Hydrometeorological Forecasting Lab:Predicting Heavy Rainfall, Flash Flooding, & Stormwater Runoff

Meteorologist Anthony Phillipswx4sno@bsu.edu | 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

Ridge

Trough

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 floodingIf 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: wx4sno@bsu.edu