Climatologically, somewhere between 15-20% of annual precipitation falls during the month of December for much of the northwest. So far, this month (thru Dec 16th), precipitation has been measured using tweezers and nano-based units:
Spokane, WA 0.04″
Boise, ID 0.00″
Portland, OR 0.06″
Seattle, WA 0.06″ [driest first half of December on record]
Redmond, OR 0.00″
This protracted mid-winter meteorological drought of sorts encompasses much of the western United States except for the southern deserts.
A strong blocking regime over the past two weeks has provided a barrier of sorts for Pacific storms across the entire west Coast. A map of upper-level winds (aka, the jet stream) shows a split flow with the polar branch veering well to the north towards Yukon/south-central Alaska; a marginalized subtropical branch well to the south (when combined with a couple cutoff disturbances) has alleviated a tiny fraction of the severe drought across the southwestern US and parts of Oklahoma and Texas. The configuration of the atmospheric circulation associated with blocking regime is accompanied by broad-scale high sea level pressure and light winds, which create all sorts of havoc from an air-quality perspective and incessant “air stagnation” advisories.
This high-and-dry regime is unusual, but not unprecedented. Protracted mid-winter meteorological droughts are well-known in parts of the western US as a nuisance and a blessing, depending whether you are below or above the inversion, and a more frequent phenomena in the California Central Valley (google “tule fog” to find out more).
I wanted to figure out just how unusual a mid-winter dry-spell like the one we are taking part in is. To do this, I examined the US Historical Climate Network (HCN) long term climate database and chose 4 stations as a proxy for the broader Pacific Northwest: Moscow, ID; Spokane, WA; Portland, OR; and Arrowrock Dam, ID. I took the mean of the daily precipitation from these four stations and searched through the 110-year record for cool-season (Nov-Mar) periods with at least 15-days in a row with daily precipitation amounts below 0.05″.
It turns out there are 20 such occurrences in the 110-year record, making the point of this being an unusual, but not exceptional, dry-spell (longest mid-winter dry-spell according to the aforementioned criteria was 22-days, Dec 10-31 1985, that resulted in a widespread dry December for much of the western US). Somewhat noteworthy is the fact that not one, but two protracted mid-winter meteorological droughts occurred during the notorious “winter” of 1976-1977, which culminated in the driest Nov-Mar in 115+ years for much of the western US. I’ll be quick to point out however, that a couple week wet spell can quickly “correct” for missing out on precipitation the last couple weeks.
So just how important is a dry spell like this to cumulative water-year precipitation? As with most questions, the answer depends on when and where. I queried the PRISM dataset from Dr. Chris Daly’s group at Oregon State University to estimate the fraction of annual precipitation that occurs in December, and divided that number by 2 to approximate the net effect of the two-week dry spell.
This map is spatially quite rich and contains several features that I won’t go into here. But the last-two week precipitation window is typically quite important in much of north-central California, up through the Cascades and eastward into the mountains of central Idaho, basically mirroring the deep red hues of the first map where next-to-nothing fell over the past two weeks. With the dry-spell expected to continue for the next 5-days or so, and longer in parts of northern California, this winter is shaping up to be quite a bit different than last winter.