Negative NAO coming?

[usedtobe]

The GEFS ensembles are suggesting that we might be trying to slip into a pattern with a weak negative NAO look to it beyond 240 hours. When looking at extended forecasts of the index, it's important to look at how the forecasts have been doing at the longest time ranges.

Note on the 14 day forecast verification the correlations between the forecast and observed is below .50, that's not a high enough correlation to use the ensemble mean forecast much at that time range. Note how often the actual observed NAO index was higher than the forecast. During that same time period the MJO is forecst to be in the dreaded phased 3-6 though that usually only makes a difference if the signal is strong. However, in a nina year, that's the ocean area that usually is on the warm side of the Pacific which can support decent convection.

Still that's no guarantee that the pattern will be warm but should give pause for those thinking the real pattern change is on the way. However, that does not mean we could see a pretty good transitory cool shot in the east. The 12Z 240 hour Euro actually builds a shortwave ridge along the west coast which usually means we get a shot of colder air behind the low that will track to our north. The surface high isn't a true arctic one as it shifts into the basin from the Pacific. Still it probably give us some cold air.

http://www.meteo.psu.edu/~gadomski/ECMWF0.5_12z/ecmwfloop.html#picture

The GFS ensemble mean is not as amplified with that ridge but that's no surprising since it would mean out the features. If I were a betting man I might go with a temporary cold shot beyond 240 hours but still think for true pattern change we have to wait for the monster AK low to get dislodged or for a strong enough great lakes storm to start initiating some ridging to its east and northeast. None of the GFS ensembles thru 276 hours has any real snow chances for DC and Baltimore.

[Deck Pic]

Thanks Wes. These transient cold shots in a bad pattern do not usually bear fruit. Maybe we can wishcast a clipper if we can get a quick transient ridge out west.

[usedtobe]

Thanks Wes. These transient cold shots in a bad pattern do not usually bear fruit. Maybe we can wishcast a clipper if we can get a quick transient ridge out west.

That's what we'll prbably need or some weak wave on the end of a front. Sort of like tomorrow morning but with a little more cold air.

[H2O]

Nice posts Wes. I trust your opinion and appreciate your thoughts on what is annually a tricky time of year. The MJO phases and the resulting warmth we seem to be in/headed towards makes me hope you are right in that we can get a shot or two of temp cold air.

[ORH_wxman]

Nice post Wes. The terrible Pacific pattern probably makes the -NAO fairly useless for most of us. Kind of like the big -NAOs in December 1996 and 2001. If the PAC can start looking better by December, then the potential -NAO that the Euro ensembles show might be able to tap into more cold air.

[usedtobe]

Nice post Wes. The terrible Pacific pattern probably makes the -NAO fairly useless for most of us. Kind of like the big -NAOs in December 1996 and 2001. If the PAC can start looking better by December, then the potential -NAO that the Euro ensembles show might be able to tap into more cold air.

There used to be an old forecasting adage, a cold alaska, warm eastern U.S (mid Atlantic). The monster Ak vortex and strong EPO makes it hard to get much going.

[Ian]

I moved this as it's not necessarily specific to the DC area. I know others would like to see Wes thoughts too!

[uncle W]

NYC had an arctic outbreak in January 1972 between blow torches...It brought the coldest temperature of that winter of five degrees...A week before it was -60 in Alaska...November 1971 had a storm on Thanksgiving...Snow north of NYC...A brief cold shot then back to warmth...That's what I think will happen into early December...I think the ao/nao got negative late in the game that year...

[CoastalWx]

I'll throw the Euro ensembles in. They also have a weak -NAO after d10 or so with a slow weakening towards the end of the run....but that could be noise too.

In either case, the AK trough is still there, but slowly weakens as heights try to rise out west. The problem is that the flow is more zonal now, shoving that Pacific air into Canada and the CONUS. It's kind of a waist of a -NAO because that -NAO doesn't have any good cold to shove south into the US. Of course this is the overall pattern...it doesn't mean a transitory PNA ridge and 522 thicknesses can't get shoved into the northeast for a brief period to allow for a winter event. The interior would probably be favored in this pattern given climo, but coastal areas could do it with the right combo...and this combo isn't all that difficult.

It's a stable pattern right now, so I'm not feeling a change anytime soon.

[usedtobe]

I'll throw the Euro ensembles in. They also have a weak -NAO after d10 or so with a slow weakening towards the end of the run....but that could be noise too.

In either case, the AK trough is still there, but slowly weakens as heights try to rise out west. The problem is that the flow is more zonal now, shoving that Pacific air into Canada and the CONUS. It's kind of a waist of a -NAO because that -NAO doesn't have any good cold to shove south into the US. Of course this is the overall pattern...it doesn't mean a transitory PNA ridge and 522 thicknesses can't get shoved into the northeast for a brief period to allow for a winter event. The interior would probably be favored in this pattern given climo, but coastal areas could do it with the right combo...and this combo isn't all that difficult.

It's a stable pattern right now, so I'm not feeling a change anytime soon.

I'm surprised no one has posted last night's Euro 192 hr forecast. I don't believe it but it does dig a shortwave far enough south to put the mid atlantic region in play for weather. It has no support from the GEFS ensembles or the euro ensemble mean but that usually doesn't stop those who want to fantasize.

[tombo82685]

I'm surprised no one has posted last night's Euro 192 hr forecast. I don't believe it but it does dig a shortwave far enough south to put the mid atlantic region in play for weather. It has no support from the GEFS ensembles or the euro ensemble mean but that usually doesn't stop those who want to fantasize.

drops some frozen precip in the mtns of vnc,va,wv, pa and lehigh valley then new england...would like to see lower thickness for piedmont and coastal areas...they are running 540-546 so thats indicative of a warm layer some where.

[usedtobe]

drops some frozen precip in the mtns of vnc,va,wv, pa and lehigh valley then new england...would like to see lower thickness for piedmont and coastal areas...they are running 540-546 so thats indicative of a warm layer some where.

That's good to know but that solution still has little support from the ensembles, at least what I can see. I don't get the euro ensembles except for the ensemble.

[bluewave]

Maybe not in the specific way that the OP Euro is showing, but we could see some energy close off over

a portion of the East in the 7-10 day range. So far this fall we have not missed many opportunities for for a

closed low under Canadian blocking. It will be interesting to see if the ensembles start picking up on this.

[usedtobe]

But there really isn't any canadian blocking right now, at least that I can see. The only model showing it is the Operational Euro.

[icebreaker5221]

Also, don't forget about the PDO. It is often difficult to get troughing over the eastern US with a persistent trough over the Pacific NW, simply due to the wavelengths of longwave trough-ridge patterns. If this trough sets up slightly inland, say, over the Rockies, you're almost guaranteed to be warm in the east under the effect of southwesterly flow ahead of the trough. Dec 1990 was a fairly typical -PDO pattern (perhaps the negative heights were a tad east of the climatological location, but you get the idea), and the east was dominated by a ridge:

However, if the west coast trough shifts further west, it is possible to subsequently shift the downstream ridge west as well. This further allows for lower heights to build in east of the ridge. We've seen this happen in Jan 2000, and again more recently in Dec 2010. Certainly a very negative NAO can help to develop this sort of pattern quite a bit, but not without the cooperation of the west coast trough:

The -PDO does not look to be going away any time soon.

There are at least hints in the long range pattern of a westward shift of the west coast trough, but as we've seen lately the pattern has been quite transient, so it's difficult to say whether or not this change will last:

[usedtobe]

I think the westward shift the the pacific ridge is the hope for any real change as then maybe we can pop a ridge over western Canada and the western U.S. Frist we have to weaken the big AK vortex and then shifting the trough west. However, I do see potential for quic shots fo cold air like we have today over the mid atlantic after storms exit the coast.

The GFS Oh Valley low after 240 hours is a different shortwave than the euro. I still think any meaningful changes won't come before the end of the 1st or second week of Dec but admit any ideas past day 5 are just wags.

[winterymix]

The GFS Oh Valley low after 240 hours is a different shortwave than the euro. I still think any meaningful changes won't come before the end of the 1st or second week of Dec but admit any ideas past day 5 are just wags.

So...December 5th is in play for the Mid-Atlantic?

[strongwxnc]

Yes. Thanks for moving it Sir!

I moved this as it's not necessarily specific to the DC area. I know others would like to see Wes thoughts too!

[tombo82685]

I think the westward shift the the pacific ridge is the hope for any real change as then maybe we can pop a ridge over western Canada and the western U.S. Frist we have to weaken the big AK vortex and then shifting the trough west. However, I do see potential for quic shots fo cold air like we have today over the mid atlantic after storms exit the coast.

The GFS Oh Valley low after 240 hours is a different shortwave than the euro. I still think any meaningful changes won't come before the end of the 1st or second week of Dec but admit any ideas past day 5 are just wags.

12z euro has a nice shot of cold at hr 240, with a nice ridge out in the pacific. Some higher hgts are trying tpo build into greenland but still looks pretty transient. Stillm the pig of a pv in alaska, thought a little weaker. New england does see some snow with the thanksgiving storm, boston even sees rain going over to snow.

[usedtobe]

12z euro has a nice shot of cold at hr 240, with a nice ridge out in the pacific. Some higher hgts are trying tpo build into greenland but still looks pretty transient. Stillm the pig of a pv in alaska, thought a little weaker. New england does see some snow with the thanksgiving storm, boston even sees rain going over to snow.

MY posts were really aimed at the mid atlantic from dc and Baltimore. New England can get snow when we can't. The changes to the euro are more how I would expect the pattern to start to change though I have little faith in its forecast at 240 hours. It pretty much lost the deep vortex it had last night at 192 hours.

[tacoman25]

Regarding the big AK vortex...Fairbanks Eielson hit a low of -41 this morning, breaking the record from 1969. This is shaping up to be one of the coldest Novembers on record for much of Alaska, as the huge vortex looks to restrengthen again over the coming week. Any ideas on what this might mean for the coming winter and blocking? 2010 had nothing of the sort. I have to admit it worries me a bit, as several -ENSO years that had very cold Novembers in Alaska ended up blowtorching the CONUS come winter: 1975-76, 1999-00, 2005-06. Then again, we have a number of -PDO phase winters that did not follow this trend: 1950-51, 1955-56, 1961-62.

[Isotherm]

Then again, we have a number of -PDO phase winters that did not follow this trend: 1950-51, 1955-56, 1961-62.

Those latter three years are much closer to now IMO in terms of the overall global regime, PDO/AMO/NAO wise. Good news is the AK vortex seems to be weakening by weeks 2-4 on the euro weeklies. Usually I'd say if the entire month of December follows a certain pattern, it's a bad omen for the rest of the winter. In this case I think we'll be looking much better pattern wise 15-25 days from now, speaking for the Eastern US.

[brad1551]

Lots of great info on why the cold air is not arriving just yet. For the snow lovers this could be a scary set up for winter. Do not worry the cold air will be coming, it's just taking a little longer then expected

[Hailstorm]

Those latter three years are much closer to now IMO in terms of the overall global regime, PDO/AMO/NAO wise. Good news is the AK vortex seems to be weakening by weeks 2-4 on the euro weeklies. Usually I'd say if the entire month of December follows a certain pattern, it's a bad omen for the rest of the winter. In this case I think we'll be looking much better pattern wise 15-25 days from now, speaking for the Eastern US.

Good post, and I agree with you. Do you think that even if the entire East Coast roasts during the first ten days of December, would the long-term cold air regime for the second half of December be pronounced enough to offset the overall positive temperature departures for December? Or maybe even cause negative temperature departures of more than -3* F below normal?

[usedtobe]

The rpna and positive epo pattern has certainly been the driving force dictating the temp anomalies during the 1st half of the month. Below is the mean 500h anomaly pattern. You certainly can get a sense of the the pacific pattern and that there has been a belwo normal heights from ak into the west and ridge over the eastern half of the country in the mean.

that has kept the temps from the northern plains eastward into new england warmer than normal.

The dreaded epo is forecast to remain positive through the month but is also forecast to ebb towards the end of the period. Note that our current cold shot is happening during the one negative epo period that we've had recently.

The warmest areas relative to normal with a positive epo is usually west of the mid atlantic states but much of the country typically ends up averaging above normal. Below is the typical 500h anomaly pattern and the corresponding temp anomalies associated with it.

One of the two important 500 features associated with a positive epo is the ak or gulf of ak vortex and a flat ridge (positive anomaly to its south and soutwest. The good news is that the euro ensembles are weakening the ak ridge as are the gefs mean as it weakens or if the vortex shifts south and west. Then our pattern can start to improve. A postive epo like pattern does not mean that every day will warm just that on average temps will be warmer than normal. Usually, it's because cold air is shut off from canada.

[Neblizzard]

Excellent post Wes, things definitely look encouraging in the long range. If the models are right weakening the AK vortex and retrograding it west, then we have the opportunity to establish an -EPO pattern within the several weeks. Maybe a full scale pattern change by 3rd week of December...

[OKpowdah]

Makes sense that ensembles have been underestimating the high index annular mode. The last ten weeks have been squarely within the positive phase of the inter-seasonal cycle (changed phase in late August ... consistent with Irene ...). We should the NAO slip into the negative phase of this cycle by the middle to end of December.

[Bob Chill]

Makes sense that ensembles have been underestimating the high index annular mode. The last ten weeks have been squarely within the positive phase of the inter-seasonal cycle (changed phase in late August ... consistent with Irene ...). We should the NAO slip into the negative phase of this cycle by the middle to end of December.

This post intrigues me. I was just going through AO tabular stuff today looking for analogs and the AO went positive about the same time in late August and has remained either positive and pos neutral since the end of August after being negative for quite a while.

Where can I read up un the high index annular modes? I started reading the stuff on the CO State website but I got lost quickly.

[mitchnick]

This post intrigues me. I was just going through AO tabular stuff today looking for analogs and the AO went positive about the same time in late August and has remained either positive and pos neutral since the end of August after being negative for quite a while.

Where can I read up un the high index annular modes? I started reading the stuff on the CO State website but I got lost quickly.

Bob, ask JI, he knows that stuff inside and out

[skierinvermont]

Based on the state of the stratosphere, I wouldn't be surprised if there is limited blocking until February, perhaps the whole winter.