Assessing Instability on the Front Range Without
Upper Air Data
Jonathan D. Finch

Sign Guestbook

Main Page
Elevated Mixed Layer
Convective Outlooks for Bengal
Special Cases for the United States
Bengal Tornadoes--background information
Meteorological Charts for Historical Tornado Cases for Bengal
Latitudinal Comparison of the Geostrophic Wind Approximation
Potential Temperature and Mixing Ratio--Contributions to CAPE on Elevated Terrain

        A few years ago(sometime in the late 90's) I came up with a simple technique for assessing instability on the front range without the
        assistance of upper air data. A lot of information about the upper air can be determined through the appropriate use of surface data.
        The best way to show how this technique works is through a case study examples.

    Case 1

        On June 25, 1942, a thermal low was situated in the northern Texas panhandle, with a nearly stationary frontal boundary stretching
        from central Wyoming into central Colorado and then east into the panhandles and finally north and east into eastern Kansas and
        Missouri. North of the boundary, moist upslope flow was noted across northern Kansas, Nebraska and eastern Wyoming. Another
        area of surface low pressure (probably a thermal low) was located in the northern Rockies. A rather diffuse surface dryline was
        located across west Texas. A cold front stretched from eastern Idaho into western Utah. Ahead of the front, a very warm plume of
        air covered much of the central and southern Rockies.

        By midday--1230 MWT(mountain war time), surface temperatures under the warm plume included 77F at Fort Bridger(7040 ft
        or 781mb), 78F at Rock Springs(6760 ft or 789mb), and 86F at Denver (elev. 5300ft or 834mb). Skies were also mostly sunny.
        These temperatures lie nearly along the same dry adiabat on a skewt-logP diagram with surface potential temperatures of 115F at
        Denver, 114.6 at Rock Springs and 116.2F at Fort Bridger.

        With mostly sunny skies and brisk south to southwest surface winds,  I assume that the thermal profiles above these stations are close
        to dry adiabatic from the surface to 500mb. One can infer south to southwest upper level flow over this region from these surface winds.
        In such a flow regime, a deep elevated mixed layer would most certainly move downstream and cap the boundary layer over Douglas
        and Cheyenne, WY.

        Now simply take the midday surface temperatures at Rock Springs, Fort Bridger and Denver(the most immediate stations to Douglas
        where hourly sfc data were available in 1942) and move these up the dry adiabat to 500mb on a skewt. This should be very close to the
        actual 500mb temperature.Why do I use midday surface data? Because in my experience, the thermal profile is closest to adiabatic from
        the surface to 500mb at this time. In the mid to late afternoon, a strong superadiabatic layer often develops near the ground. So using
        data later in the day would give a 500mb temperature that is too warm. Now take the surface temperature and dewpoint at Douglas,
        WY(elev. 4900ft or around 845mb) and lift this parcel up to 500mb(dry at first and then moist). Now subtract this parcel temperature
        from the estimated 500mb temperature above Rawlins and this yields a 500mb lifted index of around -10. There was a  killer tornado
        near Wheatand, WY that evening.

        This technique works well during the front range severe weather season(late-May to August) when 500mb temperatures are slow to
        change, especially over a span of a few hours. Also, this is the time of year when the elevated mixed layer is a given when southwest or
        westerly flow is present across the central and northern Rockies. Of course, severe local storms are rare on the front range from October
        to April anyway.

    Case 2

        On April 23, 1960, strong upslope flow was advecting moisture into the western high plains and front range of northeast Colorado
        and southeast Wyoming. Oftentimes in spring, upslope flow only results in cool temperatures and low clouds. But southerly mid-level
        flow helps warm up the750-650 mb layer, leading to steep lapse rates overlaying the moist upslope. The result in this case was
        moderate instability. Also, with easterly low-level flow and southerly md-level flow, vertica wind shear was impressive.

        At 19 UTC(noon MST),  surface temperaures had climbed into the 60s and 70sF at the regular reporting stations in Colorado
        and Wyoming. Take the sfc pressure and temperature at each location and go up the dry adiabats to 500mb. This yields the
        500mb T. Denver was north of the warm front until mid-afternoon so we used the mid-afternoon sfc observation there.
         The 500mb T was -16C at Denver,  -16.5C at Rawlins, Laramie and Alamosa, -17C at Eagle, Rifle and Montrose, and -18C at
        Rock Springs and Durango. Colorado Springs was the warmest at -15C. Note that these temps are fairly uniform, with a slight
        decrease to the west. One can roughly estimate the 500mb wind direction from the sfc wind direction under well mixed conditions.
        In this case the surface winds were generally from the south or southwest. Taking friction into account, the upper level flow was southerly
        or southwesterly. I estmate from the above information that the 500mb temp at the tornado location and at Cheyenne at 01 UTC was
        between -15C and -18C, but most likely between -16C and -17C.  Also note that the sfc T/TD at Cheyenne at 01 UTC was
        64F/46F at 802mb. Now we have all the information we need to estimate a lifted index between -7 and -9. These findings are
        corroborated by the 00 UTC 500mb chart.