Where did the moisture go?

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.

This entry was posted in Uncategorized by John Abatzoglou. Bookmark the permalink.

About John Abatzoglou

John is an associate professor of Geography at the University of Idaho. John's interests are centered around climate and weather of the American West and their impacts to people and natural resources of the West. John and his Applied Climate Science Lab at the University of Idaho have published nearly 100 papers and book chapters on climate science, meteorology, and applied climate science connecting climate to water resources, wildfire and agriculture. The research group also develops web-based climate services that connect climate data with decision makers to help improve climate readiness of societies and ecosystems.

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