Author Topic: A graphic example of orographic enhancement...  (Read 434 times)

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Offline SLOweather

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A graphic example of orographic enhancement...
« on: February 13, 2017, 11:16:55 AM »
If you don't already know, orographic enhancement is the increase in rainfall caused by clouds getting pushed up mountains by wind to higher, colder less atmospheric pressure altitudes, There the water vapor more easily precipitates out as rain. It is also seen at a lesser scale as the wreath of clouds around the top of a mountain.

Around here, I'm most used to seeing the effect indirectly, in the decrease in rain totals the farther a weather station in San Luis Obispo is from the Irish Hills.

However, last Friday, I experienced it more directly. 2 others and I were on a trip from San Luis Obispo to Santa Barbara CA in my truck. To save a little time, I took CA 154 along Lake Cachuma to San Marcos Pass and then down into SB. We traveled in rain most of the time.



The red part of the trip was along the base of the north side of the Santa Ynez range. Along there, precipitation was mostly drizzle.

As soon as we went through San Marcos Pass and started descending the south side if the range (the blue part), we were in fog (the clouds pushed up against the moountains) and the precip changed to fat raindrops.

We had just discussed orographic enhancement while passing by the lake, and why that affected the runoff into Lake Cachuma so it was pretty cool to actually experience it.

 

Offline Jstx

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Re: A graphic example of orographic enhancement...
« Reply #1 on: February 13, 2017, 11:59:36 AM »
Orographic enhancement (gotta pronounce that correctly... :twisted:) is sometimes mentioned around here in the EWX WFO AFD's.
There is a pronounced uplift called the Balcones Escarpment that crosses the NW part of San Antonio (and extends far to the west and northeast from there). It's not a mountain, but the elevation rises sharply 2-300 feet on the face of it, then up to ~1500? feet more within some miles inland.
With a prevailing, moist SE wind off the Gulf, under some conditions this OE is enough to trigger, or intensify, TSRA activity; sometimes markedly so.
The region is known as the "Flash Flood" center of the US/world.
There is a three WX condition factor that can produce devastating floods.
IIRC, it happens when an upper Pacific SW flow/LLJ collides with saturated high CAPE SE Gulf of Mexico low level flow, and they both get jammed by a NW cold frontal boundary coming down.
What might just be an ordinary wet front goes  hyper and forms a massive squall line/cyclone/MCS that can dump huge amounts of precip. Onto a landscape with many arroyos and rocky soil (somewhat like the CA area above).

Offline Farmtalk

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Re: A graphic example of orographic enhancement...
« Reply #2 on: February 13, 2017, 12:12:07 PM »
West Virginia has quite the orographic phenomenon in this small state.

With systems generally moving in from the northwest/west, clouds and precipitation move up the Allegheny Mountains. On the leeward side of the mountain chain, almost less than half of the yearly precipitation that the upslope side receives is recorded, as there is a good-sized rain shadow in the eastern panhandle.

I once was on the upslope side and hiked from one side of the mountain to the leeward side. It rained on one side - it did not on the other.  8-)

That is very cool that you go to experience that.  :-)
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Online waiukuweather

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Re: A graphic example of orographic enhancement...
« Reply #3 on: February 13, 2017, 01:26:41 PM »
You should do the drive across the main divide mountains of NZ's south island
where rain goes from an annual 8m to only 0.6m in a relatively short distance :)

but kewl you got to see if first hand like that :)
« Last Edit: February 13, 2017, 01:32:29 PM by waiukuweather »

Offline Scalphunter

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Re: A graphic example of orographic enhancement...
« Reply #4 on: February 13, 2017, 02:37:59 PM »
That effect well known in SE Alaska where the moist air coming off the gulf is lifted up over the coastal mountains. 160 inches n year in Ketchikan, 220 inches an year in Port Sir Walter, 90 inches an year in Juneau and the other side of the range in BC gets about 6 inches as the moisture gets wrung out.

John

Offline Pagosa Weather

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Re: A graphic example of orographic enhancement...
« Reply #5 on: April 04, 2017, 05:53:58 PM »
Ala... Opid's camp (just on the south side of Mt. Wilson, above Los Angeles). Recorded over 100" of rain in 2004-5. I lived and worked in those mountains for the Forest Service that year, and it was incredible.

http://articles.latimes.com/2005/feb/25/local/me-soak25