20% chance of thunderstorms

[qedcook]

How much do you trust the National Weather Service page?

http://www.wrh.noaa.gov/slc/flashflood/

There is a 20% chance of thunderstorms in Capitol Reef but the flash flood page says 'Dry'. What do you do?

[DOSS]

How much do you trust the National Weather Service page?

http://www.wrh.noaa.gov/slc/flashflood/

There is a 20% chance of thunderstorms in Capitol Reef but the flash flood page says 'Dry'. What do you do?

Go early and be out by say 3 and you should miss the thunderstorms :).. in my experience in Capitol Reef most of the thunderstorms when they do show up happen in the afternoon. and 20% doesn't give me that much to worry about :) You could call the visitors center as well and chat them up about the weather patterns recently as well.

[ghawk]

if it's 20-30% I take a good look at the sky and do a gut check before I head in. higher than that I don't go.

[bshwakr]

__

[ocanler]

I'm planning to do Fat Man Misery this Sunday but the weather looks like 20% chance of thunderstoms after noon, under partly cloudy condition sky for the day.
How prone is this slot to flash flood? If we take an early start (7am) from the Checkboard Mesa, how long does it take to go through the canyon before starting the climb back up out of the East Virgin river? There are 7 of us. Can we be out of danger by noon? I'll keep monitoring weather forecast and will make the go/no go decision that morning.

Thanks

[canyoncaver]

I'm sure you realize this, but the final go/no go decision really takes place at the first rap or climb that you can't reverse, not at the trailhead. 20% is about as good as you're going to get this time of year. 30% and moderate flash flood danger is the highest that I am willing to risk, and that comes with a pretty serious conditions re-evaluation at the first rap.

[tylerhirshfeld]

Just my opinion,
I would go. I wouldnt go if it was greater than 30% chance of showers, but Fat Mans has a lot of exits between the slots, and I think you would be safe as long as you watch what is over head. As far as being out of the canyon by noon with 7 people, probable more like 2 or 3 oclock. I can't make any promises though.:roll:

[ratagonia]

Just my opinion,
I would go. I wouldnt go if it was greater than 30% chance of showers, but Fat Mans has a lot of exits between the slots, and I think you would be safe as long as you watch what is over head. As far as being out of the canyon by noon with 7 people, probable more like 2 or 3 oclock. I can't make any promises though.:roll:

Consider the canyon - that is the main thing. Fat Mans has almost zero extended narrows, only a short section at the end. So it is a good choice in questionable conditions. In fact, for Fat Mans this time of year, I would HOPE for rain on the way out, otherwise Fat Men like me suffer the 3 hour hike out.

Tom

[mdd]

When I see these in-between forecasts, I take a look at the forecast discussion for the region. On the NWS page the link is in the lower right of the forecast page (under Additional Forecasts and Information). Here is the direct link (http://forecast.weather.gov/product.php?site=NWS&issuedby=SLC&product=AFD&format=CI&version=1&glossary=1) that covers pretty much Utah.

Based on that I'd go, probably leaving early. And of course no matter the forecast always watch the sky and check and doublecheck the weather before you commit to that first rappel.


M

[Brian in SLC]

I've seen flash flooding at a 20% chance of precip/t-showers forecast. Kind of hard to predict for a long day.

You could be super conservative and wait until there's very little to no chance, but, you probably wouldn't get much canyoning in.

Keep an eye peeled, look at the area forecast discussion stuff, learn how to read the weather, take a best guess and err on the conservative side. A nice hike in the rain is better than gettin' hit by a flash flood at the wrong time. Also, I take a look at how far upstream the catch basin is, and, how far away a burst of rain could effect me. In the case of Fat Man's, should be able to get a decent read prior to popping in. I'd probably go for an earlier start, like, first light. Better to take a nap near the river prior to the hike out, than, get an hour or two more sleep in the morning, hike all the way in, have iffy weather, then have to decide what to do.

Mackeral sky, three days dry? Never long wet, never long dry. Interesting cloud cover over SLC this morning. Spotty weather for the next few days. I'm planning on getting sprinkled on, but, nothing sustained. Won't be in a slot canyon though...

Take care.

[tylerhirshfeld]

Consider the canyon - that is the main thing. Fat Mans has almost zero extended narrows, only a short section at the end. So it is a good choice in questionable conditions. In fact, for Fat Mans this time of year, I would HOPE for rain on the way out, otherwise Fat Men like me suffer the 3 hour hike out.

Tom

Ditto

[qedcook]

My real question is do you trust the national weather service's flash flood page to be accurate?

[ratagonia]

My real question is do you trust the national weather service's flash flood page to be accurate?

Absolutely Not.

WTF QedCook? The NWS's page is the best guess based on a lot of science, and is the best prediction available. Which is still not really all that good.

Is there some reason to view it differently? It is a prediction, no more.

Tom

[ocanler]

Great advice from all. Thanks!
Looks like it's a fairly "safe" canyon based on your comments.
However, if weather looks too sketchy for the afternoon, our fall back plan would be to do Birch Hollow instead in the am. With an early start, we should be done by noon in Birch. We had plan to do Birch Hollow on Monday, so we'd just flip-flop them.

O

[spinesnaper]

The other issue with the 20% chance of precipitation is the meaning of this term. My understanding, and I am always prepared to be corrected, this does not mean that there is a 20% chance of precipitation . What it means is that there is 100% chance of precipitation over 20% of the forecast area.

The other weather tool that has been great is my Radarscope Iphone app. Here is a sample screen shot:

http://www.basevelocity.com/RadarScope/screenshots/photo.jpg

This is real time doppler radar on your iphone provided you have cell service. I have used the data for go/no go decisions on many an early morning. That canyon will always be there.

Ken

[scubabryan]

The other issue with the 20% chance of precipitation is the meaning of this term. My understanding, and I am always prepared to be corrected, this does not mean that there is a 20% chance of precipitation . What it means is that there is 100% chance of precipitation over 20% of the forecast area.

Ken

The first and only time I ever heard it defined this way was when I was watching the video at the back country desk to sign up for express permits for Zion. I shared this with a few people and they all looked at me like I was an idiot.

[CarpeyBiggs]

The first and only time I ever heard it defined this way was when I was watching the video at the back country desk to sign up for express permits for Zion. I shared this with a few people and they all looked at me like I was an idiot.
um. kind of. see here - http://www.srh.noaa.gov/ffc/?n=pop


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%. )

[ocanler]

Ok, it looks like NOAA has changed the % chance of thunderstorms from 20 to 40% for East Zion for Sunday after noon. I'm thinking Fat Man might not be a wise choice, even if the canyon offers escape routes.

How about Birch Hollow in the am instead? If starting at 7:00 and at the trailhead, how long does it take do go through all the rappels and to the merge with Orderville? I am thinking we should be there by noon at the latest. Is Birch Hollow prone to flash flooding, are there escape routes?

Any info would be much appreciated.

Gracias

O

[Brian in SLC]

The other issue with the 20% chance of precipitation is the meaning of this term. My understanding, and I am always prepared to be corrected, this does not mean that there is a 20% chance of precipitation . What it means is that there is 100% chance of precipitation over 20% of the forecast area.

I don't think so. I think it means that there is 20% chance of rain at any given point in the area for that time range.

http://en.wikipedia.org/wiki/Probability_of_Precipitation

[scubabryan]

Ok, it looks like NOAA has changed the % chance of thunderstorms from 20 to 40% for East Zion for Sunday after noon. I'm thinking Fat Man might not be a wise choice, even if the canyon offers escape routes.

How about Birch Hollow in the am instead? If starting at 7:00 and at the trailhead, how long does it take do go through all the rappels and to the merge with Orderville? I am thinking we should be there by noon at the latest. Is Birch Hollow prone to flash flooding, are there escape routes?

Any info would be much appreciated.

Gracias

O

I've been in Birch a few times during the rain and it's never been an issue for us but YMMV. The last section of rappels in Birch (5 or so) is the tightest and there is no escape if a flash flood were to happen. If your group is experienced and depending on traffic in the canyon, there should be no reason your group shouldn't be to the canyon junctions by noon. Fastest I've done it with 3 people has been 1hr 40mins but again, it depends on the experience of your slowest person. However, if there is substantial rain, you will be hating life climbing up the steep, dirt slope to get to the upper part of the gulch (if you plan on exiting up Orderville). That slope turns to a muddy mess. Plus the 4x4 road getting you back up to the corals could be in bad shape with lots of rain (mud), not to mention the soupy, muddy mess the North Fork Rd will be driving the 6 mile dirt portion back to the paved road.

[canyoncaver]

I always understood it that 40% means that 40% of the forecast area will receive rain. Saying that a certain point within the forecast area has a 40% chance that it actually will receive rain, is essentially saying the same thing.

[Brian in SLC]

I always understood it that 40% means that 40% of the forecast area will receive rain. Saying that a certain point within the forecast area has a 40% chance that it actually will receive rain, is essentially saying the same thing.

Nah, not really. Your first statement would be that there's a 100% chance that it WILL rain, but, only in 40% of the forecast area. So, if the forecast was spefically for that area, it would be 100% chance.

But, the calculation for the whole area would yeild a 40% chance, yeah, but, you can't really know that unless the forecast discussion would lead you there.

Slightly different, methinks.

But, "whatever"...ha ha.

Its all an edjumacated crapshoot anyhow.

[mdd]

Nah, not really. Your first statement would be that there's a 100% chance that it WILL rain, but, only in 40% of the forecast area. So, if the forecast was spefically for that area, it would be 100% chance.

But, the calculation for the whole area would yeild a 40% chance, yeah, but, you can't really know that unless the forecast discussion would lead you there.

Slightly different, methinks.

But, "whatever"...ha ha.

Its all an edjumacated crapshoot anyhow.

In other words, the latter is probabilistic, but the former isn't - so they aren't really saying the same thing, all because of the word 'will' in the former.

M

[Brian in SLC]

Yeah, something like that, Mike. If there were a 20% chance of rain in an area, and, it meant that 20% of the forecast area would recieve rain, then folks shouldn't be in a slot canyon anywhere downstream of that area because that interpretation would mean that it would rain, with certainty. But, I don't think it does, so...

Anyhoo. Take care out there!

[spinesnaper]

Ocanier

Keep in mind that we are now entering monsoon season for the Colorado Plateau. Thundershowers are common and the weather can be unstable. Those canyons will always be there so you might consider alternative plans if the weather looks dicey. Remember that flash floods are not simply the water coming down canyon. It also involved water and debris coming down the sides of canyons. These are inherently dangerous environments and your safest choice is to postpone your excursion to a date with better weather.

Ken