With data back to 1941, McGrath has only reached 90°F on 5 days: once in 1969, on 3 consecutive days in 2013 (reaching an amazing 94°F), and once in 2019. All were in June. However, July and August have both seen 89°F, most recently on July 8, 2019.
To dispel doubts about the accuracy of yesterday's measurement, the AWOS instruments measured 88°F upstream (on the Kuskokwim River) at Nikolai and 86/87°F downstream at Sleetmute and Aniak. As usual, RAWS thermometers in the area were higher, but they should be disregarded.
However, the heat is only half the story. Shortly after the temperature peaked at 6pm, a very intense rain shower developed, dropping over an inch of rain in a very short time. Brian Brettschneider notes that the rain rates exceeded the estimated threshold for a 1000-year recurrence interval! Here are the precipitation frequency intervals from NOAA's Atlas 14 (click to enlarge):
Between 7:23 and 7:53pm, the ASOS reported 0.78" of rain (compare to 0.73" for a 30-minute event with a 1000-year recurrence interval), and 1.22" fell in 1 hour 24 minutes.
The notion of return interval is based on historical statistics and the assumption of a stable climate, which is of course never quite true, and we do expect more frequent heavy precipitation events in a warming climate with more atmospheric water content. Nevertheless, this was clearly an extremely rare event for McGrath.
Putting the two aspects together, the combination of rare heat and rare heavy rainfall was obviously way outside the historical climate record for McGrath. For instance, the warmest day in the past 84 years with a daily precipitation total of 1" or higher was only 72°F. There have only been 3 days with a high temperature above 80°F and a daily rainfall above 0.5" (most recently - again - in 2019).
I'm curious how the "80 temp, .5 inch precip" record holds up down the Kusko in Bethel. July 4, 2007 in Bethel topped out at 80 degrees then exactly 1 inch of rain fell from a dramatic thunderstorm in the evening. Bethel's heaviest short-duration rain events always occur with hotter weather and the resulting thundershower activity that forms over the Kilbuck Mountains and moves west.
ReplyDeleteGood question, the answer is that was the only day with 80+ and 0.5" or more in Bethel (data back to 1924). There are only 3 other days with 75+ and 0.5" or more; one of them just 6 days later in 2007 (July 10).
DeleteInterestingly one of the 3 hot+wet days in McGrath was July 3, 2007. So that was quite a week of weather in SW Alaska.