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Last modified
1/25/2010 7:07:24 PM
Creation date
10/5/2006 1:53:28 AM
Metadata
Fields
Template:
Floodplain Documents
County
Saguache
Basin
Rio Grande
Title
Radar - Rainfall Reconstruction of the Saguache Creek Flash Flood of July 25, 1999
Date
6/1/2001
Prepared For
Saguache County
Floodplain - Doc Type
Flood Documentation Report
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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />When these two figures are superimposed (not shown) it indicates that <br />the only region of the country in which .bQtb the 500 and 700 MB dew points are <br />considered very moist is the Four Corners area that includes southern and <br />southwestern Colorado. This indicates how deep moisture was present just to <br />the southwest of the flooded area 7-10 hours prior to the flash flood. <br /> <br />Note that the mid-level wind direction was from the southwest and speeds <br />were 10-15 mph. This wind direction is conducive to the formation of persistent <br />thunderstorms on ridges that are oriented southwest to northeast. The low wind <br />speeds are conducive to slow storm movement. <br /> <br />These two factors work together for the formation of either a large <br />singular storm system anchored into topography by the cloud-layer winds or the <br />development of successive storm formation that results in rainfall cores "training" <br />over the same basin. In the case of the Saguache storm, the former event <br />occurred as one large, persistent storm formed over the southwest facing <br />mountain ridges just to the northwest of the town of Saguache. <br /> <br />3.0 Data Sources, Analysis Techniques, and "Gridding" Procedures <br /> <br />3.1 Data Sources <br /> <br />The procedures, techniques, and data utilized in the analysis of this event <br />will be briefly described and further detailed in the following sections. Data <br />utilized in this reconstruction includes, NEXRAD WSR-88D base reflectivity data <br />from the National Weather Service Radar located near Pueblo, Co., upper air <br />atmospheric soundings from Grand Junction and Denver, Co. along with <br />Albuquerque, N.M., surface observations from numerous stations in Colorado and <br />New Mexico, and c1oud-to-ground lightning data from the National Ughtning <br />Detection Network (NLDN). <br /> <br />In order to better analyze some of the meteorological components of this <br />event in a more efficient and accurate manner, a "grid" is established over the <br />area in which the flash flood occurred. The size of one of the squares in the grid <br />is coincidental with a pixel depicted in the NEXRAD data utilized in the rainfall <br />analysis. Use of this grid is very helpful in deriving the rainfall totals, which will <br />be described in more detail in the following section. In addition, this grid is <br />extremely helpful in analyzing the c1oud-to-ground lightning data and the <br />relationship it has with the storm <br /> <br />3.2 Use of Radar to Calculate Storm Rainfall <br /> <br />The utilization of radar to estimate rainfall has been in use for over 30 <br />years by meteorologists in both the government and the private sector. In <br />general, most current radar-derived rainfall techniques rely an assumed <br />relationship between the strength of the <br /> <br />6 <br />
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