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Thread: 2012-01-17/18 - Mountain wind storm: CO front range

  1. #1
    Member Jeff Duda's Avatar
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    Default 2012-01-17/18 - Mountain wind storm: CO front range

    A fairly typical mountain wave-generated windstorm is well underway across the Front Range vicinity in Colorado. Boulder seems to be one of the places most notably being impacted, mostly because there's more sensors there. The NCAR mesa and foothills lab sites are a good way of viewing this event. Looks like gusts up to almost 80 MPH have been reported so far there. I've seen a max gust of 92 MPH just southeast of Boulder so far. The high wind warning for this event told of possible gusts as high as 100 - 120 MPH at higher altitudes.

    I'm not 100% sure what specifically is causing this event, but I'm pretty sure the following factors are involved:
    -Strong cross-barrier (in this case, west winds across the peaks of the Front Range) flow. This event has winds that are more NWly than Wly, but speeds approaching 80-90 kts around 500 mb is apparently enough to get it done.
    -A stable layer at the altitude of the terrain so that the first parcels forced up and over the peaks cannot continue to rise, thus starting the oscillation.
    -Not too stable of lapse rates in the lowest layers to get vertical transport of strong winds
    -Some factor involving the Freude number or something relating to the gravity wave phase speed sqrt(g*H)

    The 00Z DEN sounding is showing the strong NW winds aloft, a pretty strongly stable layer about 50 mb deep centered around 600 mb (well within the range of the tallest peaks), and a stable, but not too stable (except for the lowest few hundred meters), lower atmosphere.

    Sometimes the strong winds are produced by rotors created by the shape of the waves and the differential friction in the lowest levels of the atmosphere. Not sure if this is happening with this event.

    These types of events are very mesoscale, and the E-W extent of this event is pretty thin. Generally only locations within a few miles of the really complex terrain features are seeing the stronger winds from this event. In this case, locations east of I-25 in Colorado are pretty much not being affected.
    M.S. Meteorology, Iowa State University, 2011
    B.S. Meteorology, B.S. Mathematics, Iowa State University, 2009
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  2. #2
    Member MClarkson's Avatar
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    Default

    The Froude number basically shows whether mountain flow wants to contract or expand. 1 is the threshold, with less than 1 being contracting and greater than 1 being expanding, or vice versa, I forgot. If you have stable air and flow over the terrain, the natural tendency is for the flow to be squeezed, and thus accelerate over the peaks. There are situations where the Froude number switches near the top of, because the air speed vs the gravity wave speed has changed. Then the flow continues to compress on the lee side, and the air continues to accelerate by mass continuity. Normally the flow would expand back to nearly its original thickness, and thus nearly its original speed.

    This got me one time when I was keeping an eye on one of our planes putting into Aspen or Eagle or something like that during moderate winds. For no apparent synoptic reason, the airport took a 96 knot gust without warning! Good thing the plane wasn't on short final at the time, huh?

  3. #3
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    PRELIMINARY LOCAL STORM REPORT...SUMMARY
    NATIONAL WEATHER SERVICE DENVER CO
    930 AM MST THU JAN 19 2012

    ..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
    ..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
    ..REMARKS..

    0726 AM NON-TSTM WND DMG BOULDER 40.03N 105.25W
    01/19/2012 BOULDER CO CO-OP OBSERVER

    SEVERAL 1-3 INCH DIAMETER BRANCHES DOWN ACROSS NUMEROUS
    LOCATIONS OF THE BOULDER AREA.

    0720 AM NON-TSTM WND GST 1 SW BOULDER 40.02N 105.26W
    01/19/2012 M83.00 MPH BOULDER CO MESONET

    NCAR MESA LAB

    0555 AM NON-TSTM WND GST 2 NW ROCKY FLATS 39.91N 105.23W
    01/19/2012 M79.00 MPH JEFFERSON CO MESONET

    0510 AM NON-TSTM WND GST 1 NE BOULDER 40.03N 105.24W
    01/19/2012 M75.00 MPH BOULDER CO MESONET

    NCAR FOOTHILLS LAB IN BOULDER

    0404 AM NON-TSTM WND GST 2 NW ROCKY FLATS 39.91N 105.24W
    01/19/2012 M75.00 MPH JEFFERSON CO MESONET

    0400 AM NON-TSTM WND GST 3 N ELDORADO SPRINGS 39.98N 105.28W
    01/19/2012 M79.00 MPH BOULDER CO MESONET

    NCAR MESA LAB

    0338 AM NON-TSTM WND GST 1 NW LYONS 40.23N 105.28W
    01/19/2012 M81.00 MPH BOULDER CO TRAINED SPOTTER

    1144 PM NON-TSTM WND GST 4 WSW ELDORADO SPRINGS 39.90N 105.34W
    01/18/2012 M76.00 MPH JEFFERSON CO MESONET

    1000 PM NON-TSTM WND GST S BOULDER 40.03N 105.25W
    01/18/2012 M104 MPH BOULDER CO TRAINED SPOTTER
    TELEPHONE AND/OR POWER LINE POLES BLOWN DOWN.


    1000 PM NON-TSTM WND GST 3 N ELDORADO SPRINGS 39.98N 105.28W
    01/18/2012 M78.00 MPH BOULDER CO MESONET

    0900 PM NON-TSTM WND GST W BOULDER 40.03N 105.25W
    01/18/2012 M87.00 MPH BOULDER CO TRAINED SPOTTER

    WEST OF BOULDER IN BOULDER CANYON.

    0730 PM NON-TSTM WND GST 4 SSE BOULDER 39.97N 105.22W
    01/18/2012 M92.00 MPH BOULDER CO TRAINED SPOTTER

    HIGHWAY 93 ON THE WEST END OF DAVIDSON MESA. TRAILER
    BLOWN OVER ON HIGHWAY 93 NEAR MARSHALL.


    0724 PM NON-TSTM WND GST 2 NW ROCKY FLATS 39.91N 105.24W
    01/18/2012 M87.00 MPH JEFFERSON CO MESONET

    0458 PM NON-TSTM WND GST 8 NE FOUR CORNERS 40.94N 105.80W
    01/18/2012 M78.00 MPH LARIMER CO TRAINED SPOTTER

    0335 PM NON-TSTM WND GST 1 NW LYONS 40.23N 105.28W
    01/18/2012 M77.00 MPH BOULDER CO TRAINED SPOTTER

    0317 PM NON-TSTM WND GST 1 NW LYONS 40.23N 105.28W
    01/18/2012 M74.00 MPH BOULDER CO BROADCAST MEDIA

    0200 PM TSTM WND GST 3 SW PINECLIFFE 39.90N 105.47W
    01/18/2012 M90.00 MPH GILPIN CO CO-OP OBSERVER

    COAL CREEEK CANYON


    1120 AM NON-TSTM WND GST 21 N NEW RAYMER 40.91N 103.83W
    01/18/2012 E61.00 MPH WELD CO CO-OP OBSERVER

    1040 AM NON-TSTM WND GST 10 NE PAWNEE BUTTES 40.93N 103.87W
    01/18/2012 M60.00 MPH WELD CO TRAINED SPOTTER

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