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Old 02-25-2014   #11
elkhaven's Avatar
Belgrade, Montana
Paddling Since: 1991
Join Date: Sep 2013
Posts: 1,661
As glenn said it depends largely on the condition of the snow pack. Others have noted that if rain falls on "powder snow" i.e. a fairly dry snow pack, the pack can and will absorb a lot of water. Even in the Cascades most freshly fallen snow has less than a 20% moisture content, so chances are good the SWE will go up.

If the pack is isothermal you will see much different results. In most isothermal snow packs (all layers the same temp, i.e. no thermal gradient to drive water up the pack and the pack is usually near saturation) free water in the snow pack is held by surface tension within the pores of the pack. In this state it is usually in some weak form of equilibrium where as ice melts and water moves down the pack it forces water near the base out. When it rains, this equilibrium is upset and in essence the rain increases hydraulic head in the pack pushing out much of the free water. It appears that the rain has melted a bunch of the snow, when in reality it has just pushed free water out. This often pulls the new precip, or again as glenn noted, forms channels in the pack giving the new precip preferential flow paths through the pack. The end result is the free water in the system, plus much of the new precip exits the snow pack. Thus a net loss to SWE.

Rain at this time of year will be very near freezing and will have little heat engergy to change ice to water. Very little snow is melted in these events but a lot of free water can be released.

At this time of year the snow pack should not be isothermal, it should take on more water therefore it's likely that the event will increase SWE. But given the far from normal conditions and the abysmal snowpack you guys have over there it won't be able to increase it much and the event will likely push the pack closer to an isothermal condition.

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Old 02-25-2014   #12
River City, Oregon
Paddling Since: 1995
Join Date: Jan 2013
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The latest "rain on snow" event caused a slight loss in SWE, on average, in most area snow packs. Just as predicted. Although there were a few (very few) sensors that actually registered a net gain in SWE. All sensors showed a pretty decent drop in snow depth though. Even the sensors that were flat or slightly rose in SWE still had a decent drop in snow depth. I would assume this is the snow pack consolidating and becoming more dense. Thanks again for the explanations and links. Interesting stuff to ponder in February.

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Old 02-26-2014   #13
Redmond, Oregon
Paddling Since: 1973
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U.S. Drought Portal
Welcome | U.S. Drought Portal

Soil moisture modeling from the University of Washington is based on the Variable Infiltration Capacity Macro-scale Hydrological Model. It includes soil moisture, snow water equivalent and runoff.
several interesting links.

This is more of a big picture.
As snow depth is to SWE this is SWE to two more variables with SWE.
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Old 02-26-2014   #14
Join Date: Apr 2004
Posts: 137

So up here in the Upper Klamath Basin we had a pretty large rain on snow event around valenties day with rain at very high elevations. Take a look at some of these SWE graphs

Crater Lake (annie springs) snowtel at 6600 ft. You can see the storm move in around the 13th and precip levels climb but snow depth doesnt. This is a rain event. You can see before and after that event the depth totals can increase alot but not necessarily the precip or the SWE, these are snow storms. So you can see that the SWE did increase during the rain storm. However during this storm Annie Creek (which drains the area below this snowtel) blew up and went to bankfull. The 35 inch snow pack was able to absorb the rain event and store that water. However the thinner snowpack at lower elevations could not deal with the rain and warm temps and much of that snow melted causing the high water.

This plot at the Fish Lake Snowtel, which is in the Cascades @ 4,660 ft shows what happened to thinner snowpacks at lower elevations. Around the 13th the precip began increasing as the rain fell but the snow depth and SWE both dropped. The thinner snowpack couldnt handle all of the rain and alot of it melted and the rain turned into run off. We saw pretty big peaks in most of the cascade streams coming into the Klamath Basin.

Last plot is from Taylor Butte which is east of the cascades in the high desert @ 5,030 ft. You can see that the rain even pretty much obliterated the shallow snowpack that we had there.

So the point is that (like others have stated) the condition of the snowpack has alot to do with the SWE response. But if you are looking at SWE to predict runnoff (especially in eastern oregon) you can get tricked. For example here in the upper klamath, that storm may have increased SWE at the high elevations (where most of the snowtel sites are) but it utterly wiped out most of the snow pack at the lower elevations. This is going to mean lower spring flows on our rivers out here because all of that low elevation snow that would normally come down in the spring is gone; and in our basin that is a much larger proportion of the area than the high peaks.

OK I have been rambling for a while. Hope that makes sense.

Pray for snow
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Old 03-08-2014   #15
Wadeinthewater's Avatar
Walterville, Oregon
Paddling Since: 1977
Join Date: Mar 2009
Posts: 559
Forecast for Santiam Pass Saturday night - Rain. The rain could be heavy at times. Low around 42. Windy, with a southwest wind 29 to 32 mph, with gusts as high as 47 mph. Chance of precipitation is 100%. New precipitation amounts between 1 and 2 inches possible. Watch SWE drop south of 50% for the Willamette Basin.

Real adventure is only one bad decision away.
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