All-time records may fall in the Columbia Basin with the upcoming heatwave

An unusually strong area of high pressure is forecast to build over the Pacific Northwest during the next several days. The drought in place across much of the West (including the Columbia Basin) also adds a little to what temperatures the region may see. With soil moisture values low, there is little if any water to evaporate out of the ground during the day. Evaporation itself is a cooling process that on very hot days can make a few degrees’ difference. Without it, this high pressure system (which may break records itself) is coming during a time when the rest of the environment is primed to produce extreme heat.

This has prompted the National Weather Service in Pendleton to issue an excessive heat watch for most of the lowlands in their county warning area beginning Friday afternoon and lasting to Wednesday evening. This is likely to become a warning Thursday or early Friday. Details vary slightly from location to location, but for the Tri-Cities they expect highs to climb from near 100 on Friday to between 108 and 114 for Monday and Tuesday with slight moderation Wednesday.

12z weather model output for the Tri-Cities. (Source: Storm Vista via Chris Callais)

Those high temperature values are on the conservative end of what many weather models have been showing for the Tri-Cities for much of the week. At one point, the GFS (the primary American global model) had highs peaking around 124. The above chart gives an overview of what major weather models are showing for the Tri-Cities Airport from Wednesday’s 12z run (meaning the models are based on data leading up to 12:00 UTC or 5:00 PDT). Personally, I give the chances of peak temperatures in the Tri-Cities as follows:

Temperature	% Chance
110°	85%
115°	55%
120°	15%

The hottest days will be followed by warm nights with little relief. Friday night may be our last low at or below 75° for a while, with the warmest nights possibly not dropping below 80°. While the excessive heat watch currently ends Wednesday evening, models are indicating that the heat will last beyond that time frame. It may, however, continue in a moderately subdued form. Local National Weather Service offices do not issue alerts further out than about seven days because that is the end of their forecast period. There is too much uncertainty to pin down exact details beyond a seven day period, but it may not be until Independence Day weekend before our highs dip back below 100°.

PREVIOUS RECORDS

The Tri-Cities and Columbia Basin have certainly seen their fair share of heat since records began at the end of the 19th Century but temperatures this hot are incredibly rare. Since records began in 1894, the Tri-Cities have only reached 110° twenty-one times (in fourteen individual years). The most recent time this happened was July 30, 2020. These highs are virtually unseen in June, though. Only two heatwaves have ever produced highs at or above 110° during this month (in 1912 and 2015) and the highest ever observed in June was 111°.

The all-time record high for the Tri-Cities is 115° set in 1898 and 1939. There is a chance that this heat wave will tie or break the all time record, though that is not as certain as breaking the June record.

Also worth considering is the forecast duration of this heat wave. The National Weather Service forecast for the Tri-Cities gives three days in a row above 110°. This would set a new record, beating the old record of two days in a row which has been set a couple times (most recently in 2015). As noted above, the National Weather Service forecast currently lies on the conservative end of what models are showing for this heat wave. Here are other heat duration records for the Tri-Cities (note the limitation of the NWS forecast only going out seven days):

Temperature	Record days at or above	NWS forecast
110°	2 days (most recent: 2015)	3 days
105°	6 days (1911)	5+ days
100°	10 days (1967)	6+ days

The all time Washington state record high temperature is 118°. This value was recorded twice – near present day Vernita Bridge in 1928 and at Ice Harbor Dam in 1961. While the Tri-Cities is unlikely to earn the unfortunate distinction of recording the hottest temperature in Washington, I think it is possible that a well positioned weather station nearby could meet or exceed that number. For a record like that to count, meteorologists would have to verify that the station is in an appropriate location and that the thermometer is calibrated correctly. A meteorologist at Avangrid Renewables, put it to me this way:

I’m comfortable saying if the state record isn’t at least tied [temperatures] will get within a degree of it.

Chris Callais

Oregon’s all time record is on par with Washington’s. Officially, a high of 119° was recorded in both Pendleton and Prineville in 1898. Those values are disputed and it seems more likely that the record is actually 117° set at Umatilla in 1939.

The forecast lows are nearly as unusual as the forecast highs. Thanks to low humidity and frequently clear skies, the Inland Northwest typically cools off reasonably well at night during heat waves. The record highest low temperature for the Tri-Cities stands at 82° set in 1906. All told, only 10 nights have ever been recorded with a low that didn’t get below 80°, only two of which have occurred in June. It appears possible that we will add another day to that list early next week.

This heat wave is regional in scope. Other cities around the Pacific Northwest stand a chance at breaking their own all time records and British Columbia stands a chance at breaking Canada’s national all time record which is 113°. Here are records and forecasts for select cities:

City	NWS forecast	June record	All time record
Portland	108°	102° (2006)	107° (1981 & 1965)
Seattle	101°	96° (2017)	103° (2009)
Spokane	108°	105° (2015)	108° (1961)
Yakima	111°	108° (2015)	110° (1971)