Howard Pass Again

It's time for another mention of northern Alaska's "pole of cold" for measured wind chill, i.e. Howard Pass.  As noted by reader Gary, conditions have been harsh up there in recent days.  In typical fashion, strong northerly winds funneled cold air through the pass (elevation near 2000 feet) with great ferocity starting last Saturday evening, and it wasn't until yesterday afternoon that the wind chill rose above -60°F for the first time in over 3 days.

The minimum wind chill based on hourly mean temperature and wind speed was -73°F on Sunday evening (-25°F temperature, 80mph wind).  This is respectable but not all that unusual for the location; wind chills this cold or colder are observed at least once in nearly every winter.

The MSLP analysis from Environment Canada shows the very typical setup at 4pm Sunday, with a very tight pressure gradient caused by a ridge to the north and sprawling low pressure across southern Alaska.

In the 2012-present history of the NPS instruments at Howard Pass, -60°F wind chill has been observed as early as November 17 (2021) and as late as April 9 (2013).  As one might expect, it's most common in January and February.  Interestingly, this is the first time the wind chill has stayed below -60°F continuously for more than 80 hours this early in the season.  However, late November 2021 saw a much more prolonged period of substantial cold, with 11 straight days having a wind chill reading below -60°F at some point in the day.

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.]

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:

Winter Lives

The calendar may say June, but the weather has been more than a little wintry in the past few days at Howard Pass in the western Brooks Range.  Readers of this blog are well acquainted with the extreme wind chills that occur regularly up there in the winter, but it turns out that the site can put on a good show even in "summer".

As the chart below illustrates, two multi-day episodes of wind chill below +10°F have occurred in the past few days, and early yesterday morning the wind chill dropped below 0°F for a few hours.  This is the first time a sub-zero wind chill has been measured at the site in the summer months; previously the latest was on May 19, 2013, and the earliest was on September 9, 2012.  (However, there are only 5 complete June's in the station's history since 2012, because the anemometer had been disabled by winter winds and was not yet repaired in June 2015, 2018, and 2020).

An interesting difference from the winter wind chill setup is that the cold air blowing through Howard Pass this week is not being drawn from a locally reinforced cold pool on the North Slope.  In the cold season the extreme wind chill occurs when severe cold develops across the interior North Slope under a surface-based inversion, and then a strong pressure gradient forces that cold air up and over the shallow pass.  But in this case it's just an early summer Arctic air mass; there's no serious radiative cooling at the surface in the continuous daylight of Arctic summer.

To document the nature of the air mass, here are the surface observations at 5am yesterday, and the 3am Utqiaġvik sounding (click to enlarge).

Frigid in the North

With high pressure over the Arctic Ocean continuing to funnel very cold air into northern Alaska, surface conditions have become downright frigid across the interior and eastern North Slope, with widespread -40s and some -50s today.  Umiat reached -55°F this morning according to the RAWS instrument, and saw a high of only -47°F this afternoon.

But at least there's no significant wind in Umiat.  From Deadhorse all the way across to the Canadian coast, wind chill values are -70°F or lower, with stiff offshore breezes creating seriously nasty conditions.

The lowest temperatures occurred in the sheltered valleys of the interior North Slope to the south and southwest of Umiat; satellite measurements suggest some spots may have dropped below -65°F.  Here's a satellite-observed temperature map courtesy of Twitter user wrighthydromet (click to enlarge).

 

 

Our favorite wind chill site, Howard Pass, also had a very extreme episode of wind chill yesterday, with sustained winds over 50mph in conjunction with temperatures below -40°F.  This is the first time in 7 years that the Howard Pass thermometer has dropped below -40°F, although missing data is an issue.

Back in December I looked at the strong inverse relationship between wind and temperature at Howard Pass.  Remarkably, -40°F or lower has only ever occurred with a wind speed of 37mph or higher, and the median wind is 48mph at such low temperatures.  The chart below zooms in on the low-temperature portion of the hourly distribution of temperature and wind.

It's interesting to observe that while yesterday's cold blast was a typical wind chill episode at Howard Pass, the temperature didn't recover much last night as the winds quickly died down to almost nothing.  Consequently, today's temperatures were easily the coldest on record for low wind speeds; this morning it was a full 10°F colder than previously observed at Howard Pass with a wind speed below 5mph.  This illustrates the intensity of the cold air mass over northern Alaska.

Here's a surface analysis chart for 3am yesterday; notice the very strong pressure gradient over northwestern Alaska, an obvious prerequisite for these extreme wind chill episodes.