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Flash floods associated with MCSs – observations
Petersen W. A., L. D. Carey, S. A. Rutledge, J. C. Knievel, N. J. Doesken, R. H. Johnson, T. B. McKee, T. Vonder Haar, and J, F. Weaver, 1999: Mesoscale and Radar Observations of the Fort Collins Flash Flood of 28 July 1997. Bull. Amer. Met. Soc., 80, 191-216.
Petersen et al.
Notable summertime floods of the western U.S.
Rapid City, S.D. June 9, 1972
Big Thompson Canyon, CO July 31, 1976
The Fort Collins, CO Flood – July 30, 1997
5 fatalities, 40 injuries, half of CSU library collection ruined
Most well documented flash flood – occurred close to CHILL research radar, two NEXRADS and over CSU DAS
Topography and radar locations
1600 27 July 1997- 1300 28 July 1997 1730 28 July 1997- 2300 28 July 1997
Coordinate system origin for later graphs (intersection of Taft Hill and Drake Roads)
Christman field CSU
5.3 inches/6 hr
NWS issues special statement outlining potential for flooding
Flooding north of FCL from earlier rains
Flood advisory issued
Flood advisory cancelled
Flash flood watch issued
Urban Street Flooding Advisory issued
Urban Operation Center Opened
Spring Creek Floods onto Roads
Campus buildings ands streets flood
Homes flooded
Flash flood warning issued by NWS
Train culvert breaks sending a wall of water into mobile home park
Train derails at broken culvert
Dewpoint depression < 6°C shadedTemperature contours = 2°CHeight contours = 30 m
500 mb
Summer “monsoonal” flow –moist air aloft from SE Pacific
Ridge axis
Weak flow aloft over Northern Colorado
Cloud tops < -20°C each hour from 0600-1800 MDT
Surface dewpoint > 60°F to east of line
Frontal positions at 0600 (a), 1200 (b), 1800 (c) and 2400 (d) MDT
500 mb vorticity centers
Note the easterly flow to the north of the surface front. This flow brought warm moist high e air upslope to the foothills of the Rocky Mountains
1800 MDT sounding from Denver, CO overlaid on a sounding taken during TOGA-COARE along the equator!
Atmosphere over eastern Colorado had characteristics of the tropical Pacific!
Other sounding parameters:
Extremely moist compared to climatology
Not too unstable
Flow and radar echoes at 18, 19, 20, 21 MDT
Radar echoes from flood region (red box) do not stand out as unusual
Note this bow echo – this feature modified the strength of the easterly flow into the Fort Collins storm during part of its lifetime
1416 MDT
1716 MDT
Cloud drift winds
Low level easterly winds were sufficiently deep that low level clouds drifted from east to west
Wind barbs in knots (standard)
Cross section at 2 km AGL of radar echoes from Cheyenne, WY NEXRAD for period of most intense rainfall – Spring Creek is the black line
Note strong echoes over Spring Creek basin and the redevelopment of convection and “training” of the heaviest rainfall on the south side of the complex.
Radar beam
Outbound velocitiesInbound velocities
Outflow from convection
Cell regeneration region
Radial velocities from CHILL radar superimposed on reflectivity contours
Spring creek
Cloud to ground lightning strokes between 1800 and 2300 MDT (different colors for each hour)
(little box is flood area)
Hardly any lightning!
Dual Doppler derived winds at 1 km AGL at 2110 and 2130 MDT
Flood storm Bow echo to south
It appears that the easterly flow into the flood storm accelerated as a result of some interaction with the storms within the bow echo, although the nature of the interaction was difficult to specify
Dual-Doppler derived u (easterly) wind
component
Bookend vortex at northern end of bow echo
Easterly “jet” flowing toward convection over
Ft. Collins
Strength of easterly wind (expressed as a deviation from a mean value for the time period)
Rain mass flux determined with radar
Amount of rain produced by the storm over Fort Collins highly correlated with the strength of the low level easterly inflow
Relationship of lightning strikes to mean reflectivity evolution in storm
-10°C
Lightning strikes
Rain mass flux
Reflectivity
Evidence from this data suggests that the warm rain process (collision-coalescence) dominated precipitation production during much of the storm – making the storm more “tropical-like” and unlike the storms further east on the same day (see lightning chart)
Strong echoes in “warm rain” region, rather than associated with hail in ice region
Reflectivity profiles as a function of height, normalized to profile maxima for the Fort Collins storm (bold), tropical Atlantic convection (dashed) and the tropical west Pacific (thin).
Note similarity between tropical convection and the Fort Collins storm
Reflectivity (dBZ)
ZDR – blackKDP - Blue
Vr – blackReceding - solid
LDR – blueIce fraction - Black
Polarization radar variables show that precipitation process was warm rain process
Reflectivity (color)ZDR ( black)KDP (white)
Rainrate from Nexrad Z-R relationRainrate from polarization algorithm
Microphysical processes suggested by polarization data: 2-3 mm raindrops formed by collision-coalescence rise through freezing level, freeze above about –10°C and fall to the northwest of updraft in heavy rain region.
Rainrates in flood conditions are estimated better with polarization radar compared to conventional Z-R relationships.
NEXRAD standard Z-R
Values low and did not match spatial distribution
NEXRAD tropical Z-R
Values reasonable except lower value with CHILL - matched spatial distribution
Multiparameter polarization estimate of precipitation
Slightly low maximum but best estimate across the gage network
Blend of relationships