FLOOD01363

• County: Statewide
• Title: Colorado High Plains Thunderstorm Systems - A Descriptive Radar-Synoptic Climatology
• Date: 6/1/1974
• Prepared By: CSU
• Floodplain - Doc Type: Educational/Technical/Reference Information

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3.0 An operational example - October 6, 1993

The operational setting for this event was the late fall movement of a decaying sub-
tropical disturbance over the states of Arizona and New Mexico. The upper level 700mb
to 300mb disturbance passed directly across Arizona during the period of 11 :OOAM to
6:00PM. It provided both ample lower and upper level forcing features in a ')uicy
atmospheric" environment when evaluate by the QCP2 technique.

Figure 3 shows Maricopa County in Arizona, the ALERT Automatic Rain/stream gauges
and weather stations used by the FCDMC. The ten weather stations were used as a
mesonet to initialize the QCP2 technique. Figure 4 shows the estimated Phoenix area
sounding which was estimated using the Winslow, Yuma and Tucson, Arizona soundings
at 061l200GMT and 07/oo00GMT. Figure 5 shows the QCP2 calculations made at
11 :OOAM using the weather station observations of temperature and dew point at the
Durango (D), Carefree (C) and South Mountain (S) locations on Figure 3. The key
temperature/dew point couplet to activate the warm layer coalescence mechanism
was 81F/60F as observed at the Carefree site.

Figure 6 shows the FCDMC mesonet QCP2 for the period 11 :OOAM to 12:00PM based
on using the surface temperature/dew point couplets on the composite or predicted
sounding. It shows a precipitation bulls eye over the eastern third of Maricopa County.
The arrows on the map show the low level wind flow observed at the 10 sites and it is
clear that Phoenix and Scottsdale are potentially ripe for a flash flooding event. Figure 7
is QPF product that could be issued based on the 11 :OOAM QCP2 and some limited
prediction of the passage of the upper level disturbance. The storm probability matrix is
based the observed variation of the surface moisture fields and knowledge of local terrain
effects from nearby mountains to the south, southwest, north and east of Phoenix.

Figures 8 and 9 are QCP2 products based on 1 :OOPM and 2:00PM mesonet observations.
Clearly the "bulls-eye" remains on the eastern third of the County. Note that Maricopa
County covers 10,000 square miles. Figure 10 is the 3:00PM Phoenix WSR-88D
reflectivity observation scaled to the Maricopa County area. Note the train-echo effects
in the line of strong storms aligned off the terrain to the southwest of and over Phoenix.
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The accumulated rainfall measured in the FCDMC ALERT flood detection network for
noon to 2:00PM is shown in Figure 11 while the WSR-88D Storm Total rainfall product
is shown in Figure 12. Clearly Mother Nature hit the "bulls-eye" with an urban flooding
rainfall of 2.00 - 5.00 inches in about a 3 hour period. No loss of life was caused by the
event but the evening rush hour was hopelessly snarled in Scottsdale and north Phoenix
for several hours due to flooded roads. Finally the peak 60 minute rainfall observed by
the ALERT rain gauges in the "bulls-eye" are compared to the QCP2 to show the merit
of the technique in this case in Figure 13. In all three cases the QCP2 provided a good
estimate of the quantitiative convective precipitation potential for a 3 hour period.

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