100º F Potential for Northern Maine


By: Victor J. Nouhan, Climate Team Leader


Official Weather Service and Cooperative climate records for all northern Maine sites north of Millinocket have not indicated a single occurrence of 100+ degree F heat over the past 50 to 100 years (length of record depending on site). Although most people would assume this fact to be normal for such a northern region of the country, the lack of occurrence of 100 degree heat in northern Maine is surprising when compared to sites of similar latitude and altitude above sea level, or sites even further north.


First a little background with regard to Caribou's (a site representative of many northern Maine locations located at 46.9 degrees N latitude and 624 feet above sea level) climate record. Official data collection began at Caribou in 1939 making the length of record 66 years, relatively short compared to some New England locations which began official record keeping since the late 1800s. This shorter record gives less opportunity to capture extreme weather events (such as heat waves). Since 1939, Caribou's all time record high temperature is 96 degrees first recorded on June 29th 1944. The second and last, somewhat surprisingly early season occurrence, was on May 22nd 1977. Longer cooperative records indicate four likely candidate days of near 100 degree F heat across some lower elevation sites of northern Maine prior to 1939. The first, and perhaps best documented occurrence, was August 1935, when older cooperative records indicate a maximum temperature occurrence of 99 degrees F at Presque Isle on August 18th 1935, which correlates well with the 103 F degree high temperature recorded at Bangor that day. The second and third candidate days occurred over the two successive Julys of 1911 and 1912 where Presque Isle reported maximum temperatures 99 degrees F and missing respectively with Millinocket reporting a corresponding 99 and 100 degrees F. The fourth (and oldest) candidate of 101 degrees F occurred in Millinocket on June 18th 1907 (two years before the start of the Presque Isle coop record). The last close approach of 100 degree F heat to northern Maine occurred on August 2nd 1975 when Millinocket reached 100 degrees F and Houlton reached 99 degrees F, but the extreme core of this heat wave did not reach far northern Maine locations such as Caribou which only reached 95 degrees (or less).


So what factors would need to be met for northern Maine locations to reach 100 degrees F? Well first off, site elevation would have to be less than 1000 feet above sea level (Caribou is at 624 feet above sea level), since thinner air at higher elevations at our latitude would not be able be heated to this extent. Second, the timing of the heat wave would have to be somewhere within the late June to early August time period, where sun day length is long, sun angle at solar noon high, and soil temperatures relatively warm. Third, ground conditions would have to relatively dry compared to normal (drought like conditions) before a heat wave capable of producing 100 degree F heat enveloped northern Maine. This is because dry soil/ground conditions allow for more sensible heating of the air just above the ground, rather than evaporation of excess moisture of wet soils. Fourth, the best candidate air mass would be one that originated from the northern Plains/upper Midwest and traversed over central Ontario south of James Bay within 3 or 4 days before reaching northern Maine. Very hot air masses that move into northern Maine from this trajectory would maintain greater dryness and down slope westerly component surface winds for better surface heating compared to higher moisture tropical air masses that originate near and move northeast (on more southerly component winds) from the Gulf of Mexico. Lastly, mostly sunny skies and no cold frontal passages will be needed on the day the thermometer makes a run for a 100 degrees F,  assuming the other four factors are met. No air mass will likely max out in each of these five factors, but the qualifying air mass will need to have at least some positive contribution from each factor for a successful 100 degree F day for any northern Maine location.


Have other locations in similar or cooler climate regimes near northern Maine reached 100 degrees? The answer is yes. Roberval Airport (48.5 degrees N latitude and about 570 feet above sea level) on the southwest side of Lac St Jean in central Quebec has reached an all time high temperature of 100 degrees F on August 21st, 1976. Moosonee Ontario (51.3 degrees N latitude near sea level) near the south shore of James Bay reached an all time high of 100 degrees F on July 31st 1975. The Timmins/Iroquois Falls area of east central Ontario (near 48.4 degrees N latitude and 800 to 1000 feet above sea level) have reached 100+ degrees F several times in their climate records culminating with a 106 degrees F on July 29th 1916. In fact, 100 degrees F has been recorded as far north and east as Goose Bay Labrador in eastern Canada (about 53.0 degrees N latitude near sea level).


The question that naturally arises is what has kept temperatures from reaching 100 degrees F across northern Maine since 1939 when other locations not too far have already reached this figure. Perhaps it’s just a coincidence, but some of the answer may be related to the fifth factor mentioned a couple of paragraphs ago. It appears that cold fronts have a slight preference for midday to early afternoon passage across northern Maine when the hottest day of a heat wave event is often underway, preventing the maximum temperature potential of a hot air mass. Perhaps the relative close proximity of northern Maine to the Canadian Maritimes and cooler Atlantic water temperatures brought by the Labrador Current provides a natural barrier for extreme heat, but this certainly does not explain how Goose Bay Labrador reached 100 degrees F. In any event, the fact that Caribou has reached 96 degrees F in late May of 1977 when ground temperatures were still relatively cool following the winter with highest recorded snow depth, underscores the fact that the atmosphere is capable of reaching 100 degrees F at lower elevation northern Maine sites if a similar synoptic scenario were to unfold in the late June to early August time frame.  One last interesting note: although the all time high is 96 at Caribou, there are many 95 and 94 degree record days as well. Compared to other locations with similar or longer length of records that only have a couple of record high temperatures within a degree or two of the all time record, the greater number of 94 and 95 degree record days at Caribou suggest that a more extreme heat event is likely within the next 25 to 50 years even in the absence of anthropogenic global warming.

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