Wednesday, January 18, 2012

Here It Comes

The transition over the next 12–24 hours should be impressive.  Right now, we have your classic "cloud storm" underway in which snow is falling from a layer of stratus clouds aloft, but quickly sublimating as it falls into a dry low level airmass.  Note the ragged cloud base and only slight obscuration of the highest peaks in the Wasatch at 9:20 am.

Looking southeast toward the Wasatch Mountains @ 9:20 AM
The morning sounding from KSLC shows quite well the remarkably dry airmass below 600 mb, which features dew-point depressions exceeding 20ÂșC at some levels.

Source: NCAR/RAL
The moist stable layer with a base at about 600-mb reflects the upper-level portion of a warm-frontal zone that is approaching from the west.  This warm-frontal zone will descend over time and eventually bring the Wasatch much needed snow.

In fact, the models are going berserk later today and tonight in the Wasatch Mountains.  The 1200 UTC NAM, for example, generates more than 0.5 in of SWE in portions of the Wasatch Mountains during the 3-h period ending at 5 PM.

NAM forecast valid 5 PM MST 18 Jan 2012
Point data from the NAM for Alta shows 1-h SWE rates exceeding 0.1" per hour from 3 PM this afternoon to 3 AM tonight.


As we discussed yesterday, this will likely be Cascade Concrete with a high water content.  The Alcott algorithm is presently calling for a water content of 14%.  All this heavy snow will be falling on an incredibly weak snowpack.  Not surprisingly, the Utah Avalanche Center has issued an Avalanche Warning beginning at 2 PM for the Wasatch backcountry.

Perhaps the most difficult aspect of this forecast is the snow level.  Here's why.  We have a ton of warm air streaming in aloft, but cold, dry air is entrenched at the surface.  Yesterday it appeared that cold, dry low-level air would scour out enough for snow levels would rise, but it is stingier in the latest model runs.  This leads to a deep stable layer in the latest NAM forecast that is below freezing and would enable snow to fall even in the lower elevations of the Wasatch Mountains.

NAM forecast sounding for KSLC valid 8 PM MST 18 Jan 2012
Hopefully something like the sounding above will verify as we need every flake we can get.

2 comments:

  1. The models are nearly always too cold and moist with warm frontal passages here, particularly in the Salt Lake Valley. Today seems pretty typical of this. I have some ideas about why this is the case in the SLC area, but am still amazed at how warm and dry the lower levels are right now.

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  2. The models don't really crank things up until after 2 PM and the radar is starting to fill in nicely. Things are now saturated down to mid-mountain level. We'll probably see a huge gradient in precipitation at that level to start with far greater amounts above cloud base (i.e., above about 8000 ft) than below. That's been observed in similar events (e.g., Shafer and Steenburgh 2006). Nevertheless, even with cloud base lowering, the dewpoint at KSLC hasn't budged much at all and sits still at 12F. Incredible! Such dry air is going to put a dent in accumulations in the valley, especially this afternoon, as you suggest.

    Your concern about the models and warm fronts is interesting. In SLC, one often sees precipitation shadowing produced by the Oquirrhs during warm front events, but those events usually feature SW flow. Tonight, however, the 700 mb flow is 260@5 PM, 280@11 PM, and 290@5am. As such, the shadowing might be more in the western portion of the Salt Lake Valley. It will be interesting to see how things play out.

    I do think that the high res models like the 4-km WRF run by the NWS are probably overdoing the precip amounts. Here at the U, our 1.3km WRF produced about 2.5" of SWE at Alta by 6 AM tomorrow, which seems out there. The 12-km NAM is about 2.0" by that time. I lean toward the lower value. In fact, I'm inclined to go a bit lower than the NAM, but won't be crying if we get more than that!

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