Friday, February 22, 2013

Anatomy of Our Approaching Storm

We have a fascinating storm setting up for tomorrow, one that will bring some snow to the mountains and some heartburn to meteorologists.

First, let's talk about the key ingredients.  Over the past few days, moist air, originating over the subtropical Pacific near China has streamed across the North Pacific and is now impinging on the Pacific Northwest.  One of my students dubbed this the "Soy Sauce Express" and it shows up as a filament of high atmospheric water vapor content (a.k.a. precipitable water) in the contours below, which are overlayed on the 1200 UTC (5 AM MST) satellite image from this morning.  

1200 UTC (5 AM MST) 22 Feb 2012 IR satellite imagery and GFS
Precipitable Water Analysis
The water vapor content of this plume is not unusually high, but it will help fuel tomorrow's storm.

Another key ingredient is the low center over the Gulf of Alaska, which has tapped into cold air originating over the high latitudes.  It is the confluence of this cold airmass with the warm, moist air in the Soy Sauce Express that is generating a strong cold frontal passage for us tomorrow.

The wildcard in all of this is the cyclone that was near the western tip of the Aleutian Islands this morning.  This cyclone is forming very rapidly (something meteorologists call explosive deepening, click here for more discussion) and it is going to affect the structure of the jet stream over the North Pacific and the intensity of the duration of the storm here in Utah.

So, here's how things will play out.  The snow showers we've had this morning will taper off today.  Overnight, warm-moist air associated with the Soy Sauce Express will spread into the Northwest and eventually sag southward into Utah.  This will lead to the development of mountain snow late tonight ahead of the approaching cold front.

NAM sea level pressure, surface wind, and 12-h accumulated
precipitation valid 1200 UTC (5 AM MST) 23 Feb 2013.
Things pick up tomorrow morning with the approach and passage of the cold front, which will likely be associated with strong winds and heavy snowfall in both the mountains and valleys.

NAM sea level pressure, surface wind, and 6-h accumulated
precipitation valid 1800 UTC (5 AM MST) 23 Feb 2013.
For the Cottonwoods, this probably means something like 8-14 inches through late tomorrow afternoon.  Storm skiers should be happy.

After that, it's a bit more of a crap shoot given the vagaries of post-frontal snow showers.  Older model runs were calling for a prolonged period of cold, unstable, northwesterly flow, but, due in part to that explosively deepening cyclone, the more recent runs are building the ridge in more rapidly.  As a result, the flow shifts from northwesterly late Saturday afternoon to northerly by Sunday morning.  So, periods of snow are likely in the Cottonwoods Saturday night, but we'll just have to see how productive they are and how long they can last.

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