Fire Weather Trends

Continued dry and very warm weather has allowed fire activity to persist in the central interior, and unfortunately for Fairbanks the air quality has gone badly downhill, with visibility dropping to just a mile and a quarter in smoky conditions yesterday.  This isn't as bad as last year, but it's certainly unpleasant.

Statewide total fire acreage has risen above 110,000 acres as of yesterday, with 35,000 acres tallied on Sunday alone.  It's been 4 years since this much acreage burned this late in the season, but of course there have been much worse fire seasons in the past.

Before going any further, I must make a quick note also of the record heat at Utqiaġvik, where Saturday was easily the warmest day on record.  The daily mean temperature of 66°F smashed the record of 63.5°F that was tied just a couple of weeks ago; and the daily low temperature of 56°F was also easily an all-time record (formerly 53°F).  Breaking all-time temperature records by this margin - with a period of record of more than a century - is no small feat, and it will cement this late summer heat wave as an historic event.

In my last post, I mentioned the seasonality of relative humidity as a major factor for the typical late summer drop-off in Alaska's fire activity.  Thinking a bit more about this, I decided to revisit an analysis from 2014 in which I looked at trends in "vapor pressure deficit" or VPD in Fairbanks.  From a fundamental standpoint, VPD is more directly correlated with evaporation and fire fuel drying than relative humidity; for example, low relative humidity at low temperatures does not produce much evaporation at all.

After updating my 2014 charts, here's a look at June-August average daily temperature, dewpoint, and (in columns) VPD.  Interestingly, there's only a very slight upward trend in VPD overall, although both temperature and dewpoint have trended up modestly.  As it turned out, the extreme summer of 2013 (record high VPD) did not portend a change to drier conditions, but in fact summers have generally been humid and wet in Fairbanks since then.

There's much more of an upward VPD trend for the month of May, however, and this is consistent with Alaska's rapid long-term warming in spring, and the known trend towards a longer fire season.  Last year (2022) was an example of remarkably early and aggressive fire activity.

The charts for June, July, and August illustrate the rapid seasonal drop-off in evaporative potential - see below - and of course with sunshine weakening, days shortening, and rainfall increasing in late summer as well, the odds are stacked against fire activity by the time the calendar reads August.

The chart below shows an alternative historical perspective based on the number of days that would be considered very favorable for fire activity, i.e. days with a peak VPD value above 2.5kPa in Fairbanks.  A VPD of 2.5kPa can occur with hot and humid conditions such as a temperature of 86°F and dewpoint of 58°F (relative humidity about 40%), or it can occur on a cooler but very dry day such as a temperature of 74°F and a dewpoint of 20°F (RH about 15%).  Either way, a lot of evaporation is occurring.

2013 was a remarkable outlier for high-evaporation days in Fairbanks, but it wasn't an extremely active fire year, simply because of the lack of lightning: it was too dry for widespread ignitions.  The historical relationship between high VPD days and seasonal fire acreage (see below) shows that nearly all years with more than 10 very dry days do produce at least a million-acre fire season, and conversely it's rare to get a big season without at least a handful of dry days.  For reference, 2023 is up to 8 such dry days in Fairbanks, with Sunday being the most recent.

Average VPD for June-August gives a broadly similar relationship to the high-VPD days - see below - although there are a number of very inactive fire years with above-normal VPD for the summer as a whole.  Presumably it's the really dry days that matter more, as those produce rapid fire spread and account for a disproportionate fraction of the seasonal activity.

And finally, a bit of good news, and not at all surprising for the time of year: some rain is on the way this week, and temperatures are predicted to drop off quite substantially in the next two weeks.  Fire activity should wind down accordingly.  Here's the latest 6-10 day forecast from CPC:

Fire Activity

The dramatic change in the weather pattern across Alaska has produced a belated start to meaningful wildfire activity, and according to the AICC, about 47,000 acres have now burned in the state - and virtually all of this in the last week.  This is no longer the lowest fire acreage on record for the date, and it won't be the least active fire season on record: 1995 ended up with only 44,000 acres burned for the season.

Most of the (still very limited) fire activity is south of Nenana and between Salcha and Delta Junction, but there are some fires in the hills around Fairbanks, and the Fairbanks airport reported this year's first instance of smoke in the ASOS observation on Saturday morning.

The remarkable change to persistent and unusual warmth across Alaska occurred right after the middle of the month, and obviously the drier, warmer pattern is partly responsible for the modest fire growth in recent days.

However, it was lightning that started the fires, and there was a tremendous amount of it during the recent heat wave.  According to data from the ALDN, over 40,000 lightning strikes occurred between Monday and Thursday last week, and last Monday's 19,000 or so strikes ranks as the 5th highest daily total in the modern history of the ALDN (2012-present).  It was also the highest since July 11, 2019.

The recent lightning outbreak is even more impressive considering the date on the calendar: late July is certainly past the seasonal peak in lightning activity.  It seems the electrical storms were greatly enhanced by high humidity and therefore greater convective instability.  Both Tuesday and Wednesday last week achieved the rare feat of having a daily mean dewpoint above 60°F in Fairbanks, and the week ending Saturday had very nearly the highest 7-day mean dewpoint on record (it was marginally higher in late July and early August 1994).

The chart below shows daily average temperature, dewpoint, and relative humidity for Fairbanks since May 1st this year.  The upward climb in dewpoint is typical, but the normal value peaks at about 50°F in late July.

This chart also suggests a reason why the recent fire starts haven't yet produced very dramatic acreage growth - the relative humidity hasn't dropped much below 40% on most days recently, whereas values of 25% or below are generally regarded as favorable for fire growth.  This too is typical for the time of year: it's rare to get very low humidity days this late in the summer.  See here for an old post on the seasonality of fire weather in Fairbanks:

http://ak-wx.blogspot.com/2015/07/fire-weather-climatology.html

Having said all this, we shouldn't assume that major fire problems can't or won't happen in the next few weeks.  The forecast is again very hot, with the NWS predicting temperatures near 90°F for the eastern interior again this weekend.  It would be unusual, but fire activity can ramp up in August; there have been 4 years since 2000 when more than a million acres burned in August (2002, 2004, 2005, 2009).  Let's hope this year doesn't make a run at that joining that group.

Heat Wave

Now that the heat has receded a bit, let's look back at events of the last few days.

First off, the most notable aspect of the heat wave must be the sustained warmth in Utqiaġvik - it's been nothing short of remarkable.  For seven days in a row, the high temperature reached between 64°F and 74°F, resulting in a weekly mean high temperature of 67.6°F.  This is well above the previous record of 65.3°F for the week ending July 15, 1993.  The weekly mean low temperature was also a record, and the overall weekly average temperature smashed the previous record by more than 3°F.

Here are the warmest non-overlapping weeks in Utqiaġvik history, with mostly complete data back to 1902:

58.3°F   Week Ending July 24, 2023

55.2°F   Week Ending August 10, 1989

54.7°F   Week Ending July 15, 1993

54.2°F   Week Ending August 20, 2012

The record-breaking anomaly was undoubtedly caused by persistent offshore flow combined with an extremely warm continental air mass.  Here's a map of the 850mb height (pressure) anomaly for the week: note the twin ridges, one to the south of the Aleutians that funneled warm air up into Alaska, and one near the Mackenzie River delta, creating another warm influx to the North Slope from western Canada.

Looking at the balloon soundings from Utqiaġvik, the 850mb temperature peaked at 14.6°C, which is the warmest since 2018, and slightly behind the warmth of June 2013.

The profound warmth of the air mass is illustrated by the very high daily minimum temperatures, with many sites seeing low temperatures in the 60s °F.  The map below shows midnight-to-midnight lows for July 24 - the warmth across the western Brooks Range was particularly notable (click to enlarge).  A couple of days earlier, the Umiat RAWS reported a daily low of 62°F, the highest on record for that site (2008-present).

In the Fairbanks area, low temperatures were in the mid-60s on Monday, and the Chatanika RAWS only dropped to 71°F.  As for daytime heat, Fairbanks did reach 90°F (see my last post) on Monday, and the Ester 5NE co-op site near Fairbanks recorded 92°F; that's the highest temperature measured in Alaska so far this year.

Rick Thoman noted that the combination of heat and humidity in Fairbanks on Monday afternoon was the worst since at least 1950: at 5pm the temperature was 88°F, and the dewpoint was 62°F.

Based on the Fairbanks airport balloon sounding data, Monday's air mass was clearly the hottest in the vicinity since the notorious heat wave of late June 2013.  The 850mb temperature reached 17.8°C (versus 18.4°C in 2013), and the 1000-500-mb thickness reached 5688m (versus 5694m in 2013).  Here's what it looked like on Monday afternoon: a deeply heated continental sounding, but with quite a bit of humidity for such a northern location.

Heating Up

After a summer that has generally been on the cooler and wetter side for many in Alaska so far (except in areas close to Canada), things are heating up.

Much of the North Slope has been well into the 70s and even low 80s Fahrenheit in the past few days, with Umiat reaching 82°F yesterday, and Utqiaġvik reached a rare 74°F on Wednesday: the warmest since 2009.  Rick Thoman also noted that Wednesday's daily mean temperature of 63.5°F tied the highest on record in Utqiaġvik, with only one previous day matching that level of warmth: July 13, 1993, when the all-time high temperature of 79°F was recorded.

A heat wave is also now unfolding for the interior, with both Fairbanks and Eagle predicted to reach 90°F tomorrow, according to the NWS.  This would be the first 90°F reading in Fairbanks since 2017; there have only been 36 such days in the 93 years of Weather Bureau/NWS data.  (Eagle already reached 91°F earlier this month, on the 7th.)

It's actually fairly late in the summer for Fairbanks to see this kind of heat: in the past, 80% of the 90°F days have occurred before July 15.  The latest on record was August 15 (2010), and the earliest was May 28 (1947).  The median date is June 25, which is a little before the early July seasonal peak in average temperature: it's more common to get very dry and warm weather earlier in the summer rather than later, when clouds and humidity are more abundant.

Here's a view of all 90+°F days in the Fairbanks climate record since 1930.  No trend in frequency is immediately evident, although it's slightly notable that the past 30 years have seen only a handful of such days.

The location of the official climate observing site has changed over time, so it's interesting also to look at data from the UAF farm.  It seems that before about 1970, 90°F was observed in fewer years, but when it did happen there were typically two or more days with such heat.  However, this is probably related to the time of observation, which seems to have typically been around 5pm in the earlier years - and therefore close to the time of maximum warmth, allowing the highest temperature to be recorded as the max for two consecutive days.

Here's the upper-air setup for the impending heat wave: a strong ridge near the Alaska-Canada border by Sunday afternoon, with associated clear skies, subsidence, and widespread high airmass temperatures for much of northern, central, and eastern Alaska.

Temperature Comparison at Utqiaġvik

Way back in 2014, I posted some analysis of temperature differences between the airport and CRN sites in Alaska's northernmost town, Utqiaġvik (formerly Barrow).  With more than 20 years of climate data now in the books from the CRN site, I thought it would be interesting to revisit this - and in particular, to take another look at the apparent warming trend in the town's temperature relative to the outlying CRN site.

Here are the previous posts:

https://ak-wx.blogspot.com/2014/08/barrow-area-temperatures.html

https://ak-wx.blogspot.com/2014/08/barrow-area-temperatures-part-2.html

The locations of the two thermometers are indicated here:

 

First, let's confirm the close correspondence between the instruments at the two sites on a monthly mean basis:

No concerns there.  As noted and discussed in the previous posts, daily maximum temperatures tend to be most different between the two sites in summer, and for daily minimum temperatures the greatest differences are found in winter.  In both cases, the airport is usually warmer than the CRN site.  Systematic differences are least in autumn, when there's relatively little land-sea contrast in temperature.

 

The following charts provide another perspective on the overall relationship between temperature and temperature difference.

Now for the trend over time - see below.  Interestingly, there does seem to be a small but statistically significant increase in the temperature difference between the two sites, with the airport ASOS becoming progressively warmer than the CRN site.

How does the trend vary over the course of the year?

Interestingly, the trend in temperature difference is not confined to one season, but appears throughout the year, although with some variability.  This seems to suggest it is indeed a systematic trend at the local level rather than a trend caused by changing weather patterns at a certain time of year.

The largest trend has occurred for minimum temperatures in summer, and the charts below provide another perspective.  For the minimum temperatures (blue lines), there's a slight but perceptible tendency for relatively cooler CRN temperatures in recent years.  The maximum temperatures are less affected, except in June.

What do we make of this?  Occam's Razor suggests that the growth of the town is producing a gradually increased urban heat island effect.  In my 2014 post, I speculated that urban warming was unlikely to be the cause, because the Utqiaġvik population had decreased in the 2010 census; but the 2020 census reversed that trend, taking the population to an all-time high of about 4900.  This is a modest but perhaps significant increase of 8% in 20 years.

How significant is the divergence in temperatures compared to the overall long-term warming trend?  Well, two decades is not a long time in climate terms, but based on this limited history, the linear trends in annual mean temperature differ by about 15%.

This difference isn't remotely enough to alter the picture of rapid Arctic warming, but nevertheless it does illustrate the value of having a long-term climate monitoring program that's designed to avoid problems of instrument and site changes.  In other words, I'm a big fan of the CRN program.

June Climate Data

For the month as a whole, June was really quite similar to May for climate anomalies in Alaska, with the big picture involving an ongoing trough over the Bering Sea and western Alaska.  The mid-atmosphere pressure anomaly was actually more pronounced in June than in May, and a larger fraction of the state was cooler than normal in June:

This was the third month in a row that was both cooler and wetter than the preceding 30-year median for the state overall, according to the NCEI data.

Just like last month, wind and clouds were both very abundant around the northern Gulf coast as well as the southwestern interior and the Alaska Peninsula.

Historically, the month-to-month correlation of statewide temperature anomalies is highest from spring (April-May) through summer (July-August), so it's not particularly surprising to see such persistence in the pattern.

As for the wider Arctic, there wasn't as much unusual warmth in June as I expected to see, given the extreme wildfire situation at lower latitudes in Canada, and in fact the largest warm anomaly for the month was in eastern Siberia.  In terms of standard deviations, the town of Aasiaat on the west coast of Greenland took home the prize for largest anomaly, and over there the unusual chill was also linked to a strong and persistent upper-level trough.

Severe Thunderstorms

The east-central interior saw a remarkable and rare outbreak of strong to severe thunderstorms yesterday evening, and the NWS meteorologists in Fairbanks had their hands full issuing warnings for the numerous storms.  The first severe thunderstorm warning was issued at 5:47pm AKDT for a storm about 30 miles west of Fairbanks, and 12 more warnings were issued over the next two and a half hours.

Here's the Fairbanks radar image at the time the first warning was issued (click to enlarge).  The storm cell in question is the one just to the north of the Parks Highway (the red line) to the west of Fairbanks.  The storm just to the southwest of that, closer to Nenana, dropped half an inch of rain at the airport there in 22 minutes.

Two particularly severe storms developed a bit later well to the northeast of Fairbanks, near Central, and eventually crossing the Yukon River.  Here are the radar snapshots from 7:08 and 7:24pm: note the very high radar reflectivity values, which are strongly indicative of large hail.  The Weather Service issued warnings for "half dollar size" hail, i.e. 1.2 inches in diameter.

The ingredients for the storms involved very high humidity at low levels and a powerful impulse (widespread lifting/ascent) in association with a strong cold front approaching from the west.  The dewpoint reached 61°F in Fairbanks in the early evening, which is about as high as humidity typically gets in summer there, and Fort Yukon's dewpoint made it up to 63°F.  Low-level humidity greatly adds to convective instability, which is the fuel for deep, vigorous overturning in thunderstorms.

Here's the 500mb analysis at 4pm yesterday.  Note the very sharp trough over southwestern Alaska and the strong pressure gradient to its east, with strong southerly flow between the trough and a ridge downstream over northwestern Canada.

 

That southerly flow and the strong Canadian ridge produced excessive heat over Alaska's far eastern interior, western Canada, and southeastern Alaska: here are maximum temperatures yesterday (click to enlarge).

I imagine Rick Thoman will provide more expansive historical context for the heat wave on his Substack channel, but the 91°F reported by the co-op observer at Eagle is pretty unusual: before 2012 it was about a once or twice a decade occurrence.  Father east, the 95°F at Carmacks is within a degree of the all-time high in 2004.  Canada has been breaking all sorts of heat records this summer.

Finally, here are simple radar animations from yesterday evening: split into two halves for the sake of file size (i.e. the second animation picks up where the first one ends).  What a remarkable event!  I'd be very glad to hear of any reports from those on the ground - feel free to share in the comments.

Daily Temperature Records

Interior, south-central, and southwestern mainland Alaska have had a relatively cool year so far overall; by "relatively" I mean in comparison to the previous decade.

One way to look at this is to consider the number of daily temperature records that have been set in recent months: for example, Anchorage hasn't set or tied any daily high temperature records so far this year, compared to 12 instances in 2022 and at least 4 instances in each year from 2013-2022.  Bethel set one high temperature record on March 6, compared to 6 records last year.

Of course, low temperature records are a rare breed these days, so there haven't been many of those either (Nome's record cold in April notwithstanding).

I thought it would be interesting to use ERA5 gridded data to look at daily records in a continuous spatial framework.  I don't have as much confidence in the ERA5 temperature data prior to 1979 (the start of the modern satellite era), so I calculated calendar-day records for the 1979-2022 period of record.  Here are maps of the number of days this year that have set new records compared to that 44-year historical period.

The absence of new high temperature records is quite notable for central and southern mainland Alaska, but there have been several record-warm days along the Arctic coast and in the Aleutians.  As for (post-1978) record cold, the epicenter has been in northwestern Alaska to the south of the Brooks Range.  Ground-truth data from Kotzebue fully confirms this, with 4 all-time daily cold records broken in April (April 7 and 10-12), and 10 days with post-1978 cold records.

We can create an objective measure of the "number" of records each day by calculating the grid area that sets a record, and then we can take the average for any window of time.  On an annual basis, this is the result:

 

On the warm side, 2019 was the "winner" for daily warm records, which makes sense as it was the warmest year of record statewide for Alaska (1925-present) - the only year on record with an annual mean temperature above freezing.

It's no real surprise to see that 1992 had the most daily cold records, although it wasn't the coldest year since 1979 - both 1999 and 2012 were colder on the basis of the annual statewide mean.  But 1992 saw astonishing cold relative to normal in September, and May was also very cold.

The increase in coverage of warm records is quite dramatic, as we'd expect, but the decrease in cold records is even more significant.  Notice that the last couple of years have been close to "normal" for coverage of warm records, but cold records haven't reached anywhere near normal.  This is consistent with the well-known fact that daily minimum temperatures are generally warming faster than daily maxima around the world.

How about the summer season?  Here we see that 2004 really stands out, and indeed it was the warmest summer on record statewide (1925-present), and also the worst fire season on record.

 

Here's a map showing the locations (blue shading) where 2004 was the warmest summer on record, based on ERA5 data since 1979.  But 2019 was the warmest summer in south-central and southwestern Alaska, and some other years show up for Arctic Alaska.

If we look at the area where 2004 had the most daily record highs, there's a good correspondence with the mean temperature map overall, as we'd expect.

But the fickle nature of daily records means that there's also more noise - for example, Fairbanks shows up as having the most daily record highs in 1994 (see the tiny yellow patch in the central interior above).  Somewhat remarkably, this is correct: Fairbanks did have its most (broken or tied) daily record highs in summer 1994, whether you look at 1979-2022 or the entire period of record.

Here's the number of daily record highs for five major sites based on the 1979-2022 period, using ground-truth station data (click to enlarge):

 

For Anchorage, 2019 stands out by far (it was by far the warmest summer on record), but 2004 comes out on top for Juneau, Nome, and Utqiaġvik.  The latter is interesting, because summer 1989 was much warmer than 2004 in Utqiaġvik, but it produced hardly any record highs.

The overall consistency between ERA5 and ground-truth data is quite encouraging.  Going forward, it will be nice to have this new tool for analyzing the "daily records" aspect of climate variability on a complete spatial basis.