July was a turbulent month across the inland northwest with a barrage of disturbances during the first three weeks of the month, due to a combination of pesky cutoff upper-level lows and monsoonal moisture as described in a previous post. Unlike weather systems during the cool season that are associated with widespread precipitation, summertime precipitation is associated with convection that often is not widespread. What this means is that one town gets slammed with hail the size of chicken eggs, while it’s all bark and no bite for a town 10 miles away. Great time of the year to watch the Doppler radar.
While the general timing of such convective events is fairly well forecasted by the National Weather Service, do not expect a spot forecast for your backyard. This is where a forecast like a 30% chance of rain can be interpreted as a combination of the probability of precipitation within a region and the fraction of the area would observe measurable precipitation.
July is typically a dry month for much of the inland northwest, with relatively more precipitation climatologically across the eastern part of the region with a more favorable environment for thunderstorms. However, last month much of the eastern half of Washington was the target of the convective plumes with a sharp demarkation to the south and east of below normal precipitation.
This continues the run of sogginess across Northeastern Washington and the Idaho panhandle. Spokane county, WA as a whole had its 2nd wettest Mar-Jul on record, with last year being the 3rd wettest on record. Oh what parts of the heartland would do for some of this surplus moisture.
Temperatures last month were above normal, with most locales between 2 and 6 F above 1981-2010 normals. In Spokane, overnight lows did not drop below 65 between the 8th and 14th of the month. Boise, ID hit 108 on both the 9th and 12th of the month, only 3F cooler than its all time high and the warmest since 2003.
The wet + warm conditions in July across eastern Washington into northwestern Montana were a peculiar combination. And the map below helps illustrate why. It shows the Pearsons correlation coefficient between July temperature and precipitation, plotted is the r-value.
In July, precipitation anomalies are negatively correlated to temperature anomalies across much of the country, including the northwest. This shouldn’t come as a surprise as straightforward way to have cooler summer days involves (i) blocking out as much sun as you can from warming the surface, and (ii) having wet ground that allows for energy absorbed by the surface to go toward evaporating water rather than warming up the temperature. So cool and wet summer months, and warm and dry summer months, generally go hand-in-hand. Not the case for parts of the region this past month. Instead, we had a very moist and warm air mass in place, also known as mugginess.
No need to complain though as this warmth pales in comparison to the oppressive heat across the midsection of the country as all time record high temperatures set during the dust-bowl continue to be topped. The combination of heat and paltry precipitation amounts have had many compare the current drought situation to the drought of the mid-1930s.