May was a relatively cool month across the inland northwest with temperatures a couple degrees Fahrenheit below normal while much of the rest of the continental US was well above normal. This continues the run of the warmest year-to-date in observational record for much of the eastern US.
During the month of May, most locales in the inland northwest see a 10F increase in daily high temperatures climatologically speaking. This occurs due to a combination of the longer days and the general poleward retraction of the storm track associated cloud cover that inhibit daytime heating.
While the potential solar radiation increases throughout the month due to basic Earth-Sun geometry, realized solar radiation at the surface is well, partly cloudy. Observed hourly solar radiation this past month from a couple remote automated weather stations (RAWS), one in the Idaho panhandle just north of Coeur d’Alene (Hoodoo) and one in western Montana just west of Missoula (Ninemile), show the preponderance of cloud cover during the latter half of the month.
The sequence of days from May 5-16 where the peak hourly solar radiation exceeds 750 W/m2 is indicative of mostly clear skies. Which brings us to precipitation…
May was a rather dry month, with most places receiving below normal precipitation and in the bottom tercile. More notably was the prolonged dry-spell that tempered spring runoff and flooding concerns following the record precipitation we previously discussed across the Idaho panhandle and northeastern Washington state. Spokane saw 17 consecutive days without measurable precipitation, with comparable dry spells in Missoula, MT, Boise, ID and Lewiston, ID. Pocatello, ID did not observe measurable precipitation until the 24th of the month. In Moscow, my pesky spider friends decided to set up shop in my cocorahs gauge like they did in August last year. I never figured I’d assess climate through the number of spiders in my rain gauge, but here’s for another indicator.
On the positive side for water supply, the dry month of May did little to hinder the well above normal precipitation numbers for the water year 2011-2012 as most locales are still 10-25% above normal. You can explore some of these maps for yourself through this link.