Tag Archives: polar jet

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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Weak Elevated Convective Instability Associated With Dynamic Height Falls

There is always something interesting going on in weather.  What initially may seem run-of-the-mill can become more interesting upon closer inspection.  Sunday featured a migratory baroclinic wave, cutoff from the primary westerlies, passing through the intermountain west and “ejecting” into the central plains during the afternoon.  What was particularly interesting was the interaction of a deep frontal circulation within a region of “dynamic” height falls aloft which supported the release of elevated instability.  The co-location of a mesoscale divergent jet stream aided in the rapidly increasing cloud field and elevated “gusty” virga showers.

The wave in question is quite clear in WV with the mid-level trough axis over Colorado at 12Z Sunday:

The 300 hpa upper air analysis clearly shows the upper wave and jet stream level winds as well as the large amounts of divergence (plotted yellow):

The 500 hpa analysis at 12Z.  Note the lower amplitude of the wave at 500 hpa as compared to 300 hpa (this is important!), suggesting this was largely an upper tropospheric wave.  Also note the large values of cyclonic vorticity near the base of the trough owing largely to horizontal shear (of course curvature vorticity is also present, but it does not play as large a role), simply expressed by :

in the natural coordinate system.

This is typical of low amplitude baroclinic waves owing to the amount of shear.  Why do low amplitude waves propagate at a greater speed than longwave troughs?  The rapid forward propagation of the wave is explained (compared to a longwave trough) by the dominance of cyclonic vorticity advection and subsequent height falls ahead of the shortwave trough.  Planetary vorticity advection on the backside of the wave is minimal due to the short wavelength, therefore height falls on the upstream portion of the wave are much smaller than height falls downstream.

The advection of cyclonic relative vorticity by the geostrophic wind dominates over f in the QG Chi equation regarding shortwave troughs:

And the 12Z GFS, once again, note the high values of cyclonic vorticity near the base of the shortwave:

Note the thermal pattern at 500 mb.  There is little to no cold air advection at 12Z:

Note that, by 18Z, as progged by the GFS, the cold air overspreads much of Colorado with little to no advection.  Why?  “Dynamic” height falls:

A combination of QG Chi and the hypsometric equation can help explain this.  As mentioned earlier, large values of cyclonic vorticity are being advected by the geostrophic wind near the base of the shortwave.  The more cyclonic vorticity and/or the stronger the wind, the greater the height falls.

The winds at 300 hpa are on the order of 70-90 kts:

And 40-60 kts at 500 hpa:

This suggests the geostrophic wind at 300 hpa is advecting more cyclonic vorticity than at 500 hpa due to the strengthened flow aloft (I don’t have the map, but the amount of cyclonic vorticity at 300 hpa is similar to 500 hpa).  Heights are falling faster aloft than they are at lower levels (this makes more sense now…remember the higher amplitude 300 hpa shortwave compared to 500 hpa?  This implies heights must fall at a greater rate aloft than regions below with a forward propagating wave).  As a result, because the atmospheric column is shrinking from some level below 500 hpa to the upper troposphere, temperatures must cool in response, implying forced ascent.

The effects of these dynamic height falls results in cooling of the upper levels and steepened lapse rates, which can be enhanced by diurnal insolation in the lower levels.

By 18Z, note the large convective cloud field which has developed post-front over the Colorado Rockies.  Also note the plume of high cirrus associated with the divergent jet stream (green) with a crudely drawn streamline:

By 20Z, the front has progressed into the plains, but note the still cloud free region in western NE:

The 12Z GFS @ 18Z over western NE shows the influence of dynamic induced height falls in the model Skew-T.  Note the mid-level lapse rates:

BY 21Z, note the steep mid-level lapse rates and the deep cold front (red).  Also note the GFS upper level winds are mainly S-SW.

Air parcels lifted to the top of the frontal circulation will be able to ascend freely somewhere around (after calculating a crude elevated LFC) 500 hpa before reaching the Equilibrium Level near 400 hpa.  Also note the very narrow and shallow zone of CAPE–likely around 50 j/kg or less.  Note the rapid expanse of the cloud shield over western NE along with the small pockets of weak convection from 20z to 23z:

Also note the continued SE flow aloft.  The rapid expanse of the cloud field was likely enhanced by the mesoscale jet circulation.  Also note how cold the cloud temperatures are in western NE:

The 12Z GFS @ 0Z is forecasting winds at 300 mb to be nearly due SSW (180-200 degrees).  Also note the ridge axis upstream.

Air parcels exiting the jet streak would become supergeostrophic and flow to lower heights aloft, in this case towards the NW.  This enhances the ageostrophic wind field/divergence and mesoscale forced ascent.  Remember that, in the case of jet stream circulations, this must be considered in addition to the large scale synoptic vertical motion field.  Jet streaks and their associated circulations are mesoscale and are not in any way related to synoptic scale ascent and the QG equations of vertical motion (which can be shown through a scale analysis).   Note that, in the 0Z analysis, which employs upper air soundings, the flow is indeed S-SE (also indicated in the cloud flow pattern and in satellite derived winds).  In this case, the 12Z model runs were likely underestimating the strength of the jet circulation and associated vertical motion field.  While the impacts were relatively minimal here, in winter, that could result in models significantly underestimating the potential for heavy snow banding (just one of many potential high impact events), for instance.

Also worth noting here is the 18Z 300 hpa wind forecast at the same time (compare this to the 12Z forecast above).  Satellite derived winds ingested by the numerical model data assimilation systems were able to adjust the upper level wind field to the satellite observations.  This is a good example showing “off” hour runs are not worthless and can have operational significance if used intelligently by the forecaster.

A large area of showers formed over the region including heavy convective showers.

 

Surface observations suggest most of the shower activity over western NE never hit the ground and cloud bases (AGL) remained high at around 10-12000 feet.  The main effect of the showers was to enhance horizontal momentum transport downwards and increase wind gusts along the frontal circulation.  Most surface observations showed peak wind gusts with the arrival of the showers, some in excess of 40 mph, hence the name “gusty” showers.

This case is a good example of the importance of “dynamic” height falls in meteorology, especially in terms of summer convective potential when deep, moist convection is often initiated/and or enhanced by very low amplitude waves/upper level “impulses” due to cap erosion, steepening lapse rates (increased CAPE), and regions of low level mass convergence (surface based and/or elevated).  Also important is the co-location of differing meteorological circulations (e.g. mesoscale jet circulations, frontal circulations, regions of synoptic ascent, etc.), especially during winter storm events.  Being able to diagnose and forecast these regions is of utmost importance, even more so in the short-term forecast and NOWcast.


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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