2330  EST  18 JAN   …. STARDATE   201201.18

I love forecasting snowstorms  in the Pacific Northwest.    It doesn’t happen very often  but   when it does  happen… is really quite interesting and very different from what we get in the northeast US.  Back in the winter of 2006-7  or  2007-8    a really great pattern set up for the Pacific Northwest around Christmas time and that  region  got   hammered with significant snow and ice storms –even  Christmas Day in Seattle and Portland and much of the northwest.  My experience over the past few decades has been that if you are not getting the Pacific Northwest right   –the West Coast and general but especially the Pacific Northwest– than the rest   of your  forecast is going to suck Moose  balls.

I am not going to spend a lot of time talking about the moderate snow event which is likely to move through portions of the upper Midwest the Great Lakes and the interior of the Northeast US  on JAN  20-21.  In fact if this was a regular winter this should be a pretty ordinary event and not get any intention are all.  Essentially what happens is that a wave of   Low pressure develops on the front as the large cold high over the north these slides off  the Maine  coast.  The Low tracks   into  WVA  then    through PA.  It’s a pretty weak system and a dozen have a lot of moisture with it and it’s moving pretty fast.  Still for the snow lovers  in the NE…  some snow for folks in the Northeast is better  than NO snow.
Now I know many of you in the eastern U.S. and especially in the northeast are probably shrieking or thinking    “Hey   DT  you  ignorant geek … who  Gives  a rat ass  about  the  the   Pacific Northwest?…”     If you are asking that question or thinking those thoughts…  then you’ve got a problem because you  are missing something really important about this particular snowstorm.    Not so much  about   the Pacific NW   snowstorm  per se… but  what is causing it …the implications for the entire pattern as  we  move into  February.  So pay attention there   sparky and let me tell you how Big Bertha  and her huge Butt plays into this whole thing .
By now I am sure  some of you have heard or read about the potential for significantly different pattern setting up  over  the next two weeks so that February and perhaps March 2012  end up being colder than normal or… at least  the threat or the possibility of it being colder  than normal over the eastern CONUS .    There are strong indications are  this MIGHT     happen but we have to be careful here.  We  have been  down this road a couple times before…  and every time we have gotten close or looks like the pattern is starting to change… it  turns to     Moose  Crap.    So if you have your snow boggles on and you all excited because somebody in Pennsylvania or in New England is telling you that this time it’s really really really gonna happen…  be careful.

A word to the wise.  Saying the same thing over and over and over again… or in this case forecasting the pattern change the pattern change the pattern change…  only to see the pattern finally change…  is not REALLY  forecasting weather.   What you are doing is saying the same bullshit  over and over  and over.   In other words if your  forecasting skills in the extended and long range   have been really good  in the last two winters…  But now you seem to have trouble figure out was going to happen and when / IF the pattern change is going to occur…    that  is  probably a clue  you are  doing something wrong.
This first image shows us the long  terms trends  of the NAO and the AO since September. 

 As you can see from the image it is readily apparent  that the NAO and AO have only drop into the negative territory twice since the up SEPT 15 !!!    But more importantly since December there had been a several tries or surges where both the  NAO and the AO have reached the Neutral Line  — which I have  Number as  you can see.   And every case the model data showing the AO or the NAO   would  drop to  negative territory but in every instance  -  EVERY    INSTANCE–      both the  AO and the NAO bounced back up strongly into positive territory and all of the model data turned out to be disastrously wrong.      So again if you’re absolutely certain that the pattern to  turn around  and that it is  going to  turn out  to be a great February and March 2012… BE  CAREFUL.  The trend here is not your friend.  Of course that’s not to say the trend can not be broken but it does need to be respected .

BIG  BERTHA…
This next image shows the overall pattern back on January 15.   As you can see from the image itself I’ve highlighted the massive explosion in the WPO  as a turn negative and built a huge rage into the Bering sea….  and Alaska…  that expanded all the way into the western and Asian  side of the Arctic circle.   This  explosiove   Ridge developemnt  was caused primarily by the stratospheric warming event.  Also keep in mind that when the models of first detected this development   –  back in late DEC and  early JAN– most of them showed this would lead to a much colder middle and the latter January…  Which as we  all now know…  did NOT  happen.    Instead… the huge Ridge in the Bering sea and the western side of the Arctic circle moved into eastern Siberia.    This in turn allow the polar vortex to move into western Canada and eventually into the Gulf Alaska were it is currently  situated.  It was this very movement of the  PV which is  WHY the arctic air came pouring over the Rockies in the continental divide into the Pacific Northwest and the Seattle Portland metro areas!!!   

But more importantly is how the pattern slowly evolves.  This next image shows the  72  hr — 12z  JAN 18  run-   valid  JAN 21.  I  have highlighted the individual disturbances or impulses and Pacific jet stream and you can clearly see the amazingly cold bitter arctic air mass over Northern Canada  that is associated with this  super   positive  AO.    This is one of the reasons why having such a strongly positive arctic oscillation is so bad for those wanting winter storms over the central or eastern CONUS.  As you look at this image    you can  clearly see   that  in this sort of  flow there  is very little way to get that cold air to come southward .

96 HRS ECMWF…  We can see a new development which is a  core  or center of the  PV reemerging or developing over anchorage or in the Gulf of Alaska.  Meanwhile more strong impulse is a batter the West Coast move across the Rockies into the planes bringing in much milder air

120 HR  ECMWF   VALID   JAN 23…  In this image I have highlighted the upper air map at 500 MB  and the  SURFACE map.  The piece of energy in the central Plains   develops into a significant winter storm for the Great Lakes which pulls up a lot of warm air into the northeast U.S.  Chicago   COULD have another  significant snowstorm however.   More importantly however it is this buildup of warm air along the East Coast which is driven northward into the high latitudes and forces the Heights to rapidly warm or increase over Eastern Canada.  Sometimes is buildup of warm air and increasing Heights can cause the NAO   to flip especially when it’s followed by several surges of warm air from the eastern U.S. into  eastern and southeastern Canada.

That is exactly what happens over the JAN  24-25-26   time frame.

This  image is a hemispheric shot of the European model at   DAY 6 .. JAN 24 .  I  have highlighted some important features which are occurring at this time over the hemisphere.  The most important  is  feature  #1 which shows a another Ridge exploding northward into the arctic regions from the northern Pacific however this is significantly different than what we saw back on January 14-15.  That ridge   exploded northward over the Bering sea and we did not have a PV  (Polar Vortex)   in the gulf Alaska  which will  likely have   JAN 24-25.     This  Ridge develops over eastern Siberia…  because of the new vortex which forms over the Sea of Japan.  The development of a deep vortex / 500 Low  in the Sea of Japan is  text book teleconnection for a much colder pattern over the eastern U.S.    

   ASSUMING   this is correct   the Ridge in Eastern   Siberia will begin to drive the  PV in the gulf Alaska…  to the   east into Western Canada which begins to alter the entire pattern.

At Day 7  JAN 25…  The Great Lakes Low is now over southeastern Canada but it’s still feeding up warm air into southeastern Canada.  However right behind that we see a major trough amplification over the entire Plains  and the Midwest   ( see  THICK  BLACK Line).  This forces another surge of very warm air into the East Coast.    Meanwhile over the West Coast and the eastern  north Pacific   the  Gulf of Alaska  PV is being driven east   while a  ridge is beginning to form off the California Coast.

DAY 8 …    Again I have  matched up  the upper air map at 500 Mb with the surface  map   so YOU  can follow how things develop.  The new  trough over the Mississippi Valley has caused   a surface Low to form over Alabama…  while warm air surges from the Deep south all the way into the Northeast U.S.     Again I cannot stress how important this warm areas with respect to feeding the developing  or  Building   500 Mb heights over  eastern  Canada …  Which in turn brings about the face change in the   NAO.  So even though I know winter weather lovers in the eastern U.S.  will hate these warm temperatures the two serve a purpose! 

In this next image DAY 9 …  We can compare the European vs. the GFS from a hemispheric perspective.  Both models or a remarkably good agreement as  they  BOTH  show the PV in the gulf Alaska…  They both show some sort of Ridge trying to form off the Southern California Coast in the eastern Pacific…  And they both show some sort of building or increasing Heights over Eastern Canada.

By DAY 10   –lets  use the   EURO ENSEMBLE 500 mb maps – the changes are underway big time.  Big Bertha has now left the gulf Alaska…  or rather is leaving the gulf Alaska and sliding into Canada.  It’s doing so because of the developing ridge  over the Bering sea and eastern Siberia.  In addition the Heights are rising rapidly over Eastern Canada and southern Greenland which indicates the possible development of a -NAO.

The 12z GFS  ensemble – JAN 18 run — strongly agrees with this overall development of the pattern.  In this image we can see the 6-10 day    500 MB  height  CHANGES.  This is useful in showing us where things are trending..  We can see a very strong negative anomaly over the gulf Alaska  which is the strong Polar Vortex.         But we can also see the Heights beginning to increase over Eastern Canada.

Looking at the same product but now for the 11 to 15 day…  We see major changes.  The GFS  ensemble shows the vortex that was in the gulf Alaska  has now shifted east into western or  Central Canada.  The Heights are rapidly building in the Gulf of Alaska and we see increasing Heights over Greenland as part of a  -NAO.

 
If we look at the  12z GFS  ensemble itself   — the 500 mb  mean –  at   DAY10…  We can see a very close match to what the European ensemble model is showing.  Note the strong vortex over the Sea of Japan!     Note the    ridge over   far eastern  Siberia and the Aleutian islands…  And notice the  dark purple the long date and shading extending from North Central Canada down  to NW Ontario Canada.  This tells me that the model is showing the  PV is splitting apart or redeveloping.

312  HR  the 12z   GFS  Ensemble has evolve into a completely different and full blown winter pattern over the north American continent.  As I have highlighted here…  There is a new deep  PV  over the Sea of Japan…  A very strong Ridge e over the Bering sea and  new PV   with 2 centers: one over  far N  central  Canada  and the  otther over  James bay .  If we accept this model as verbatim or being fairly close to the truth…  We now have a raging +PNA  pattern  a   – AO   a  – NAO and the eastern U.S. is now in business for February winter storms.

WHAT CAN GO WRONG? 

This is probably the most important part of the whole discussion given what’s happened this winter.  There are so many things that could go wrong with this sequence of events as shown by the weather models.  But it’s important to do this because you don’t want  get caught up in the mental loop  or  trap of believing that the pattern change HAS  to occur because the weather models say it has to occur.

1) the development of the deep   500 Low that becomes a new polar vortex feature in the Sea of Japan.  If this feature does not develop   THEN the entire pattern change will fall into a pile of  Moose    crap.      It is this feature which causes the Ridge to develop over eastern Siberia and the Bering sea.  If there is no big  500  mb Low  in the sea Japan in the last few days of January then   the ridge  will act  TOTALLY different…  And it will not cause the changes to occur across North America.

2)  this is related to  #1  above.   Suppose the   PV in the Gulf of Alaska it does not move.  Say goodbye to the pattern change.

3)  THE  WARMTH  over the   eastern  US does NOT   reach into   eastern  Canada …  Which in turn would mean a much weaker  -NAO.  Or it might only force the NAO   to go to briefly neutral…  which as I stated at the beginning of this essay  has been the trend all winter long.