Monday, February 6, 2017

Expectations Through Tomorrow Afternoon

So far today, things have been "quiet" in the mountains.  It's windy on the ridges, but the chairs appear to be turning at the resorts.  Since midnight, Alta-Collins has recorded 0.21" of water equivalent.  Snowbasin-Middle Bowl about 0.13".  Let's call it the calm before the storm.

The ingredients for the storm that will develop later today and tonight are a potent atmospheric river, strong southwest to westerly flow, and a warm front that will push through northern Utah tomorrow.

A great site for examining atmospheric rivers is the Atmospheric River Portal maintained by the Scripps Institute for Oceanography/UC San Diego.  We'll take a look here at the GFS integrated water vapor transport forecasts from the GFS using graphics from their site.  The forecast for 1800 UTC (1100 MST) today shows a corridor of strong integrated vapor transport extending from the eastern Pacific to the California. coast.  The strongest transport, however, is still well upstream of the coast.  Some weaker tentacles of stronger transport extend inland to the north and south of the High Sierra, but northern Utah is in an area of weaker transport that doesn't meet the threshold typically used to identify atmospheric river conditions (250 kg/m/s).

That will change over the next 24 hours.  By 0600 UTC (11 PM) this evening, the strongest area of vapor transport has reached the California coast and atmospheric river conditions have developed over northern Utah has moisture pushes onshore and is able to survive transit across the northern Sierra and southern Cascades. 

 Water vapor transport maximizes tomorrow morning over northern Utah.  A strong warm front will be pushing through northern Utah then as well.

All in all, this is a pattern highly favorable for heavy precipitation in the northern Wasatch and the Bear River Range, including Ben Lomond Peak, Powder Mountain, and Snowbasin.  If we look at last nights NCAR ensemble forecasts for Powder Mountain, we see most members pick up the action after 0000 UTC this afternoon (07/00Z).  A couple of members are somewhat unenthused, but 8 out of 10 members generate at least 2 inches of water for the 24-hour period (right hand half of the figure below) ending at 0000 UTC (1700 MST) tomorrow afternoon and some go much bigger than that.  

A quick look at the NAM shows it is putting out about 2 inches of water in the area around Ben Lomond Peak, Powder Mountain, and Snowbasin for the 24-hour period ending at 0000 UTC (1700 MST) tomorrow afternoon.  This all suggests 2-3 inches of water equivalent is likely for that region, with potentially higher values at Ben Lomond Peak.  Most of that precipitation will fall as wet, heavy snow, but temperatures climb enough tomorrow that snow levels could reach 7000-8000 feet tomorrow afternoon.  

The numbers for Alta-Collins are lower.  The NAM is going for 1.36" for the 24-hour period ending at 0000 UTC (1700 MST) tomorrow afternoon.  The flow direction is an odd one as sometimes WSW Alta gets skunked a bit.  My take is that 1-2" of water for upper Little Cottonwood probably makes sense for that 24-hour period.  Some areas in the central Wasatch favored by SW flow may do better unless the moisture plume ends up a bit further north.  

The situation for snow density is ideal for your classic upside down snowfall.  Temperatures and snow levels are rising and water contents are likely going to be quite high tomorrow.  Our alogirhtm is going for something like 14% water content (think Cascade Concrete) for tomorrow afternoon.  I wouldn't be surprised if it is higher.  Winds will probably maximize tomorrow afternoon as well.  Note the strongest 700-mb (crest-level) winds in the time-height section below occur around 0000 UTC Wednesday (1700 MST tomorrow afternoon).  That's about as strong of a flow as you'll see produced by this model at that level.  

There's a chance of more after tomorrow afternoon, but that is strongly dependent on how quickly the ridge builds upstream as discussed in the previous post.  I'll tackle that tomorrow.  

No comments:

Post a Comment