After a historically anemic start to the water-year for much of the Northwest, things are about to get interesting. Everyone likes a good comeback story, and the story for water year 2011-2012 is far from over.
First, let’s recap the last 60-days, during which typically of 25-40% of the annual precipitation falls for most of the western US and Northwest.
On a related note, I happened to post a blog entry on rain-gauge calisthenics just before the valves to the sky were turned off and resulted in the paltry total you see in the image above. Rather than take the blame for some anti-rain dance I performed, let’s take a look at a rather unusual circulation (through the 300 mb winds, a proxy for the jet stream).
A stubborn long-wave blocking regime and split flow forced the polar jet northward bombarding much of southeast Alaska (record snowfall in spots like Valdez where 250 inches of snow have been recorded in the last 45-days), leaving much of the western US and the northern tier of the US high and dry.
Earlier this week, the NOAA (NOHRSC) reported that only 15% of the continental US had snow-cover (the number changes rapidly this time of the year with the passage of weather systems across the midwest and eastern US). This contrast to last year around this time when 70% of the continental US had a white blanket. A far more relevant accounting of water stored in mountain snowpack comes from snow survey’s and SNOTEL that are a much better integrator of snow information. Today’s SNOTEL map is colorful; problem is that the warmer hues that dominate the West reflect downright pathetic numbers, including all-time lows in much of the Sierra Nevada where brown ski-slopes equal grumpy skiers and no green for ski resorts. Numbers in the Inland Northwest are not much better, and generally around 65-75% of normal (or average for this time of year) in the Cascades and Rockies and 30-50% of normal in the northern Great Basin.
Now, for the turn in the story.
The flow pattern is morphing into a more typical zonal flow that should direct the storm track back across the Pacific and Inland Northwest. This will be accompanied by a very cold air mass developing over western Canada migrating southward as warmer air invades the polar regions. As this colder air dives southward toward the northwest and interacts with moisture-laden storms traversing the Pacific we should fully expect a good comeback story to materialize. Here is a medium range forecast of 300hPa winds for early Wednesday when the real fun begins and our first shot of significant low-elevation snowfall accumulation for much of the inland northwest except for the western Snake River plain near Boise. Note the strong jet stream across the inland northwest with a strong tap to the Pacific.
An extended forecast of snow-depth from a global weather forecast model for the next 8-days and more detailed animation for the PNW shows reason for optimism. The week-two forecast (somewhat of a fantasy forecast), suggests a continuation of the comeback. It would not be unreasonable for much of the northwest to end the month with normal or above normal precipitation and snow for the month of January. Even if we are par for the course in January, much of the western US will still be in a precipitation deficit for the water year.
Water year precipitation is a lot like a basketball game. We’ve had a dry-spell for most of the game thus far and we’re currently down 15 points with 4 minutes to go in the 2nd quarter. The game isn’t over, but we’ve dug ourself quite a deficit to overcome. The next two weeks is our chance to mount a comeback and end the half within striking distance. With La Nina still in place and her influence typically being most pronounced for late winter across the PNW, there is good reason for optimism even beyond the end of January. So far, this winter has been far from a typical La Nina.