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December 11 Frontal Wave Snow Event


WxUSAF
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11 minutes ago, Scraff said:

In GooFuS we trust. Looking good!

DFBF0923-02F5-4217-A742-FD177B2460A6.thumb.png.2c0e1e3ae1ba32d8d6316b1b3d34be58.png

I’ll hug that for now. Again I won’t really start to feel confident until we’re inside 24 hours and even then only somewhat.  I’m not saying I don’t think this is a legit threat, just the hit zone is so narrow with these progressive waves and slight changes in the amplified of the wave, baroclinicity, or the location of the boundary can have huge impacts and none of those are easily modeled.  That said things look about as good as they can right now so I’ll stay cautiously optimistic we get our first widespread event. It would be nice to get everyone on the board before Christmas for a change. 

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Cmc and gfs are very similar. Good sign when globals are that close 72 hrs or less from an event. Euro will prob look similar. 

Blend of mesos and globals gives much of the area between .2 - .4 qpf as snow with some lollies .6+. I don't expect my yard to be efficient with stickage. Hopefully 1-2" if things break right. 

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You may be wondering why we are seeing such a difference on the precip post frontal between the Euro and the GFS. Has to deal with the shortwave we see riding through the south. Below we have the Euro's depiction and it is a very weak feature.

Euroshortwave.gif.6aec58e8eed2249af877aaa6070ddc3c.gif

But notice on the GFS it has a somewhat stronger shortwave. So what that is doing is providing some push back to the cold air (which we don't have on the Euro) in the mid and upper levels so we are seeing a better moisture feed over top the colder air. So basically it is raising heights somewhat in front of the shortwave in it's natural tendency to try to force ridging.

GFSshortwave.gif.6a65bbe3c76f0e05e9ffc74f7734b2ac.gif

You can see the impact this has when looking at the Precipitable water maps. Note with the Euros weaker shortwave we are seeing a pretty much straight line.

Euromoisture.gif.aae090f4a4a965c4ee1c569c4cad1301.gif

But note the slight bowing of that precipitable. we see on the GFS. This is due to the stronger shortwave providing more push back to the colder air. 

GFSmoisture.gif.83b7d7ae99a75f1a046421d69e980872.gif

 

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One thing that makes this a possible accumulating snow vs. a white rain scenario is what we are seeing in the lower levels. Below are the soundings for central Carroll county about 6 hours before we should see a flip. If you note the lower levels are torching.

 

42hrsounding.gif.a58b6c3289527513a0e9832d975d6820.gif

 

But this is 6 hours later just shortly before we would expect the flip to begin. Notice that the lower levels have cooled significantly. What we are seeing here is a dry air intrusion into the lower levels centered roughly on the 850 mb. What this is doing is dropping the dews (green line) below freezing for all but the surface. So what we have is good lift (far left diagram) in the moister air (700 mb and above) creating precip that is in turn falling through the drier air below and rapidly cooling it. Now checking soundings around the DC/Balt corridor all show a similar signature of a lower level dry air intrusion.

***Now I don't have the soundings for the Euro but looking over the different maps suggests to me that it also sees this lower level dry air intrusion.***

48hrsounding.gif.7b43e890879e879ac1547e587f11d3fb.gif

So what does the above mean? It means that the difference between white rain and accumulating snow will be highly dependent on rates. Well DUH, right? We will need the rates to cool what is significant warming from 800 mbs and down, especially at 950 mbs to the surface. Don't get the rates and we never see the cooling needed at the very lowest levels of the atmosphere to get anything more then white rain. This could once again be a case where those that get into the heavier banding shown on the precip maps above score a few inches and yet those just north or south get a mulch covering or just white rain. 

Considering that this will be highly rate dependent the potential for bust, one way or the other is high as well for any particular locale. Especially when you note the difference between the Euro and the GFS from my previous post on precip totals as well as banding.

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