The summer that doesn’t want to end could send highs to 90 degrees at some locations in the Inland Northwest today, and there is a chance that some daily records will be broken.
An unusually strong system of higher air pressure is repeating a pattern that’s been the dominating the weather for the past three months, especially in September. Highs will be running about 20 degrees above normal in the Spokane area on the first full day of autumn.
The forecast calls for highs of 90 in downtown Spokane and downtown Coeur d’Alene and 89 in Spokane Valley and Post Falls with sunny skies and light east winds.
The high Tuesday at Spokane International Airport was 80.
The warmer temperatures today will come from a tongue of warm air that typically forms in the lower elevations beneath strong high pressure systems. Hot air rising from the ground reduces lower-elevation air pressure and creates what’s known as a thermal low, or “trough,” beneath the upper-level high. That low is moving into the Inland Northwest later today, and should be persistent enough to keep the heat around through Thursday afternoon.
Forecasters said earlier this week that the current weather pattern, if it happened in July, would result in scorching temperatures.
On Thursday, a weak Pacific frontal system is going to bump into the high pressure system and its underlying thermal low, dampening the strength of the high and pushing the thermal low eastward. As a result, expect breezy conditions late Thursday and then cooling temperatures into the 80s Friday and Saturday and then upper 70s on Sunday.
Computer forecast models are calling for a low pressure system to arrive early next week, although National Weather Service forecasters have not pinpointed the time of its arrival. Otherwise, expect fair weather through Monday and even into Tuesday, although cooler air may be on its way by Tuesday with highs in the lower 70s..
Normal for Sept. 23 is 69 for a high and 43 for a low in Spokane. So far this month, the average temperature is 4.2 degrees above normal.
At 7 a.m., temperatures were very mild for late September. It was 57 at the airport, 62 in Pullman and 54 in Deer Park.
But the seasonal pooling of colder air at the surface at night is showing up, too. Felts Field in a low spot along the Spokane River at Millwood was 49 degrees, and Coeur d’Alene near the lake was 48. Sandpoint was 45, and Newport, 42.
The colder temperatures at night also reflect the low humidity that is allowing the air to radiate its heat back into space during the lengthening period of darkness that comes with autumn.