flash flood likelyhood

[qedcook]

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.

[Bo_Beck]

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.

I learned something interesting recently. When NOAA predicts precipitation for a particular region, I had always thought that it meant that it was 20-30% chance for that region?! What it actually means is that 20-30% of that region will recieve a measurable amount of precipitation. Sooooo....Flash floods could definately be a reality depending on what area within that region recieves the precipitation?

[Scott P]

I had always thought that it meant that it was 20-30% chance for that region?!

Actually Bo, your first assumption was correct. This comes directly from the main site at NOAA:

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.

[Bo_Beck]

I had always thought that it meant that it was 20-30% chance for that region?!

Actually Bo, your first assumption was correct. This comes directly from the main site at NOAA:

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. [/i]

Thanks Scott, but I see a discrepancy in interpretation? If confidence is 100%, but exact geographic isolation is just so-so, or if the cell is measured to cover only 40% of the area, how can it possibly be the same as 50% confidence over 80% of the area? I would hate to think that 2 people getting different readings are simply combining the 2 readings and coming to a compromise? I would like to err on the side of the 100% confidence over a certain portion of the area (particularly if I'm in a slot canyon with large watershed). I would think it might be important to have a NOAA forecast for adjacent areas as well assuming that they are part of the watershed feeding the canyon I might be visiting? Not too sure if NOAA grids it out, but will now have to take a gander. Seems to me that the Antelope Canyon Tragedy sort of highlights what I'm trying to get at.

[ratagonia]

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.

Spring flashes occur with rapid warming perhaps combined with rain falling on snow. Requires very specific circumstances.

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

[rockgremlin]

I love how Tom sticks an old geezer emoticon after every post...

[ratagonia]

I love how Tom sticks an old geezer emoticon after every post...

"moses"

it seems like they made it just for me!

T

[Iceaxe]

I was thinking.... greybread...

[Bo_Beck]

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)!

[ratagonia]

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)!

A good day for training...






















with SNORKLES!

T

[Scott Card]

Hey Bo, are those the Behunin photos? If so, that was a crazy story with fortunate results.

[Bo_Beck]

Hey Bo, are those the Behunin photos? If so, that was a crazy story with fortunate results.

Yeah Scott! If you look very closely in the first picture you can see 2 rappelers simul rapping up near the top.

If you look very closely at the second picture........................................... ............you wont see 2 rappelers simul rapping up near the top

[Bo_Beck]

Hey Bo, are those the Behunin photos? If so, that was a crazy story with fortunate results.

Yeah Scott! If you look very closely in the first picture you can see 2 rappelers simul rapping up near the top.

If you look very closely at the second picture........................................... ............you wont see 2 rappelers simul rapping up near the top

Actually though...if you look closely at the second picture you can see a rappeler just under the right side of the tree about to touchdown into the swirling pool of water between the rock and the other hard place I was giving conditional belay at the time and was floating in water at the normal LZ for that final rap.

[Scott Card]

Hey Bo, are those the Behunin photos? If so, that was a crazy story with fortunate results.

Yeah Scott! If you look very closely in the first picture you can see 2 rappelers simul rapping up near the top.

If you look very closely at the second picture........................................... ............you wont see 2 rappelers simul rapping up near the top

Actually though...if you look closely at the second picture you can see a rappeler just under the right side of the tree about to touchdown into the swirling pool of water between the rock and the other hard place I was giving conditional belay at the time and was floating in water at the normal LZ for that final rap.

I think I see the legs in that second photo! That is just crazy. That would have absolutely scared the bajeebees out of me. Luck/skill was with you that day. I remain a wuss about flash floods.

[snap101]

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!