Friday, April 13, 2012

Finding the Goods and the Graupel

Today was one of the more enjoyable days of the year.  Conditions were better than expected, and we skied a couple of primo runs first thing this morning while the sun was peeking through the clouds at Alta.    It was a nice way to conclude our vacation.

Amongst the particles falling from the sky around noon were some heavily rimed snowflakes.

Some might even call this graupel.  Chances are we will see more of it at times through tomorrow.  For graupel (or heavily rimed snowflakes) to form, you need two things.  The first is supercooled cloud water, very tiny droplets that are below 0ºC, but freeze on contact with an ice crystal.  Most clouds contain supercooled cloud water, but warmer clouds typically have more cloud water than colder clouds.  With free-atmosphere crest-level temperatures near -4ºC, we're dealing with relatively warm clouds today and tomorrow [compared to most (but not all) mid-winter storms].

The second are strong updrafts that can keep ice crystals suspended as they are rimed and gain mass.  We should have those as well, especially during the day.  For example, the NAM produces 50–250 J/kg of CAPE over northern Utah tomorrow.

CAPE is one of many diagnostics used by meteorologists to forecast convection (see wikipedia for gory details).  In this instance, the CAPE forecast over northern Utah suggests we'll see unsettled conditions in the Wasatch Mountains with periods of snowshowers and possibly even a thunderstorm tomorrow.  Given the strong updrafts and warm temperatures, the snowshowers will likely include heavily rimed snow crystals or graupel at times.

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