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Big Snow threat, what will it do, part II


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There is no question the euro is usually the better set of guidance, but this is a sort of in your face to those who were foolishly dismissing the GFS as garbage last night. There is a reason why we have guidance. You have to weigh everything.

Scott not to beat the horse....was exhausted last night so I wasn't sure where you were on the thought that ncep was having feedback issues....but yet another case where here and many other places feedback was raised as a reason for what we saw in other models....when in reality although it may play a role in qpf distribution it has no role possibly in what we were seeing.

4-8 or 6-12 still seems like a good number for many particularly in eastern SNE

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Safe bet right now and I'm more conservative than most here.

The weenie GFS/NAM snow maps give most of us ~12"+

Not sure why some are so down on this system. 

 

 

http://wxcaster.com/gis-gfs-snow-overlays2.php3?STATIONID=BOX

http://wxcaster.com/gis-snow-overlays2.php3?STATIONID=BOX

 

Simple answer, Some want to see a 970 mb low pass the BM, Instead accepting that it may just end up a long duration event that yields basically the same results

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Scott not to beat the horse....was exhausted last night so I wasn't sure where you were on the thought that ncep was having feedback issues....but yet another case where here and many other places feedback was raised as a reason for what we saw in other models....when in reality although it may play a role in qpf distribution it has no role possibly in what we were seeing.

4-8 or 6-12 still seems like a good number for many particularly in eastern SNE

There still could've been some feedback on those runs. At d3-4 who knows if it existed or not? Maybe the GFS would've pulled a euro op from the other night if it didn't. The GFS is the boy who cried wolf wrt this issue though. I don't think anyone totally discounted it (maybe John did...idk), but when you see multiple lows out to sea with small convective vorts while there's a strong s/w and PVA ripping through the SE it has to at least enter your mind that the GFS is doing it again.

We have more of a lobe from the PV hanging back now and the flow is just too progressive for this thing to wind up and amplify.

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SREFs look phenomenal for eastern areas. At KBOS, there are 3 outliers that spit out 1-3" but the remainder are in the 8-20" range with a solid cluster of 8-14" I really like the setup for Boston Metro. It's just going to be an awesome drawn out storm with snow from various mechanisms that doesn't have crazy 2-3"/hr monster band CCB but rather multiple different facets.

 

I'm feeling 8-12" here and >6" regionwide

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Scott not to beat the horse....was exhausted last night so I wasn't sure where you were on the thought that ncep was having feedback issues....but yet another case where here and many other places feedback was raised as a reason for what we saw in other models....when in reality although it may play a role in qpf distribution it has no role possibly in what we were seeing.

4-8 or 6-12 still seems like a good number for many particularly in eastern SNE

 

No I said it is a real situation, but I saw many calling the solution garbage and completely tossing it. That's not a wise thing to do. Recall that I said I really couldn't care less what it is showing until we get all the guidance in. I'm certainly not good enough to deduce if the GFS or any model is suffering from feedback. Sure I can perhaps look at a solution and perhaps deduce that it may be an issue...but I'm certainly not good enough to say toss it. I would like to see the whole suite of guidance. That's what WPC does when they do the model discussions.

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There still could've been some feedback on those runs. At d3-4 who knows if it existed or not? Maybe the GFS would've pulled a euro op from the other night if it didn't. The GFS is the boy who cried wolf wrt this issue though. I don't think anyone totally discounted it (maybe John did...idk), but when you see multiple lows out to sea with small convective vorts while there's a strong s/w and PVA ripping through the SE it has to at least enter your mind that the GFS is doing it again.

We have more of a lobe from the PV hanging back now and the flow is just too progressive for this thing to wind up and amplify.

IMO it's far more likely that there were gaps or errors at init than propagated data errors caused by feedback. We can see every run the changes at even 24 hours are dramatic vs the previous runs. That has zero to do with coastal convective feedback at 72 hours but is immediate and traceable.

That won't stop someone from saying it was feedback that caused the wobbles but it's an overused term and doesn't fit the old standard definition of there being an associated mega qpf hotspot that spins up spurious mL features. Those still happen but at a much lower frequency than earlier model versions.

Model error does not equal convective feedback all the time.

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Well now I'd have to think lol. So a model like the GFS isn't made to handle real life convection. It's given parameters to help it handle convection because convection is too small to handle on its grid scale. So it's given these complicated approximations to help it handle growth and decay of convection. Now these can go wrong And as a result the latent heat released messes up the MSLP fields, heights etc and can cause a weak spurious low to form when in reality it should not have formed.

What the NAM does is that it is allowed to try and handle convection like it thinks it would happen in real life. The problem here is that it starts to go nuts if it doesn't handle it properly and now it develops these intense lows that begin to take on a life of its own. There are braking terms to cyclogenesis in real life. Think about it. When you have warm air advection in general it rises and cools. That is an offsetting process. When you have cold advection in general it sinks and sinking air warms. Offsetting process. The NAM will sometimes ignore some of the processes going on to slow down cyclogenesis. But it has become much better than it used to

Be. I probably should research it more, but that's off the top of my head.

 

Right, basically boils down to convection allowing versus convective parametrization. The GFS doesn't allow convection, so when certain mass field requirements are met it simulates convection by dumping a ton of QPF. But in order to do that it must release a ton of latent heat. And if it does that it must balance the mass fields again by creating locally lower pressure. The parametrization is always being tweaked, but the problem essentially remains the same and really can't go away if the model is to avoid spinning out of control.

 

We know the normal caution flags with it, lowest pressure right over highest QPF, spontaneous development of vort max at H5. It's not as simple as it looks like the low is too far SE.

 

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No I said it is a real situation, but I saw many calling the solution garbage and completely tossing it. That's not a wise thing to do. Recall that I said I really couldn't care less what it is showing until we get all the guidance in. I'm certainly not good enough to deduce if the GFS or any model is suffering from feedback. Sure I can perhaps look at a solution and perhaps deduce that it may be an issue...but I'm certainly not good enough to say toss it. I would like to see the whole suite of guidance. That's what WPC does when they do the model discussions.

Scott gotcha. Ncep at one time had a paper on convective feedback with particular examples. Most of the time when I hear the term tossed the criteria isn't close to met. But I'm sure it all plays a part I just think most of the time it's a diminishing part/problem.

Often when we see these long drawn out solutions it's the models way of a compromise solution pending a movement towards one of the extremes. iE multi day storms seem to be the new inverted troughs

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