Reloading

Two cyclones are interacting over the eastern Pacific to "reload" the atmosphere for this weekend's storm.  As shown in the loop below, there is a slow moving cyclone over the Gulf of Alaska and a second, faster moving cyclone moving across the Pacific further to the south.  The merger of these two systems enables the penetration of cold air from the high latitudes into the mid latitudes, as indicated by the southward penetration of deep cumulus clouds (a.k.a. open cellular convection) behind the cyclone late in the loop.  That kind of airmass is the skiers friend and it should begin to move in during the day on Saturday and give us some lower water content snow.

1600 UTC 18 Jan – 1600 UTC 20 Jan 2012 IR satellite and sea
level pressure (contours every 4mb) loop (3-h increments)

The two cyclones also contribute to the advection of high precipitable water air into the western United States.  In this regard, they are also contributing to some of the higher water content snow we will receive between now and Saturday.

1600 UTC 18 Jan – 1600 UTC 20 Jan 2012 IR satellite and precipitable
water (contours every 5 mm) loop (3-h increments)

The large-scale perspective above helps to place what we will see between now and Sunday into context.  Meteorologists typically use something called the forecast funnel to predict the weather.  That funnel starts with the largest scales and then zooms into smaller scales.  An analysis of the large-scale context, such as done above, is the first part of the forecast funnel.