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