diffrence between NAO an PNA

[1954Dodge]

Can someone explain to me if the PNA has to be negative also when the NAO is negative to get snowstorms on the east coast. I understand that the NAO has to be negative to get the big storms but what does the PNA have to be. Also does the AO and AAO have an effect.

Thanks

[wxmeddler]

So, these indices are just values.. its good to know what they are for a long term pattern but if your forecasting for a specific storm your better off looking at the 500 mb charts to determine steering and blocking issues.

For east coast snow storms, its good to have a -NAO, + PNA, and -AO. But it is not completely necessary to have all these to get a storm there can be alot of exceptions. A lot of the time it depends on other factors.

[weatherwiz]

Well for the Northeast a combination of a -NAO/-PNA can be a very good one, having the -NAO in place can help with locking in the cold air and the -PNA coupled with a -NAO can mean a fairly active storm track and with an active storm track you'll have your chances for snow.

The PNA doesn't have to be negative though in order for us to get snow here in the Northeast, just like the NAO doesn't have to be negative...we can still get snow with +NAO's...it just depends on the setup/placement and how the state of the NAO is affecting the 50/50 low placement, if there is one.

One thing to always look for as well is sometime our biggest storms occur when the NAO is switching phases...this doesn't mean we see something everytime but if there is a system coming up the coast at about the same time this is occurring it can definitely be a huge help.

As far as the AO goes since it's basically correlated with the NAO chances are if one index is one sign, chances are the other index will be of same sign, this is not a guarantee though and there are plenty of instances where they are opposite, especially when the phase is a weak signal.

As far as the AAO goes I'm not sure of the correlation to anything.

[Thunder Road]

Well for the Northeast a combination of a -NAO/-PNA can be a very good one, having the -NAO in place can help with locking in the cold air and the -PNA coupled with a -NAO can mean a fairly active storm track and with an active storm track you'll have your chances for snow.

The PNA doesn't have to be negative though in order for us to get snow here in the Northeast, just like the NAO doesn't have to be negative...we can still get snow with +NAO's...it just depends on the setup/placement and how the state of the NAO is affecting the 50/50 low placement, if there is one.

One thing to always look for as well is sometime our biggest storms occur when the NAO is switching phases...this doesn't mean we see something everytime but if there is a system coming up the coast at about the same time this is occurring it can definitely be a huge help.

As far as the AO goes since it's basically correlated with the NAO chances are if one index is one sign, chances are the other index will be of same sign, this is not a guarantee though and there are plenty of instances where they are opposite, especially when the phase is a weak signal.

As far as the AAO goes I'm not sure of the correlation to anything.

don't you mean +PNA?

[weatherwiz]

don't you mean +PNA?

For what part?

[Thunder Road]

For what part?

"Well for the Northeast a combination of a -NAO/-PNA can be a very good one"

don't we like a +PNA because it dirves a west coast ridge and therefore east coast trough. I though the -PNA was what was killing our snow chances right now. I could be wrong though.

[SnowGoose69]

A +PNA is better, but a -PNA can work quite well if you have a-NAO...see 93-94 as the best example...even 95-96 had stretches with a -PNA although many of the winter weather events occurred during the +PNA periods...I believe the November 95 and 12/19/95 events occurred during a -PNA/-NAO phase....a -PNA with a +NAO will generally not work since the track of the storms will go north...-PNA/+NAO would be jackpot for places like Detroit and Toronto.....I should correct the fact that a -PNA/+NAO won't always kill you...93-94 the NAO was very positive but the EPO and a ridiculously massive polar vortex aided in the shifted storm track south and plenty of low-level cold air for icing events in the MA.

[weatherwiz]

"Well for the Northeast a combination of a -NAO/-PNA can be a very good one"

don't we like a +PNA because it dirves a west coast ridge and therefore east coast trough. I though the -PNA was what was killing our snow chances right now. I could be wrong though.

Well ORH wxmann compiled a list of -NAO/-PNA winters for us here in the Northeast and most of the years on the list were pretty good winters for us.

[JBG]

A +PNA is better, but a -PNA can work quite well if you have a-NAO...see 93-94 as the best example...even 95-96 had stretches with a -PNA although many of the winter weather events occurred during the +PNA periods...I believe the November 95 and 12/19/95 events occurred during a -PNA/-NAO phase....a -PNA with a +NAO will generally not work since the track of the storms will go north...-PNA/+NAO would be jackpot for places like Detroit and Toronto.....I should correct the fact that a -PNA/+NAO won't always kill you...93-94 the NAO was very positive but the EPO and a ridiculously massive polar vortex aided in the shifted storm track south and plenty of low-level cold air for icing events in the MA.

I'm confusled by your post. Was 1993-4 a negative or positive NAO period?

[usedtobe]

I'm confusled by your post. Was 1993-4 a negative or positive NAO period?

It was very positive, that's why places like DC didn't get much snow and instead got lots of ice. The epo by contrast was very negative and delivered arctic air into the east.