Extreme Ridge

Records have fallen as upper-air heights have risen over Alaska today, reflecting a really extreme ridge of high pressure aloft (i.e. elevated "heights" of constant pressure surfaces like 500mb).  Here's a view of the mid-atmosphere situation at 3pm today:

The ridge is centered near Yakutat, and accordingly this afternoon's balloon sounding observed Yakutat's highest 500mb height on record for November, by some margin.  It was very close to the record for winter (November through March) as well; that record is just a smidge higher, from February 1989.

This afternoon's Fairbanks sounding also saw the highest November 500mb height of record, and again it was only just behind the all-time winter record, set in late December 1983.

The most obvious effect of the monster ridge has been to deliver extreme warmth to the coastal periphery of Alaska, from the southwest to the North Slope.  Strong winds lifted temperatures to around the freezing mark across the entire North Slope, and above freezing in areas with additional downslope warming.

Three of the four CRN sites on the North Slope rose above freezing:

Deadhorse: 34°F

Toolik: 37°F

Ivotuk: 39°F

It looks like the entire length of the Haul Road from Galbraith Lake northward rose above freezing, and there are even a couple of 40°F readings at USARRAY instruments in coastal ANWR.

Of course it's very warm at higher elevations of the interior as well - Denali's Eielson Visitor Center is currently sitting at 44°F - but the strong inversion has kept valley-level temperatures lower, as is typical in the absence of a chinook setup.  Here's this afternoon's sounding from McGrath: below 0°F at the surface, but +45°F about 1800 feet above the surface.

Vortex Records

I need to make just one more comment on the Arctic vortex that dropped down to the Bering Strait region nearly a month ago now: after putting together some new charts of upper-air data from Alaska's sounding sites, it is even more clear just how unusual the event was.

Consider the following chart, showing Nome's recent 850mb temperature observations in comparison to the historical range of observations.  The weather system we're talking about arrived early on July 18 with a dramatic 12.5°C temperature drop in 12 hours at 850mb.  Remarkably, the -6.7°C measurement early on July 18 was the lowest 850mb temperature that's been observed at Nome between June 25 (1949) and August 12 (1973); the previous coldest on record for July was -6.1°C on July 27, 2000.

Even more remarkable is that the 500mb height early on the 19th was the lowest on record between June 6 and August 22.  We can say pretty unequivocally that this was the strongest mid-summer trough in the modern climate history of Nome (1946-present).

The subsequent strong vortex event of two weeks later also shows up on the chart above, with 500mb heights very close to record low levels for the time of year.

The sounding observations from Bethel (below) show the cold blast on August 8, with the -4.3°C measurement sitting right at the record lows for mid-late summer.

I'm working on setting up an interface to view these graphics in realtime for all of Alaska's sounding sites.  It should be handy for quick context on major climate anomalies.

Is Alaska Cold Home-Made?

My post from a couple of weeks ago about cold on the North Slope elicited a comment from a reader about the origin of the very low temperatures.  Specifically, the comment asked whether it was accurate to suggest that the cold was imported from farther north in the Arctic; my post stated that high pressure was acting to "funnel very cold air into northern Alaska".  Surely, the reader asked, Alaska's cold develops in situ when there's no competing warm influx to prevent it.  After all, there's not much severe cold over the Arctic Ocean these days, so how can the North Slope cold have its origin farther north?

The answer to this question is that both ideas have validity.  Valley-level surface cold across interior and northern Alaska certainly is generated locally when conditions are right (clear skies, calm winds), but it's also true that surface conditions are powerfully influenced by the thermal quality of the large-scale air mass.  When it's warm aloft, temperatures at the surface are usually warm and are never severely cold; but when it's cold aloft, then the surface is usually very cold and is rarely warmer than normal.

The overall correlation can be illustrated with a scatter plot of December-February 850mb temperatures and daily minimum temperatures at Fairbanks:

Slightly more than half of the variance in daily minimum temperatures is explained by temperatures aloft (850mb is around 1200-1400m above sea level).  Taking -40° as a benchmark for severe winter cold, this only happens when 850mb temperatures are below normal, and more often 850mb temperatures are significantly below normal.

The correlation with temperatures higher up in the atmosphere is smaller, of course, but it's still significant.

Similarly, the recent cold spell at Umiat (as low as -56°F 10 days ago) was associated with colder than normal conditions aloft.  The chart below shows the rather close correspondence of surface and 850mb temperatures in the past several weeks - but note that I've used upper-air data from the nearest sounding site at Utqiaġvik, about 170 miles to the northwest.

Here are the scatter plots of Umiat daily minimum temperatures and Utqiaġvik upper-air temperatures:

I also looked at the correlations in terms of daily departure from normal, rather than absolute temperatures - see below.  As it turns out, it makes very little difference in this analysis.

To summarize, historical data show that about half of the variance of winter daily temperature is explained by the temperature of the air aloft, which in turn is largely dictated by the source region of the air mass.  So while it's fair to say that Alaska's cold can be greatly amplified in situ - for example under a strong inversion with clear skies and calm winds - the coldest episodes are never just home-made; you need both cold air aloft AND favorable surface conditions to take the thermometer down to its lowest levels.

Cold and Dry

The past few days have brought a fairly notable cold spell to the eastern and northeastern interior, with the first sub-minus 50°F temperatures of the season emerging at the usual cold spots.  Today is the fifth consecutive day with a daily minimum of -50°F or lower at the Chalkyitsik RAWS site near Fort Yukon, and this appears to be the most since January 2010 for that particular site (although there's a lot of missing data from 2012-2015).

It's been pretty chilly in Fairbanks too, with daily low temperatures below -30°F for three days, and a high temperature of -30°F on Saturday.  Of course solar heating is essentially zero at the winter solstice.  As an aside, here's a link to a very attractive solstice video from the other day.

Here's a time-lapse taken yesterday of the short daylight hours around the winter solstice in Fairbanks. Today, Fairbanks will only see 3 hours and 41 minutes of daylight, and the sun will only rise 2° above the horizon. #akwx pic.twitter.com/mCBQH8DKC1

— NWS Fairbanks (@NWSFairbanks) December 22, 2019

It has been a little unusual to see cold like this at valley level without colder conditions aloft.  According to the Fairbanks balloon soundings, the 850mb temperature did not drop below zero Fahrenheit; this temperature is colder than normal at 850mb, but not dramatically so - a colder air mass prevails about a quarter of the time in deep winter (per data since 1981).  In contrast, only about 5% of days see a high temperature of -30°F or lower in Fairbanks at this time of year.

This means, of course, that the surface-based temperature inversion has been strong; see Saturday morning's sounding below.  The surface-850mb temperature difference has been about 20°C in recent days, and this is exceeded on only about 7% of days in December and January - and more often than not it happens when conditions are warmer than normal aloft.  Big inversions tend to occur under high pressure, when clear skies and calm winds allow for strong radiational cooling, and at the same time subsidence aloft produces relatively warm, dry air above the inversion.  In contrast, cold air aloft tends to occur in association with a trough, which also typically brings cloudier and less calm conditions that don't promote a strong inversion.

So the unusual aspect of recent days has been the juxtaposition of relatively clear and calm conditions with a cool air mass, allowing surface cold to "outperform" over the northeastern interior.  One important reason for the absence of cloud is that the air aloft has been very dry indeed; the total column water vapor ("precipitable water") dropped to about 1mm late last week, which is down in the extreme lower tail of the historical distribution.  Less than 2% of soundings are this dry in Fairbanks in December and January.  The dryness of the air probably has to do with its origin in the high Arctic, as it seems to have been transported southward on the west side of a persistent and intense "polar vortex" near Canada's Banks Island.  This is illustrated below by the 500mb charts from last Friday morning and this morning respectively, courtesy of Environment Canada:

If we look at the history of precipitable water and surface-850mb inversion strength in Fairbanks, there is an inverse relationship, and this stands out particularly clearly when cold air is in place - see the figure below.  The blue markers show precipitable water versus inversion strength for the lowest 10% of 850mb temperatures in December and January; notice the uniformly low water content when the air is cold both aloft and at the surface (i.e. blue markers and strong inversion).

In conclusion, the combination of very cold and very dry air aloft is a recipe for extreme cold at the surface in Alaska's interior, and the last few days fulfilled the dry aspect of this.  There's a chance the "very cold" part may show up too in the near future; and this would be quite a shock to the system after all the warmth of recent years.   At the very least, the coming weekend looks to bring the coldest weather of the season so far.

Bering Ridge Wrap-Up

As a postscript to my earlier analysis of the intense ridge over the Bering Sea and western Alaska (see here and here), the charts below indicate the magnitude of the recent 500mb anomaly at 15-day, 30-day, and 45-day time scales compared to the Northern Hemispheric extremes since 1958.  As before, we're looking at the standardized 500mb height anomaly, i.e. the departure from normal divided by the standard deviation, and I've removed the (seasonally-varying) long-term linear trend at each location.  The charts show the daily maximum and minimum values of the standardized anomaly across the entire Northern Hemisphere since 1958.  Click to enlarge the images.

Based on this analysis, the upper-level ridge that affected western Alaska in recent weeks was most anomalous on a 30-day time scale.  Remarkably, the 30-day standardized height anomaly at 500mb just to the south of Nome was the most extreme in the global reanalysis history back to 1958, for either Northern or Southern Hemisphere, and for either positive or negative anomalies.  Here's a map of the peak 30-day height anomaly:

In the earlier post I noted that the record 15-day ridge was an extreme high-pressure block over northern Greenland in November 1965.  But interestingly the record event for a 45-day time scale was also over Alaska, in the late winter of 1989.  This event appears to have been related to a major disruption of the stratospheric polar vortex (a "sudden stratospheric warming") in February 1989.

Finally, here's a chart showing Southern Hemisphere extremes on a 45-day basis.  The record for most anomalous ridge occurred near the southern tip of South America in the early austral winter (late April - early June) of 2016.

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.

Snow and Upper-Level Wind

I mentioned the other day that it was unusual to see southerly flow aloft during a heavy snow event in Fairbanks-land, because southerly flow is normally associated with downsloping in the interior, and this precludes sustained, significant precipitation; chinook winds are dry.  However, chinook winds also involve deep southerly flow from near or at the surface to levels far aloft.  In the recent storm, significant southerly flow was confined to levels above 700 mb, as shown by the wind vectors in the following Fairbanks soundings from Tuesday morning and Tuesday afternoon.

In the recent event, despite southerly flow in the middle troposphere, widespread deep ascent was generated by intense synoptic-scale forcing across the eastern interior.  The series of maps at the end of this post shows the evolution of the flow pattern at several levels and documents how the storm system evolved.

Looking at historical data from Fairbanks, it's clear that heavy snow events are much more commonly associated with upper-level winds from directions between southwest and northwest.  The chart below shows the wind vectors at 3pm AKST for each day since 1957 on which at least 6 inches of snow was observed.  Note that the markers show the direction the wind is coming from, so the wind vector points from the marker to the chart's origin.  Tuesday's event is represented by the only marker on the vertical axis (500mb wind from due south). Not only are winds more typically westerly during heavy snow events, wind speeds tend to be higher than they were on Tuesday, often 50 knots or higher at 500mb.

The next two charts show corresponding results for 700mb and 850mb winds.  There's a nice clustering of wind vectors from a direction just south of west, with quite substantial velocity.  On Tuesday afternoon the 700mb wind was almost calm (3 knots from 130°) and the 850mb wind was a light 7 knots from the northwest.

The 500mb chart above showed two historical snow events with an easterly component to the flow; the reanalysis 500mb maps for these events are shown below, along with the recent event (third map).  The October 1963 and November 1996 events look rather similar to the most recent one, with a strong trough over western Alaska.

Below are the Fairbanks wind scatterplots for less substantial snow events with 3-5" in a calendar day.  The main difference seems to be that wind speeds aloft are usually less when snow amounts are smaller.

To document the recent event in more detail, here is a series of maps showing the pressure and wind patterns at several levels.  First, the MSLP evolution (times in AKST):

3pm Monday
9pm Monday
3am Tuesday
9am Tuesday
3pm Tuesday
9pm Tuesday
3am Wednesday

Next, the 500mb analyses: what a potent trough!

3am Tuesday
3pm Tuesday
3am Wednesday

Here are the 250mb analyses:

3am Tuesday
3pm Tuesday
3am Wednesday

And the 700mb maps; the stippling shows relative humidity - note the trough and high-humidity frontal zone over eastern Alaska on Tuesday afternoon.

3am Tuesday
3pm Tuesday
3am Wednesday

Finally, the Wednesday afternoon 850mb analysis shows the extremely strong temperature gradient and frontal zone over the far eastern interior.  The dashed contours are at 5°C intervals and the 850mb temperature goes from +14°C in western Yukon to -6°C at Fairbanks.  The deep ascent that produced the snowfall was directly associated with the broad (and vertically tilted) zone of temperature contrast.