THE DOCUMENTATION OF EXTREME EVENTS: TWO CASE STUDIES
IN UTAH, WATER YEAR 2005
Tim Bardsley and Randy Julander 1
ABSTRACT
The Natural Resources Conservation Service (NRCS), formerly the Soil Conservation Service, has
monitored mountain snowpack and precipitation in the Western United States since 1934. The automation of
measurement sites began in the late 1970s and now over 700 SNOw TELemetry (SNOTEL) sites are installed, most
reporting hourly. The established length of record and hourly records now make it possible to evaluate and
document extreme hydrometeorlogical events. Two recent events in Utah brought attention to the current lack of
protocol for documenting extreme events. During October 20 - 22, 2004 the twenty four hour precipitation
intensities for six Utah sites exceeded the National Weather Service estimated 100-year average return interval, and
eleven of seventy seven Utah sites with 15 years or longer record, measured the maximum twenty four hour
precipitation intensity of record. A second event Jan 8th to 12th hit SW Utah with high intensity rains and snow
leading to the flooding of the Santa Clara River and the destruction of over twenty homes. The Utah NRCS Snow
Survey office is working towards a Snow Survey system wide protocol for the documentation of extreme events
recorded by SNOTEL stations that will provide validation and documentation of extreme events, easily accessed by
current and future data users. (KEYWORDS: floods, rain on snow, SNOTEL, Snow Survey, precipitation intensity)
INTRODUCTION
In response to the Dust Bowl of the 1930s, the Soil Conservation Service (SCS) was mandated to measure
mountain snowpack in the Western U.S and to forecast the water supply for this region. The SCS has since been
renamed the Natural Resources Conservation Service, and since the late 1970s has automated nearly 700 of its
manual snow courses with automated hydrometeorlogical stations called SNOTEL for SNOw TELemetry. These
stations consist of snow pillow to measure the snow water equivalent of the snowpack, and a precipitation gage, as
well as air temperature, and in many cases snow depth, and soil moisture. Most SNOTEL sites and all sites
maintained by the Utah Data Collection Office (DCO) are now measured hourly. The Utah DCO includes Utah,
Nevada, and portions of eastern draining California. The SNOTEL network represents the largest network of near
real-time hydrometeorlogical stations in the mountains regions of the Western U.S. As such, data from the
SNOTEL system are used for a large variety of operational, research, and recreational applications. These data
users need to have confidence that an event that seems questionable has been validated, and is not simply a “glitch”
in the data set. These “outlier” events should hopefully be found by DCO staff during normal data quality assurance
work, and investigated as soon as possible after the event. Two such events were apparent during the first third of
the 2005 water year in Utah, and are the motivation for this paper. They are the events of October 17 to October 22,
2004, and January 8 to January 11, 2005. The October event will be presented first, followed by the January event.
OCTOBER 2004
A very significant storm system impacted most of the state of Utah starting between late October 17 and early
Oct 18, 2004 and tapering off between Oct 21 st and 23rd. Storm totals for October 17th through 22nd ranged
between 31.5cm and 2cm with an average of 88 sites being 12.7cm (Figure 1). This storm was followed by another
significant, but lower intensity storm with little or no gap at many sites before the beginning of November. Of the
77 SNOTEL sites with a period of record of 15 years or greater, 51 had the cumulative October precipitation of
record, and two tied the record. A breakdown of record October precipitation by basin is represented in (Table 1.)
The period of maximum precipitation intensity was on the 20 th or early 21st at most sites, with the notable exceptions
of the north slope of the Uintas, The Wasatch Plateau and South Eastern sites which were approximately one day
later. Twenty-four hour precipitation intensity frequency analysis revealed 12 of 86 sites (two missing) in
______________________________________
Paper presented Western Snow Conference 2005
1
USDA, NRCS Snow Survey Program, Salt Lake City, Utah, randy.julander@ut.usda.gov
Figure 1. Storm total accumulated precipitation in centimeter for 88 Utah SNOTEL sites from October 17 th through
October 22nd 2004.
excess of 25 year return intervals with 6 of those sites exceeding 100 year return intervals. Five of the six highest
intensity sites were in the South West region of Utah.
Basin
Bear River
Weber-Ogden Rivers
Provo R.-Utah LakeJordan R.
Toole Valley-Vernon
Creek
Green River
Duchesne River
Price-San Rafael
Dirty Devil
South Eastern Utah
Sevier River
Beaver River
Escalante River
Virgin River
# of
Records
# of
Sites
7
10
8
14
8
11
3
2
11
2
1
1
8
2
0
7
3
6
12
5
3
3
15
2
2
7
Table 1. Cumulative October precipitation records per
basin.
Figure 2. 700mb Geopotential height anomalies
for 10/20/2004, scale is in meters and anomalies
based on date from 1968 to 1996. Image
provided by NOAA-CIRES/Climate Diagnostic
Storm Analysis
Center
A very deep and slow moving low pressure system centered over the SW U.S. brought warm moist air over
Utah for the days of this event. Highest intensities precipitation mostly occurred on October 20 th, while Utah was at
the leading edge of the trough and with the greatest instability (Figure 2). Most SNOTEL sites in Utah showed this
event occurred mostly as rain. Significant local variations occurred, but in general sites above approximately 2900
meters showed the majority of precipitation as snow, while sites below this elevation showed a mix of rain and snow
with rain dominating. Most sites below approximately 2200 meters collected all rain.
A feature of this storm that appeared to be unique in addition to the total precipitation was the precipitation
intensity. Analysis of the return frequency of the maximum 24-hour intensities was performed for all sites with
continuous hourly records for the storm period. Two methods were used to evaluate the 24-hour intensities. First,
the National Weather Service produces a Precipitation Frequency Data Server (NOAA Frequency 2005,
http://hdsc.nws.noaa.gov/hdsc/pfds/sa/ut_pfds.html). This site uses historic precipitation stations to calculate the
average return interval for storms of various durations. When a SNOTEL site has a record of 15 years or more, the
site creates a frequency analysis for that point. The product also interpolates values for areas where no gage has
existed, or a gage has insufficient data. Each SNOTEL site was evaluated on this webpage to determine the return
interval of the maximum 24-hour intensity. The results of this analysis are displayed in Figure 3. The greatest
return interval is for the newly installed Gutz Peak in the Santa Clara drainage with greater than a 1000 year
estimated return interval. An additional simple analysis compared the maximum 24-hour intensity to the record 24hour intensity of all sites with a 15 year or greater record. There were 12 sites indicating a 24-hour intensity record
from this analysis, half of which were located in southwest Utah.
Stream Response
In general stream response was relatively benign for the majority of the state with the exception of the South
West Corner of the state. Outside of this area most streams and rivers showed moderate increases in flow. The most
notable exception was the Virgin River and its tributaries which showed increase in flow on the order of 100 fold.
The Paria and Escalante, and Upper Sevier also showed similar scale increases, but these flows were still well below
flood stage, and low reservoir levels allowed much of this water to be captured.
SUMMARY
The Storms of October 17th through 22nd, 2004 and January 8th through 11th 2005 had a very significant impact
on immediate hydrologic conditions in the state of Utah as well as on the water year as a whole. Combined
precipitation totals from these two events were near the average water year total for several sites in the Southwestern
region of the state. The extreme nature of these events will draw additional interest in hydrometeorological data
from these time periods. These data users will have access to an event summary report through the Utah data
collection office or its web page.
ACKNOWLEDGEMENTS
Thanks to Mike Bricco for his assistance in producing the GIS graphics.
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