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Thread: Post Frontal again this week?

  1. #1
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    Post Frontal again this week?

    Similar pattern to the week before last.
    Coolest days earlier, warming later in the week.
    Fcst high at Sacto is 63 deg for Wednesday.
    68 Thurs
    70 on Fri
    74 sat

    Precip on Tuesday/Wednesday and it moves towards the mountains on late Wednesday.
    Flow from the N on Thursday.
    Trouble is, things may be 12 hours out of sync, with the front passing at the "wrong time".
    Take a look at BLIP tonight - since it will give us some idea about what might happen on Thursday.
    Peter Kelly

  2. #2
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    Re: Post Frontal again this week?

    Good point on front passing at the "wrong time". This is the best way to describe when the front passes during the day vs at night. When it passes at night it usually promise a classic post frontal day the next day. When it passes during the day, you often get short window of classic post frontal in the late afternoon followed by too dry the next day. Will see how this one will turn out.

    Ramy

  3. #3
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    Re: Post Frontal again this week?

    Update 10 am Tuesday.

    BLIP is available for review. Looking good for Thursday. I may fly - but surface wind right now is fcst to be stroing from the north - based on these soundings - so that may cancel my plans. I'll look again tomorrow (but I may or may not post - since you can look at blip and rasp on your own).

    Here are the sounding forecast for Thursday...
    (You can get to these by following instructions on the VSA weather page)

    Look carefully at each sounding below. Jump back and forth. Start out with the first one at 11 AM...

    We'll start out with high cirrus (red and blue close at high altitude), but thinning as the day progresses...

    Wind is backing from N to NNW between 5 and 10,000 ft (an indication of frontal passage)

    Even at 11 am it is unstable up to about 2,500 ft, with a trigger temp of 54 F (12 = 54F)

    At 2 PM trigger temp is 61F (16 = 61F) and you can see good thermals up to 5,000 over WSC at that time - but air is dry so you may not see the cu (pretty big spread between red and blue lines below 10,000 ft).
    That concludes today's lesson!

    Here is the 11 AM sounding (as fcst for thursday) ...
    Click image for larger version. 

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    2 PM
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    5 PM
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    Last edited by Peter Kelly; 03-19-2013 at 10:42 AM.
    Peter Kelly

  4. #4
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    Re: Post Frontal again this week?

    I actually see a classic "front passing at the wrong time" scenario, as Peter suggested in his first post. Front will be passing during the day on Wednesday, and by Thursday it is already too dry and stable to provide a classic post frontal valley day. I agree with Peter that it is looking good for Thursday, but the question is how good. The current forecast is a far cry from what I would expect for a spring post frontal day... No cu, and relatively low BLTop (7K-8K) over the mountains. The valley will likely be soarable as Peter says, but I dont see it quiet getting to 5K. At least this is what the NAM blipmap is showing now. It can still change significantly between now and Thursday.

    Ramy

  5. #5
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    Re: Post Frontal again this week?

    I agree with Ramy that the "front" will pass on Wednesday, but I don't know if Ramy is using the models available at the moment (as of noon on Tues).
    In any case we would nat have "post frontal soaring conditions" with this "front", even if it came through at 9 AM because it dissolves as it passes. The low (and or front) does not hold together, and pressure patterns shift too much.

    Here are current models showing the front as is shown in the forecast models that are current (as of now)...

    5 AM WED
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    9 AM WED
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    1 PM WED
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    4 PM WED
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    Peter Kelly

  6. #6
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    Re: Post Frontal again this week?

    AND THE LAST COUPLE...

    7 PM WED
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    9 PM WED
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    So with all of that said, the thing to do is look at the BLIP and then the RASP and see what will happen.
    The BLIP and RASP take most things into account - more than we can do by looking at models, in any case.

    And always keep this in mind....

    The fcst is always most accurate 15 minutes before you launch... you can quote me on that.
    Peter Kelly

  7. #7
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    Re: Post Frontal again this week?

    Update Wednesday morning

    I spent the last several hours revising the VSA wx page... better format of the info on the page and provided a better tool for looking at fcst soundings...
    http://www.valleysoaring.net/?page_id=18

    BLIP and RASP are available for tomorrow (for Thursday)

    Here is the sounding for 3 Sisters at 2 PM (for Thursday)
    I put the cursor on 61 deg at valley floor altitude, hit the sounding about 3,000 ft - no cu clouds. Sure enough, the wind is blowing. It all agrees with what we see on the RASP. Based on all of this, I will not fly tomorrow (Thursday).

    Click image for larger version. 

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    Last edited by Peter Kelly; 03-20-2013 at 03:34 PM.
    Peter Kelly

  8. #8
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    Re: Post Frontal again this week?

    Update Wednesday night.

    BLIP shows it should be nice soaring on Saturday - for those pilots that are available to fly on Saturday.
    I won't be able to make it myself, but I thought I'd mention it.
    Peter Kelly

  9. #9
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    Re: Post Frontal again this week?

    Peter,

    It will be interesting to see if the BLIP map holds up for Saturday. I'm a little skeptical with the high pressure building in so strong on Friday. I hope, the BLIP is correct. Time will tell...
    Pete Alexander -- 98

  10. #10
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    Re: Post Frontal again this week?

    But what about Friday?
    Hint: I am sure if Kempton was paying attention he would only be looking at Friday...

    Ramy

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