Dramatic Swings in Weather This Week

If the computer models hold, we are in for quite a roller coaster this week as far as weather is concerned, especially in the mountains.

First, we have two troughs moving through the area, one that went through last night, the other scheduled for tomorrow (Tuesday).  Temperatures will really bottom out in the wake of the 2nd trough and by Wednesday we are looking at 700-mb (10,000 ft) temperatures near -18ºC over the Wasatch Front.  That means temperatures below zero fahrenheit above 10,000 ft.

GFS 700-mb temperature forecast valid 1800 UTC (11 AM Wednesday MST) 7 December

If you look at the forecast above, however, you will also see that it is quite warm over the north Pacific off the coast of California, where 700-mb temperatures are above 0ºC.

If we look at the precipitable water forecast for that time, we also see a tongue of vapor-rich air just upstream of the California coast.  

GFS precipitable water forecast valid 1800 UTC (11 AM Wednesday MST) 7 December

That airmass pushes inland very rapidly, and by 0000 UTC (5 PM Thursday MST) we've rebounded to -5ºC.

GFS 700-mb temperature forecast valid 0000 UTC (5 PM Thursday MST) 9 December

So, our flirtation with brutally cold air should be brief and it appears we'll be in for milder weather later in the week through the weekend.

How much moisture will get into northern Utah will depend, however, on the inland track of the atmospheric river and the impacts of the Sierra Nevada.  A direct track across the southern high Sierra is usually bad for moisture transport into the Intermountain West due to the blocking of low-level moisture and the loss of water vapor on the western (windward) side of the range.  As is often the case, and suggested in the precipitable water and precipitation forecast for Thursday afternoon (below), there is a major decrease in precipitable water across the Sierra due to these effects.

GFS precipitable water forecast valid 0000 UTC (5 PM Thursday MST) 9 December

The NAEFS plume diagram for Alta is, however, fairly optimistic for some base-building water-laden storms late Thursday through the weekend.

That would be good news for the upper elevations of the Wasatch, but let's take a closer look in a couple of days as this is still too far out for me to get excited about details.