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John1122

December 2020 Medium/Long Term Pattern Discussion.

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We are hitting well into the window for December discussion. My best event of winter was on December 12th last year and I believe in 2019 it was a December event for the far Eastern areas.  That was as the pattern of a very cold November bled into December. To get there this year we are going to have to count on the pattern to switch in December itself. Hints that it may, at least for a short time, as we head into the last few days of November and first week of December. 

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The GFS d9/10 has a little Miller A action going after having a cutter earlier.

It got our Kentucky areas and was close to more of us north of 40. 

The storm keeps showing up, but we will see if the GFS is up to its usual tricks. The Euro had some snow showers around in the same time frame at 12z. 

 

sn10_acc.us_ov.png

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Ensembles past day 10 look pretty similar over NA this AM to what they have looked like the past few days. Really unique tilt to the ridge over the western US, at least as long as I've been looking at ensembles (not too many years). 

Kinda positively tilted. Using the CMC ensemble this AM since there weren't really any big changes to the GEFS or EPS.

giphy.gif

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The 6z GFS has that red herring -NAO at the end of its run.  But yeah, the GEFS and GEPS have trended nicely for the SE.  Cold source is lacking but the GFS at 6z shows repeated storms with just enough cold air.  Almost all cut just enough West to put E TN out of the game, but long way to go.  The Canadian has been leading the way on this.  Their operational got an upgrade I think last winter.  Did the ensemble?  Again, we sit in a very similar position to last winter...two respected ensembles showing change.  The EPS is almost there.  At some point we have to reel in a winter pattern, right?  Great discussion above.  Was nice to get up and read it.  

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Ensembles all followed cooler overnight. Euro, GFS, Canadian. I don't look at the teleconnections weekends. More on Monday. Fingers crossed!

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EPS sees a window for eastern forum areas around Dec 1:

giphy.gif

GEFS has some southern lows too, at that time (sorry couldn't find its city charts on Weathermodels)

giphy.gif

I would still rate it at something like a 10% or less chance, but it is a window, at least for now. 
 

EDIT to add that I think this is the window John was talking about last night. 

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12z EPS is on the boat now with the 18z GEFS and 12z GEPS.   Beginning around 156, trough slides into the SE and holds. Fingers crossed, we are not tracking a d10+ change now.  The post Thanksgiving trough is the beginning of however long this lasts - I refuse to say pattern change any more.  LOL.

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That is a beast of a storm the Euro is advertising in John's post above. Surface Pressure drops a little over 20 mb in 24 hours and our temps would really crash as the front came through. Even though it doesn't bomb out in a location to give us a ton of snow, I would love to watch it develop if it came it pass in the way the Euro shows. It would look at feel very wintery and just be fun to track. 

It looks like four pieces of energy ultimately phase over the course of several days:

giphy.gif

 

Each piece, as it rotates through, would produce some upslope enhancement for the typical areas. 

EPS members aren't loading on weathermodels, so I can't see how enthusiastic or not the members are. I wouldn't say the GEFS members are wildly supportive of the Euro's solution, but there are some similar hits. Timing seems to be the key, as it usually is with these big phasing storms, when the models see them 8 - 10 days out. 

But, here is a mean of the 3 major ensembles, GEFS, EPS, and GEPS:

giphy.gif

all show a ridge building in that time period across the Rockies, which would allow energy to drop in and pool somewhere in the OH Valley and/ or Great Lakes, it just a question of whether or not the pieces of energy catch each other and where they do that. Overnight Canadian model shows what could happen if all the timing is off. 

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Regarding the pattern after d5...

Looking briefly at teleconnections this morning, looks like the initial ridge out West will be +PNA driven with the potential for it to shift to an -EPO rigdge after that.   The one think I am wary of is that modeling during the fall advertised big cool downs in the East and verified out West.  A silver lining is that they did "see" the NA shake-up.  This has a different look than those false alarms.  Those involved really cold anomalous air masses and (as noted prior), they often had a very erroneous -NAO.  Now, a -NAO is showing up on ensembles right now but thankfully it is no overly strong AND the pattern appears to be driven by ridging out West and not changes over Greenland.  In fact, the changes over Greenland are a result of the ridge popping out West and changing down stream wavelengths.  One constant is that the Aleutian's low is back on modeling.  Last winter, massive ridging in that area resulted in winter ending in the East...it just never left it felt like.  Something to watch for, and it is important, is weather we can get cold air transported into NA with the Alaskan BN heights leaving that area.  Overall, the pattern to begin December looks quite stormy.  The last two days of the Euro operational are quite cold(winter type cold).  If we can get 10-15 days of winter in December followed by a thaw...might be able to fit-in one more cold shot after that - but that is getting ahead of myself.  

On a side note, this is kind of opposite the last two years as John has noted.  When we hit Thanksgiving during those years, we could see modeling beginning to show the storms which would end the November cold.  This time, we may be tracking the onset of cold weather(unspecified length).  I do want to be super clear that I am very unsure of the duration of this potential western ridge.  During the past two winters, they have not stuck around for long.  We will see if that changes.  The warmth in the NE PAC might work to offset the La Nina - we hope.

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12z CMC is sharply colder with middle teens(for lows) in E TN during its d8-10 run and single digits in the western forum areas.  It also has snow showers deep in Dixie as well.  If Holston's powerhouse storm develops, going to feel sharply colder after the nice temps of the past weeks.  

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CMC was an extreme run for sure.  The GFS at 12z also amplifies but later in the run.  We have been here before....could easily be a head fake as the CMC operational develops a massive block(and I mean massive) over the top.  Those types of runs have tended to not materialize.  So proceed with caution.  That said...have to think that stable vortex would be jostled severely if anything even close to those two operationals occurred.

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12z Euro operational also is quite cold after day 8.  Almost hate even talking about cold after the last head fakes, but it is there on modeling.  So worth a mention to quote a friend.

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IIRC my first winter here, November was mild until it got cold after Thanksgiving, and culminated with a good January snow. Oh yeah White Christmas too 2010 before that January 2011 snow.

Past performance will not help future results, lol!

Oh crap April 2011. Nope! Even me the severe wx enthusiast. Nope!

OK getting serious, since this is the true December thread, I figure it'll take a few cold fronts to set the table for anything after mid-December. Except for at elevation, I doubt much happens through first third of December. 

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12z EPS looks good verbatim, control looks even better.  That said, we need to be wary of the trough slipping every so slightly westward as a trend during this run.  The past two cold shots out West were depicted in the East, but came west with time.  Not saying that is going to happen...but the -NAO is starting to show up, and that makes me very wary.  Good looking 12z suite....let's see if we can get something good to verify finally.

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48 minutes ago, Carvers Gap said:

12z EPS looks good verbatim, control looks even better.  That said, we need to be wary of the trough slipping every so slightly westward as a trend during this run.  The past two cold shots out West were depicted in the East, but came west with time.  Not saying that is going to happen...but the -NAO is starting to show up, and that makes me very wary.  Good looking 12z suite....let's see if we can get something good to verify finally.

Really until the GFS gets on board with such a deep trough im very skeptical of it. Like you have noted we have seen this same song and dance before.  Lets get under 5 days then I might fully buy in.

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3 hours ago, Met1985 said:

Really until the GFS gets on board with such a deep trough im very skeptical of it. Like you have noted we have seen this same song and dance before.  Lets get under 5 days then I might fully buy in.

It looks like we are going to get some sort of banana high over the top.  Probably a little early for that to work outside of the mountains as Jeff notes.  That said, the cold source is a big mystery.  Is there a decent source or not.  I tend to lean on the Candadian right now as it has done a really good job with this change in modeling....that said, I think it is likely too cold.  Has the GEFS been upgraded?  I know the operational was upgraded and the ensemble was to follow.  It it has been upgraded since last winter, it may have some new winter biases.

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The Canadian went all in again and is stirring echoes of 1989. It has temperatures in the 10s on the Plateau and mountains in the afternoon on December 2nd. 20s elsewhere. Freezing weather and snow showers all the way into the deep south, 31 in northern Florida. In the period right after that the GFS went nuts and dropped 2 feet of snow in the Smokies, 6-10 inches on the northern Plateau and SEKY. Basically fantasy land stuff on that GFS run, but always fun to have winter weather back to talk about. Lets us get a look at model biases early and see if they hold up through winter. 

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2 hours ago, John1122 said:

The Canadian went all in again and is stirring echoes of 1989. It has temperatures in the 10s on the Plateau and mountains in the afternoon on December 2nd. 20s elsewhere. Freezing weather and snow showers all the way into the deep south, 31 in northern Florida. In the period right after that the GFS went nuts and dropped 2 feet of snow in the Smokies, 6-10 inches on the northern Plateau and SEKY. Basically fantasy land stuff on that GFS run, but always fun to have winter weather back to talk about. Lets us get a look at model biases early and see if they hold up through winter. 

Would not take much to beat last year; heck, at this point, I would like one hard freeze so I could stop mowing grass. 

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0z EPS moved the lower 500 heights back eastward on this run.   While no ensemble has a perfect set-up, they do provide a great example of a pattern which was originally a trough in the East, then a ridge, and now a BN heights in the SE.  As John noted, the Canadian is very cold.  I think it gets this solution by driving a strong storm west of the area.  The GFS seems to lack some organization at 6z.  Seems like the Euro is similar to the GFS.  I could honestly see either outcome as a reality.  It does look like the Monday to Wednesday after Thanksgiving are going to have the potential to be much colder than we have been experienced of late.  Again, I kind of like the GEPS right now as it caught the changes about 48 hours earlier than other global ensembles.   That said, they all look pretty similar now.  I will say, I am not a huge fan of the present, but temporary -NAO.  Again, in the past that has been a harbinger of a head fake.  In this case it may just be one which is forged by the heights over the East being forced to higher latitudes.   The one signal that appears to be legit is a suppressed storm track and a stormy EC(not necessarily snow...).

 

edit:  Post has been checked for typos.  I was in a hurry earlier, and some of that made zero sense.  Should be fixed now.  

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12z GFS starting to move in line with the cmc in terms of cold air moving in after thanksgiving.  LP swoops In and causes storms to rotate counter clock Wise From the Midwest down into Tennessee.  Multiple storms have that counter clockwise rotation on modeling.  Something fun to talk about at the very least, we will have to see how things progress over the next few days 

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12z Canadian was a bit warmer but still very cold with snowy weather across the Tennessee Valley and well down into the Deep South.

The GFS is also very cold with the weird long duration snow event due to the storms rotating through as mentioned by AMZ where it's snowing from December 4th - 8th in at least some part of the Valley region.

The Euro is seasonable transitioning to  cold and suppressed with a snow/frozen event in the Eastern Carolinas with a massive cold 1050 hp and a storm gathering in Texas at the end of the run. It's quite a tease to say the least. 

 

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Well hello there ECMWF weeklies! And I see you CFS weeklies. Can they be right?

Well if the MJO actually progresses, it would be supportive going into cooler phases (Southeast US). Multi-month +ABNA background is slightly below normal heights here, but AN everything Canada. A cooler version of that (progged normal heights Great Lakes) would verify cold down here.

Both weekly products have that colder look weeks 3-4. AN precip East Coast with BN precip Mid South is not a bad look. Remember snow carries lower liquid content. That AN precip can stay southeast of here as rain. No promises. It just feels nice to write about something other than blowtorch or covid.

BONUS: CFS weekly snow charts get the Mid South week 3 or 4 depending on which presentation. PATTERN recognition is good for the higher elevations too (Mountains, Plateau).

Then Christmas severe verbatim. ! week 5-6 forecasts are disputed.

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Somebody gif that run.  Might not see a model run like that in the history of modeling.  That cutoff forms around 210 SE of Memphis and precip doesn't leave SW VA completely until 312.  Pure wildness.  Snow totals from that run should be pretty high.

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I have to get with the plan.  The 12z GFS run had a similar cutoff as well.  Do I think that happens?  Likely not as other models don't have that.  But with that blocking over the top in Canada, it is a good lesson on how storms cannot climb(add latitude) with AN heights over the top.  Almost like bowling ball season...  Might be some slow movers as steering current are lacking. 

edit:  accumulation map is in banter since we are looking at a storm from 210-300.  

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That one will be a big time clown map from about Murfreesboro east. Especially with snow sticking around Eastern areas another 36 hours after it starts.  Looks like the Euro from earlier except the Euro was so suppressed only Eastern NC was hit. 

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