Hawaii’s Warmest Summer on Record (and Alaska's Second Warmest)

Although 2800 miles of open ocean separate them, both Anchorage, Alaska and Honolulu, Hawaii experienced their warmest climatological summers (June-August) on record this year. It appears that this was Alaska’s second warmest summer (following that of 2004) but it is likely that it was Hawaii’s warmest. It is hard to confirm this outcome,  since NOAA does not provide average temperatures for Hawaii as a whole (the only state they don’t do such for). However, data for a handful of specific sites—Honolulu, Hilo, Lihue, and Kahului—make a strong case that this summer’s heat across Hawaii has been unprecedented.

Here’s a closer look at the summer temperature records set in Hawaii (and Alaska in less detail).

The summer of 2019: Hawaii

There are just four sites in the Hawaiian Islands that have reliable long-term period of record (POR) for temperature records: Honolulu (Oahu), Lihue (Kauai), Kahului (Maui), and Hilo (Hawaii/Big Island), all essentially located at sea level. Below are this past summer’s statistics for all four sites, with the new monthly and/or summer records highlighted in bold:

Honolulu (POR 1890-)

MONTH           2019 AVG       RECORD WARMEST (YEAR)
June            81.7°           82.7° (2005)
July            84.1°           previously 83.3° (2016)
August          84.3°           ties record of 1994
Summer          83.4°           previously 83.1° (2005)

SPECIAL NOTES FOR HONOLULU:

—95° on Aug. 31 ties all-time record high last set on Sept. 19, 1994
—August’s 84.3° ties August 1994 as warmest single month on record

Of the 92 days of summer (June 1-Aug. 31), 29 saw daily record highs set or tied. A total of 37 daily “max/min” records (warmest daily low temperature) were set, including minimums of 81° on Aug. 2, 3, and 26 that tied the all-time records for such (set on three other occasions in August and September 2015). On September 5 the all-time max/min temperature record was broken when a daily low of 82° was observed.

Lihue (POR 1905-)

MONTH           2019 AVG       RECORD WARMEST/YEAR
June            79.8°           ties record of 1981
July            81.7°           previously 81.1° (2017)
August          83.5°           previously 82.7° (2017)
Summer          81.7°           record/previous 81.1° 2017

SPECIAL NOTES FOR LIHUE:

—91° on August 25 and 31 tied the all-time record high last set on seven other occasions (twice in July 1918, once in September 1936, and once each in October 1926, 1930, and 2012).
—August’s 83.5° was the warmest single month on record (previously 82.7° in August 2017)
—Of the 92 days of summer (June 1-Aug. 31), a total of 31 set or tied daily record highs. An amazing 21 of these were in August alone, which may itself be a record for so many daily heat records in a single month at any site in the U.S. with a long POR! A total of 33 daily max/min records were set this summer in Lihue, including all-time record-warm minima of 81° on Aug. 3, 12, 21, and 24  (beating 80° from multiple previous occasions).
—As of September 9, every day since August 24 has either broken or tied the site’s daily record high—17 consecutive days and counting. For a site with a POR since 1905, that is astonishing! Furthermore, five consecutive days of the all-time record of 91° were measured on September 4-8.

Figure 1. A table of daily record maximum temperatures for Lihue on Kauai Island. Note the astonishing string of daily record highs set in August this summer (21 out of 31 possible days). With a period of record (POR) dating back to 1905, this is a truly astonishing feat, perhaps unique in the annals of U.S. climate data. The table is updated through September 9. Image credit: NWS/Honolulu NOWData.

Hilo (POR 1949-)

MONTH           2019 AVG        RECORD WARMEST/YEAR
June            77.3°            77.6° (1980)
July            77.9°            79.4° (2015)
August          80.1°            previously 79.7° (2015)
Summer          78.4°            78.6° (2015)

SPECIAL NOTES FOR HILO:

—August was the warmest single month on record (previous was 80.0° in September 2015)
—Of the 92 days of summer (June 1-Aug. 31), 15 saw daily record highs set or tied, and 25 daily max/min records were observed, the warmest being 76° on August 20 and 21, 1° shy of the all-time record for such (77° in August 2015 and October 2015).

Kahului (POR 1954-)

MONTH           2019 AVG        RECORD WARMEST/YEAR
June            80.9°            previously 80.7° (1981)
July            82.5°            previously 81.7° (2015)
*August         81.8°?           82.9° (2015)
*Summer         81.7°?           81.8° (2015)

*Note: There were four days of missing data in early August, so the August average and summer average may not be accurate.

SPECIAL NOTES FOR KAHULUI:

—97° on July 29 ties the all-time heat record last set on August 22, 2015 and August 31, 1994. This is also a contender for the highest reliably measured temperature in Hawaiian climate records. See more about the all-time hottest temperatures observed in Hawaii in a post of mine from 2011.
—Of the 92 days (actually 88 because of missing data) of summer (June 1-Aug. 31), 41 set or tied daily record highs, and 11 daily max/min records were set, the warmest being 78° on July 18, which is 1° shy of the all-time record for such (79°) set on October 17, 1979.

In conclusion, it would seem that the summer of 2019 was the warmest such on record for the Hawaiian islands, with the only other contender being 2015. What was remarkable was how many daily record highs were tied or broken. That being said, it takes a temperature of only about 3°F above the normal daily high to set a record in Hawaii, since the climate is so equable and extreme variations of temperature from day-to-day during the summer months are practically non-existent.

Figure 2. The principal cause of Hawaii’s long hot summer may be unusually warm sea surface temperatures. Though these were especially significant around Hawaii this summer, warming oceans are not limited to the area around Hawaii. Averaged globally, August 2019 saw the warmest sea-surface temperatures on record. Image credit: Brian Brettschneider, from ERSST data.

Why has it been so hot in Hawaii this summer?

The simple answer is that the sea surface temperatures have been running about 1.0°C-1.5°C (1.8°F-2.7°F) above average this past summer. As noted above, that is enough to push land temperatures (especially at sites near the ocean as all of the four listed above are) into record-breaking territory. The local press has also mentioned (anecdotally) that the normally brisk trade winds have been weaker this summer than usual, allowing the air temperature to climb higher than is normally seen and allowing heat to build near the top of the ocean surface.

September is usually the warmest month for the state, both for sea surface temperatures and land locations, and so far (as of September 8) record temperatures (both daily highs and lows) have continued unabated

Alaska has its second warmest summer on record

Although August was another record breaker for Anchorage, a large portion of the state saw below-average temperatures in August that prevented this past summer from becoming the warmest on record for the state as a whole (the summer of 2004 still holds that honor).

Figure 3. Although this past August was the warmest such on record for Anchorage, and at most sites in south-central and southwest Alaska, a large part of the state actually experienced cooler-than-average temperatures as detailed in the map above. Image credit: Brian Brettschneider, from NCEI data.

Although the final data have yet to be released, it appears that the summer of 2019 will end up being the second warmest for Alaska since records for this began in 1925. It was, however, by far the warmest summer on record for Anchorage, with a June-August average temperature of 62.8°. This crushed the previously warmest summer of 2016 (60.7°) by a whopping 2.1°! Incredibly, Anchorage has not had even one day with a below-average temperature since May 30. A statistic that really stands out (among many) is that Anchorage has had 31 days of temperatures 75° or higher so far this year. The previous record for this was just 15 (in 2015)!

It should also be mentioned that this has also been the driest summer on record for Anchorage, with a June-August precipitation total of only .90” (normal would be 5.86”). June with just .06” and August with just .04” were both the driest such months on record. It should be noted that August is usually Anchorage’s wettest month of the year, with 3.25” of rainfall. A serious drought continues in Southeast Alaska as well where Yakutat (1.05”) and Sitka (1.63”) also had their driest Augusts on record.

Figure 4. It was by far the warmest summer (June-August) on record for the city of Anchorage. Every month was respectively its hottest ever observed: June averaged 60.5° (previous record 59.5° in 2015), July averaged 65.3°, the warmest single month on record (previously 62.7° in 2016, and August averaged 62.6°, the third warmest single month on record (previously 61.2° in 2004). Image credit: Brian Brettschneider, from NCEI data.

The ”warmest summer on record” moniker also held true for most of the sites in south-central Alaska (Kodiak, Homer, etc.). Outside of this region, Kotzebue—north of the Arctic Circle on the Chukchi Sea—also had its warmest summer with an average of 59.3° (previous warmest summer was that of 2004 which averaged 57.3°). As of September 5, the last time Kotzebue saw a single day with a below-average temperature was March 21.

Figure 5. Temperature departures for Alaska during the climatological summer (June-August) 2019. Virtually all of the locations in the two darkest red zones in the map above (departures of +4°F or more) experienced their warmest summer on record. Image credit: Brian Brettschneider, from NCEI data.

In summary

It is alarming that two such disparate regions (Hawaii and Alaska), with hugely different average temperatures and amounts of variability, simultaneously endured their hottest or near hottest summers on record, although separated by 3000 miles of ocean. Why?

Sea surface temperatures heavily influence both state’s climates. A massive zone of warm SSTs called the Northeast Pacific Marine Heat Wave of 2019 is likely a big player in the heat in Hawaii. Further north, warm ocean currents have kept sea ice extent near record lows around Alaska for much of 2019.

Being at almost the same longitude, Alaska and Hawaii may share to some degree the large-scale climatic element of a pervasive upper atmospheric ridge (see below), although this ridge has had a far greater impact on Alaska’s weather than Hawaii’s. It would seem, hypothetically, that a very strong and stubborn high-pressure ridge has established itself in the Alaskan region, a phenomena we have seen in recent years (remember the Ridiculously Resilient Ridge or RRR that resulted in California’s worst drought on record between 2013-2015?). We shall see if this continues into the fall and winter.

KUDOS: Thanks to Brian Brettschneider, climate researcher in the International Arctic Research Center at the University of Alaska Fairbanks, for the Alaskan graphics used in this blog.

Christopher C. Burt
Weather Historian

NOTE: For Tuesday's tropical weather update, see the post from Jeff Masters from earlier today.
 

Figure 6. A Ridiculously Resilient Ridge (RRR) has predominated over Alaska during the summer of 2009, resulting in record heat across the state. Image credit: Brian Brettschneider, from NOAA/ESRL data.