Avalanche Advisory
Advisory: Ogden Area Mountains Issued by Mark Staples for Saturday - April 7, 2018 - 7:13am
bottom line

Today the avalanche danger is CONSIDERABLE at mid and upper elevations. Rain yesterday and heavy rain today will cause both wet loose and wet slab avalanches.

Most avalanche activity will be wet loose avalanches. However, larger wet slab avalanches may occur as well. It's hard to forecast for wet slab avalanches but they could break 3-5 feet deep and be very destructive.
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current conditions

Since yesterday morning the Ogden area mountains received 0.8-1.1 inches or precipitation. Some of this was snow in morning, but most fell as rain in the afternoon as temperatures warmed and the rain/snow line rose above 8000 feet.

This morning mountain temperatures are mostly in the upper 30's F. Temperatures are just barely below freezing above 9000 ft. Westerly winds are averaging 10-15 mph gusting 15-20 mph at 9000 ft ridgetops. The rain/snow line will rise throughout the day with heavy rainfall. See mountain weather at the bottom.

Our Week in Review can be found by clicking here, including coverage of a recent avalanche cycle in the Provo mountains.
recent activity

Yesterday, ski patrols easily triggered shallow, wet avalanches as the snow became more and more saturated through the day.
Avalanche Problem 1
type aspect/elevation characteristics
LIKELIHOOD
LIKELY
UNLIKELY
SIZE
LARGE
SMALL
TREND
INCREASING DANGER
SAME
DECREASING DANGER
over the next 24 hours
description

Heavy rain today will continue to saturate the snowpack and cause wet loose avalanches. The period of greatest instability may be this afternoon when rainfall is heaviest. These will be good triggers for larger deeper slides. See the next section about Wet Slab Avalanches.
Avalanche Problem 2
type aspect/elevation characteristics
LIKELIHOOD
LIKELY
UNLIKELY
SIZE
LARGE
SMALL
TREND
INCREASING DANGER
SAME
DECREASING DANGER
over the next 24 hours
description

Rain yestereday combined with heavy rain today should cause some large wet slab avalnches. These types of slides are hard to forecast, but we know they are often triggered by small, wet loose avalanches.

Some wet slabs may break less than a foot deep on all aspects within several different ice crust and snow layers that formed during the last week. Check out these layers in the photo below.

Some wet slabs may break much deeper on buried faceted layers. These layers gained a lot of strength and the snowpack was mostly frozen and strong after rain on March 22 followed by very cold temperatures. Despite this increase in strength, water percolating through the snowpack could awaken these layers and cause wet slab avalanches to break 3-5 feet deep mostly on NW, N and NE aspects where these faceted layers have existed for most of the season. Check out this video from the Sawtooth Avalanche Center in Idaho showing what kind of damage wet slabs can do.

HEADS UP - Wet slab avalanches may be hard to predict, but we know they have the ability to run very far downhill, often much further than you'd ever expect.
weather

Today's weather forecast is simple in terms of avalanches. Heavy rain will fall today. The heaviest rain should occur this afternoon. By tomorrow morning 1-1.5 inches of rain should fall. At 9000 feet, temperatures should reach the upper 40's F. Winds should increase some as the cold front approache and blow 10-20 mph gusting 30-40 mph from the SW. Unfortunately, the cold front will pass over the area tonight and not much additional precipitation is expected once temperatures start dropping. Only a few inches of snow should fall by early tomorrow morning.

In meteorological terms, this weather event is WAY outside of what is normal. Rain this time of year is normal, but rain up to 11,000 feet is not. Jim Steenburgh, University of Utah Atmospheric Sciences Professor, wrote a blog post about this event worth reading. If you don't have time or interest to read his blog, at least look at the graph below. All you need to know is that the red squigly line is the previous record high values for precipitable water. This weather event is the yellow dot and well above that.is well above that.
general announcements

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This information does not apply to developed ski areas or highways where avalanche control is normally done. This advisory is from the U.S.D.A. Forest Service, which is solely responsible for its content. This advisory describes general avalanche conditions and local variations always occur.