30" of snow lost in 21hrs @ Snoqualmie?

Amar Andalkar for the win!
Well explained, I learned something new today.
Always a bonus.

Thanks for the kind comments.  I'll try to answer some of the questions.

...now help me reconcile the Tinkham site.

That's a tough one, it appears as though the density has not increased significantly because the SWE is dropping roughly in concert with the snowdepth.  My guess is that this is a damaged or malfunctioning SWE sensor, looking at its sensor history log the Tinkham site has a recent history of SWE sensor troubles in late 2008, which they apparently did try to repair.

The SWE sensors can be very finicky and behave in seemingly strange ways at times, since the pressure of the snowpack on the pillow is not a simple function. The sensor is very slow to adjust to new loads, such as a heavy snowfall, often taking 1-2 days to reach equilibrium. It is also affected by ice layers in the snowpack, which can act as bridges spanning across the pillow and reducing the weight felt by it. Rainwater can sometimes pool above the pillow (depending on local geometry), causing a temporary increase in the reading until the water drains away off to the side. In addition, they are sometimes prone to leaks which ruin all subsequent data as the antifreeze slowly drips out.  The Tinkham pillow may be leaking??  Here's an example of a SWE sensor showing a spike, comparing data at Cougar Mountain (updated version of plot above) and nearby Meadows Pass.  There's an obvious strange spike in the Meadows Pass SWE, which is slowly going away over the course of a day. This looks like it could be caused by water pooling and then slowly draining.




Other than the weird spike, both of these sites are fairly consistent with other.

How'd you learn about the inner workings of the snow measurement equipment?

Reading, lots of reading, and being obsessed by snow and snowfall. Here's some useful links with info:

www.id.nrcs.usda.gov/snow/siteinfo/typical_snotel.html , nice photos of SNOTEL sites
www.webs.uidaho.edu/epscor/snotel/Teache...de/teachersguide.htm , click on "Selecting A SNOTEL Site to Adopt"

Fascinating post, Amar!  Isn't there also the possibility of some mechanical erosion of the saturated snowpack surface by runoff at inclined sites?

Don't know. Clearly we see big runnels in the snow surface after major rain-on-snow events, some of which may be caused by erosion, but probably more so by drainage channels within the snowpack. I would guess that surface erosion is only significant when rain falls on a very hard firn or ice surface (such as bare glacial ice), since a seasonal snowpack is usually too porous for water to flow along the surface and erode it. Small streams are often seen to be eroding into the surface of most glaciers or permanent snowfields during late summer.

But what about the heat of condensation when the water vapor in moist, warm air condenses on cold snow?  If condensation is possible, that should be a much bigger transfer of heat than conduction.  And if I understand what the dew point signifies, if the dew point is above freezing then condensation on the snow will be occurring?

I'm not sure how much this contributes. The heat of vaporization (condensation) for water is almost 7 times as great as the heat of fusion, so every millimeter of water condensing onto the snow surface could potentially melt almost 7mm of SWE from the surface of the snowpack. But even 1mm of condensation is a large amount, far more than even the most dew-drenched field has on a fall morning. I don't know how much vapor condenses onto the surface of a snowpack under normal rainstorm conditions, and a quick search didn't turn up any good leads for finding the answer. Maybe someone else knows?

I don't have any numbers to put science up like Amar, but from my East Coast experience when we really watched every inch of snow melt away, fog was our biggest enemy. We might have heavy rains for days and find that our snowpack had only lost a half inch. One warm foggy day, though, and we could lose several inches. That seems to support the idea that condensation is a major contributor to SWE loss.

I don't have any numbers to put science up like Amar, but from my East Coast experience when we really watched every inch of snow melt away, fog was our biggest enemy.  We might have heavy rains for days and find that our snowpack had only lost a half inch.  One warm foggy day, though, and we could lose several inches.  That seems to support the idea that condensation is a major contributor to SWE loss.

snow sucking fog, a skier's worst nightmare...

An overview of the snow densities puts the current snowpack layers in the upper-30 to mid-40% range.

Storm total through this evening was 14.71", with 11.38" as rain.

The snow lysimeter measured 3.57" water leaving the snowpack.

A few inches new snow this evening on Snoqualmie, rather calm as the storm passes. Wicked windy over here in Ellensburg, looks like Mission took a hit from the wind.

snow sucking fog, a skier's worst nightmare...

That's gotta be a bad horror film...

Yesterday at Summit West and Alpy the snow was realy dense and heavy. Around 3PM it started snowing pretty hard, and between 3PM and 5:30 we got about two and a half inches of beautiful snow. I went home then, so I'm not sure how much snow they got in total.