Numerical weather model progs have consistently suggested the potential for an east coast long wave trough for the past 2 weeks. One significant area of model inconsistency has revolved around the potential for deeper cyclogenesis as the long wave trough interacts with the warm and moist Gulf of Mexico/Gulf Stream air masses. For this potential event, we will discuss the possible roles in moist latent heat release and the “self-development process” on the development of a possible significant cyclone October 19-21st across portions of the Great Lakes and northeastern U.S.
Many authors and research scientists have discussed the role of latent heat release and cyclone development. Both simplified QG equations and PV discussions take into account the role of moist latent heat release (a diabatic process) on cyclone development, and much research revolves around the role of low level PV anomalies. Under the right circumstances, the phasing of PV anomalies in the vertical (low and upper level) can result in mutual amplification of the anomalies through the “self development” process. I have discussed the self development process in greater detail in previous blog articles, but the general process can be summed up as such:
- The interaction of an upper level wave disturbance/PV anomaly/jet streak with a low level surface wave/PV anomaly which incites surface convergence
- Further surface convergence results in increasing low level wind fields and subsequent warm air advection ascent through the WCB (Warm Conveyor Belt)
- Warm air advection ascent in the WCB results in increasing surface pressure falls/low level convergence
- Warm air advection in the low levels forces upper level height rises and the development of a shortwave ridge axis ahead of the main trough/upper level PV anomaly
- The presence of an upper level ridge axis results in a greater magnitude of differential cyclonic vorticity advection and subsequent forced ascent above the level of non-divergence (LND)
- Greater differential cyclonic vorticity advection aloft forces even greater SLP pressure falls and increased low level convergence
- The process continues and the cyclone begins to rapidly develop (go back to step one and repeat)
This discussion, however, ignores the many other significant processes that may contribute including potential deep, moist convective processes aiding surface pressure falls, mesoscale forcing along fronts/jet streaks, and moist latent heat release/diabatic processes. Here we will briefly discuss the possible influence of moist latent heat release and the feedback to the self development process discussed above.
The three day forecast involves several important “players” including an early low amplitude wave acting as a “cyclogenetic trigger”, an intense Pacific jet streak which will eventually “carve” out a long wave trough across the central CONUS, and a tropical plume of moisture in the Gulf of Mexico. How these features interact with one another will have significant impacts on the cyclone development by day 3.
The first feature of interest is the low amplitude upper level wave across the intermountain W.
06z NAM 250 hpa heights/vorticity:
Which is clearly seen on the WV channel:
The second feature of interest is a Pacific jet streak just off the west coast of Canada.
The third and possibly the most important feature is a tropical plume of moisture across the southern Gulf of Mexico.
CIMSS satellite derived precipitable water:
This plume is also associated with an area of organized deep convection (seen on the WV image above) and a weak surface low as analyzed by HPC at 06z:
The intermountain W upper level wave is projected to eject across the central/southern plains with weak surface development as it interacts with an E-W oriented baroclinic zone. Southerly flow is expected to advect the tropical plume of high moisture northward across the GOM and eventually over the Gulf Stream.
Significant numerical model differences begin to arise by day 2.5 as numerical guidance begins to show discrepancies regarding the interaction between the aforementioned synoptic low pressure system and the tropical plume/low pressure across the GOM.
Note by day 2 both the NAM/GFS and ECMWF (not shown) all carve out a significant long wave trough across the central CONUS.
Subtle synoptic scale differences are evident including placement/timing, but the general pattern is quite similar at day 2.
It has been shown by numerous research studies (and proven by dynamical equations) that moist latent release can have substantial impacts on the evolution of synoptical scale cyclogenesis.
Using a PV approach, where Potential Vorticity is defined as,
where PV is the product of absolute vorticity and the gradient of potential temperature.
Latent heat release through the process of condensation can have significant impacts on both the static stability of the atmosphere (cloud bearing layer) as well as the distribution of theta (and potential vorticity). It has been shown through research and observation that significant diabatic heat release can result in the development of strong low level PV anomalies, which in turn, can have significant effects on low level vorticity and mutual amplification/self development of synoptic cyclones. Moreover, in terms of the topography of the US and its orientation to the Gulf Stream, the low level wind field response to a PV anomaly can result in increased inland moisture advection which can further hasten/supplement the feedback process through increasing condensation across the WCB as well as result in extreme precipitation maxima (potential flooding). It should also be noted that this process can have a negative influence on synoptic scale development, especially when the low level PV anomaly (associated with significant latent heat release through condensation) is displaced away from the parent low.
NAM modeled precipitable water values in the GOM are progged to be in excess of 2.5″ by day 2:
It is pretty clear that significant amounts of moisture will be advecting northward out of the GOM, but how will the eventual cyclone develop?
Note by day 2.5 (hour 60) the NAM has a coupled low level PV anomaly/synoptic surface low near the greatest precipitable water gradient across GA/SC:
Note that the 06z GFS at hour 60 has the synoptic scale surface low displaced from a secondary coastal low (which is associated with the greatest magnitude precipitable water gradient).
Is this difference important? And why do these differences show up? Let us investigate more.
Remember that warm air advection through the WCB results in synoptic ascent, and with high moisture values present, condensation (and possibly deep, moist convection) will be present. The development of latent heat release induced low level PV anomalies can have significant influences on mutual development through changes in both the wind fields as well as thermodynamic characteristics of the air mass.
Note the rather significant differences in convective precipitation in the 06z NAM and GFS at hour 66 (previous 6 hr QPF total):
Note how the NAM has a separate precipitation maximum near the SC/NC border. Also note that much of the precipitation of the GFS is well off coast. Areas of significant convective precipitation can imply regions of significant latent heat release in the low/mid levels of the atmosphere. These areas are possible regions where low level diabatic induced PV anomalies may develop.
Note the significant difference in the low level vorticity by hour 66:
Also note the farily significant differences in the low level mass fields in response to the low level PV:
Note the NAM has the strongest 850 hpa low level jet near the region of high low level absolute vorticity (implying high PV) when is then tied to the location of the greatest latent heat of condensation. These differences begin to have significant impacts on cyclone development as local wind fields near the low level PV anomaly greatly influence the behaviour of warm air advection (and the sea level pressure falls).
Note how the 850 thermal ridge “curves” inland and into the low level circulation on the NAM.
Whereas on the GFS WAA is mazimized off coast with little inland advection:
So what does this all mean? Going back to the bulleted list above, mutual development/self development is much stronger in the NAM than the GFS, and this can be tied to subtle differences regarding the development of the diabatic induced low level PV, the interaction of the low level PV with the synoptic low, the associated warm air advection regime, and associated precipitation processes (due to both model convective schemes as well as the aforementioned differences).
Once this process is underway, rapid and intense cyclogenesis commences. Using the NAM as an example, classic self development/mutual development occurs as the upper level PV anomaly interacts with the low level diabatic induced PV.
By hour 57 note the relatively “flat” 250 hpa height field in the NAM simulation:
Note by hour 72 the significant shortwave ridge axis which has developed (pink line):
Note the “destruction” of PV (relative vorticity in this case, but it can imply PV) and the development of an anticylonic shortwave ridge axis):
This further implies greater values of differential cyclonic vorticity advection above the level of non-divergence and subsequent pressure falls. This further increases low level surface convergence (and if the low level PV anomaly induced via diabatic latent heat release is co-located with the synoptic low) can result in an additional feedback preocess: increasing inland low level moisture advection. This can further increase low level latent heat release and force even greater surface pressure falls as the process repeats.
NAM precipitable water loop. Note how the diabatic induced PV anomaly and the synoptic surface low are “co-located”, thus enhancing the process of LHR, WAA, and DCVA (differential cyclonic vorticity advection):
This is classic mutual development/self development at work, and it also shows the sensitivity to various model processes such as the model convective scheme, boundary layer schemes (associated with air-sea interaction and the feedback to the low level moisture/theta-e fields), precipitation schemes, etc.
The final solutions, while relatively similar, do vary significantly in the placvement and intensity of the surface low.
06Z GFS @ 84 hours:
06Z NAM @ 84 hours:
06Z NAM animation:
Note the GFS low level theta/SLP (925 theta) in the animation below how the latent heat release/thermal advection induced PV anomalies (seen in SLP maxima) never co-locate with the parent synoptic low (resulting in weaker mutual amplification/self development) and a weaker (albeit still quite strong) overall solution (NAM 976 mb/GFS 990):
Why does all this matter? Having an understanding of the processes involving cyclone development as well as potential model bias will give the forecaster a huge advantage in interpreting the possible final solution. Knowing what features to track (i.e., the position of the tropical moisture plume), understanding model bias such as convective paramaterization schemes, boundary layer schemes, etc., the forecaster will both be able to diagnose potential failure modes within the individual models as well as better forecast the eventual impacts. Of course we are not even considering the usage of dprog/dt analysis, other numerical models beyond the NAM/GFS, MOS guidance, ensemble guidance, etc.
Once again, meteorology is a beautiful thing when it makes sense. The art of meteorology is understanding how to interpret the mass amount of information and develop it into a forecast with clear impacts to the public/user. What do I believe will happen? It does seem the latest NAM guidance is a bit too “amped” up, and that is likely a product of its convective generation due to its over-amped convective scheme and liberal development of DMC near the cyclone low. Using the latest dprog/dt of the global guidance, it is clear the global models are still “catching up” the observations and trending a bit stronger (the NAM was clearly the best at simulating the current tropical development in the GOM), not to mention the global models will typically be less skillfull at simulating the processes involving DMC than various non-hydrostatic models such as the NAM (and less aggressively filtered guidance such as the RGEM), although in this case, it does seem the NAM is a bit too aggressive given the overall synoptic wave setup. Were I to make a forecast now with impacts listed, I would likely make heavy use of more intense guidance such as the NAM and less use of the global guidance (for reasons listed above) and would consider the extreme end for possible future updates to impacts. Were this a winter weather event, the impacts would be even greater as blizzard conditions would be a likely scenario. It should also be noted rather significant run-to-run inconsistency will likely occur as small errors will have rather large impacts on the eventual forecast so relying on any one individual model run would likely be a poor choice (but also waiting for the models to “agree” would result in a poor lead time).
A followup blog post will be made after the fact to discuss the eventual impact and the correct forecast.
Further reading for the inquisitive: