Thursday, September 12, 2013

The West Comes Unglued!

Hydrometeorological insanity prevails over the western U.S. as the monsoon goes out with a bang rather than a whimper.  A good rule of thumb for meteorology is to beware when the atmosphere is in outlier mode, and that is certainly the case this week as a monsoon trough amplified over the southwest and ridging built over the northwest, leading to a pronounced high-over-low pattern over western North America.  These features, along with the abundant moisture and the development of crest-level easterly to southeasterly flow across much of Utah and Colorado can be seen in the 0600 UTC (0000 MDT) analysis for last night.

0600 UTC (0000 MDT) 12 September 2013 Analysis of 700-mb geopotential height (contours), wind (barbs), and relative humidity (color filled at 10% increments beginning at 70%).
Let's start in southern Utah where a flood warning now extends over a huge chunk of terrain from the high plateaus running down the central part of the state to the Colorado and Green Rivers to the east.

Source: NWS
Reports to the NWS for Monday morning through yesterday evening include 3.95 inches at Buck Flat on the Aquarius Plateau north of Escalante, 2.85 inches in Escalante, 2.73 inches at Bryce Canyon, 2.13 inches at Bullfrog Marina, 2.95 inches in Zion National Park, 2.91 inches in Capitol Reef National Park, and 2.77 inches in Caineville.  To put those numbers into perspective, the average September rainfall at most of those sites varies from about 0.66 to 1.25 inches.  

The net impact of all this rain has been tremendous stream flows.  The Dirty Devil near Hanksville topped out at almost 10,000 cubic feet per second (cfs).  Note that on Sunday, the flow was around 30 cfs.  

Source: USGS
The hydrology of the western US is snow dominated, but in this case, all of this rain and runoff appears to be having a small but measurable impact on reservoir storage, with a slight uptake in the level of Lake Powell.

And it doesn't stop there.  Incredible rains hit the foothills above Boulder, Colorado last night with more than 6 inches observed at the National Center for Atmospheric Research.

Source: NCAR

All of this and I haven't even discussed the modest downslope wind event along some portions of the Wasatch Front (gusts to 47 mph at Centerville and 45 mph at the University of Utah).

Yup, this is quite an event.  And it isn't over yet.  


  1. Amazing what moist flow from the "wrong" direction can do once it hits high terrain. Orographic enhancement of the precip along the front range contributed to the flooding, as shown in the Denver radar loop from the past 24 hours or so.

    1. That is a really terrible situation in Boulder and other surrounding areas...still raining. The orographic component is interesting. You can see 3 spots for preferred initiation right now. One to the west of Denver with downstream precipitation to the NNE, one to the east of Denver with precipitation east of I-25, and one to the west of Colorado Springs. This could end up being an interesting case study.

    2. Don't worry...whenever something of note happens in Boulder, you can count on a case study...

  2. Some of the areas east of Lake Powell should have had a flood warning as well. I didn't see one yesterday and I don't remember about previous to yesterday. Gauges in Natural Bridges and all around the Abajos and Canyonlands were in the 2.5 inch range though. All of the canyons in those areas must have badly flooded including some popular ones for hiking that have large drainage basins like White Canyon and Dark Canyon.

  3. The clouds yesterday (September 11) were pretty interesting during the late afternoon and evening. Even though it didn't rain that much around the SLC area, I saw some impressive cumulus/cumulonimbus, with rapid vertical development and a bit of rotation in some of them. I couldn't put the camera down, shot some beautiful photos. There was also an impressive lightning show to the southwest after dark from strong cells in southeastern Tooele County.

    1. The cell in the early evening over the southern part of the Great Salt Lake should have had a severe thunderstorm warning...not sure if it eventually did, but it didn't when I checked and it had 70 dBZ echoes with the radar algorithm predicting up to 2 inch hail.

  4. Currently there is a thin layer of fog of some sort hanging on top of the GSL, its really interesting and I don't recall ever seeing something like it in my years here, especially on a warm afternoon. Any ideas what it is?

  5. Thanks for keeping such a great weather blog. I am interested to read more analyses of how anomalous this event has been, particularly for Colorado's front range.

    I'm just an amateur weather enthusiast, and I am mindful of and cautious about the complexities of connecting weather events to climate change, but, it seems totally off the charts that some of the *event* totals in CO are approaching *average annual* precipitation.