8:40 AM

I’m sure everyone has seen the extended forecast on the GFS that dumps a significant arctic air mass into the Rockies and West coast.  This naturally leads to a large ridge over the eastern third of the nation and rather warm conditions.  The ECMWF is not as aggressive but does have a storm track through the southern Great Lakes and into New England.  So what exactly has changed in the overall pattern to support such an outcome and more importantly, why is the GFS so aggressive?

I examined all the data that would point to what should drive this pattern.  The NAO, AO and EPO index values are all rapidly falling towards negative or are already in a negative state.  The PNA values continue to rise.  So what would support a large trough in the West?  Well, the answer lies in the MJO forecast on the GFS.  This is the

GFS Ensemble MJO forecast

ensemble forecast of the MJO wave on the GFS to the left.  For weeks now, the GFS, ensemble guidance, and various other long range guidance continues to try to drive the MJO into the nuetral zone of this chart.  This was supposed to happen when the MJO was in phase 3 earlier this month, however clearly that did not materialize.  So what do we have this morning?  The MJO is now forecasted to go nuetral once again and then double back into a phase 5 state.  A phase 5 state would support a more La Nina pattern and thus a Southeast ridge.  So we can see where the GFS is going here.  I don’t have guidance for the ECMWF take on this, but I’m guessing this forecast is not nearly as aggressive as the GFS family of guidance.

So let’s step away from the model guidance and let us think on our own for a moment.  We have the major indices falling into place that would point to a trough int he East.  We have the MJO in phase 6 (a phase that supports pattern change) that is moving towards phase 7, which supports a El Nino pattern.  Stages 7, 8, and 1 support a trough in the eastern two-thirds of the nation.  All of this data should point to the brunt of the Arctic air that will be driving south to head towards the Plains and bleed towards the East, not West.  The ECMWF starts to suggest such an out come in the model guidance in fact with a cold air mass in place through Monday over the East.  As the negative EPO tanks, a large ridge forms over Alaska.  We can see exactly how there is a difference in the guidance going forward.

GFS

The next three images are the GFS, the PARA GFS, and ECMWF at 240 hours.  Note the key differences that will make a significant difference for the extended period.  The operational GFS does support a very strong ridge over Alaska, but conditions to drive the Pacific jet stream undercutting the ridge.  Meanwhile, a strong polar vortex (PV) dives into the central Plains of Canada.  As a result, the ridge builds over the East with very warm conditions.  The GFS does attempt to reposition the trough axis further east with a shortwave over the Mississippi Valley, however the PV is clearly focused over western and central Canada.

Next we have the Para GFS.  The new GFS has some noted differences.  For one, the Pacific jet stream is not blasting through the

PARA GFS

southern end of the negative EPO ridge and there is no clear large PV over central Canada.  Instead, a weak upper low is over the US/Canada board as a surface low drives towards central Ontario.  The trough axis is further east, over the easter Rockies rather than the central Rockies and the ridge is also further East.  I believe the GFS guidance for this period is wrong and way to aggressive with the cold dump in the West and is directly connected to the problems this model is having dealing with the MJO.  If the MJO went into phase 5, as suggested.  This pattern (given the other developments in the stratosphere and the convection at the date line supporting a negative EPO) would be the result.  Given the track record of the GFS forecast of the MJO however, how much confidence can on have in this solution.  One to the ECWMF!

ECMWF

There are stark differences in this model guidance compared to the GFS.  Since I don’t have guidance on the ECMWF MJO forecast, I’m going to assume this guidance is based on the MJO being in stage 6.  Why?  Well, the pattern displayed to the left continues the pattern change that is starting today.  Note the differences in the guidance.  The ECMWF produces an Omega ridge complex over the Pacific Ocean, which supports a strong ridge over Alaska while leading to a trough off the West coast.  A RIDGE forms over the Rockes and much of the West, however the amplitude of this ridge is stunted due to the position of the upper low.  As a result, the trough in the East is not as deep and the storm track is further north.  However, note that a trough is in the East.  Note the a weak upper low (which is supported by the warm stratosphere, which leads to a weak PV signature) is over the Hudson Bay.  The cold air is focused over the Plains and bleeds east.  This solution is significantly different from the very warm GFS in this time period.  Further, if one takes a step further beyond 240 hours and considers the trends of the other features that will drive this pattern, a large ridge could form over the West and refocus the cold air from the Plains into the East coast.  This would be called a “roll over” ridge development.  The idea of the roll over is that the upper low off of British Columbia slides southwest towards Hawaii while the ridge slides slight to the East from eastern Alaska to the western Rockies.  The result is a deep trough in the East.

A word of caution with this time period as well.  If this upper low off the British Columbia coast is slight further south and west than depicted this morning, that would lead to a high amplitude ridge over the West and thus a deep trough over the East.  As a result, the storm track would be much further south and would lead to much colder solutions, just in time for December 10th.