Darrin's Sophomoric FAQ

[Darrin Rasberry]

Here are a few questions on forecasting that have been burning in my mind for a while. Any kind of help would be appreciated!

1. What are the usual keys for cyclogenesis for the southern plains during the chase season? In specific, if a low isn't marching in from the Pacific, how does it form? I have a hint it's from WAA "converging" (if that's the right word) at frontal boundaries caused by some other low. Think April 10, 1979 with the low that developed in Mexico and caused the Wichita Falls F4.

2. If the surface layer during a storm is (significantly) not stabilized, will a storm always draw from this layer in typical storm setups, i.e. will storms always be surface-based if SBCAPE exists? If not, how does one determine whether a storm will be surface-based if sufficient SBCAPE exists?

3. Is an elevated tornadic cell a contradiction in terms, i.e. if an elevated storm produces a tornado, does this mean it is now surface-based by definition?

4. What causes low-topped supercells?

5. The RUC shows a very dry 850mb layer eventually intruding above a very moist ground layer for a day in early June in Iowa, but the surface layer still stays moist (i.e. the situation I saw didn't involve "mixing out" at the surface even though the intrusion at 850mb was VERY dry). Does this mean a cut in instability and low storm potential like I inferred about this day in question?

6. If a storm moves counter to projected motion, like Greensburg after the split or like (to use an extreme case) Jarrell, TX, do shear progs go out the window entirely due to the storm's relative position to the winds? Will the storm "pick up" winds at the surface differently?

Thanks in advance for your help!!

 

[Patrick Marsh]

1. What are the usual keys for cyclogenesis for the southern plains during the chase season? In specific, if a low isn't marching in from the Pacific, how does it form? I have a hint it's from WAA "converging" (if that's the right word) at frontal boundaries caused by some other low. Think April 10, 1979 with the low that developed in Mexico and caused the Wichita Falls F4.

Low pressure areas develop wherever the greatest pressure falls are located. Somtimes this is from areas of strong WAA inducing ascent, other times its lee cyclogenesis, and many more. There isn't 1 way for a low-pressure to develop.

Just for the record, low pressure systems do not cause tornadoes. Tornadoes are spawned from thunderstorms.

2. If the surface layer during a storm is (significantly) not stabilized, will a storm always draw from this layer in typical storm setups, i.e. will storms always be surface-based if SBCAPE exists? If not, how does one determine whether a storm will be surface-based if sufficient SBCAPE exists?

No. A storm will be "rooted" in whatever level yields the most CAPE with the least amount of CINH. It is possible (and common) to have SBCAPE over 3000 but have SBCINH over 100. Remember CAPE is the accumulated energy over the depth of the ascent. It is possible to have a surface based parcel that has 3000 J/kg of CAPE at its disposal between the surface and the equilibrium level...however it must first be lifted through 100 J/kg of CINH. Meanwhile, it is possible for a parcel atop the CINH to be lifted much more readily...leading to elevated thunderstorms even if SBCAPE is extreme.

(Now in the case above, it certainly is possible that the elevated storm would become a supercell. Then the pressure purturbations underneath the mesocyclone could possibly create additional lift to aid in getting the surface based parcels to their LFC. This would lead to the elevated supercell becoming surface based wth time.)

3. Is an elevated tornadic cell a contradiction in terms, i.e. if an elevated storm produces a tornado, does this mean it is now surface-based by definition?

I feel I can argue this both ways...

There have been reported (killer) tornadoes in OK that have occured north of an arctic cold front in February. The tornadoes occurred when the surface temperature was in the 30s. This cold layer was very shallow and the thunderstorms were "elevated" above this shallow layer. The pressure purturbations underneath the updraft were so strong that the updraft was able to lift the extremely stable surface parcels up into itself (i.e., the tornado was able to penetrate the stable layer). However, I feel this is a very localized event and that the majority of the parcels being ingested into the storm originated above the stable layer. So the question is asked, do all the parcels have to originate in the same layer for the storm to be rooted in that layer? Do a majority of them have to be? Or should we invent a new term "near-surface" based convection to account for these situations with localized surface-based parcels being ingested into an elevated storm?

4. What causes low-topped supercells?

All the necessary ingredients for supercells, plus the addition of low equilibrium levels.

5. The RUC shows a very dry 850mb layer eventually intruding above a very moist ground layer for a day in early June in Iowa, but the surface layer still stays moist (i.e. the situation I saw didn't involve "mixing out" at the surface even though the intrusion at 850mb was VERY dry). Does this mean a cut in instability and low storm potential like I inferred about this day in question?

Depends on a lot of additional factors, including what is the main forcing.... Dry air is not necessarily a bad thing...when it is aloft. It can actually aid updraft speed by allowing for latent heat release.

6. If a storm moves counter to projected motion, like Greensburg after the split or like (to use an extreme case) Jarrell, TX, do shear progs go out the window entirely due to the storm's relative position to the winds? Will the storm "pick up" winds at the surface differently?

By definition, storm relative helecity is defined to be dependent upon storm motion. Slight deviations in storm motion and speed can cause huge changes in storm-relative helecity...not always increasing them.

 

[nickgrillo]

Probably the best example that I can think of (that I personally chased) was the mid-late morning supercell that traversed through northern KS/MO/IL on 3-12-2006. The storm developed and maintained itself in a weak baroclinic zone well north of a surface warm front that was drapped across southern MO. There was zero surface-based CAPE in it's inflow sector, and for that matter, when I modified local observed soundings and RUC forecast soundings I couldn't get any more than ~300j/kg. That was usually after I lifted parcels well above the shallow, very stable boundary layer that the storm was thriving in. This particular supercell produced six confirmed tornadoes (1 F0 in Leavenworth Co. KS, 4 F0's and 1 F1 in northern MO). The last tornado that occurred with the storm formed in Ralls Co. MO approx. ~15 minutes before I intercepted it on the MO/IL border. Once I got on it, and to no surprise, it appeared to be extremely cold and obviously "elevated" in nature. Now, deep-layer shear was extremely intense (0-6km bulk shear in the order of 70-90 kts). As Patrick said, vertical pressure perturbation gradients (via the updraft interacting with an environment characterized by substantial deep-layer vertical wind shear) were so strong that the updraft was occasionally able to (locally) breach the layer of stability (CINH) and draw in surface-based parcels, and consequently, produce short-lived and weak tornadoes. VPGF's (vertical pressure gradient forces) associated with sustained and well-established supercells can have the same effects on updraft velocity as CAPE. Hence, most of the nocturnal tornadic supercells that you see occurring in "large" CINH environments (after the lower-levels cool due to nocturnal stablization and the boundary layer de-couples; often generating a thick layer of CINH atop it) occur when deep-layer vertical shear is particularly strong. One event that comes to mind is the 11-12-2005 killer (E)F3 Evansville, IN tornado. CINH for surface-based parcels was generally 100-150j/kg throughout much of the warm sector. That particular supercell matured in southern IL and moved into a localized region of more favorable kinematic profiles in southwest IN (in addition, I analyzed a mesolow just to the south, which could have augmented that storm's intensity as it approached EVV). The vertical pressure perturbation gradients associated with that supercell's updraft allowed it to penetrate the layer of stability (CINH) that existed on top of the boundary layer and allowed it to feed of surface-based air parcels. Low-level hodographs were particularly large and the associated SRH was very favorable for strong tornadoes (widespread 150-250j/kg in the lowest 1km).

As far as CINH and its impact on tornadogenesis, Jon Davies has written a plethora of stuff on the topic (both formal and informal papers; all of which are available to read on his website).