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Thread: flash flood likelyhood
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03-03-2009, 05:58 PM #1
flash flood likelyhood
How likely are flash floods this early in the spring. I was under the impression that they are extremely unlikely, even with a 20-30% chance of light rain this weekend.
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03-03-2009 05:58 PM # ADS
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03-03-2009, 06:09 PM #2
Re: flash flood likelyhood
Originally Posted by qedcook
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03-03-2009, 07:04 PM #3I had always thought that it meant that it was 20-30% chance for that region?!
Precipitation Probabilities (PoP): It is defined as the likelihood of occurrence (expressed as a percent) of a measurable amount of liquid precipitation (or the water equivalent of frozen precipitation) during a specified period of time at any given point in the forecast area.
In more detail, here is what NOAA has to say (example on a specific local site):
Explaining "Probability of Precipitation"
Back to FAQ's (Frequently Asked Questions)...
Forecasts issued by the National Weather Service routinely include a "PoP" (probability of precipitation) statement, which is often expressed as the "chance of rain" or "chance of precipitation".
EXAMPLE
ZONE FORECASTS FOR NORTH AND CENTRAL GEORGIA
NATIONAL WEATHER SERVICE PEACHTREE CITY GA
119 PM EDT THU MAY 8 2008
GAZ021-022-032034-044046-055-057-090815-
CHEROKEE-CLAYTON-COBB-DEKALB-FORSYTH-GWINNETT-HENRY-NORTH FULTON-
ROCKDALE-SOUTH FULTON-
INCLUDING THE CITIES OF...ATLANTA...CONYERS...DECATUR...
EAST POINT...LAWRENCEVILLE...MARIETTA
119 PM EDT THU MAY x 2008
.THIS AFTERNOON...MOSTLY CLOUDY WITH A 40 PERCENT CHANCE OF
SHOWERS AND THUNDERSTORMS. WINDY. HIGHS IN THE LOWER 80S. NEAR
STEADY TEMPERATURE IN THE LOWER 80S. SOUTH WINDS 15 TO 25 MPH.
.TONIGHT...MOSTLY CLOUDY WITH A CHANCE OF SHOWERS AND
THUNDERSTORMS IN THE EVENING...THEN A SLIGHT CHANCE OF SHOWERS
AND THUNDERSTORMS AFTER MIDNIGHT. LOWS IN THE MID 60S. SOUTHWEST
WINDS 5 TO 15 MPH. CHANCE OF RAIN 40 PERCENT.
What does this "40 percent" mean? ...will it rain 40 percent of of the time? ...will it rain over 40 percent of the area?
The "Probability of Precipitation" (PoP) describes the chance of precipitation occurring at any point you select in the area.
How do forecasters arrive at this value?
Mathematically, PoP is defined as follows:
PoP = C x A where "C" = the confidence that precipitation will occur somewhere in the forecast area, and where "A" = the percent of the area that will receive measureable precipitation, if it occurs at all.
So... in the case of the forecast above, if the forecaster knows precipitation is sure to occur ( confidence is 100% ), he/she is expressing how much of the area will receive measurable rain. ( PoP = "C" x "A" or "1" times ".4" which equals .4 or 40%.)
But, most of the time, the forecaster is expressing a combination of degree of confidence and areal coverage. If the forecaster is only 50% sure that precipitation will occur, and expects that, if it does occur, it will produce measurable rain over about 80 percent of the area, the PoP (chance of rain) is 40%. ( PoP = .5 x .8 which equals .4 or 40%. )
In either event, the correct way to interpret the forecast is: there is a 40 percent chance that rain will occur at any given point in the area.
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03-04-2009, 03:51 AM #4Originally Posted by Scott P
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03-04-2009, 09:26 AM #5
Re: flash flood likelyhood
Originally Posted by qedcook
It is also possible to have thundershowers, and the usual "summer" form of flash flooding.
Perhaps what you should be more concerned with is spring flooding. It is warm out (here in Zion) and snow up high is melting and producing flooding. Subway? Might be in flood right now. Narrows? Peaking around 100 cfs at 3 am this morning.
Know what is above you in the watershed. Many canyons do not have higher elevation terrain feeding them, but some do (especially in Zion).
Tom
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03-04-2009, 09:29 AM #6
I love how Tom sticks an old geezer emoticon after every post...
It's only "science" if it supports the narrative.
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03-04-2009, 09:33 AM #7Originally Posted by rockgremlin
it seems like they made it just for me!
T
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03-04-2009, 09:37 AM #8
I was thinking.... greybread...
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03-04-2009, 08:19 PM #9
First picture is 30% and second is 70%...not really! It was a 70% forecast by NOAA for heavy rain in main canyon that day. It was nice in the other parts (30% of the canyon)!
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03-04-2009, 10:12 PM #10Originally Posted by Bo_Beck
with SNORKLES!
T
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03-04-2009, 11:13 PM #11
Hey Bo, are those the Behunin photos? If so, that was a crazy story with fortunate results.
Life is Good
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03-05-2009, 05:21 AM #12Originally Posted by Scott Card
If you look very closely at the second picture........................................... ............you wont see 2 rappelers simul rapping up near the top
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03-05-2009, 05:28 AM #13Originally Posted by Bo_Beck
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03-05-2009, 03:12 PM #14Originally Posted by Bo_BeckLife is Good
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03-08-2009, 10:46 PM #15
bo how was behunin flowing?? we have always wondered how those first few raps would be with water and that last one with the approach right down the water way it seems like a nice place to get swept off the edge!
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