A Synoptic Climatology of Winter Season Flooding Precipitation in Complex Terrain

A synoptic climatology of the last 57 years of atmospheric flow and thermodynamic analyses are performed to determine the large scale features responsible for extreme Truckee River daily mean river flows at Reno, Nevada to the lee of the Sierra Nevada Mountains. During this time period 18 events from November – May produced river flow rates exceeding 5000 cubic feet/second (cfs). Many (8) of these events were associated with major devastating river flooding along the Truckee River in Reno when in excess of 10,000 cfs stream flow rates where observed. Most notable of all events with >15,000 cfs stream flow rates were events in 1950, 1955, 1963, 1997 and 2005.

The synoptic analyses covered the region extending from northern British Columbia to the west central coast of Mexico and from ~3000 km west of the Pacific coast to the Front Range of the Rocky Mountains. NARR-A and NCEP-NCAR Reanalyses data were employed for at least 72 hours prior to peak stream flow periods every 6 hours in an effort to diagnose the location and intensity of the mid-upper tropospheric jet stream, midtropospheric moisture flows, lower-midtropospheric potential instability as well as midtropospheric thermal advection patterns. Additionally, composite rainfall analyses were also performed to determine when, where and to what extent all 18 events were the result of heavy rainfall as well as snowmelt within the Truckee River Basin. Composite charts of these fields as well as composite soundings were constructed just upstream from Reno, Nevada near the headwaters of the Truckee River Basin.

Results of these analyses indicated that in virtually every one of the 18 case studies heavy rainfall exceeding at least 25 mm/day (and often several times that amount) occurred to the lee of the Sierra Crest prior to flooding. Additionally, and most importantly to the study, the following atmospheric features were in place at least 24 hours prior to and during the heaviest rainfall that contributed to the flooding: 1) a very strong (>75 m/s) and elongated 250 hPa polar jet streak oriented southwest-northeast from ~2000 km west-northwest of the flood region and extending from the base of a deep synoptic scale trough over the Pacific, 2) a secondary midtropospheric (~700 hPa) wind maximum with an elevated “atmospheric river” (e.g., Nieman et al. 2008) of significant water vapor flux extending from the flood region upstream to the base of the synoptic scale trough over the Pacific, 3) a matching conduit of 850-500 hPa potential instability nearly coincident with the elevated “atmospheric river” of relatively high water vapor flux and 4) a very strong zone of 500 hPa warm air advection juxtaposed to cold air advection flanking the upstream trough and rapidly approaching the flood zone. The composite sounding indicated that warm moist air advection over the Sierra Nevada at and above 700 hPa established a deep midtropospheric sounding structure approaching saturated moist neutrality. This is consistent with the larger scale features observed including strong mid-upper tropospheric airflow, midtropospheric water vapor flux and midtropospheric warm air advection.

These features strongly suggest that to achieve primarily cold season lee side (spillover) precipitation rates often substantially exceeding 25 mm in 24 hours, that cause extreme rainfall and subsequent flooding in the lee side river basins of the western U.S., intense mid-upper tropospheric dynamics are essential and these dynamics are strongly coupled to midlevel jets and rivers of moisture much higher than the low-level jets and low-level rivers that have been shown (e.g., Nieman et al. 2008) to cause extreme rainfall and subsequent flooding in the Pacific coastal mountains of the western U.S.