Carvers Gap

The 12z CMC definitely shows the potential for the trailing system around the 20th.

The 12z GFS was very cold and is well out of sync with its ensemble.  Could we be seeing modeling beginning to sense blocking?  Maybe.  This occurred in early January(when modeling missed the mid-Jan cold shot), and that isn't the first recent GFS run where the TPV got trapped.  I suspect that there is one really cold shot embedded in the pattern between the 20th-30th.

Meanwhile, the 12z Euro does have some talking points.  The 17th system is more of a slider as confluence is helping there from the NAO block.  North of I-40 is light snow.  There is another system right on its heels.

Here is a great article about the NAO.  It does a great job into really looking at all sides.  Be sure to read the part in conclusions about correlations to snowfall along the East Coast.  That could also be read as areas just inland from the coast as well such as E TN - but I am extrapolating.    But overall, it covers a ton of great material, and is not one sided.  Kocin is mentioned in this.  I will try to get some of his stuff later.
 

https://wcd.copernicus.org/preprints/wcd-2020-20/wcd-2020-20.pdf

The big thing to watch with NAO climatology, the better storms IMO often occur towards the end of the NAO cycle.  We have definitely had more -NAOs during the past 4-5 winters.  2-3 years ago it became more apparent that we were headed back towards a -NAO winter scheme.  We really had gone maybe a couple of decades without seeing consistent winter-after-winter -NAO stuff.  (Now, that is off the top of my head.)   So, it is important to really dig into storms versus actually seasonal or month by month analogs.  NE TN can score from a DC pattern that is snowy.   We can also score from an EPO pattern.  Where we have trouble scoring is with a moderate to strong ENSO of either type.  We need ENSO forcing to be weak but not nada either.  
 

As for middle TN during the past three years, I could make more of a case for La Niña driving those patterns as you all often need a bit of a SER to score.  It sucks in E TN, but that SER often produces a storm track along the Apps or through the eastern Valley.  Whether that SER is a product of an EPO or the EPO is the product of the SER...now that is a fun debate.  Why?  Well, the SER is basically a standing wave and maybe we could also call it a block.  When that SER sharpens along the SE region it backs flow again much like the NAO, and it consequently (one could argue) buckles the jet.  I will have to go back and look, but one of the big recent storms for middle TN or the ice storm had a stout SER which maybe was connected to the NAO.  It seems like one of those storms came directly at the end of NAO blocking or at the beginning.  I am not in a place where I can check NAOs currently.  
 

Either way, when the Atlantic has a block form (SER or NAO), that often forces the upstream ridge to pop.  Placement of the mid-continent trough is highly dependent on where the block sets up shop.  And it makes a big difference here.

Now, prior to these recent current -NAO winter episodes, the Pacific often drove the pattern as the NAO was absent.  However, that was not what my original post which was about re: whether the EPO drove this recent January outbreak.  The EPO during this recent outbreak was a likely by-product of the NAO.

So rule of thumb.  Some EPO pattens are a direct result of NAO blocking, but not all EPO patterns are a result of NAO blocking.  When a couplet of NAO and western blocks form...pretty good chance the NAO forced that.

This is the current run at 12z.  Look at the NAO region.  It is (edit)weaker than four days ago.  Notice the upstream (upside down) omega with the trough is not sharp compared to four days ago(below).   The western PNA is great, but without the Atlantic block...nada.  

This was just four days ago.  Notice the NAO region(Greenland) is marginally better as the heights are higher over a larger area of the Davis Straits.  The omega contains the BN heights which is carved out better.  Notice the storm is sharper and a better cold air supply.  Look at the difference on the surface.  Boom.  The NAO below has actually backed the flow just enough that the PNA ridge is in EPO territory.  This is what I am talking about when I say "NAO driven."  That western ridge buckles so that it is a bit more in the eastern Pac.  The NAO forced that IMHO.  Without the stronger NAO, the ridge on todays run (above) pushes slightly eastward (in the West), and we lose confluence and our cold air supply to boot.

Just now, Daniel Boone said:

Yeah, very good description. Actually started to mention yesterday that the blocking should provide enough buckle to be sufficient to support at least ine event this month. 

And even with this system on 17th-20th(it has moved around a bit), if one goes to Tropical Tidbits at 500, all one has to do is toggle back until they see the heights over Greenland get stronger.  I think those runs are around Feb 7th at 6z(off the top of my head...I breezed through that a minute ago).  Then, go look at the surface for that storm. As the NAO has weakened during subsequent runs, the storm has gotten warmer (until 6z today which was a good run). One thing the NAO block also does is it also often turns the AO negative, not always but often.  That opens the door for cross polar flow from Siberia.  The NAO block then often forces that flow into the East - if it doesn't hook into the SER!  LOL.  

Modeling was likely in error about the upcoming NAO intensity which was forecast to be stronger than it is now for the second half of February.  As the NAO diminished...the ensuing runs have warmed dramatically.  But one thing I tend to note, modeling will often struggle with HL blocking up until the last minute.    My guess is the QBO has reversing or is about to...that is correlated to negative NAOs as well.  I haven’t looked at it recently, but it is due to flip.  The NAO might be one of the most difficult things for current computer modeling to effectively portray.  But let’s see where the pattern goes after the potential system on the 18th.  My guess is base warm(with cold interludes), but I am not completely sold on that. 

The other driver during that January timeframe was the MJO relented and rolled into phases 1-2-3.  It wasn’t overly favorable, but it wasn’t hostile either.  For an induced NAO block to form a ridge out West, the Pacfic jet extension had to move westward.  As soon as that jet extension relented, the NAO buckled the flow.  
 

This time around, not sure that is going to happen as the IO and MC are muddying the MJO waters.  The NAO doesn’t play nicely at all times.  Sometimes it hooks into an eastern ridge, and sometimes it can’t overcome a jet pounding the West Coast.  But...it is a common ingredient in many, many great EC and E TN storms, and it was this past January.
 

For now, it looks as if we have potential for a storm on the 18th...might be our last shot. 

The downstream NAO ridge/block often forces the western ridge to pop by buckling the jet, and that is likely what occurred during January.  Without the NAO, the flow would have been zonal.  The NAO sharpens western ridges(forces the upstream jet to buckle) and brings cold southward but allows storms to gain latitude along the EC. The debate for me is not whether the the EPO was there,  I just don’t think it would have been there without the NAO buckling and forcing the trough into the East. That trough in the East forced the western ridge to pop.   I just don’t think the January pattern was EPO driven....the EPO was a byproduct of downstream blocking and a retraction of the Pacific jet.  Without the NAO, the EPO ridge doesn’t form in a meaningful way IMHO.  That is my point.  As soon as the NAO disappears, the forcing for the EPO is lost...and it disappears almost immediately.  This has been a common then during the past 4-5 winters.  The cold during the past few winters has often been coupled with a brief NAO episode.  Think of it like a water hose on high pressure.  When the end is blocked, the hose will often buckle.  That is exactly my point.  Without the block - no upstream EPO block this winter.

We can get into previous winters and storms later, my original comment was specifically about the January cold being driven by the NAO block, and it was.  I stand by that comment 100%.  You will find that is often a common theme during big EC storms.  And that has to be fished out of data (almost on a day by day basis) as the NAO (of late) appears for 2-3 weeks and won’t show in a monthly 500 pattern on reanalysis as it gets washed out.   Again, what the NAO does is it forces the jet to buckle in an upside down omega pattern and forces confluence over the eastern half of the country.  E TN, more often than not, needs it.  

That’s what I have, the defense rests.

I will add some scientific journal articles about the NAO itself.  Its influence on EC wx/climatology is well documented and well studied.

But again, the 6z GFS has hopefully a trend towards a colder Feb 18 system.

You all can dig up my old NAO posts.  It is a key driver for E TN snowstorms.   There isn’t a ton of debate on that.  It is also a key driver in many Kocin storms.  For middle and west TN, the EPO is more important as those areas just don’t benefit very often from coastals.  -NAO has driven many snow storms during the past 4-5 years. It produces confluence.  Without it, everything races out to sea.  When we lost the NAO during late Jan, we torched.  
 

This is key...The NAO will often force the Western ridge to form as the upstream NAO ridge backs flow, forcing a trough over the East, and forces an almost upside down omega by buckling the jet northward as it crosses the West coast.  Without the block over Greenland, the upstream ridge will often not form over the eastern PAC.   It buckles the jet.  Without the NAO, the jet doesn’t buckle this winter.  That is just atmospheric physics.   January’s pattern was NAO driven as it forced the upstream jet to buckle.  And I do have the saved maps for that sequence and also many other recent storms - whenever I ran out of memory on Am Wx...I moved my graphics to cloud based servers.
 

Because E TN is closer to the Atlantic, it makes sense that the Atlantic is our main driver.  When I get time, probably over the summer as I am getting next winter’s forecast ready, I will pull some analytics.  (Now, we haven’t talked about a PNA which is different.).  I will take individual storms and look at those.  Seasonal readings don’t tell the story.  They have to be system by system.  Often NAOs won’t last the entire season.  They can sometimes be brief.  What the NAI does is buckle the jet.  That backs up flow and will often force or sharpen the ridge over the eastern PAC.   Plenty of evidence of that in many great eastern snow storms.

1 hour ago, Itryatgolf70 said:

For the time being, the tpv over southern Canada keeps me intrigued. In all seriousness, models should be colder than indicated. Also, Raindance saying beginning of March should resemble mid Jan with a cold blast. Doubt it, but anything is possible. 

The beginning of March is within the window that we have been talking about since mid Jan.  Glad he is discussing that.

We just need the Pacific not to have massive jet extensions....that has been a problem for several winters.  I have my theories on why this is occurring(one of you knows my thoughts), but I do wonder.  

46 minutes ago, Daniel Boone said:

Very true. The last upgrade apparently implemented too much warmth adjusting. 

Modeling was completely blind to the mid-January even until just days prior.  Big storm wen through the Plains and modeling flipped.  I think we are going to have to get this system worked out on the 18th, before we know for sure the hand we are dealt.  

I honestly don't buy the EPO stuff this winter.  We got a decent rotation of the MJO and the NAO fired.  It got cold as the NAO pattern matured.  The EPO helped some, but without that NAO, the cold wouldn't have been there at all.  Same was true last winter.  The reason we have seen severe cold during the last 4-5 winters is the NAO is beginning to fire again.  We went a couple of decades without a true NAO negative trend.  In E TN especially, the EPO is not always a driver.  The storm that brought snow to this area just three weeks ago was driven there by the NAO block.  The EPO would have not allowed for confluence.   E TN almost always does better for snow w/ an NAO block, and that was true this winter w/ Knoxville seeing that record snowfall.

Right now it is the GEFS vs EPS/GEPS....there is not consensus that a warm-up is permanent.  It may well be the reality, but that is not the case right now.  Pretty substantial model war under way.  The GEFS has had a really bad warm bias this winter. 

Also, my general rule for pattern durations is 4-6 weeks.  I really just didn't' want to admit that at the time(Jan21st), but that has come to pass.  If that rule holds, the can kicking into late Feb make sense.  That means the actual pattern is Jan 21-end of Feb.  I can almost use that rule and never look at an MJO cycle, and I should have done that this time around. It works almost as well as "thunder in the mountains."  What we thought was a thaw, was really a true pattern change.  I think on modeling we are actually watching models struggle w/ the next pattern change later this month.  The beginnings of that change begin next week.  Remember when the pattern was can-kicked by several days during December?  Same deal IMHO.  

To be clear, pattern changes don't mean better snow chances.  I am simply talking wx at this point.  We need pretty incredible cold come March at this latitude.  But I do think we are going to see a -NAO build.  Is the EPS wrong?  Possibility.  But it really deepened that feature on the 12z EPS run.  Normally, I would just say that warmth is coming and mail it in.  But I am skittish after what happened during January, and modeling missed a monster cold shot.

The PDO has been a pain.  We discussed that in depth last fall as a potential thorn.  It certainly(more times than not) puts the default trough in the West.  

That said, the Mountain West has been STRUGGLING w/ precip deficits.  Could be a very rough summer there.  Fortunately, I think Jackson, WY, got some snow this week.   So, it is not helping them a ton.  There are snow machine businesses that haven't been able to run their machines in the dead of winter.  Cooke City, MT, had bare ground two weeks ago, and their main street was pavement.  I have never seen that during winter.  If the PDO was fully on board, there would likely not be bare ground there.   But.......it is incredibly common to have severe cold in the eastern Valley, and then that be it for winter.  Same was true for me as a kid.  I did wonder when it got so cold, if that might be it.  

But I tell ya where it has been cold - Alaska.  I follow Stan Zuray (Yukon Men) on Facebook.  He is a great follow on social media.  He is a fisheries expert.  He has been posting about how cold the interior has been there.  

I think the main culprit this winter was the December GOA low on steroids that created that monster chinook.  It scoured Canada of snow, and made it tough to rebuild the cold there.  To me that has been a problem.  If that had not occurred, we would have been cold from the last week of December through the third week of January.  We managed to get some of that snow/cold deficit back, and even get back into positive territory in terms of snow...but I think that chinook put us in the hole from a source region standpoint.  

But let's see....January changed-up unexpectedly.   It certainly looks like a warm signal, but the atmosphere doesn't have to do what modeling says.  The storm/no-storm on the 18th could change things up much like the Plains storms did in early Jan.

1 hour ago, Holston_River_Rambler said:

I'm probably gonna

A) be wrong

and

B ) get a comment about slinging spaghetti on a wall

but I think we are about 7 - 10 days away from seeing a meaningful end to this cluster             of a pattern we've been in. I was really hoping about this time last week we'd see some better signs by yesterday or today and yeah the Euro above is nice, but I'd like to see all ops at least showing a way out.

Doesn't have to be an epic look. Doesn't mean it has to show snow. All I'm asking is for two weeks without a trough slamming into California. Hopefully the Euro is the first hint of changes brewing. 

 

That satellite imagery Jeff posted yesterday is not too encouraging, but if we can believe the GEFS RMM today (updated late) the mean splits the difference of 30 members between several big phase 8-1-2, several CODs, and several, I guess, status quos: 

 

I especially like the one member that goes back to phase six and stays there for a week. 

GFS OP likes the COD. I still think it lo amp/ borderline traverses COD 8-1-2-3. If it plays out like it did in January, our best chance for a January redo may be around the Raindancewx date of March 1. 

I think the NAO is the gonna be the driver.  We did this dance during January.  Once the NAO got its act together, things started shaking.  Plus, we are in the zone where the SSW (not the current one, but it could be I guess), but the one before could be showing up on mid range modeling and long range.

edit:  all we need is the MJO(like you have there) to just be not so much a pain in the neck, but just let the potential NAO drive the pattern and force confluence.

The 12z Euro control went freaking bonkers w/ the cold.  I doesn't even look like the 0z run.  The ensembles does provide some support for that.  Man, this reminds me of the mid-Jan cold snap.  The models didn't have it...until they did.  Time will tell.  

LOL.  I knew that would happen.  Send me a Christmas card.

The EURO (aka Dr No) pours cold water on early spring talk....man. It traps a piece of the TPV, and there she goes.  That is a classic SSW response.  That is a -EPO/-NAO couplet.  

And ha, ha...right on schedule the 12z Euro brings in a massive cold front around the 20th.  Not a "stop the presses" look, but a "similar to early January look."

But I will say, extreme cold IMBY is often followed by winter breaking quickly for spring.  84-85 did that.  

Just now, Holston_River_Rambler said:

I think we've been fighting that since I've been typing on this site, lol. 

 

Truth.  We just need the Pac to be neutral as I think we are in a -NAO cycle.  LOTS of discussion as to why the water is so warm east of Australia.  That has caused absolute chaos this winter in regards to the SOI and MJO.  I can't find a great explanation for it, but that area has a lot to say about our weather.  What we need is a true weak la Nina or a true weak el Nino.  Those weaker phases work...moderate, strong, or nada don't work.

1 hour ago, midwoodian said:

Reading between the lines on recent posts…fair to say it’s 50/50 we get another cold shot or prolonged cold snap in the Tennessee Valley?


.

Between Feb 15-20th, yes...almost a certainty.  As to whether it is prolonged? I think that is reaching the "highly unlikely" range of options.  I tend to think of prolonged being 7-14 days.  I don't see that at the moment, but you never know.  Nothing is a certainty in the weather, except what is occurring right now.  I would say 70/30 that we don't see a prolonged cold snap, but we could see mini-cold snaps.  Looks like a pretty typical spring pattern setting up for March with swings to warm and cold. But again, modeling is all over the place right now....the mid-January cold shot was not seen until about 5-7 days out.  It is looking likely that the LR predictions to a return to cold are likely going to bust pretty badly....but not for certain at this point.