Wednesday, January 4, 2017

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Throwback to 1997? Or just 2005?

The 2016-2017 season has been pretty wild to start, with an extremely wet October up high, a dry November, and a somewhat classic December (mix of wet/cold/warmer but not too dry thankfully!). Then January comes along, keeps the light and dry pow coming for a few days, but follows it up with a potentially devastating round of very strong atmospheric rivers and high snow levels. Depending on how the snow levels work out, ski season could be in deep doodoo for the lower elevations and off the hook up high (high snow level scenario) or world class all around (lower snow level scenario).

The last two proper flood events are similar to the forecast event in time of year and general setup. They were both right around New Years and preceded by colder storms, which setup an excellent scenario for rain-on-snow flooding. This year we could see even more productive flooding due to the certainly saturated state of soils (the end of 1996 had some good rain events as well, but I can't recall 2005). The ultimate flood events include a coupling of the polar jet, subtropical jet, and the narrow, elongated plume of concentrated low to midlevel water vapor flux (atmospheric river). Shown below is the moisture transport and 500 hPa height field from the gold standard flood event in 1997. The moisture source originates near Hawaii, leading to the Pineapple Express term, whereas when the moisture plume originates further westward it is sometimes called the Mango Connection.

Get ready for a repeat! Below is the 72 hour integrated IVT.


The GFS Ensemble IVT forecast shows the Friday break and the weekend demolition derby of moisture flux. These values have come down a bit in recent forecasts, yesterday they were off the charts. Atmospheric river (AR) thresholds start at 250 kg/m/s (but are rightfully arguable as a function of latitude, just talk to some Arctic/Antarctic meteorologists) and strong ARs are typically in the 400-700 kg/m/s range. The weekend event will be memorable.

The pan-Pacific upper level atmospheric patterns conducive to the most extreme storms are well-known and characterized by a blocking high near the dateline that is undercut by a strong zonal jet (see figure below from Underwood et al. 2009 J. Hydrometeor.). Contours are 250 hPa heights, filled contours are 250 hPa winds.

Figure from Underwood et al. (2009 J. Hydrometeor.)

Looks a lot like what is forecast for Saturday!

The same signal appears in a composite of 500 hPa heights and water vapor transport anomalies I made for days characterized by upside down storms and high avalanche hazard in the northern Sierra (Hatchett et al. in prep). The thick black lines show positive height anomalies (ridging) developing near the dateline, and as one gets closer to the day of the event, the anomalous trough develops offshore of California with strong vapor transport favoring copious precipitation (panel d, star shows the Tahoe Basin). Right now, we are looking like panel c, about three days out (see above GFS forecast). UDSE means 'upside down storm event', or layering of more dense snow upon less dense snow. As Donald Trump would end his tweet on this: "Excellent for avalanches! Nice!"
Figure from Hatchett et al. in prep.

The forecast for precipitation is outrageously high and not worth showing. It will precipitate jaguars and wolves like a good old hurricane of the west (see Ralph and Dettinger 2012 Bull. Amer. Meteor. Soc.). The biggest question for hydrology and snow enthusiasts is WHERE IN THE WORLD WILL SNOW LEVELS BE?

First order of business is 700 hPa temperatures, even though there are a multitude of physical processes that control the snow level. Before we look at 700 hPa temperatures, here's a diagram I made highlighting just a few of the snow level/precipitation processes that are occurring at a range of scales. 
You can see that there are a lot of things going on. This version is also lacking the potential Sierra Barrier jet (Parish 1982 J. Appl. Met. Clim.) which strongly controls orographic precipitation gradients in the Sierra Nevada (Lundquist et al. 2010 J. Hydrometeor.) and is a key reason for the bullseye in precipitation that often is seen in Plumas County (e.g., Ralph et al. 2016 J. Hydrometeor.). If you are into snow science and look at crystals and snowpack stratigraphy, try to relate what you see to processes that produce those layers and crystal habits. It will help you understand the snowpack and mountain weather that much better.

Things look OK Saturday (above). It is Sunday where things get bad, and both the GFS and NAM agree on this.

After a 12 hour period where several billion gallons of rain will fall, we will shift back into a cooler regime late Sunday evening/early Monday. Hopefully there will be some snow left!

Stay tuned!


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