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When do you know it's going to rain?

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When do you know it's going to rain?

Postby overheadx2 » Wed Aug 29, 2012 7:04 pm

I saw a post a while back where some one stated that if the cumulus clouds aren't building by 10:00, the chance of a significant afternoon rain is pretty low. What are some relatively good early indicators that tip you off ahead of time? When do you guys start to feel pretty sure it's going to dump or not?



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Re: When do you know it's going to rain?

Postby kpeter » Wed Aug 29, 2012 7:14 pm

At the first sight of cumulus clouds, at any time, I think the odds go way up. I don't think I would put much stock in a 10am cut off. I'm fairly certain I've been in afternoon rain storms that are much more sudden than that.
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Re: When do you know it's going to rain?

Postby lostcoyote » Wed Aug 29, 2012 7:59 pm

red sky in morning
sailor take warning
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Re: When do you know it's going to rain?

Postby SSSdave » Wed Aug 29, 2012 8:07 pm

The 10am statement is worthless. Sierra summer thunderstorm weather is more complex than one might be able to size up with a few generalizations.

You'll know its going to rain when you can see rain in the distance and those clouds are moving in your direction. Best to Google "thunderstorms mountains", read some reasons why storm build over mountains, regularly read the Hanford and Reno NWS forecast discussions, and look at NWS radars on days when storms occur. Over time you will become more knowledgeable. Also frequently read what this Mammoth forecaster has to say.

http://mammothweather.com/
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Re: When do you know it's going to rain?

Postby Wandering Daisy » Wed Aug 29, 2012 8:42 pm

The northern Sierra, north of I-80 can have quite different weather than southern Sierra, particularly in Spring and Fall. The monsoonal flow from the south on the east side often does not get to the northern part of the Sierra. Pacific systems often hit the northern Sierra and miss the south.

It is going to rain when a huge black broiling mass of clouds is quickly moving up the valley headed right for you! My experience has been that storms seem to take a while to build up. First you have clear days. Next day more clouds. Next day nearly overcast. Next day rain. A big storm system in Baja usually means a higher chance of rain. For short trips, I find the weather reports pretty useful. Cumulous clouds are "fair weather clouds" and do not necessarily mean rain. More concerning are high stratus and "makeral" sky - those little puff dots all over - this usually means a front is moving through. But I agree, it is more complex than a simple rule of thumb.
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Re: When do you know it's going to rain?

Postby tim » Wed Aug 29, 2012 8:54 pm

As Daisy indicates, its also important to remember that your generalization applies only to convectional thunderstorms (monsoonal moisture). While that's the primary source of summer (Jul/Aug) rainfall in the southern Sierra, frontal rainfall can occur at any time of the day or night, and is far more common early or late season.

It can even occur in mid-summer as we found out on our trip last week (viewtopic.php?f=1&t=8265&start=0) when there was no evidence whatsoever of potential precipitation when we went to bed, but a huge thunderstorm occurred in the middle of the night with a full hour of rain, hail, thunder and lightning.
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Re: When do you know it's going to rain?

Postby overheadx2 » Thu Aug 30, 2012 2:03 pm

Thanks, I kind of thought that seemed a little simple. I guess once your there, it doesn't matter much anyway. Just put your head down and hike.
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Re: When do you know it's going to rain?

Postby lostcoyote » Thu Aug 30, 2012 5:11 pm

"Cumulous clouds are "fair weather clouds" and do not necessarily mean rain."

an exception to this is the humid monsoonal flow... where cululus clouds are the seed clouds for the thunderheads .

it is true that with the northern fronts, cumlulus clouds often follow a front.... but monsoon flow is not that normal front. it's moisture buildup as the cool pacific air meets up with the warm humid air usually in the afrternoons during this sort of flow.


so when will it rain....

well, if you hear thunder close by, you might want to hurry up and find a spot to hunker down.... or walk through it if you are not on top of a ridge, mountaintop, or pass, or even above timberline all exposed and such.... and if exposed above timber, don't set up camp next to a snag - lol


speaking of monsson flows - there's a small surge occuring yesterday & today.
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Re: When do you know it's going to rain?

Postby ucangler » Tue Sep 04, 2012 2:09 pm

Just came from humphrey's basin (will TR tonite or tomorrow when I catch up with work) and it rained and hailed on us when we where about a few hundred ft below the pass. Morning looked fine and as we climbed higher the clouds started coming in. I was personally looking for the grey globs and yes when those grey clouds came they let us know they were rain clouds. We hunkered down the first 20 minutes of rain by hugging the cliffs. About a few minutes after that it started to hail pea sized hail for a good 10 minutes. After the hail, we saw a break in the clouds and we decided to book it over the pass.

I think the thing you have to also consider color of the clouds. The white nimbus ones seem to be under the threshold point for rain while the grey nimbus ones were past h20 holding capacity and dumped rain on us.

My 3 cents.
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Re: When do you know it's going to rain?

Postby Herm » Sat Sep 08, 2012 11:16 pm

You know that it is going to rain when- after watching the forecast for a whole week, and the forecast being quite good for the whole week, then pulling the trigger to go on a trip, then the day before leaving the whole forecast changes to include 20% chance showers/thunderstorms for the duration of the trip - well, then you have an inkling that it might rain. My wife, being the optimist that she is, quickly points out that there is an 80% chance that it will not rain.
On a more serious note, I seem to sense the change in humidity, as I really start sweating noticibly more. Soon thereafter, rain often follows. Just happened this week on a trip to the high country above Sabrina Lake. And many of the most menacing of skies produced nothing more than visual delights. Ah, the range of light.
I am not in a hurry, so don't be hasty.
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Re: When do you know it's going to rain?

Postby ndwoods » Thu Sep 13, 2012 10:11 pm

Actually that statement is not so wrong....by 10 or noonish, if the cumulus are forming and are sharpedged on the top....that's a pretty good indicator you have rain coming. Cumulus clouds in the morning mean there is a good chance...you just gotta watch em. And if they get sharp edged and growing, that's a rain indicator. If they are fuzzy on the edges, then they are probably not rain clouds...especially at the top. If you see them towering up on the top and then they start blowing over that means the wind is blowing up there and they won't get sharp and form a thunderhead. If they continue to go up and flatten out...that's a thunderhead. And you'll know by then ....laugh....cuz you'll hear them in the distance...
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Re: When do you know it's going to rain?

Postby Cross Country » Sat Sep 15, 2012 7:43 pm

lots of good info here. I've seen it raining a half mile from me coming my way and it never arrived. At other times I've been rained on when one hour earlier I thought there was no chance of rain. There's an old saying about mountain weather. If you don't like the weather, wait a couple of hours. It'll change.
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