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A Subtle Heavy Rainfall Signature (SHARS) Event in Southeast Arizona: Ramifications of Major Flash Flooding in an Urban

A Subtle Heavy Rainfall Signature (SHARS) Event in Southeast Arizona: Ramifications of Major Flash Flooding in an Urbanized Desert Environment. Michael Schaffner and Erik Pytlak NOAA / National Weather Service Tucson Weather Forecast Office. NWA 2006 Annual Meeting. The Motivation….

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A Subtle Heavy Rainfall Signature (SHARS) Event in Southeast Arizona: Ramifications of Major Flash Flooding in an Urban

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  1. A Subtle Heavy Rainfall Signature (SHARS) Event in Southeast Arizona: Ramifications of Major Flash Flooding in an Urbanized Desert Environment Michael Schaffner and Erik Pytlak NOAA / National Weather Service Tucson Weather Forecast Office NWA 2006 Annual Meeting

  2. The Motivation… • Unusual Subtle HeAvy Rainfall Signature event for SW U.S. • Other flash flood patterns are much more likely, mostly from subtropical upper lows/IVs or decaying tropical cyclones • Heavy rain/flash floods common in summer, but usually isolated • Areal coverage usually determines the difference between run-of-the-mill and major flash floods in SE AZ • What if the Brawley Wash Flash Flood occurred over Tucson (just 25 miles east) instead? • How can we make this relevant for forecasters/emergency managers/media?

  3. Meteorological characteristics of SHARS events From Pete Manousos, SOO HPC: 1999. Presentation to the COMET Hydromet Course, UCAR, Boulder, CO. [ http://www.comet.ucar.edu/class/hydromet/07_Jan19_1999/html/qpfprimer ].

  4. August 14, 2005 meteorology • Classic SHARS sounding • High PW • Unusually high CAPE for 5am MST • Low aspect ratio (tall/skinny CAPE) • Weak shear, but wind veering with height • Rather shallow EL • Some room for insolation • A little drying above 400mb

  5. August 14, 2005 meteorology • Upper low formed on monsoon baroclinic flank Aug 12 • Dynamics transformed from IV/subtropical upper low to more traditional westerly short wave trough • Warm top convection fires at nose of dry slot • MCS migrates into the comma head • Exact structure not well depicted by models • Missing MMGM sounding

  6. August 14, 2005 meteorology

  7. August 14, 2005 hydrology • Brawley Wash near Three Points • Area of 2 to 4 inch rainfall immediately upstream produced 1st peak • Basin average rainfall produced 2nd peak

  8. Water broke out of channel and flooded several properties 1 vehicle-related fatality on a side tributary due to flash flooding at a low-water crossing Numerous low water crossings downstream were closed Impacts downstream from peak flow

  9. Ramifications of major flash flooding in an urbanized desert environment Alamo Wash Brawley Wash

  10. Stream data (Pima County) examined from 50 events spanning 2000-2006 Several older events (USGS gage) from 1986-1992 Rain data from a network of 4 to 10 rain gages Alamo Wash in Tucson

  11. Peak flow and unit discharge increase due to urbanization • Urbanization results in a unit discharge increase of 2.6 times natural Sonoran desert conditions • Translating the 2.5 inch rainfall over Alamo Wash would result in a FEMA 100-year flood with multiple channel breakouts and several hundred structures flooded

  12. Questions? Rillito Creek in Tucson, July 31, 2006

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