Some maps and images from 4-21-2001 event near the California-Nevada-Arizona triple point:

Synoptic situation:
    Low pressure at the surface over eastern Riverside County (CA) is a reflection of the strong upper low centered near Las Vegas (LAS). System is translating slowly eastward. Pronounced lapse rate (temperature decrease with altitude) is enhanced as clear slot develops on the southern edge of upper low permitting insolation (surface warming by incoming solar radiation) which further destabilizes the atmospheric profile. The temperatures aloft as measured in the 1200UTC (5am PDT) sounding at Desert Rock, NV (DRA) were:
+3.8 deg C at 850 mb, -7.7 deg C at 700 mb, and -25 deg C at 500 mb.


1800 UTC Visible Satellite Image:
1800 UTC (11am PDT) visible image from GOES-10. Sorry, the source image doesn't permit much zooming. But perhaps you can make out the discrete cumulonimbi located south and southwest of the southern tip of Nevada.

1700 UTC Surface Plot & Mixing Ratio Analysis:

Surface data plot and mixing analysis from 10 am PDT

The mixing ratio is one way of looking at the available moisture in the air at a stated level. The mixing ratio is plotted in grams (of water vapor) per kilogram (of dry air). Note the mixing ratio maximum located south of the southern tip of Nevada at this hour. Compared to the values normally seen on the plain, these values are paltry. However, remember that a lot of the terrain around this area is elevated, and that temperatures aloft are very cold.


1700 UTC Surface Plot & Convergence:

Surface data plot and divergence (convergence) analysis from 10am PDT

Divergence of the surface wind can be computed mathematically (convergence cannot be computed directly; fortunately, negative divergence equals convergence). Note the strong convergence (i.e. negative divergence) signature in the area between Blythe and Needles, very close to the area of interest. This pattern would support development of deep convection in an otherwise favorable atmospheric structure, particularly if the convergent flow was adding low-level moisture. Note that there appears to be higher dewpoints southward down the Colorado River valley.


Radar Images from KESX (Las Vegas) nexrad:
    I have obtained radar data (base reflectivity and base velocity) for two separate scans that seem to show the unfloding of this event.
What we know about mini-supercells (which I believe was the convective mode) in this event is that they are smaller than ordinary supercells both in lateral dimensions and in the vertical. The smaller lateral dimension makes the detection of circulations features (i.e. mesocyclones) much more difficult because the entire mesocyclone can sometimes fall within one bin of radar data, making detection of the feature with velocity data impossible. Another problem with the data I could obtain is that the velocity data is base velocity (i.e. not storm relative), so the displayed velocity is contaminated by the motion of the storm. However, the storm motion at the time of this event was mostly toward the east, which is mostly perpendicular to the beam from KESX in the Needles-Bullhead City area, so some data reflecting storm-scale motion can be detected.
 


    1840UTC (11:40am PDT) Base Reflectivity


    1840UTC (11:40am PDT) Base Velocity


    1857UTC (11:40am PDT) Base Reflectivity



    1857UTC (11:57am PDT) Base Velocity
Zoomed Radar Images:


1840UTC Base Reflectivity


1840UTC Base Velocity


1857UTC Base Reflectivity


1857UTC Base Velocity

Summary:
    In the works ... page will be updated soon.


Credits for imagery:
    Thanks to CIRA-CSU for the satellite archive and to UCAR/RAP for the nexrad archive from which the satellite and radar images were obtained. And many thanks to Tim Vasquez for developing and perfecting Digital Atmosphere(c) from which the surface plots were created.