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Thread: Wave Alert: Nov 21-22 (Thu-Fri)

  1. #1
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    Wave Alert: Nov 21-22 (Thu-Fri)

    A "backside slider low" as Doug Armstrong use to call them, is forecast for late day Thu Nov 21 and morning of Friday, Nov 22:
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    So called because they "slid in on the backside of the mountain range". Strong ENE offshore winds. This setup of a strong NE to ENE is ideal for a cross country wave hop to the Bay Area, and possibly return. Gunard & I flew similar conditions from Goat to St. Helena, then in mild wave down the Napa valley to nearly the Delta, cross at 17K to Diablo. And, if you are more adventurous, you could then hit the waves at Golden Gate, Santa Cruz range, Fremont and return to Diablo. If this holds, and with forecast wind speeds of 50knots at 18K, the return upwind jump from Diablo to Napa would cost little.

    FNX is in the shop, but this flight is at the top of the to do list.

    Retirees, we need conscripts, I mean data points!
    Kemp
    ASH-25 (FNX)

  2. #2
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    Re: Wave Alert: Nov 21-22 (Thu-Fri)

    "backside slider low" one of my favorite type of low pressure systems.

    Hopefully, this forecast will hold. Standing by...
    Pete Alexander -- 98

  3. #3
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    Re: Wave Alert: Nov 21-22 (Thu-Fri)

    I always loved the colorful descriptions that Doug came up with, to help us to understand the atmosphere. I too heard him announce the approach of the backside slider on more than one occasion, and when I pressed him to explain it in terms of the upper air, it was easier to understand. I translated the atmospheric movement into this:

    The Low pressures (at our latitudes) are routinely on the north side of the jet stream, as the jet wraps around the lower half, spinning the massive vortex of air in a counterclockwise rotation. A dust devil of ascending air that is 300 miles wide. When the west to east jet stream gets a split and the low starts to become a cutoff low, it heads southward, and over our area is coming at us from the North with winds from the NE as the low passes overhead.

    I see the low passing overhead on Wednesday, and the winds on the NE side of that low (being from the NE) will be here on Thursday.

    But, the low is often quickly absorbed by the Pacific high as it moves southward and the jet stream that was wrapping around the sough side will dissipate. That happens in the late summer or early fall, but now that it is getting closer to winter it may hold together.

    This 300 mb chart show the fcst position for Thursday at 5 PM PST
    You can see there is still some strong energy from the lower part of the polar jet, thus it's holding together pretty well.
    The added benny of having the air from the NE is that it is often dry air. After the rain on Tue and Wed, I'd expect a drying out.

    I wish I were available to fly this week.


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    Peter Kelly

  4. #4
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    Re: Wave Alert: Nov 21-22 (Thu-Fri)

    As of this morning's charts, Thu afternoon thru Fri afternoon's wave outlook has improved, with consistent direction winds at 20-25knots at 850, 30-35 at 700mb, and 40-50 at 500mb AND running all day on Friday. However (Peter K, pls comment..) I have never seen such easterly direction winds, like 050 swinging to 090 at 850mb. The June 12, 2010 flight where FNX flew down the Napa valley had these winds from 040, but only at 850mb, it turned to direct northerly higher up:
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    Again, this is June 12, 2010, NOT the current forecast. I don't have any historic data on this situation, but it seems that this upcoming wave could be an unusually good wave because:
    - The winds are orthogonal (right angle) to the Mendo ridgeline and the Napa valley.
    - The winds are modest, I mean, 40-50 knots at 18K, so penetrating upwind is disproportionally better than a 70 knot wind.
    - The wave could work through out the day on Friday
    These are the Fri morning charts:
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    I don't want to get overly excited, so let's wait for the RASP, including for the North Bay. The only increased risk is that if you don't find the wave, you'll have a 15-20 knot headwind to get back to WSC. So, get established in the wave first, THEN release.
    Last edited by Kempton; 11-19-2013 at 08:47 AM.
    Kemp
    ASH-25 (FNX)

  5. #5
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    Re: Wave Alert: Nov 21-22 (Thu-Fri)

    Seems to me we are dealing with too small of an area.
    The pressure gradient north to south will be strong in a few areas, but it is not very wide spread and thus difficult to forecast or it will be difficult for the models to reliably predict the wave lift - IMHO - but they are good models, but the data going in can't be changing after the model runs or the fcst becomes invalid.

    Here are two charts - I will comment after I upload them...
    They are both for Thursday at 5 PM
    Top one is 500 mb - 18,000 ft
    Lower one is 700 mb - 10,000 ft

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    Last edited by Peter Kelly; 11-19-2013 at 10:05 AM.
    Peter Kelly

  6. #6
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    Re: Wave Alert: Nov 21-22 (Thu-Fri)

    On that first chart, the top one, mostly dark blue - that is for Thursday at 5 PM at 18,000 feet - 500 mb

    The yellow snake is the high level of upward vorticity. It is showing the line of the front as it is overhead the Eastern border of Montana, cuts diagonally across Wyoming, and over to Northern CA. Very graphic!
    But the point is not the upward velocity, but the pressure gradients. The isobars are sharply curved over Nevada, and that could change easily, and there is little pressure gradient due east of the Low. Along the coast from the center of the low ( off the coast of Santa Barbara) to the OR/CA border on the coast there are only two pressure lines - not much of a gradient, thus not much of a wind ( east to west) but that is as Kempton noted on the models. "....he winds are modest, I mean, 40-50 knots at 18K, so....."

    The lower chart in the previous post is the 10,000 ft level for the same time - Thursday at 5 PM.
    Certainly a tight Low with four isobar lines between the bay area and northern ORegon, indicating strong winds east to west and lots of vertical velocity as noted by the colors,

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    but the lines have sharp bends and they could easily smooth out as the ridge to the NE of us becomes weaker or the low to the SW of us becomes weaker, etc.

    It is all very changeable and difficult to predict more than 12 hours in advance.

    Best to curb your enthusiasm.

    Let's see what develops.
    Last edited by Peter Kelly; 11-19-2013 at 10:08 AM.
    Peter Kelly

  7. #7
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    Re: Wave Alert: Nov 21-22 (Thu-Fri)

    Quote Originally Posted by Peter Kelly View Post
    It is all very changeable and difficult to predict more than 12 hours in advance.

    Best to curb your enthusiasm.

    Let's see what develops.
    For us non chart readers I find the discussion very informative, wish I had more weather background.

    I am hoping to get a wave flight in, and was even optimistic enough to load the WSC Wave window airspace file in my flight computer, and made sure it has a transparent colored fill per PK & 6DX recommendation in the WSC website Wave Window pages.

    OK, perhaps too optimistic, but at least it is loaded and tested, ready to activate when needed.

    Thanks for keeping the weather forecasting on going, I have curbed my enthusiasm, but got the OK for a couple days off from the 'boss'. (Yes, I am retired).
    ~ Larry Roberts

  8. #8
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    Re: Wave Alert: Nov 21-22 (Thu-Fri)

    I've not given up... However, I'm starting to lean towards Friday. I'm hoping the next run of the models help to support a fine wave day on Friday. Staying tuned...
    Last edited by Pete (98); 11-19-2013 at 09:45 PM.
    Pete Alexander -- 98

  9. #9
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    Re: Wave Alert: Nov 21-22 (Thu-Fri)

    A "backside slider low" sounds delicious. Can I get fries with that? I find the discussion tasty as well - in spite of the curbed enthusiasm. If Friday continues to look good, I think I'll order up a tow to go from the Williams Soaring Center grill - errh I mean Pawnee.
    Like Larry, I am a confirmed 'non-chart reader' but enjoy the thoughts of those who are bold enough to do so. If it weren't for threads like this one, most of my decisions to fly would be based on the Cal model.
    Jim D. - 1B

  10. #10
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    Re: Wave Alert: Nov 21-22 (Thu-Fri)

    You guys got my attention. I am starting thinking about trying from Byron on Friday.
    Both Thursday and Friday looking good for velocity and wind direction on the NAM. The wind seem to drop off quickly south of Mt Diablo, so I may try to fly north. If you guys planning to try to fly south to Mt Diablo, the ideal wind direction for Diablo is N to NE. I dont recall ever flying in wave in winds further east than NE, but I expect it will still work well and perhaps the whole range south of Mt Diablo as well. The only problem with the Diablo wave is that the more easterly wind the further west the wave is which usually puts it right in class bravo. If you arrive there with a comfortable altitude above the 10K ceiling than not a problem, further low you need to look for it between Wave1 waypoint (in Hollister database) to the edge of class bravo and hope for the best. If not found than go to Wave3 which is downwind of Los Vaqueros towers and usually has good wave as well and far enough from class bravo. The other challenge with Mt Diablo is Norcal Approach. While not required, they expect you to call them. You will be assigned a discrete squawk code and will be handed from one sector to another. Sometimes it goes smooth, but last time when I tried to fly further north and south above 10K they asked me to maintain below 10K.
    Kempton, what do you plan to fly?

    Ramy

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