Tag Archives: height tendency

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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Central/Northern Plains Cyclone Event (11/12-14/2010)

All Images Can Be Clicked to Enlarge

An interesting weather event is shaping up for late this week and into the weekend.  A strong PV Anomaly over the intermountain west is going to “eject” into the plains initially propagating along a quasi-stationary frontal zone over the plains before developing into a cyclone as it tracks NE.  The global numerical guidance has been very consistent modeling the general pattern that can be expected, but as always significant run-by-run (and model by model) inconsistencies and large model spreads exist.  Even as we reach the “short-term” (2-4 day forecast period), a lot of variability is still persistent amongst the models.  However, since this is partially a blog about weather forecasting, I thought it would be appropriate to actually make a weather forecast.  In reality, this is the challenge all forecasters need to make on consistent basis, but decisions still need to be made so the appropriate weather risks can be assessed and disseminated to the public (and private) in a timely manner and with sufficient lead time.

As of late this afternoon, a stationary front is parked over the southern plains with a positive tilt trough slowly propagating eastward.  The warm sector across the southern plains is moist with surface dewpoints ranging from the upper 50s to upper 60s.  Moisture is going to play a key role in the eventual cyclogenesis of the storm.

The 0Z upper air map depicts the positive tilt trough:

The 0Z 500 hpa analysis depicts a strong vorticity maxima at the base of the trough:

The NAM 0Z analysis also depicts this clearly in its shaded vorticity fields:

A strong PV anomaly is positioned over the 4-corners with tropopause heights as low as 500 hpa.

The interaction of this strong vorticity maxima/PV anomaly with the warm and moist air mass in the warm sector is likely going to result in a strong cyclogenesis event.

This graphic shows the surface low track of each numerical model analyzed at 12Z yesterday morning (11/11/2010).  It is easy to see the model guidance has significant spreads in both surface low intensity and surface low track.  Note the NAM track in green and the GFS in red.

The 18Z NCEP guidance converged a bit, but the spread is still significant amongst the NAM/GFS.

The Short Range Ensemble Guidance is not much better:

The first 24-36 hours of the forecast is generally pretty clear as all guidance has the upper PV anomaly ejecting into the southern plains and lifting NE along the front.

At 12 hours, (12Z) note the still positive tilt to the trough.  The PV anomaly remains at the base of the upper trough.

Of course, positive tilt troughs are not very conducive to surface cyclone development.  Under this configuration, most of the the time the main belt of westerlies would cut off as the cold air remains well to the north.  Almost all vorticity associated with this trough is due to shear vorticity on the downstream side of the trough.  Cyclonic vorticity advection is minimal ahead of the trough (therefore height falls are not induced…see last post for the QG Chi equation) with any relative vorticity advection offset by planetary vorticity advection on the backside of the trough.

Note that the main jet level winds are on the downstream portion of the positive tilt trough.  Once again, from the thermal wind relation, strong jet level winds reside over regions of enhanced thermal gradients:

What would eventually happen is the trough would slowly “de-amplify” with time as the jet stream propagated northward with weak surface development.  The leftover baroclinic zone over the plains would moderate with time leaving (resulting in a weakening horizontal thermal gradient) an area of enhanced shear vorticity aloft over the region.

With this system though, a couple things are different.  First, as the main belt of westerlies cuts off, cold air advection behind the stationary front ceases to exist.  Weak warm air advection along the stationary front continues as air (on the warm side of the front) weakly advects NE due to the pressure gradient developed by the departing cyclone well into Canada.

Note that, at 850 hpa, cold air advection slackens on the cold side of the stationary front and warm air advection begins to dominate along the warm side as air continues to advect NE in association with the departing cyclone (blue line).  Our stationary front has now developed into a warm front (see inside the red circle).

The front begins to propagate northward as a result of this flow configuration.  Subtle height rises develop aloft due to the weak low level warm air advection decreasing with height (QG Chi equation).

BY 21Z, note that a weak downstream ridge axis has developed in association with the differential warm air advection.  Also note the existence of the upstream shortwave.  Kicker trough?

Why do cutoff lows “kick-out” with an approaching upstream “kicker” shortwave?  There are  a number of differing reasons, but I find the QG approach simplistic and succinct.  (To help differentiate…kicker is the shortwave that “kicks-out” the cutoff low, the cutoff low that “kicks-out” is the kickee).  Note with the incoming  kicker shortwave heights have fallen upstream of the main trough.  This acts to “flatten” the upper level height field in between the kicker and kickee (in between the two systems).  Going back to the QG Chi equation we love so much:

the amount of planetary vorticity advection by the geostrophic wind decreases (remember that f decreases equatorword) on the backside of the main trough (the kickee).  Note in the 12Z upper level height map the trough is advecting increasingly large values of f.  By 21Z (the previous image), with the approach of the shortwave trough upstream, the ridge (over the intermountain west) has flattened and now little to no f is being advected on the backside of the cutoff low.  Heights do not fall on the upstream side, and the small amount of cyclonic vorticity advection on the front portion of the kickee results in slow height falls downstream which results in forward propagation.  The cutoff has now been “kicked-out”.

By 0Z, the downstream ridge has continued to amplify and a lead shortwave at the base of the trough has now developed as large values of cyclonic vorticity are being advected with increasing height falls:

Now the feedback process begins as warm air advection continues in the warm sector.  In a moist system such as this, diabatic affects can be significant as warm and moist air condenses and releases latent heat, especially along the warm front.  This process can act to increase the along-front thermal gradient therefore acting in a frontogenetic manner.  Moreover, the release of low level latent heat due to condensation can act to decrease the static stability of the atmosphere.

In the omega equation, note the location of the static stability parameter in the various RHS forcing terms:

Low static stability acts to enhance the cyclogenesis process during the initial stages of development.

Without considering every forecast hour, skipping ahead to hour 33, the NAM now features a significantly amplified upper ridge downstream and the lead shortwave has now taken on a negative tilt.

Both the NAM/GFS are illustrating an upper level jet coupling combined with the significant low level warm air advection/diabatic heating and subsequent upper level height rises ahead of the main shortwave.  It is likely mesoscale jet circulations will play a prominent role in enhancing divergence (and vertical ascent) and the subsequent cyclogenesis process as well as strong convection along the cold front.  Rapid cyclogenesis and occlusion is likely with this system.


What does this all mean?  First the significant model differences.  The 0Z guidance is in and the spread remains signficant.

The NAM is a significant outlier with its significantly farther W track while the GFS is on the opposite end of the spectrum.   Interestingly, it seems with increasing resolution of the numerical model being considered, the farther W its surface low track is.  After the NAM, the ECMWF (blue) is next followed by all the other global models (CMC, NOGAPS, UKMET, GFS).

Even the 21Z SREF spread is rather large:

Once again though, it seems the W tracks are dominated by higher resolution guidance while the farther E tracks are dominated by the RSM (pink/red), a variant of the GFS used for the SREF data.

It should also be mentioned the GFS and the rest of the global guidance has continued to shift westward with time to match the higher resolution mesoscale models.  First, consider the size of this storm.  In terms of the Rossby Number R, it is rather small compared to a typical synoptic scale system which yields larger values of R:

QG theory works nicely with large synoptic systems, but as R increases, sub-synoptic scale forcing becomes more prominent in the development of the system.  Numerous studies have shown this…and various omega equations have been developed to account for sub-synoptic scale forcing.  Mesoscale circulations are more prominent, and often times the higher resolution models can more effectively forecast these systems.

Lets just illustrate this with a simplistic comparison by arbitrarily choosing the 700 hpa pressure level.

First the 0Z NAM @ 33 hours:

and the GFS at the same time:

A couple things worth noting.  First, the NAM 700 hpa heights are much lower than the GFS.  The low level mass response is stronger in the NAM than GFS, and the low is displaced slightly farther W.  It is common for rapidly developing cyclones to develop farther W than expected.  Why?  It mostly relates to the position of the warm front and zone of warm air advection.  Think of the propagation of a surface low.  The region of surface low pressure will propagate towards the region of strongest surface pressure falls.  It makes sense then that developing surface lows propagate along the surface warm front.  From our earlier discussion, remember rapid cyclogenesis results in more amplified upstream ridging owing to thermal advection/diabatic heating in the warm sector.  Also remember differential cyclonic vorticity advection (along with mesoscale jet circulations), which is a dominant forcing method aloft, results in vertical motion fields which are displaced farther W from the surface low.  Therefore, as surface occlusion begins, low level warm air advection decreases (and subsequent differential warm air advection decreases) resulting in less height rises upstream of the shortwave.  Differential vorticity advection and forced ascent become stacked with the surface low and intensification slows significantly (surface low can still deepen due to forced ascent above the surface but still below the level of non-divergence…in other words, baroclinic processes may still result in forced ascent in the low levels above the surface occlusion…hence why surface lows still deepen after occlusion).  This also results in a much slower track.

FInishing this post up, rapid intensification almost always results in a system displaced farther W.  Global guidance continues to shift W towards the higher resolution model solutions, and the high resolution models are all developing a rather significant TROWAL (Trough Of Warm Air Aloft).  This makes sense as the low level mass fields are much stronger in the high resolution models due to the increased deepening and more rapid intensification.  A strong closed low level circulation results in warm air “wrapping” around the main upper low and a region of enhanced warm air advection ascent on the north then backside of the upper low (this also enhances the development of the surface low farther W and sometimes NW).  Large TROWALS are obviously efficient snow machines in winter since the precipitation falls on the cold side of the storm (in the low levels).

Subsequently, the NAM has a much larger QPF field farther west into the cold side of the storm and a farther W track of the surface low.

With the GFS farther E and with a less defined TROWAL of QPF:

Given the information I have given and the current trends (which further support the dynamic assessment above), and without doing an in-depth analysis of the thermal fields (for brevity), it seems an early season snow event across portions of eastern MN/western WI is likely.  Track wise, I think it will be slightly farther E of the NAM but a tad farther W of the ECMWF which would take it slightly inside the SE Minn border or right along it.  I am also leaning towards the NAM intensity which would yield a mid-level TROWAL well displaced over the low level cold air.  Given this information, it is quite probable a band of accumulating snow can be expected across central and eastern MN into W Wisconsin into the Arrowhead of MN.  It most certainly will be fun to watch.

Finally, as a quick mention, it is worth noting IPV thinking yields similar conclusions.  IPV thinking also deals with the diabatic effects even though potential vorticity is being conserved along isentropic surfaces.  For instance, significant diabatic effects on the warm sector (definitely not adiabatic!) results in the destruction of the upper level vorticity (in the QG perspective…upper level height rises) with increased development of the cyclone in the low levels below the region of latent heat release (strong low level mass response) .


Cold Air Advection Over the Northern Plains

A large intrusion of cold air into the central and eastern portion of the United States this weekend ushered in the coldest air of the season across the Northern Plains and Great Lakes region.  It was slightly unusual in that there was no significant surface low/intense shortwave trough and associated sharp cold front.  What starts as a mid-upper level weakly baroclinic stationary trough centered over the Hudson Bay eventually develops into a case of lower to mid-level frontogenesis, incredible cold air advection, and synoptic subsidence incited by a low amplitude upper level wave disturbance.

Let’s investigate.

The analysis fields of the GFS on the 30th September, 12Z depict the trough over the Hudson Bay:

The upper tropospheric wave disturbance (12z) is seen here over northern Alberta:

By 0Z the 31st, the analysis fields of the NAM/GFS nicely capture the intensifying wave disturbance as it begins to interact with the larger trough over the Hudson Bay:

The disturbance is quite evident in the WV loop:

This is also captured nicely in the RUC Analysis field at 0Z.  Note the lack of a low-level wave as no low level mass response has developed at this point in time.

Since there has yet to be any low level mass response, it makes sense there is a total lack of cold air advection in the low levels (850 hpa) along the southern Canada border into northern MN:

By 18Z the 1st October, the disturbance had progressed over the Great Lakes with the weak phasing essentially complete.  Note the significant subsidence behind the wave disturbance.

Here is how the 12Z GFS has it analyzed (at 500 hpa) 6 hours later (same time as the WV image):

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