Questions for the mets around here.

[Wnghs2007]

What is causing the pv to remain on the other side of the pole right now.

Also what is causing the nao to be somewhat positive right now..

Thanks to all who can answer. I need to learn how things like this work. Thanks

[Wnghs2007]

Also do yall have any speculation as to when these things might switch. Thanks

[yoda]

What is causing the pv to remain on the other side of the pole right now.

Also what is causing the nao to be somewhat positive right now..

Thanks to all who can answer. I need to learn how things like this work. Thanks

The NAO is currently negative (I think), and should remain such throughout the winter, although it will have some forays into the positive range.

PV = Polar Vortex. That is something I am not good with in forecasting. You need SF or Wxguy to answer that... or Don.

[Wnghs2007]

No right now the nao is positive. No blocking high over Greenland and Wxrisk.com(DT) says it is positive as Shitaki mushrooms right now.

[yoda]

No right now the nao is positive. No blocking high over Greenland and Wxrisk.com(DT) says it is positive as Shitaki mushrooms right now.

Then it must have shifted over the past few days, cause it was negative not too long ago. Right, there is no blocking high over Greenland right now.

[Wnghs2007]

Yeah I do believe that had it not shifted we would not be in a transition warm cold pattern right now. The troughs would be stuck for a few days and it would not be so transitional if the nao was neg.


Also ther was a map of nao showing it went positive but I cant find it. Wxguy25 posted it a few days ago I believe

[Pinwheeler]

Plymouth State plot of 500mb height- Hemishpere


Climate Diagnostics Center analysis that plots height anomalies at 500mb in just the Arctic region

A polar vortex is - basically any 500mb rather cirucular low below 5200 meters, below 5100 meters, or below 5000 meters, something like that, or -35 degrees C at 500mb. The lowest heights in the hemisphere. These things are old occluded lows from the mid/high latitudes and have lost a large amount of the thick clouds they used to have. Why they move slowly - occluded lows lose some ability to move themselves in any particular direction very fast.

Actually right now there is one in the northern Hudson Bay, but also a stronger one in Arctic Ocean in the direction of central Russian coast.

I think the idea is to bump a bunch of the cold air into central Canada and dump it on us. It takes time for the large scale flow to change.

[wxguy25]

The POLAR VORTEX (PV for short) is analyzed at h500 and is the core of the coldest air found in either the northern or southern hemisphere. For our purposes we will take a look at the northern hemispheric PV.

Cold air is more dense and compressed, so it has a low thickness (we will use 1000-500mb thicknesses for this example). The density of the cold air causes heights to fall and thusly the reason why lower heights = colder temperatures, lower thicknesses, and higher heights = warmer temps and higher thicknesses.

So b/c the cold air is very dense and has a low thickness, heights at h500 will lower. The center of the lowest heights and temperatures in the northern hemisphere is regarded as the PV. The PV moves in the direction of arctic airmass. At the surface however a polar airmass is represented by high pressure.

Here is an example from JAN 18, 1977 during one of the coldest winters on record. In this situation, the PV is located over the Northeastern part of the United States. 500mb heights are sub-499 meters from Nova Scotia back to the northern tip of Lake Ontario.

http://vortex.plymouth.edu/cgi-bin/gen_ ... m=&dd=&hh=

Temperatures at h5 were as low as -44 C underneath (fig. 1) it which corresponded to extremely low 1000-500mb thicknesses (fig 2).





Also due to the significant temperature gradient between the very cold airmass underneath the PV and areas surrounding it, the jet intensifies in response.

As for what’s keeping the PV on the other side of the pole for now is the lack of the Caspian connection. w/o a ridge near 70 E to force the PV over the pole on to our side, it has remained camped out near the north coast of Siberia unless it were to re-form on our side of the pole.

[Stormsfury]

To add onto wxguy25's post about the effects of the severely displaced PV in the coldest winters on records, January 1977 was an extremely cold month, and still stands as the coldest month of record for many locations across the nation ... including at one point, snowcover covered the ground in all 50 states ... below pictured, is the 850mb temperatures from Jan 18th, 1977 (0z) ...

[wxguy25]

We had the perfect teleconnective set-up that winter.

December 1976-February 1977 Geopotential Height Anomaly Map-Click here for MAP

Edited Text/Link by Stormsfury (for browsers with no wraparound).

[Stormsfury]

ON a side note, there was a small slug of moisture that went thru CHS on the 19th, with -12C 850mb temperatures ... If my memory serves me correctly, there was a small sliver of snowfall that evening (18th into the 19th) ... but hey, I was only 4 ...

But back on track ... the map creation that wxguy25 supplied shows it all .. cross-polar flow, very amplified PNA, strong height anomalies over Greenland and the Caspian Sea Region, as well as the North Pole, basically forced the bitterly cold air well into the Eastern US with little limitations, especially with the lowest height anomalies located over Eastern Canada/NY State (a severely displaced PV) ...

[wxguy25]

ON a side note, there was a small slug of moisture that went thru CHS on the 19th, with -12C 850mb temperatures ... If my memory serves me correctly, there was a small sliver of snowfall that evening (18th into the 19th) ... but hey, I was only 4 ...

But back on track ... the map creation that wxguy25 supplied shows it all .. cross-polar flow, very amplified PNA, strong height anomalies over Greenland and the Caspian Sea Region, as well as the North Pole, basically forced the bitterly cold air well into the Eastern US with little limitations, especially with the lowest height anomalies located over Eastern Canada/NY State (a severely displaced PV) ...

http://www.stormsfury1.com/Temp/DJF1977.png

the North pacific SSTA configuration during the winter of 1976-77 was classic--there was a pronounced cold pool stretching from East Asia all the way to the Aleutian islands and w/ warm water surrounding the west coast of NA. This couplet allowed a pronounced Aleutian low to develop which pumped the downstream PNA ridge forcing the trough to remain in the EUS. This in combination with the factors that SF touched on (Caspian connection, -NAO in the means etc…) resulted in a historically cold winter or much of the EUS.

Folks, what you see here is about as good as it gets!

[yoda]

Good posts by you two!! Now can you guys make it come back?

Most interesting discussion. I have been looking at the 1976-77 winter for a while. It is one that interests me and I have been doing some research on it for the DC region.

[wxguy25]

Good posts by you two!! Now can you guys make it come back?

Most interesting discussion. I have been looking at the 1976-77 winter for a while. It is one that interests me and I have been doing some research on it for the DC region.

It might look exciting but 1976-77 was NOT a snowy winter in DCA.

11.7 at DCA, BWI and 10.6 at IAD.

1976-77 was also a classic example of a winter where the cold overwhelmed the pattern. Obviously this was NOT a great snowy winter in the DC area, but the good news is that this is NOT an analog.

Even Winchester out there in the Shenandoah Valley didn’t have a great winter either. 16.1”

[yoda]

I know. I am doing some research on why this was so.

[Stormsfury]

I know. I am doing some research on why this was so.

Cold overwhelmed the pattern and the principal storm track thrown way south ... in fact, for precip has a whole, below normal ... any s/w generally approaching towards that PV would have been sheared out and the moisture dried up ... very hard to get something going when dewpoints at the 850mb are -40ºC or lower ...

SF

[yoda]

I know. I am doing some research on why this was so.

Cold overwhelmed the pattern and the principal storm track thrown way south ... in fact, for precip has a whole, below normal ... any s/w generally approaching towards that PV would have been sheared out and the moisture dried up ... very hard to get something going when dewpoints at the 850mb are -40ºC or lower ...

SF

Well SF just answered it for me then...

Thanks!