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PD3.5 Lite Obs and Nowcasting


Bob Chill

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Looks like I was posting in the wrong thread this morning.  Here's the 06z Swiss model.  It has been good with temps the couple of times I've looked at it, and it really shows the benefits of elevation.

7trBXJ5.png

I also put up last night's RGEM ensemble, which basically looks like a low-res version of the above map.

https://www.americanwx.com/bb/topic/50960-weekend-snowfestrainfestmixed-mess/?do=findComment&comment=4826411

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17 minutes ago, clskinsfan said:

We need that though. We need it to be heavy with the marginal temps.

We do, but the timing now is pretty bad.  Precip commences at the warmest part of the day and ends about the time we enter diurnal cooling.

See altocumulus overspread to my west, but still under clear skies and blazing sun.  35F according to nearest PWS, but feels much, much warmer.

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P&C shows 5-9”...currently under a Winter Storm Warning.  27 degrees and mostly cloudy.  

URGENT - WINTER WEATHER MESSAGE
National Weather Service Pittsburgh PA
408 AM EST Sat Feb 17 2018


MDZ001-WVZ512-514-171715-
/O.CON.KPBZ.WS.W.0003.180217T1900Z-180218T0900Z/
Garrett-Eastern Preston-Eastern Tucker-
Including the cities of Mountain Lake Park, Oakland MD,
Grantsville, Terra Alta, Rowlesburg, Hazelton, Davis, Thomas,
and Canaan Valley
408 AM EST Sat Feb 17 2018

...WINTER STORM WARNING REMAINS IN EFFECT FROM 2 PM THIS
AFTERNOON TO 4 AM EST SUNDAY...

* WHAT...Heavy snow expected. Total snow accumulations of 5 to 7
  inches, with localized amounts up to 8 inches, are expected.

* WHERE...In Maryland, Garrett County. In West Virginia, Eastern
  Preston and Eastern Tucker Counties.

* WHEN...From 2 PM this afternoon to 4 AM EST Sunday.

* ADDITIONAL DETAILS...Plan on difficult travel conditions. Be
  prepared for significant reductions in visibility at times.
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27 minutes ago, Fozz said:

What's the difference between the Ferrier map and the normal 10:1 map? Why the huge disparity?

      The NAM keeps track of what percentage of falling hydrometeors are snow and sleet.    It's called the % of frozen precip.    As the model is running, it continually multiplies QPF x % of frozen precip to get its "snow" (really snow + sleet) accumulation.    So in a situation with 0.6 liquid and 70% snow flakes or 70% snow and sleet, you get  .6 x .7 = .42 liquid as snow+ sleet.    Slap a 10:1 ratio on it, and you have 4" of snow, even though we all know that a situation with 70% of what's falling from the sky being snow (or sleet) is not likely to pile up on the ground.    That's what the maps with snow depth are useful in marginal situations, as they realize that snow isn't really accumulating.

       The Ferrier maps take advantage of the NAM microphysics and apply a ratio based on the rime factor which accounts for pure snow (low value of rime factor) or heavily rimed snow (high rime factor).    The more rimed the snow (supercooled water is accreting on the ice nuclei), the less it will stick, and the Ferrier method cuts down on accumulations.    

       It's clear here based on the Ferrier and snow depth maps that the NAM doesn't think that this is a big accumulating event despite the generous 10:1 maps.

 

   

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Call me a pink elephant dancing in a toto but I actually like the northward trend of the heavy southern band over the past 24 hours. I'll gladly take the heavy rates and worry about precip type later. Anything to get me up to a 1-2% chance of seeing the T word rather than a 0% chance.

ETA: I also like the radar trends over Tennessee right now for the heavy bands to be over/closer to the metro area.

PS: (Hopefully not too bantery)

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4 minutes ago, Interstate said:

So what model should we look at or are you talking about all snow maps in general 

      I'm saying to use the 10:1 maps in conjunction with the snow depth maps (and the Ferrier map for the NAM3).     With the setup is "right", they'll be in close agreement.   When they differ a lot, use the 10:1 maps with extreme caution.     I explained it a little bit a few posts back.     

      It doesn't mean that the 10:1 maps won't end up being right if the model has some errors with the setup, but those maps are not consistent in these marginal, complicated setups with what the model really thinks is going to accumulate.

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2 minutes ago, high risk said:

      I'm saying to use the 10:1 maps in conjunction with the snow depth maps (and the Ferrier map for the NAM3).     With the setup is "right", they'll be in close agreement.   When they differ a lot, use the 10:1 maps with extreme caution.     I explained it a little bit a few posts back.     

      It doesn't mean that the 10:1 maps won't end up being right if the model has some errors with the setup, but those maps are not consistent in these marginal, complicated setups with what the model really thinks is going to accumulate.

Isn't the "Positive Accumulated Snow Depth Change" a better indicator of accumulations? On TT, I mean.

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5 minutes ago, nw baltimore wx said:

Isn't the "Positive Accumulated Snow Depth Change" a better indicator of accumulations? On TT, I mean.

     yes, and in the post of mine that you quoted, I mentioned the snow depth maps (I should have used the full "accumulated snow depth" term).     They sometimes run a bit low, so I don't treat them as gospel, but I always view them in conjunction with the 10:1 maps.    When they're radically different, the model is trying to tell us that it's not nearly as favorable for big accumulations as the 10:1 maps show.    In the "no doubters", they'll be in the same ballpark.

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1 minute ago, high risk said:

     yes, and in the post of mine that you quoted, I mentioned the snow depth maps (I should have used the full "accumulated snow depth" term).     They sometimes run a bit low, so I don't treat them as gospel, but I always view them in conjunction with the 10:1 maps.    When they're radically different, the model is trying to tell us that it's not nearly as favorable for big accumulations as the 10:1 maps show.    In the "no doubters", they'll be in the same ballpark.

I'm hoping that if you and I actually stay all/mostly snow, this will be a good test comparison of the 3.  This is the first winter where they've been widely available, but we haven't really had a storm where they can be evaluated.  

39/22 here

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