Transcript Predicting Heavy Rainfall, Flash Flooding, and

Hydrometeorological Forecasting Lab:
Predicting Heavy Rainfall, Flash Flooding, & Stormwater Runoff
Meteorologist Anthony Phillips
[email protected] | www.wx4sno.com
 Recognize & understand basic synoptic features on weather maps
 Become acquainted with links to weather forecasting charts and
graphics on the internet
 Determine when and where heavy rainfall will occur & if flooding
could be a concern
 Learn how to calculate stormwater runoff problems
Review of 2/28 Severe Weather
 0.56” of rain fell in Muncie on Friday, 25-Feb-2011
 Rain changed to snow which accumulated to 5.8” by late Friday
 Strong to severe storms developed across central Indiana late
on Sunday, 27-Feb-2011
 Thunderstorm “training” effect produced torrential rainfall
 Combined with already saturated ground and left-over snow
pack, flash flooding occurred quickly across much of Indiana
Review of 2/28 Severe Weather
Review of 2/28 Severe Weather
Review of 2/28 Severe Weather
Surface Weather Analysis
 http://aviationweather.gov/adds/progs/nav.php?current=0
Surface Weather Analysis
 High pressure systems: H
 Associated with sinking air & clear skies
 No precipitation
 Low pressure systems: L
 Associated with rising air, cloudy conditions
 Usually produce precipitation
Surface Weather Analysis
Upper Air Charts
 http://www.nco.ncep.noaa.gov/pmb/nwprod/analysis/
 Time of day will determine which UTC time to choose
 If it’s currently morning: use 00 UTC column & select “fine”
 If it’s currently evening: use 12 UTC column & select “fine”
Upper Air Charts
 Different graphics/maps are across top row
 Forecast days into the future are listed on the left
 Hours forecasted into the future are listed as links
Upper Air: 850 mb Chart
 Temperature at this height is represented by solid colored
lines
Blue: < 0° C (frozen)
Purple: 0° C (rain/snow line)
Red: > 0° C (liquid)
Upper Air: 300 mb Chart
 Used to locate the Jetstream, ridges, and troughs
 Winds are plotted and color-coded
 Troughs are associated with cloudy weather with potential for
storms along right side of the Jetstream
 Ridges are associated with high pressure, clearing skies and
no storms
Upper Air: 300 mb Chart
Trough
Ridge
Hydrometeorological Prediction Center
 http://www.hpc.ncep.noaa.gov/
 The main source for information about upcoming flood
conditions
Hydrometeorological Prediction Center
 HPC Products:
 QPF: Quantitative Precipitation Forecast(s)
 Provides a forecasted amount of precipitation over a certain amount of time
 Useful for locating areas of heaviest precipitation 1, 2, 3+ days in the future
 Excessive Rainfall:
 If extreme rainfall is expect, this product outlines areas that are at risk
 Levels of risk are outlined as follows:
 PQPF: Probabilistic Quantitative Precipitation Forecast(s)
 % probability of certain amount of rainfall occurring over a particular area
 Flood Outlooks
 Winter Weather Outlooks, Heat Indicies, Air Quality
USGS Stream Gage Network
 http://water.weather.gov/ahps/
USGS Stream Gage Network
 Allows for monitoring of stream and river stages
 Current, past, and sometimes forecasted stages
 Also provides flow in cubic feet per second (ft3 sec-1)
 Location information provided about the gage




Latitude/Longitude
Google map
Photographs
Trouble spots
WSR-88 Doppler Radar
 http://radar.weather.gov/
 WSR-88D: Weather Surveillance Radar 1988 Doppler
 More simply, Doppler radar or just radar
 100+ Doppler sites
 Provides a look at current precipitation,
as well as previous amounts
 Also can measure wind speed by the
Doppler effect
 (hence the name)
WSR-88 Doppler Radar
 Radar emit microwave energy in pulses as it spins
horizontally
 When a pulse strikes an object
(rain, hail, birds, etc), energy
is reflected back to the actual
radar site
 The location of this strike
is plotted on a 360° circle
WSR-88 Doppler Radar
 After the radar makes one horizontal revolution, it points or
tilts slightly higher up and makes another revolution at this
higher angle
 It repeats this several times to get a 3-D scan of the
atmosphere
 How large or dense the reflected energy determines the
value measured in dBZ.
WSR-88 Doppler Radar
 Advanced, hi-resolution radar images are available
 Must have a radar viewing program such as GRLevel3
 Available for a one-time fee at: http://www.grlevelx.com/
 I currently host hi-resolution radar images for the Muncie
area at: http://www.wx4sno.com/radar/
 WSR-88D’s are being replaced with Dual-Pole radar
Flash Flood Guidance (FFG)
 http://www.srh.noaa.gov/rfcshare/ffg.php
 FFG: Flash Flood Guidance
 Provides an estimate of how much precipitation (in inches)
within a certain amount of time is needed to cause flash
flooding over a particular area
 County-by-county coverage
 Mostly available in 1-Hr, 3-Hr, 6-Hr, 12-Hr, 24-Hr time frames
 Graphical or tabular formats
Flash Flood Guidance (FFG)
 An example:
 What state does this FFG map cover?
 For what time-frame is this FFG?
 Locate the Great Smoky Mountains
 Along the middle of the Tennessee/North Carolina border
 What is the FFG value for the Smokies on the Tennessee-side?
Flash Flood Guidance (FFG)
 An example, cont:
 From the previous
slide, we determined
the 6-hr FFG value to
be: 3.0”
 To the right is a radar
image showing rainfall
for 6-hours across the
same region.
 According to this
image, about how
much rainfall has
occurred near the
white line
Flash Flood Guidance (FFG)
 Follow these rules when comparing FFG to either
radar values or to QPF:
If FFG value > QPF/radar value = no flash flooding
If FFG value < QPF/radar value = flash flooding
 So, to recap:
 If the FFG map indicates a larger number than what is
shown on radar or is forecasted on QPF maps, then
flooding is unlikely
 If the FFG map indicates a smaller value compared to
what’s shown on radar or forecasted on QPF maps, then
expect some sort of flooding/flash flooding
Stormwater Volume Calculations
 Example:
Assume the HPC is forecasting 0.50” of rainfall over the
Muncie area on Day 1. We later observe this to be true by
looking at Doppler radar total rainfall for the 24 hour
period on Day 1. Knowing that 0.50” of rain fell over one
township in Muncie, how many gallons of runoff will
there be, assuming that the entire township is an
impermeable surface?
Stormwater Volume Calculations
 Example, cont:
 Step 1: Determine how many gallons are in 1 square mile
 Convert your units to match one another:
Stormwater Volume Calculations
 Example, cont:
 Now that everything is in the same units of feet, find the
cubic volume:
 Now convert this cubic feet value to gallons (recall the
conversion factor: 1 ft3 = 7.48 gal)
Stormwater Volume Calculations
 Example, cont:
 So we know that this value of 8.75 million gallons is
equal to the amount over 1 square mile
 Step 2: Determine how many gallons of rain there are
over 36 square miles (1 township)
Stormwater Volume Calculations
 Example, cont:
 From this example above, we can see that 0.50” of rain
over one township in Muncie produces 315 million
gallons of runoff.
Stormwater Volume Calculations
 In-class Exercise:
 Weather observation equipment at Muncie recorded 2.68” of
rainfall on Monday, February 28, 2011. How many gallons
of runoff occurred as a result of this rainfall across Center
Township in Delaware County? Assume 70% of the
township is impermeable.
Comments/Questions???
 If you find yourself in need of help, please drop by my office
in room CL 427 within the Geography Department, Cooper
Science Complex (just down from West Quadrangle).
 My office hours:
 I will not be on campus during Spring break
 You can also email me at: [email protected]