The importance of the 50/50 low

[Nikolai]

The importance of the 50/50 low and resultant -NAO cannot be overstated enough, and their significance comes to light when viewing the upcoming pattern. Let's take a look!

First, an image. Yes, it's the 6z GFS, but I believe 12z will continue the trend--the image contains today's 6z on the left and yesterdays 12z on the right. The differences are not that obvious at first, but at closer look, fairly striking.

Downstream of our system, the differences are clear even earlier in the 6z run--the low that kind of serves as our 50/50 is notably stronger (a wide area of sub 516dm versus a small area of sub 522), and notably further west. The significance of this cannot be understated!!

During our last event, models intermittently showed a hit for the Northeast. Only as we approached did blocking show up stronger than anticipated, and indeed, our quasi-50/50 low ended up giving us a screwjob. Instead of letting the system come out and play, everything was shunted south--and every run of the models generally pushed the clipper further and further south, crushing it into oblivion over NC instead of letting a secondary blossom east of the Delmarva. The reason? The low that retrograded into Maine/the Canadian Maritimes became stronger and stronger each run, crushing the system.

While this worked out against us last time, it has the potential to be beneficial now.

Each run of the GFS shows the low south of Greenland further south and further west. If the trend continues--and if this last event has any significance, it should--the low could trend strong enough and far enough south and west that our next event, in turn, will not be able to amplify to the degree currently shown as far west as it is.

The difference in surface low positions, when comparing yesterday's 12z with today's 6z, could not be more dramatic. As shown by the previous image, the system does not go negative tilt as quickly, first off. Secondly, the system's surface representation is dramatically changed--the 12z run closes off the system and has it going negative tilt by 06z 12/12, while the system remains unclosed even through 6z 12/13 on today's 6z run. More importantly, the system waits longer to go negative--resulting in this surface contrast:

This post is no guarantee of a snowstorm--all rain is still a distinct possibility, and the GFS could reverse its trend. My main point in this post is to be aware of the continued underestimating of the 50/50 and -NAO, as well as a flatter shortwave. Should the quasi-50/50--if it were in the right spot, we wouldn't be having this problem--continue to trend more favorably, wintry precip could be possible.

In the span of 18 hours, the GFS has gone from putting our system over Philadelphia to putting it over Asheville, and if the trend continues, the system could stay progressive enough to give us a wintry event. That's basically the crux of this post.

[A-L-E-X]

Nik, this is excellent and well thought out.  We shouldnt shortchange this run just because its the 6z... I just saw a post by OSU where he said the verification scores for all the runs of the GFS are very similar to each other because new data assimilation occurs with each of them now.

[MJO812]

The Ukie,Euro ensembls and 6z GFS are all east. Maybe a trend? We will see.

[SnowGoose69]

The Ukie,Euro ensembls and 6z GFS are all east. Maybe a trend? We will see.

These things do not often turn around, especially once inside 7 or 8 days but with the -NAO and 50/50 low at least being present gives it a chance....there was an event last winter...I cannot recall which it was that initially was forecast in the Ohio Valley by the Euro and not the GFS....the GFS then also went west but suddenly about 5-7 days out both models went to a coastal system within one run of each other.

[isnice]

The Ukie,Euro ensembls and 6z GFS are all east. Maybe a trend? We will see.

You forgot GGEM ensembles

[Alpha5]

Great post!

[Sundog]

The fact that we are writing this off at 7 days is the scary part. I've seen entire storms disappear off the models within a couple runs and we already concluded that this WILL go way to the west?

[earthlight]

Very well thought out and a good post. That being said, the fundamental problem with this setup goes deeper than the 50/50 or quasi-50/50 low that is being detailed. First, the upper level low currently over our area is forecast to completely erode the NAO block. This is where the problem really stems from initially as it creates a snowball effect of unfavorable developments. The wavelength is really terrible, especially between the first clipper shortwave (which actually does work to raise the heights, who would have thought right?) and the big shortwave that eventually is over the Central US. Additionally, the Polar Vortex over Canada is in an awful position. That is a position conducive for the jet streak developing in the wrong area--too far west. Is it possible that the 50/50 low trends stronger and further south? Absolutely yes. But even with that development, if the guidance is even remotely close with the positioning of the PV and shortwave strength over the Central US, the most potential we would realize is a few inches on the front end in the interior. We would need some significant changes. That's not to say that they can't happen--the system is still 130+ hours out. But we need a change soon.

[tmagan]

The Ukie,Euro ensembls and 6z GFS are all east. Maybe a trend? We will see.

Without any sort of 'banana high' in early December, forget about it.

[Weathergun]

The 50/50 low is important. And I think we do better along the coast if it strong and centered over Newfoundland. But the postion and shape of west coast ridge and PV is also important. I think we also need the ridge flat out over the Northwest US/SW Canada to prevent more PV phasing and the shortwave energy from closing off before reach the coast. If not, we still probably get strong primarly low in the Ohio Valley.

[earthlight]

Without any sort of 'banana high' in early December, forget about it.

You certainly don't need a banana high to get snow in mid December (which it will be by the time this storm is occurring). I would definitely welcome it's presence, though.

http://www.meteo.psu.edu/~gadomski/NARR/2009/122006.png

[dbc]

Very well thought out and a good post. That being said, the fundamental problem with this setup goes deeper than the 50/50 or quasi-50/50 low that is being detailed. First, the upper level low currently over our area is forecast to completely erode the NAO block. This is where the problem really stems from initially as it creates a snowball effect of unfavorable developments. The wavelength is really terrible, especially between the first clipper shortwave (which actually does work to raise the heights, who would have thought right?) and the big shortwave that eventually is over the Central US. Additionally, the Polar Vortex over Canada is in an awful position. That is a position conducive for the jet streak developing in the wrong area--too far west. Is it possible that the 50/50 low trends stronger and further south? Absolutely yes. But even with that development, if the guidance is even remotely close with the positioning of the PV and shortwave strength over the Central US, the most potential we would realize is a few inches on the front end in the interior. We would need some significant changes. That's not to say that they can't happen--the system is still 130+ hours out. But we need a change soon.

That just says it all right there. Things will need to change big time if we are going to have any shot at snow.

[Chris L]

That just says it all right there. Things will need to change big time if we are going to have any shot at snow.

The Pacific is pretty bad too..

That is one of the main reasons we can't get the bonafide nor'easter.

[tombo82685]

other issue is, absolutely no cold air to begin with. You would need the storm to drop far south turn into a coastal and strengthen to pull the cold air into it.

[dbc]

The Pacific is pretty bad too..

That is one of the main reasons we can't get the bonafide nor'easter.

other issue is, absolutely no cold air to begin with. You would need the storm to drop far south turn into a coastal and strengthen to pull the cold air into it.

Yup. Frusrating but it is what it is: A raging La Nina.