Wasatch Weather Weenies

We wrapped up our probing of this weekend's storm cycle late this morning.  Looking forward to some sleep.  Click here for KSL's coverage of our weekend activities.  Here's some of our photos as well. Prepping a prefrontal sounding on Saturday. DOW examining orographic precipitation in southwesterly prefrontal flow on Saturday. DOW with cold-front approaching on Saturday.  This might be our most "sinister" photo of the entire field program. Prepping a weather balloon launch in strong post-frontal winds. Jon checking out the storm in the cozy confines of the DOW. DOW under clearing skies on Sunday morning.

Following up on the previous post's discussion of RHI scans , here's one looking directly up Little Cottonwood Canyon.  Check out the high radar reflectivities in the upper canyon. The scientific word for that is.... dumpage !!!  Yup, things look like they are picking up at Alta and it looks like it will be coming down hard for another hour or two.  Then the large-scale precipitation band will be through and we'll have to see...

Lots of you have been asking what it is like to look at storms with the DOW radar. We spend a lot of time looking at what we call Range Height Indicator (RHI) scans, which are essentially vertical scans through the storm. Here's one from today taken from our location just west of Daybreak through the mountains near the University of Utah.  We see very high radar reflectivities where the radar intersects the mountains, but also deeper and stronger radar reflectivities near and over the mountains, which usually is correlated with a higher precipitation rate. In the low-levels over the valley and nearer the radar you can see that the radar reflectivity decreases near the ground.  This is likely because of the sublimation and evaporation of snow and rain as it falls into the dry low levels.  Such sub-cloud sublimation and evaporation is one of the causes of the climatological difference in precipitation between the mountains and valleys of Utah. The color tables ...

Our decision to leave a crew at the DOW really paid off last night.  The latest storm came in fast and they started operations at 1 am.  For the past couple of hours, a broad, stratiform region of precipitation has been moving across northern Utah. Precipitation rates at Alta have picked up nicely, with .04" and .08" of SWE from 0200-0300 and 0300-0400 MST, respectively.  Snow depth sensor suggests about 2" of snow. That would be fairly low water content snow (6%), although the accuracy of the snow-depth sensor is insufficient that we can have high confidence in that number.  If, however, we're getting lots of dendritic growth with light winds, it's a possibility.  We'll have to see what the ice crystal imager says. Looking at the loop above, I see somepersistent periods of lower reflectivity over the southern Salt Lake Valley, suggesting that the Oquirrh rain shadow is still present, whereas, there is persistent enhancement on the windward side o...

We've opted not to move the DOW tonight.  We're shutting it down for a couple of hours to sleep, but tonight's storm is expected to come in overnight and we don't want to miss it.  Thus, we have a couple of students catching some shut-eye in the truck and they will get up early to fire it up once the next storm begins to move in. The storm today (Saturday) was a bit of a bust at Alta.  Looks like a 6 inch storm total at Alta-Collins with .75" of snow-water equivalent.  Most of that snow fell in dribs and drabs, with the heaviest stuff coming in the late night and early morning hours on Saturday.   As discussed earlier, precipitation shadowing by the Oquirrh Mountains appeared to play an important role in today's event. Signing off for the night...

The Oquirrh Mountains appear to have had a dramatic impact on the precipitation today.  Over the past two hours, we have been clearly in a precipitation shadow at the DOW site, and even the area around the Cottonwoods had weaker radar returns than the mountains just to the north.  In fact, Alta-Collins only had .03" of snow water equivalent from 3-4 pm.  The frontal band has simply been unable to move into the south valley with the same intensity it had to the north. The precipitation shadow is evident in the KMTX radar image below as a west-east oriented line of low reflectivity over the south Salt Lake Valley.  I'm using the KMTX image as it provides better context. The precipitation shadow was oriented from SW-NE, but veered as the winds have shifted to westerly at crest level.  Fascinating stuff.  Not what we were looking for, but interesting nonetheless.

The storm is finally here in earnest.  Nice vertical scan below of heavy snow on the western slope of the Wasatch.  The mountains are the sloping bright yellow feature.