Tag Archives: Gulf of Mexico

The Importance of Model Timing: Ohio Valley and SE Rain Event

Click on images to see an enlarged view.

The timing of upper level features in numerical models is crucial to the eventual weather patterns they subsequently simulate.  There are times, however, when the difference in timing can have significant feedback effects with errors which grow rapidly with time.  The forecast for the Ohio Valley and SE U.S. shows significant model divergence within the first 48 hours amongst the current 0Z NCEP model guidance.  The GFS is illustrating a large rain event while the NAM is much weaker with eventual cyclogenesis and keeps precipitation much farther south.  Let’s take a look why they are so different and why the current 0Z NAM is likely going to be wrong.

All numerical guidance is more or less the same by 24 hours with the large scale synoptic features.

Both feature a large scale upper trough over the central CONUS with a low amplitude shortwave embedded near the base of the trough.

Fast forward to 33 hours and things still look mostly the same.  However, upon closer inspection, it is clear the NAM has the leading shortwave at the base of the trough displaced further W than the GFS–in other words, it is slower.

The GFS, shortwave circled:

NAM:

Also note the slightly higher amounts of shear vorticity upstream of the shortwave in the NAM compared to the GFS.  Essentially the mid-level speed max is displaced farther W in the NAM.  Also note a very low amplitude and subtle downstream ridge is developing in the GFS ahead of the shortwave.  Why?

Note in the GFS 850 hpa theta-e field a large region of warm air advection has developed ahead of the upper level shortwave (circled) with a stronger low level circulation.

NAM:

Note the NAM features a much weaker wave as opposed to a developed low level circulation.  While the theta-e profile is similar to the GFS, the NAM features no warm air advection as the 850 hpa winds are mainly parallel to the theta-e gradient.  I can’t hammer the point home more, but low level warm air advection decreasing with height lends itself to upper level height rises.

Stronger cyclogenesis is a positive feedback process.  As was shown in the previous post as well, an upper level baroclinic wave interacting within a region of low level baroclinity results in developing cyclogenesis.  Vorticity advection by the geostrophic wind in a shortwave trough results in height falls aloft and forced synoptic ascent.  This forced ascent, if above the level of non-divergence, and because the atmosphere follows the laws of mass continuity, will result in a low level mass response and increasing low level convergence/cyclogenesis.  Low level diabatic heating (see the previous post for a more in-depth reasoning) mainly owing to the release of latent heat as low level moist air rises and condenses will only hasten the process–and this system has ample amounts of Gulf moisture to process.  Meanwhile, the thermal gradient in the low levels tightens and frontal boundaries become more defined owing to processes such as horizontal deformation (of the many which can result in frontogenesis).  This is all due to the increasing low level convergence/mass response to upper level forced synoptic ascent.  Mesoscale ascent/convergence along the fronts increases owing to the increasing frontal thermal gradient which results in even more low level mass convergence and increasing surface pressure falls.  Meanwhile, owing to continued synoptic ascent in the upper levels (differential cyclonic vorticity advection) and subsequent cooling, upper level heights begin to fall at a faster rate.  Because the thermal gradient in the lowel levels is tightening, the thermal wind relation

states upper level winds must increase with height.  So not only does the jet stream increase, but upper level heights continue falling at an increased rate, therefore, the amplitude is increasing.  Jet stream divergence increases due to increased cyclonic curvature in the upper level height field and a stronger jet max (as well as a shorter wavelength if the system takes on a negative tilt), therefore, stronger mesoscale ageostrophic jet circulations develop.  Cyclogenesis is now increasing rapidly; this positive feedback loop continues until the low level baroclinic zone has been sufficiently processed.

With that in mind, it is much easier to understand why timing is crucial.  In most cases, the speed of an upper level shortwave just means the timing of cyclogenesis may develop at a different time, but it will develop in a similar fashion regardless of the timing.  In this case, however, a delayed upper level response to a shortwave trough (the NAM) with the large scale trough propagating eastward will result in less warm and moist Gulf air to interact with.  That is, because the NAM is slower with the shortwave, cyclogenesis will be delayed and the positive feedback loop will not be present ( or will play a much smaller role).

Skipping ahead 6 hours, note how much things have changed.

By 39 hours, the upper level shortwave has now “ejected” into the Ohio Valley with an increasingly amplifying downstream ridge ahead of the shortwave.

The NAM, however, features a flat height field ahead of the shortwave with the shortwave much farther W.

As one would expect, the low level mass fields are completely different with the GFS developing a much more intense and deep surface low by 45 hours as deep cyclogenesis has developed strong cyclonic rotation through the depth of the troposphere.  In the mid levels, the GFS features a strong closed circulation while the NAM has a broad open wave.

NAM:

The surface fields are even more dramatic as the GFS has a strong sub 996 mb surface low while the NAM has broad and weak ~1008 mb low.

These differences result in a vastly different precipitation field:

GFS:

The differences are vast.  The GFS solution yields moderate to heavy precipitation over much of Indiana and Ohio associated with a large TROWAL (associated with the strong and deep cyclonic rotation) while the NAM is almost completely dry only 48 hours out!  You can’t really make a compromise because the solutions are so vastly different and would yield a cruddy forecast.  In my forecasting experience, when the NAM features a slower propagating low amplitude shortwave trough than the GFS, it is wrong ~ 90-95% of the time.  Under certain circumstances (as was shown with last storm…read the previous post), the NAM can be right with a slower solution under rapid cyclogenesis events.  However, those cases usually feature much more amplified and intense shortwaves and/or intense PV anomalies.  In this case, I would give the NAM a less than 10% chance of being right.  Because of that, I would simply not even include it in the forecast.  Under these circumstances, it is not unheard of for the NAM to not simulate a realistic solution until the system has already developed.  In other words, it is wrong all the way leading up to eventual cyclogenesis.  I suspect the 12Z NAM will correct a lot, but I doubt it will completely fix it.  As for the GFS, I do believe it is a bit too intense and far west with its surface low track and precipitation field, but it is most definitely the better solution.  The GEM seems like a more reasonable solution with most of the heavy precipitation staying across southern IN/OH with lighter amounts farther north.

In my experiences, the regional GEM is a far more reliable model than the NAM under most circumstances.

This post goes to show how important timing of upper level features can be on the forecast, even in the short-term (in this case only 48 hours).  It also shows how rapidly feedback effects hasten the process of cyclogenesis (IPV thinking explains this very nicely).  Most importantly, this example illustrates why forecasters must analyze both the synoptic and mesoscale features present as opposed to simply reading the model output without interpreting it.  Simply looking at model output QPF or surface fields (i.e. surface pressure fields) without considering the meteorological processes developing those fields will result in less accurate (worse) forecasts.  Learning model biases takes time and requires attention to detail.

The butterfly effect?  Chaos Theory?  Dr. Lorenz proved himself to be many years beyond that of his peers–a genius amongst geniuses.

http://eapsweb.mit.edu/research/Lorenz/Deterministic_63.pdf

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Incredible Jet Stream Divergence

No amount of superlatives can describe the storm taking shape over the central and northern plains.  My projections of surface intensity in the previous post were completely wrong (I believed 966 was too low).  With any sort of extreme weather system, any particular dynamic and/or kinematic field is expected to be impressive.  This storm, however, is displaying an incredible amount of jet stream divergence which shows up in spectacular fashion on satellite imagery.    Let’s take a look.

IR satellite image at 15Z with the center of the jet stream noted at 250 hpa with the red line.  The green circle denotes the upstream jet streak in excess of 160-180 knots.

12Z 300 hpa analysis with winds and divergence plotted (thanks to http://www.patricktmarsh.com/, I never knew jet stream level winds with divergence plotted existed!):

As analyzed by the 12Z GFS @ 18Z (250 hpa):

Note the increasing jet level winds on the eastern side of the upper level trough from 12Z to 18Z.  Let’s investigate further.

The 12Z HPC surface analysis has the cold front analyzed in northern CO:

Note the well defined lee cyclone in the Front Range of CO extending into New Mexico, an atmospheric response due to the cross-barrier flow blocking effect of the Rockies.  Large and long mountain ranges block the otherwise orderly flow of cold air advection, resulting in a geostrophic adjustment process.  Lee cyclogenesis acts to enhance the low-level south flow and, in the case of the US, the flow of warm and moist Gulf air northward.  The “blocking” of cold air into the plains acts to “displace” the cold air aloft from the low level warm air in the plains in the vertical.  Mentioned in the previous post as well, the thermal wind equation comes into play here.  http://amsglossary.allenpress.com/glossary/search?id=thermal-wind-equation1

The change in the geostrophic wind with height (vertical shear) is related to the thermal gradient.  The jet stream, therefore, is a manifestation of intense baroclinic zones and upper level fronts, not the other way around.

Let us put it together a little more.  Take a look at the GFS 12Z analyzed 1000-500 mb thickness fields:

The location of the surface trough is noted with the green line with strong surface ridging behind the front (as expected).

Note that, at 12Z, the upper level cold air isotherm packing is lagging behind the low level cold air:

The effects of large scale flow blocking become much more apparent here as we put things together.  The effect of the broad and high Colorado Rockies is to block or retard the low level progression of otherwise orderly cold air advection.

BY 18Z, the region of cold air aloft has now become superimposed over the region of lower level cold air associated with the low level front, currently being blocked by the high terrain of the Rockies.

Oh, but wait.  What did the thermal wind equation state?  The picture is becoming slightly more clear now.  The juxtaposition of cold air aloft and at low levels along with the continued effect of lee cyclogenesis due to cross barrier flow results in southerly warm air advection in the low levels of the high plains.  These processes work to enhance the baroclinic zone along the mountain barrier.

18Z GFS forecast shows how much tighter the 1000-500 mb thickness field has become due to the aforementioned processes.  Also note the high-low pressure couplet that has developed across CO with the decrease in the surface pressure of the lee cyclone, now to 984 mb.  Cold bora winds downslope into the plains as the cold air “pours” over the Front Range.

As expected from the thermal wind equation, our jet stream has now become stronger on the eastward side of the curved jet stream over our now enhanced baroclinic zone across the high plains (circled).:

Also worth noting here are some of the terrain flows that can develop under such circumstances.  In the case of the Front Range, mesoscale terrain flows can develop around or over regions of decreased height in the Rockies.  Extreme pressure gradient forces are relaxed through relatively narrow regions of the terrain, resulting in terrain enhanced gradient forces.

Both the Ferris Mountains and the Laramies reach elevations above 10,000 feet with the Snowy Range (named the Medicine Bows in Colorado) extending to over 12,000 feet.  Gaps in the terrain extend down to 7500 feet in Laramie, WY before reaching approximately 4600 feet in Akron, CO.  With I-80 along southern WY being the only large scale “outlet” for subsident air over the Great Basin, winds can become rather extreme.

The obs from Akron, CO clearly show frontal passage (boxed red) with the typical pressure falls preceding the front followed by rapid pressure rises.  Of course, peak winds occur during the time period of rapid pressure rises (boxed green) and strong descent due to efficient mixing in the convective boundary layer acting in conjunction with descent on the backside of the frontal circulation (circled red).

Let’s move on.

Curved jet dynamics result in regions of strong ascent/descent (ascent on the exit region, descent on the entrance region) on the poleward (cold) side of the jet stream.

Also note the increasing amplitude of the trough and the “digging” nature of the jet.  Is this a result of QG Chi interepreted height falls associated with abnormal thermal advection patterns noted earlier?  Think about that.  Do jets “dig” or do heights fall?  I will let the readers decide.

Goes satellite derived WV winds at 18Z suggest both the NAM and GFS are under observing the jet streak winds on the downstream portion of the trough which would result in even greater values of jet divergence.  Circled isotach at 120 kts (18Z GFS peaked at 90 kts from 300-200 hpa).

This jet stream divergence was manifested in spectacular fashion on satellite imagery:

And on multi-spectral satellite imagery:

Here is an animation of the cloud patterns associated with this divergence over Colorado.  This is the best way to see the divergence pattern and associated cloud field:

Also note the “folds” oriented perpendicular to the flow (easily seen in the visible sat images).  Personally, I have no explanation for these features.  It seems plausible the N-S oriented CO Rockies have an influence, but I personally have no reasoning.  Anyone with ideas or explanations please let me know.

Update:  The expected smooth nature of the jet cloud pattern over WY is typical earlier in the day.  As the system interacts with the Front Range of Colorado, the folds seem to originate in the region where enhanced vertically propagating mountain waves often develop.  This seems like a plausible explanation, but I will have to do more of an analysis before coming to such a conclusion.

This analysis ends here, but note this is just one explanation (also the more simplistic and less mathematical approach and reasoning) of lee cyclogenesis and further baroclinic development associated with an intense jet stream (lee troughing is possible with little to no jet stream/weak baroclinity).  Other authors have proposed a QG explanation (Bluestein uses this approach in his Synoptics in Midlatitudes) as well as potential vorticity reasoning.  In general, differing “theories” and interpretations seem to come to relatively similar conclusions (in the last 20 years at least).

Meteorology is a beautiful thing when it makes sense.

Additional reading for those interested.

http://journals.ametsoc.org/doi/pdf/10.1175/1520-0493(1989)117%3C0154:NAOTIO%3E2.0.CO;2

http://www.atm.helsinki.fi/~dschultz/pubs/19-SchultzDoswell00.pdf


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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