Report for Contract

ENC #C154213







State of Colorado

Dept. of Natural Resources







Project entitled:


Development of New Methodologies

for Determining Extreme Rainfall









Prepared by


William R. Cotton, P.I.

with Co-Investigators

Ray L. McAnelly and Travis Ashby

Colorado State University

Dept. of Atmospheric Science

Fort Collins, CO  80523-1371







November 5, 2001




1. Introduction


Since our last report and our meeting with DNR personnel in Denver on 20 July 2001, we have performed additional perturbation simulations of two cases that we have previously described: the Fort Collins flood event of 28 July 1997, and the Big Thompson flood event of 31 July 1976. Because these cases and our initial simulations have been described in some detail in earlier reports, these ensemble runs are only summarized briefly below. However, their results will be incorporated into our aggregate results and the final report.


We have also performed simulations of two additional extreme precipitation cases: the Dallas Divide flood of 31 July 1999, and the Park Range heavy rains over 18-22 September 1997.  Although both of these events occurred more recently than the cases surveyed by McKee and Doesken (1997), Doesken (personal communication) says that both events should be included in an updated "recommended final list of storms for consideration in investigating extreme rainfall potential in the Rocky Mountain region of Colorado" (their Table 5). These cases are overviewed and the results of the simulations performed to date are presented below.


2.  Additional simulations of the Fort Collins event of 28 July 1997


Six simulations of the Fort Collins event were performed with the parallel version of RAMS 4.29. All of these simulations were initialized with the NCEP reanalysis pressure level data from 1200 UTC 28 July 1997. The control simulation was initialized with a horizontally and vertically homogenous soil moisture field corresponding to 50% saturation (0.21 m3/m3).  Four simulations utilized an elevation dependent soil moisture field of 50% and 30%, 50% and 70%, 30% and 50%, and 70% and 50%, respectively, for mountain and plain regions. The elevation separating these two regions was chosen to be 6000 ft. The purpose of these simulations was to examine the effect of high- and low-elevation soil moisture changes on simulated high- and low-elevation accumulated precipitation maxima. The primary conclusion drawn from these sets of simulations was that the soil moisture variations within an elevation category most closely controls the maximum accumulated precipitation within that category. However, the partitioning of total domain precipitation between high and low elevation categories is more heavily influenced by low-elevation or plains soil moisture variations, particularly in the case of dry (30%) plains soil moisture. The final simulation of the Fort Collins event was one in which the initial relative humidity was raised to 98% below 700 mb.

This simulation did not produce larger Grid 4 accumulated precipitation maxima.  However, the initial low-level moisture field was already nearly saturated over northeastern Colorado and this perturbation resulted in an increased of less than 1 g/k of water vapor at the surface.  Maximum total precipitation in these runs ranged from 16.7 inches in the control run to 11.4 inches in the final run, with their positions occurring at various locations over the pains and at higher elevations.





3.  Additional simulations of the Big Thompson event of 31 July 1976


Three simulations were also performed for the Big Thomson flash flood event, initialized with NCEP reanalysis pressure level data from 1200 UTC 31 July 1976. The first utilized horizontally homegenous 50% saturation soil moisture initialization. The second simulation utilized 35% soil moisture initialization below 5600 ft and 50% above 5600 ft. This was done in an attempt to improve the low-level surface temperature and dewpoint forecast.  The third simulation incorporated surface observations in the initialization as well as the NCEP reanalysis data, in an attempt to improve the intial near-surface temperature and moisture fields. Precipitation maxima in these three runs were 11.2, 7.0 and 5.9 inches, respectively; the first value is close to the observed maximum value.  However, none of these simulations reproduced the flooding event over the Big Thompson drainage, and the inclusion of surface observations did not improve this aspect of the forecast. 


4.  Dallas Divide flood of 31 July 1999


The Dallas Divide flood occured on the afternoon of 31 July 1999. We classified this storm as a Local Convective event according to McKee and Doesken's (1997) classification scheme, and a Type IV event according to the synoptic classification scheme of Maddox et al. (1980).   Observational information available to us for this event includes a case study by National Weather Service forecasters in Grand Junction (Avery et al., 2001), a detailed analysis of radar and lightning data of the event by Henz (2000), and a survey of the hydrogeological effects of the flood by Jarrett (personal communication).


Four simulations have been performed for the Dallas Divide event. For this and other more recent cases, 40-km ETA model analyses are available and can be utilized for initialization, providing much greater horizontal and vertical resolution than the NCEP reanalysis data. The four simulations consist of the following: (A) CONTROL -- initialized at 1200 UTC 31 July 1999 using the 40-km ETA model analyses for the atmospheric fields and 70% homogenous soil moisture initialization; (B) ETASOIL -- Same as "A" except initial soil moisture and temperature is based on the ETA, 4-level soil model analysis; (C) DAL-PAR -- Same as "A" except that the initial fields and Grids 3 and 4 were shifted northward to the Parachute, Colorado area; and (D) Same as "B" except Grids 3 and 4 were relocated near Colorado Springs, with no shifting of the initial fields. One further simulation with increased atmospheric water vapor was performed, but increased cloudiness and reduced solar heating prevented significant convection from developing, so those results were discarded. Only the control simulation A is described here, while the results of simulations B, C and D are also included in our aggregate results.


The event occurred in a moist monsoon environment, in deep southwesterly flow ahead of a weak mid-level shortwave in Utah. Figure 1 shows the initial pressure, relative humidity, and wind fields at about 4.5km MSL (about 600 mb), on the RAMS coarse


Figure 1. Relative humidity, pressure and wind fields on Grid 1 at 4.5km MSL, at initial time of simulation A for the Dallas Divide event. Nested Grids 2-4 are indicated by dashed boxes. 


grid, along with the nested grid locations, for simulation A. The observed storm system consisted of a series of several cells on the upper reaches of the Dallas Creek drainage. They translated and propagated northeastward down the watershed, with subsequent cells forming successively further up the drainage toward Dallas Divide and thus raining into the same watershed (Henz, 2000).   Avery et al. (2001) reported that during the latter stages, the heaviest rain progressed slightly southward. Maximum observed rainfall was almost 3 inches (Avery et al., 2001), while Henz calculated peak rainfall amounts of 4-5 inches based on radar data. We do not know whether frozen precipitation was observed or diagnosed with radar data.


Figure 2 shows the total precipitation accumulation on Grid 4 in simulation A (heavy precipitation contours overlaid on shaded topography).   The simulated storm system in


Figure 2. Total precipitation contours (heavy contours beginning at 1mm and at 25mm or ~1-inch increments) overlaid on shaded topography for simulation A of the Dallas Divide event.


"A" evolved in a very similar manner to the observed evolution described by Henz (2000) and Avery et al. (2001), except the similated scenario occurred 15-18 km to the southeast of Dallas Divide (DDV), in the higher elevation region surrounding Mount Sneffels.  Several distinct cells originated near Telluride (TEL), intensified as they tracked over Mt. Sneffels, and subsequently weakened as they moved downslope toward the upper Uncompahgre River above Dallas (DAL) and Ridgeway. Most of these had almost identical tracks, with the multi-cell system eventually propagating southeastward away from the previous dominant track. The resultant maximum total precipitation simulated on Grid 4 (1.67-km grid spacing) is 19.2 cm (7.6 in.), just to the lee (northeast) of Mount Sneffels. This accumulation occurs in the 4.5 hour period between 2100 UTC 31 July and 0130 UTC 1 August, just a little later than observed.


Because the simulated event is at higher elevations than the observed storm, the precipitation pattern in Fig. 2 includes both liquid and ice precipitation. The rainfall pattern (not shown) has two distinct maxima, one on the upstream side of Mt. Sneffels where the cells intensified, and a larger maximum of 14.5 cm (5.71 inches) several kilometers to the northeast of the precipitation maximum in Fig. 2, at an elevation of about 2800m. Between these two rain maxima, hail was the predominant form of precipitation over the highest terrain, with a maximum hail accumulation (liquid water equivalent) of 16.8 cm (6.6 in.). Graupel accumulations were less than 0.1 mm. Thus the simulated rainfall maximum was just a little  more than that estimated by Henz (2000), although its location is not  over the main Dallas Creek watershed as observed.  


5.  Park Range heavy rain event of 18-22 September 1997


The Park Range storm system was classified as a GLC event (General storm system with embedded Local Convection) and a Type III event, respectively, using the classification schemes of McKee and Doesken (1997) and Maddox et al. (1980). The several day precipitation event was due to a synoptic scale wave that dug into Nevada, closed off, and gradually moved eastward through Utah and southern Wyoming. Like the previous large-scale event we simulated that impacted the San Juans on 4-6 September 1970, this event also was influenced by moisture from a tropical storm (Linda) in the eastern Pacific.


The control simulation for this fairly recent event was initialized with the 40-km ETA analysis at 0000 UTC 18 September 1997, for both the atmospheric fields and soil moisture and temperature fields. Figure 3 shows the 500 mb relative humidity, height and wind fields on Grid 1 at 1200 UTC on the 19th, 36h into the simulation, when the low was digging into central Nevada. As in the San Juan simulation of 4-6 September 1970, we found that with a larger Grid 1 extending well into the Pacific, the wave did not developed sufficiently; thus it was necessary to draw in the western boundary onto the U.S. mainland in order to directly nudge the wave into our domain. For the finest nested Grid 4 in this GLC event, we used a 2km grid spacing, slightly coarser than the 1.67km spacing we use in the LC events but finer than the 3km spacing used in the General event over the San Juans. This compromise enables the widespread rains affecting the entire Park Range to be included on Grid 4, while still reasonably resolving convective processes of the embedded convection. 


 We aren't aware of any detailed precipitation analyses of this event.   However, Doesken (personal communication) provided us recording raingage data from two National Atmospheric Deposition Program (NADP) precipitation sites in the Park Range north of Steamboat Springs: the Tower site on Buffalo Pass and the Dry Lake site about 700m lower in elevation and 8km west of the Tower site. These offer excellent time-resolved data, and are supplemented by on-line daily rain amounts at other NADP sites and


Figure 3. Relative humidity, height and wind fields on Grid 1 at 500mb, at 36h into control simulation for the Park Range event. Nested Grids 2-4 are indicated by dashed boxes. 


by routine daily and hourly NWS and cooperative precipitation data. These data indicate five-day total rains of at least 2-3 in. over most of the northwest quadrant of Colorado, 4-5 in. over intermediate elevations such as in the Flattops, and with maximum observed amounts of 6.19 and 8.08 in., respectively, at the Dry Lake and Tower sites in the Park Range. No data are available to indicate whether any frozen precipitation occurred.


Only the control simulation of the Park Range event has been completed thus far; another simulation with enhanced lower-tropospheric moisture is underway. The control simulation was run through 0000 UTC 24 September 1997.   The maximum of the accumulated precipitation field (Fig. 4) is 24.2 cm (9.53 in.), occurring approximately 20 km to the north-northeast of Steamboat Springs (SBS) in the Mount Zirkel Wilderness at about 3200m (10,240 ft.) elevation. Approximately 13-14 cm (~5.5 in.) of this maximum is hail. The maximum rain accumulation (not shown) of 18 cm (~7 in.) occurs several hundred meters lower in elevation, about 2-3 km to the north of the Dry Lake site (DLK). Graupel accumulation is only 3-7 mm at the location of maximum total precipitation and is less than 1-mm at the location of the maximum rain accumulation. The time distribution of the simulated precipitation is in reasonable agreement with the observations, although the synoptic wave remained somewhat stronger and produced precipitation in the Park Range for a day longer than observed.


Figure 4. Total precipitation contours (heavy contours beginning at 1mm and at 25mm or ~1-inch increments) overlaid on shaded topography for control simulation  of the Park Range event.




Avery, B.A., C.N. Jones, J.D. Colton, and M.P. Meyers, 2001:  A southwest Colorado mountain flash flood in an enhanced monsoonal environment. Online paper at,  National Weather Service, Grand Junction, Colorado.


Henz, J.F., 2000: Cloud-to-ground lightning relationaships to flash flooding in western Colorado monsoon thunderstorms. Poster presentation, Southwest Weather Symposium (Tucson, AZ), National Weather Service, University of Arizona, and COMET.


Maddox, R.A., F. Canova, and L.R. Hoxit, 1980:  Meteorological characteristics of flash flood events over the western United States.  Mon. Wea. Rev., 108, 1866-1877.


McKee, Thomas B., and Nolan J. Doesken, 1997:  Colorado extreme storm precipitation data study.  Final Report. Summary of accomplishment and work performed February 15, 1995 through October 31, 1996.  Climatology Report 97-1, CSU, Fort Collins, CO, May, 107 pp.