up to 35mph).

Up at the pass, a single warm episode occurred, with the temperature holding not far below freezing for more than 5 hours in a southerly breeze; then the wind reversed and the temperature dropped an amazing 42°F in 10 minutes.  In reality one can imagine the temperature drop being almost instantaneous as the boundary passed: air masses of such different density are nearly immiscible, like oil and water - hardly mixing at all.

Similar events occurred about 15 miles to the south, at Fielding Lake (elevation 3000'):

Here's the surface/MSLP analysis at 3pm, courtesy of Environment Canada.

Low pressure was pushing up from the Gulf (producing another big snowfall in Anchorage), and the impulse associated with this storm is what drove the temporary warm air intrusion across the higher elevations.

What an interesting meteorological event!

Cold Moves West

The axis of the cold trough aloft has shifted westward in the past couple of days - towards a more climatologically favorable position - and therefore southerly flow has developed across the central and eastern interior, bringing clouds, widespread light snow, and relatively warmer temperatures.  This morning's 500mb analysis:

Accordingly, the core of the surface-level cold has moved to western and southwestern Alaska.  One of the more notable cold spots this morning was Unalakleet on the Norton Sound coast: the temperature dropped to -45°F with a light southeasterly breeze as cold drained out of the interior.  This appears to be the lowest temperature in Unalakleet since February 1999.  The all-time record there (with data since 1941) is an almost unfathomable -59°F, which occurred almost exactly 35 years ago during the great 1989 cold snap.  A number of interior spots dropped into the -70s in that event, making the current cold snap look paltry.

There's more cold to come later this week as an arm of the trough reloads over eastern Alaska, but so far the coldest readings I've seen are -57°F at Fort Yukon, Bettles, and the Norutak Lake RAWS; -58°F at Tanana; and -59°F at the Allakaket FAA site.  (As an aside, the North Pole 1N co-op site has been reporting ridiculous numbers below -60°F and is not credible at all.)

So far I've not seen a daily high temperature of -50°F or lower.  Bettles came close yesterday, with a daylight high of -50°F, but clouds rolled in before midnight, lifting the temperature by a full 20°F after dark.

Here are some of today's low temperatures (the online map is too large to fit in one screen capture): 

One more interesting note: the deep southerly flow aloft has brought temperatures to nearly the freezing point a few thousand feet above the surface, as seen in this afternoon's sounding from Fairbanks:

The 32.4°C temperature inversion between 916mb and 800mb is one of the greatest inversions ever observed in the history of Fairbanks soundings.  The all-time record (data since 1948) is 33.4°C, observed in December 1956 and February 2018.  Here's my post from the 2018 event:

https://ak-wx.blogspot.com/2018/02/extreme-inversions.html

Cold High Pressure

A strong ridge moving across the Bering Sea and western Alaska since Friday has produced very high pressure at the surface for interior and eastern Alaska, and so, like clockwork, we have cold in the valleys, with strong temperature inversions.

Here's the MSLP chart from 3am AKST this morning:

In Fairbanks the airport reported MSLP of 1050.2mb at 6am, which according to Rick Thoman is "just shy of the November record of 1051.4mb set in 2010".  Interestingly, 2010 had a number of similarities to this year in terms of global climate, including a strong La Niña with a negative Indian Ocean Dipole in autumn, and terrible monsoon flooding in Pakistan in late summer.  It may be no coincidence that we're seeing a similar weather event for Alaska at the moment.

Rick commented on the 2010 MSLP record at the time:

http://ak-wx.blogspot.com/2010/11/big-high-coming.html

Here's an animation of the 500mb ridge progression from Friday morning to Tuesday morning (predicted):

Not surprisingly, temperatures dropped off impressively last night, with -20s seen widely in the valleys of the eastern interior.  As usual the coldest spots around Fairbanks-land were Goldstream Creek (-20°F), UAF Smith Lake (-23°F), and North Pole (-24°F).  Farther afield, the CRN site southeast of Northway reached -29°F and Chicken saw -31°F.

The morning sounding from Fairbanks shows the hefty inversion, and indeed the temperature hasn't dropped below 0°F at the higher elevations around Fairbanks (Cleary Summit, for instance).

 

Surprisingly, the Yukon is not yet frozen upstream at Dawson:

Below is a chart of temperatures over the last two weeks at Smith Lake on UAF's North Campus, illustrating the sharp drop-off yesterday.  Note too the difference between the blue and green lines: blue shows the temperature at 8m above ground, and green the temperature at the standard 2m above ground.  A difference of about 5°F is typical on clear, calm, cold nights, demonstrating how shallow the strong surface-based inversion is in these particularly cold locations.

 

Inversion Season

Fairbanks has seen some notably strong temperature inversions lately, thanks to an abundance of clear skies and windless conditions with high pressure aloft.  This is, of course, the time of year when valley level weather tends to become persistently colder than conditions aloft, even during the middle of the day; the sun provides very little daytime heating now.

Here's a chart showing the seasonal cycle of afternoon temperatures at the surface and at the 925mb level, which is about 1800-2000' above valley level (but varies).  I've used data from 1992-2019 for the calculation, because the Fairbanks balloon soundings didn't regularly report 925mb conditions before 1992.  Click to enlarge the figure.

Fully one-third of the year (November through February) sees a temperature inversion, on average, at the 3pm observation time.

At 3am, on the other hand, an inversion is typically present between these two levels throughout the year.

Notice that the mid-winter temperature profiles look about the same at 3am and 3pm; there's not much systematic diurnal temperature range, especially above the surface.

It's interesting to consider how these curves vary with El Niño and La Niña; we're in a La Niña this winter, so might this explain the recent strong inversions?  The answer seems to be no; La Niña winters are often colder, but perhaps counter-intuitively, the inversion is not stronger - see below.

Here's the corresponding chart for El Niño: slightly warmer than normal temperatures, but the departure from normal is not as large as during La Niña.

A direct comparison of the surface-925mb temperature differences reveals that the inversion actually tends to be stronger in El Niño winters - see below.  It seems this is because El Niño winters tend to be drier, with more southerly flow over the Alaska Range owing to a strong Aleutian low; so with more clear skies over Fairbanks, inversions are more often pronounced in the El Niño pattern.  In contrast, La Niña tends to bring colder air but also more cloud cover from the west and northwest; temperatures aloft are considerably colder, but there's not as much opportunity as one might expect for inversions to amplify the cold.

Finally, here are the recent 3am observations in comparison to normal.

Last week's surface-925mb temperature difference of 39°F is an elite-level inversion; this is only observed on about 1% of days in November through February, and the earliest it's ever been observed is just a couple of days earlier, on November 16, 2014 (with data back to 1992).  The 1992-present record is 53°F on February 10, 2018; see this post for discussion of that event and a climatology of inversion strength at all of Alaska's upper-air observation sites.

Extreme Inversions

This is a few days late but still worthwhile as an interesting note, I think: late last week some very extreme temperature inversions were observed over Alaska's interior as a strong upper-level ridge moved eastward.  On Friday morning the balloon sounding at McGrath measured a temperature difference of 62°F between the surface (-21°F) and 3500' above ground (+41°F); this is the strongest inversion measured at McGrath since March 2008.

At Fairbanks the inversion strength peaked on Saturday morning, right under the ridge axis, with a temperature difference of 60°F between the surface (-19°F) and 3000' above ground (+41°F).  This is essentially a tie with the strongest inversion ever measured by radiosonde in Fairbanks; the record inversion was just 0.1°C stronger in December 1956.  Here's the so-called skew-T diagram.

Here are the 500mb maps from Friday and Saturday mornings (top and bottom respectively), showing the slow progress of the ridge eastward; the peak inversions were closely aligned with the central axis of the ridge.

Rick Thoman posted a very remarkable map of Saturday morning temperatures around Fairbanks, showing a full 60°F of temperature difference at the surface in a distance of under 10 miles.  Locations in the hills were above freezing while some valley locations were in the -20s.

Looking at historical sounding data from Alaska's upper-air observing sites, the state record for inversion strength is 72°F at McGrath on January 13, 1966 (-41°F to +31°F).  McGrath has seen quite a number of occasions with inversions of more than 65°F, but on a typical winter day the inversion strength is greater in Fairbanks.

The chart below shows the all-time records (blue columns) for 13 sounding sites in Alaska that have been continuously active since around 1950, and the red columns indicate the 1981-2017 median inversion strength for morning (3 am) soundings in winter (December through February).  Only Annette Island has a median of zero, i.e. the average winter 3am sounding does not show an inversion.

How about the depth of inversions?  In general, stronger inversions are deeper, and so Fairbanks, McGrath, and Utqiaġvik (Barrow) are the winners for both strong and deep winter inversions.  The chart below shows the record and median values for inversion depth.

An interesting side note is that upper air observations were made at Northway from 1948 to 1955 and at Barter Island from 1953 through 1988, and both of these sites observed some very strong inversions.  In just a few years at Northway a number of 60+°F inversions were observed, with the record being 68°F of inversion on January 24, 1952.  The record at Barter Island was 64°F on January 25, 1983.

Cold Snap

Clearing skies allowed temperatures to drop to very low levels in most parts of the interior in the past 48 hours, with Fairbanks airport reaching -51°F yesterday morning.  The high temperature for the midnight-to-midnight period was only -41°F.  These are the coldest conditions in Fairbanks since late January 2012.  Many other locations, distributed very widely across the interior, dropped well into the -50s, with the "winner" being the Kanuti Lake SCAN site at -61°F as of 5am today.

The following map, courtesy of NOAA/NWS, shows reported low temperatures below -50°F in the 24 hours ending around 5am today.

As we noted before, the deep trough over Alaska has imported very cold air aloft, so the low surface temperatures are entirely to be expected under clear skies with very little solar insolation and light winds.  The Fairbanks soundings have measured 850mb temperatures around -20°F in the past couple of days, and it's not unusual to see an inversion of 30°F or more from the surface to 850mb at this time of year.  Under the right circumstances the inversion can reach 40°F or more, but that's more common under high pressure.  The sea-level pressure (MSLP) has actually been considerably below normal in the past several days.

The relationship between MSLP and temperature inversions is illustrated by the pair of charts below, which show the 850mb and surface temperatures on high-MSLP days (top chart) and low-MSLP days (bottom chart) according to the winter-season Fairbanks balloon soundings since 1981.  There is considerably more scatter in the relationship when MSLP is high, because the surface conditions are more decoupled from the free atmosphere, and accordingly strong inversions are more common; notice that there are quite a number of days in the top chart with surface temperature more than 40°F colder than 850mb temperature.  In contrast, this almost never happens when MSLP is low, and it's not coincidental that surface temperatures almost never get below -40°F when MSLP is low.  Interestingly, yesterday had one of the lowest MSLP readings on record for a -40°F surface temperature in the Fairbanks sounding, and this attests to the strength of the upper-level trough.

Here's the 500mb analysis from yesterday morning, courtesy of Environment Canada:

Finally, here are some interesting FAA webcam photos of shallow ice fog layers yesterday at Chalkyitsik, Arctic Village, and Livengood.

 

Cold Spell Ends

After nearly two weeks of significantly colder than normal temperatures in Fairbanks, clouds finally rolled in this morning and produced warming at the surface.  The temperature rose from -21°F at midnight to +9°F at 3pm as infrared radiation from the clouds above warmed the air at the surface and thereby weakened the surface-based inversion.

The chart below (click to enlarge) shows the hourly temperature data from the airport in the past 19 days, along with the 850mb and 925mb temperatures as well as the sea-level pressure (MSLP).  The strong inversion began on the 3rd when the pressure rose up through 1030mb, and the inversion weakened today as the pressure dropped back below 1030mb.  Of course the pressure itself is not directly responsible for the temperature changes, but it's correlated with the cloud cover: high pressure conditions tend to be clear and calm, allowing for large net radiation losses from the surface.

With the temperature staying below 0°F for 17 consecutive days and below -13°F (-25°C) for 11 days, this cold spell marks quite a change from recent warm winters; the last comparable cold period was in early December 2012.

It's interesting to note that the PDO phase is still moderately positive; its influence has certainly not been felt lately over interior Alaska.

Here's a chart of recent daily high temperatures, which have been more unusually cold than daily low temperatures.  Prior to today, 27 consecutive days had a high temperature below normal; that's the most since the very cold spring of 2013.  It will be interesting to see if the exceptional and persistent warmth that began so dramatically in May 2013 has ended for now, or if warmth will soon return.

Inversion Season Dates

A late-season cold spell has descended upon Alaska, bringing -30 °F temperatures to Fairbanks airport on the past two mornings.  Temperatures this low are a bit unusual for this late in the season, occurring in fewer than 3 out of 10 years after March 10.  Rapidly strengthening sunshine is making a big difference, however, as daytime temperatures would be much lower with an air mass this cold in the depths of winter.

With solar insolation rising quickly, Fairbanks will soon reach the date at which mean daily surface temperatures rise above mean daily temperatures at 850 mb.  According to the 1981-2010 normals, that date is March 19.  Based on the history of radiosonde data from several stations around the state, we can look at how the surface-850 mb temperature difference varies throughout the year; the charts below show the results.  Note that I would prefer to look at the 925 mb level as a measure of low-level inversion characteristics, but the 925 mb was not regularly reported in radiosonde data until 1992.

It's not surprising to observe that more northerly locations in Alaska generally spend a greater fraction of the year with surface temperatures lower than 850 mb temperatures, i.e. with an inversion in place on average; Anchorage sees this condition only during a mid-winter period of about 6 weeks in length.  An interesting feature of the charts is that the surface-850 mb temperature difference is nearly constant during most of the warm season at each of the locations outside the Arctic, so the low-level vertical stability of the atmosphere is little changed for a period of several months during summer.

It's also interesting to note that the Fairbanks and McGrath curves follow each other very closely indeed, despite a fairly large distance separating the two locations; this illustrates the relatively uniform nature of some aspects of the climate in interior Alaska, as opposed to the strong spatial variation near and along the coasts.

Weak Inversion

The brief cold spells in interior Alaska over Christmas and earlier this week brought strong low-level temperature inversions to the Fairbanks area, as is typical for the time of year; but since the beginning of the climatological inversion season (October 19), far more days have had weak or non-existent inversions than strong inversions.  The chart below shows the daily inversion strength based on the daily mean temperatures at Keystone Ridge and Fairbanks International Airport.  (Note the missing data from Keystone Ridge in early December.)  Remarkably, there were seven consecutive days in December with a positive mean temperature lapse rate (no inversion); the average number of such days in December is only two.

Looking at the mean inversion strength from October 19 through January 13, this winter so far has the weakest mean inversion since records began on Keystone Ridge in 1996 (see chart below).  Over the same dates, airport temperatures have averaged 4.7 °F above normal while Keystone Ridge has been 1.8 °F above normal; and so the inversion has been 2.9 °F weaker than normal.

Extreme Inversion

[Update 5 pm AKST: this afternoon's sounding from Fairbanks showed an inversion of 31.7 °C or 57 °F as warming continued aloft.  This has only been matched on 7 previous occasions since 1948.]

This morning's balloon sounding from Fairbanks airport revealed a temperature difference of more than 30 °C (55 °F) between the surface temperature and the temperature at 5300 feet above ground level.  As shown in the chart below, deep warming has occurred in the past few days, and at 3 am this morning the temperature was only just below freezing at around 4000-5500 feet.  Computer models suggest that the temperature is probably now above freezing at the same levels, despite valley surface temperatures below -20 °F.

An inversion of 55 °F or greater is quite unusual in Fairbanks; the last time it was observed was in January 2005.  In the history of upper-air observations since 1948, the strongest inversion was 60 °F on December 4, 1956.  The chart below shows the maximum inversion strength by winter since 1948-49, with the red line indicating the 10-year trailing average.

An even stronger inversion was observed in McGrath this morning (31.9 °C or 57 °F), but this is slightly less unusual there, as very warm air aloft can reach McGrath more easily.  The strongest inversion observed in McGrath was 40.0 °C (72 °F) on January 13, 1966.

Cloud Warming

Valley-level temperatures in and around Fairbanks are being held above the precipice of radiative cooling (so to speak) by low clouds, as seen in the infrared satellite image below: relatively dark shading in parts of the southern and western interior indicates relatively warm temperatures at the top of low clouds.  In northern areas, a mix of high and low clouds is also keeping temperatures from dropping as low as they otherwise would; the cold temperatures (white shading) in the satellite image show areas of cold high cloud in the northeast, not surface-level temperatures.  However, skies are generally clear in the southern Yukon Territory, and there temperatures have dropped to the -20s F and colder.

The low-level clouds have destroyed yesterday morning's surface-based inversion in Fairbanks, as shown in the two soundings below, and so Fairbanks airport has warmed by 15 °F despite continued cooling aloft.  When the clouds break up in the next day or two, the inversion will return and valley-level temperatures will drop to their lowest levels of the season.