Big Pattern Change Next Week


The first large fall storm across the western and central U.S. is looking more likely with the breakdown of the mean ridge across the western U.S. and an increasingly active Pacific Polar Jet.

What we do know.

An Aleutian Low will deepen near 175 W and will act to strengthen the baroclinic zone across the central Pacific as cold air streams in along the backside of the stationary low from Russia and the Barents Sea.

Current IR imagery and the western tip of the Aleutian Islands:

The 6 hour forecast from the 0Z 20th Oct GFS:

CIMMS analyzed precipitable water over the Pacific.  Increasing cold air streaming in from the north associated with the stationary Aleutian Low will act to enhance the baroclinic zone (boxed).  Also note Typhoon Megi east of China.

A low amplitude upper tropospheric wave disturbance, partially visible in this WV satellite loop off the coast of China and south of Japan, will set in motion the amplification of the Polar Jet over the Pacific, the development of a rapidly intensifying surface low off the west coast, and the subsequent intrusion of cold air over the northwestern U.S.  Also note the tropical systems east of Megi in the WV image (this is important!).

84 hr GFS forecast of 1000-500 mb thickness fields and SLP forecasting the rapid intensification of the surface low over the Pacific (circled) and the cold air reinforcing Aleutian Low (boxed).

Note the significantly high precipitable water values associated with the system, forecast to be around 2 inches:

What we don’t know.

A fair amount of variability exists even in the first 100 hours of the forecast period (as expected).  The large stacked upper low over the Gulf of Alaska is projected to slowly translate eastward, deamplifying with time.  An embedded shortwave in the base of the stacked upper low is projected to amplify over the existing baroclinic zone, developing a compact surface low ahead of the larger incoming Polar Jet.

The Large trough south of Alaska and the already developing embedded shortwave at the base of the trough (circled):

The 24 hour 0Z GFS forecast projecting the development of a surface low over the leftover baroclinic zone associated with the mean trough (circled) and a surface wave and developing triple point low (boxed) associated with the occlusion from the aforementioned Aleutian Low.

The development of these two systems will have profound impacts on the development of the mainland U.S. storm system next week, especially the latter system.

The 60 hour GFS 500 hpa vorticity fields clearly show the forecasted development of these systems.  Circled is the former stacked upper level low with embedded shortwave deamplifying into a compact shortwave trough as noted above in the previous WV image (circled), the development of a surface low along the Aleutian Low occlusion as noted in the previous WV image (boxed), and the incoming Pacific Polar Jet as mentioned earlier (pointed line).

By the 72nd forecast hour, note the rapid disintegration of the upper low clearly visible above in the 60 hour forecast of 500 mb vorticity fields (circled).  Why?  First, the presence of the long Coastal Range along the British Columbia coast disrupts the otherwise orderly flow of air (will go far more in-depth on this topic at a later date) across regions of even terrain (the ocean, in this case), and second, the lack of reinforcing cold air associated with this rather compact low (essentially an occlusion), and the total lack of baroclinity along the mainland of the U.S. results in rapid disintegration of the system by 72 hours.

The second system is also of significant interest.

Note that, by 84 hours, the GFS forecasts the closed upper low (earlier associated with the Aleutian Low occlusion noted above) interacting with the coastal ranges of the United States.  Once again, rapid weakening ensues as the mountains “perturb” the orderly flow of air, resulting in a region of disorganized vorticity.

This weak upper level low will have profound impacts on the development and amplification of the Polar Jet over the Northern and Southern Plains as the main storm system crashes on shore.

As forecasted by the 0Z GFS at 126 hours, the above mentioned system has now progressed over the intermountain west as an open wave, and the intense cyclone (as mentioned earlier in the post) is now quite evident over the northern B.C. coast (a track which is still highly uncertain at this point, boxed in this image).  The secondary jet on the backside of the occlusion is pointed to with the green line.

Why am I keying in on the open wave across the intermountain west?  Baroclinity.  Without a reinforcing snow pack across the Canadian Prairies and the mountain west, the lack of a significant baroclinic zone will not support the development of a significant surface cyclone even with the presence of an intense Pacific Polar Jet (which, of course, will weaken due to the lack of significant baroclinity as defined by the thermal wind equation… http://amsglossary.allenpress.com/glossary/search?id=thermal-wind-equation1).  With no reinforcing snow pack, we need to look elsewhere.  Where?  The Gulf of Mexico!

Note, by forecast hour 132, the GFS has now positioned the open wave over Texas (circled) while the powerful Polar Jet above now plows across the intermountain west.

Note that, under this flow, the low level flow off the Gulf of Mexico is limited to the SE U.S. (surface theta-e for simplicity):

Looking back at the 12Z GFS run, note here at the same forecast hour (144 here) as the 0Z run, the position of the open wave is projected to be over the SE U.S. instead of Texas:

This vastly different solution supports a prolonged period of lee troughing and subsequent low level S-SE flow off the Gulf of Mexico, and the establishment of a much more pronounced baroclinic zone over the plains.  The 12Z GFS goes on to blow up a 966 mb surface low over the northern plains by forecast hour 180 while the 0Z run develops a still strong but much tamer 978 mb surface low slightly farther east.  I am going out on a limb here, but it is highly unlikely the 12Z GFS solution verifies across the plains due to no reinforcing snow pack across the intermountain west and the Canadian Prairies (some air mass modification is likely) and what is looking to be a closed Gulf of Mexico due to the slower progression of the upper low across the intermountain west.

The 12Z’s rather generous surface low:

Why does all this discussion matter?  It goes to show just how complex weather can be even 5-6 days out.  Small deviations in the simulations of rather “insignificant” features (as we have shown here) can have far reaching effects with time as errors rapidly amplify.  Don’t forget models are ingesting hundreds of different data observations at differing times and all with varying errors associated with them–these errors will also grow with time (hence ensemble modeling and the perturbation method).  Think of everything going on in this scenario: a low amplitude wave over China, three tropical systems over the Pacific, the development of two compact surface lows over the Pacific, the interaction of those systems with the coastal range, etc. etc. etc.  For this post, we won’t even talk about the models themselves, all the parameterizations and assumptions they are making, the complete lack of a turbulence solution in the Navier-Stokes Equations, lack of infinite and continuous observations, model filtering, etc etc etc.  Maybe a post on numerical models is in the making…

I hope this post illustrates why it is not a good idea to rely on one operational model run for longer range weather forecasting.

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About jason ahsenmacher

I am a meteorologist, and I like baroclinic instability. View all posts by jason ahsenmacher

One response to “Big Pattern Change Next Week

  • Inudaw

    In the short term, the current water vapor imagery shows a rather classic blocking structure with a mean ridge over the western portions of the United States with a upper level low poised right under the ridge in southern California.

    That low finally moving out from southern California and pushing east with the attack on the ridge from the west out of the Atlantic are two key features to breaking down the ridge in the west to lead the pattern change over the next week or so.

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