[NevadaARES] Notes from NNC-AMS Meeting, 11/19

[Matthew Parker]

Northern Nevada Chapter, American Meteorological Society
November Meeting

Climatology Report:

John James, retired Climatologist for the State of Nevada, gave a 
presentation regarding the severity of the drought in Nevada, primarily 
Southern Nevada.  Although the drought is moderate in Northern Nevada, it 
has redefined the term "extreme" in the southern portion of the 
state.  Many areas in that region received only around 10% of their normal 
yearly precipitation totals during the last water year.  In one case, that 
refers to an official measuring site receiving only 1/10" of precipitation 
for the entire season.  Steve Goldstein of the National Weather Service 
commented later that during an October hiking trip in the Spring Mountains, 
only one of four springs listed as reliable water sources on the map 
showing the trail he was hiking on was actually producing water.

Weather Briefing:

Although the El Nino in the eastern Pacific remains moderate, the weather 
pattern for the remainder of November into the December does not look 
favorable for precipitation.  A very strong ridge remains over the western 
U.S. with a trough to the east and a low presure system over the Pacific to 
the southwest.  Although the ridge will oscillate back and forth over the 
region, it will continue to block significant storms for the foreseeable 
future.  Toward the end of November/early December the ridge will shift far 
enough west to bring in some cold, but dry, air from the north - this will 
result in a period of cooler temperatures but still no precipitation.

The El Nino in the eastern Pacific, although classified as moderate and 
forecast to remain so, has been increasing in strength contrary to 
forecasts.  Regarding the severity of drought, Mr. James noted that strong 
El Ninos have typically produced drier years in the southern portion of the 
state.  Long-range forecasts continue to call for an elevated chance of 
above-average precipitation for the 2002-2003 season for all but extreme 
Northern Nevada.

Presentation - Spanish Springs Flash Flooding, June 20-21, 2002:

Flash flooding occurred over a two day period in the Spanish Springs area 
on June 20 and 21, 2002.  Although conditions elsewhere in the Reno CWFA 
were favorable in advance of afternoon thunderstorms on these days, 
allowing watches to be issued in those areas, atmospheric signatures were 
not indicative of such heavy rainfall over Spanish Springs prior to the 
events of either day.  Heavy rainfall and significant hail on June 20 
produced flash flooding of roadways and some pasture land with reports of 
flooding first coming in around 5:30 that evening.

The events of June 21 were much more severe and noteworthy because again, 
review of atmospheric conditions that morning did not indicate such heavy 
rainfall in the area where the event occurred.  Atmospheric conditions 
elsewhere in the CWFA, primarily over Mono County in California as well as 
Mineral and Lyon Counties in Nevada, were indicative of heavy rainfall and 
Flash Flood Watches were issued in those areas.  As the time approached 
5:30 P.M. on June 21, a sudden increase in dewpoints in the area where 
flash flooding subsequently occurred was noted.  This area was within the 
outflow boundaries of two clusters of heavy thunderstorms; one to the north 
and one to the south.  Cells producing very heavy rain quickly formed, and 
it is estimated that in excess of 3" of rain fell in the area with one 
hour.  The result was significant flash flooding downstream from the 
torrential rainfall.

Some interesting facts about the June 21 event:

*  A "checklist" has been developed contain certain key atmospheric factors 
that are commonly associated with rainfall events that produce flash 
flooding.  Virtually none of these factors existed in the Spanish Springs 
area on the morning of June 21 when afternoon forecasts were being developed.

*  A 100 year flood event in this area would be expected to result from a 
rainfall amount of 1.22" within one hour.  This event is estimated to have 
produced in excess of 3" of rainfall within one hour.  The event therefore, 
"greatly exceeded" a 100 year flood event.

*  Several retention basins exist upstream from the Spanish Springs High 
School campus, which received significant damage from the flood.  All but 
one filled and overtopped.  It was fortuate that the dams forming the ones 
that overtopped were not washed out.

*  Observation of several of these basins with the last few weeks showed 
that they still contain significant amounts of sediment deposited by the 
June 21 event, reducing their capacity to contain runoff generated by 
future events.

*  The construction of Eagle Canyon Drive resulted in the funnelling of 
flood waters toward the SSHS campus that normally would have flowed off to 
the northeast.  Culverts under the roadway in several spots were not 
sufficient to handle the amount of runoff generated in this event.

*  The SSHS campus was constructed with a drainage system intended to 
capture runoff from the uphill side of the campus and funnel it under the 
school, discharging it on the downhill side of the campus.  This system was 
designed to handle runoff generated by a 100 year flood event.  In this 
event, the system quickly becamed clogged with silt and failed, causing the 
runoff to flow over the campus.

*  Flows in some drainages within the flood areas are estimated to have 
reached 1,200 cfs.  By comparison, the Truckee River at Mogul was flowing 
at 346 cfs this morning.

* In terms of volume, one basin within the flood area is estimated to have 
handled the equivalent of 3,000 cfs/sq. mile.

*  During observation of drainages upstream from the SSHS campus shortly 
after the event, the largest debris moved by the flood waters noted was a 
boulder approximately 2' X 4' in size and estimated to weigh 1,500 
pounds.  This size of this rock is much more substantial than much of the 
rip-rap used to shore up runoff channels and retention dams downstream.

*  This flood resulted in damages estimated to exceed $1 million in the 
Spanish Springs community.  Approximately $600,000 of this was to the 
one-year-old Spanish Springs High School.

*  The facts presented are the results of investigation of the event up to 
this point.  The case study into this event is still ongoing.



Matthew B. Parker, N7TOD
Chairman, Northern Nevada Amateur Radio Services
http://www.qsl.net/nnars
ARES Emergency Coordinator - Washoe County
32 South Amanda Circle
Sparks, Nevada   89436
(775) 425-6102
(775) 858-8536 (Pager)