Storm Relative Helicity

[Randy Jennings]

From https://www.weather.gov/oun/spotterglossary

"Beaver('s) Tail - [slang] a particular type of inflow band with a relatively broad, flat appearance suggestive of a beaver's tail. It is attached to a supercell's general updraft and is oriented roughly parallel to the pseudo-warm front, i.e., usually east to west or southeast to northwest. As with any inflow band, cloud elements move toward the updraft, i.e., toward the west or northwest. Its size and shape change as the strength of the inflow changes. See also inflow stinger.
Spotters should note the distinction between a beaver tail and a tail cloud. A "true" tail cloud typically is attached to the wall cloud and has a cloud base at about the same level as the wall cloud itself. A beaver tail, on the other hand, is not attached to the wall cloud and has a cloud base at about the same height as the updraft base (which by definition is higher than the wall cloud). Unlike the beaver tail, the tail cloud forms from air that is flowing from the storm's main precipitation cascade region (or outflow region). Thus, it can be oriented at a large angle to the pseudo-warm front."

"Tail Cloud - A horizontal, tail-shaped cloud (not a funnel cloud) at low levels extending from the precipitation cascade region of a supercell toward the wall cloud (i.e., it usually is observed extending from the wall cloud toward the north or northeast). The base of the tail cloud is about the same as that of the wall cloud. Cloud motion in the tail cloud is away from the precipitation and toward the wall cloud, with rapid upward motion often observed near the junction of the tail and wall clouds. See Fig. 7, supercell. Compare with beaver tail, which is a form of inflow band that normally attaches to the storm's main updraft (not to the wall cloud) and has a base at about the same level as the updraft base (not the wall cloud)."

 

[Brandon Centeno]

Also, other question, what is the difference between Bulk Shear 1/6 km and Bulk Shear effective? Which is more helpful and what should I look at to get the general idea of how good supercells that day will be? Which do you use to determine a good chase day?

Completely unrelated question: how do you tell if the day is leaning towards LP, CLASSIC, or HP? Whenever I chase, there’s usually someone else doing it.

April 13 might look like a good chase day. Upper trough, Southwest jet, a low, and a cold front

0-1 km bulk shear is the bulk value of shear in that layer. Ie, at the surface if you have:

Surface = (-10,0) [10 knot wind straight to the west]
1 km = (10,0) [10 knot wind straight to the east ]

Then the 0-1 km bulk shear is:
du/dz = 10 - (-10) = 20 knots

So the 0-1 km bulk shear is 20 knots, and the vector is oriented to the east, since we are dealing only with u-component (E-W, with E positive).

You can imagine it is the same thing with 0-6 km. I believe technically the calculation is done with a 500m average (so something more like .5 - 6.5 km shear).

Effective bulk wind difference/eff shear, is the same calculation, only it takes into account the effective layer. It is calculated by taking the base of the effective layer (can be the surface, or it could be 1.5 km AGL) as the lower bound and the upper bound (replacing 6 km) is the halfway point to the equilibrium level. The idea is that this helps gauge organization potential for elevated storms, where 0-6 km shear doesn't really matter.

The value for 0-6 km tends to be similar to the EBWD value depending on the time of year. Typically I use EBWD, and for supercells, you want to see 40 knots or greater to be safe, though 35 knots paired with adequate instability/steep lapse rates and "sufficient" low-level SRH will definitely still yield supercells.

If you don't know what the effective layer is, in simplest terms, it is the layer considered to contribute to the storm updraft, in a very loose sense. The requirements are: > 100 j/kg and <150 j/kg (magnitude) of inhibition. You calculate a parcel originating at the surface, for example, and it has 1000 j/kg of CAPE and -50 j/kg of CIN. That's the base of the effective inflow layer. You do this for every level of data you have until you reach a level, say 850mb, where you either don't have 100 j/kg of CAPE (through lifting the parcel) or you have 150 j/kg or more of inhibition.

 

[Michael Wilkinson]

https://www.weather.gov/media/lmk/soo/SvrWx_Fcstg_TipSheet.pdf

The tip sheet looks like a great tool for a beginner trying to visualize what impact each parameter is having.

 

[MikeD]

Just a question: What should I do when my car gets stuck in mud on a dirt road in the middle of nowhere and a wedge is half a mile away and heading directly for me?

 

[Rick Schmidt]

Dive into a ditch and pray.....oh, and point your camcorder at it!

 

[Warren Faidley]

If you have put yourself in front of dangerous storm, especially driving on a muddy road, expect poop to happen.

 

[MikeD]

Dive into a ditch and pray.....oh, and point your camcorder at it!

I don’t think I will survive by doing that.

If you have put yourself in front of dangerous storm, especially driving on a muddy road, expect poop to happen.

You didn’t answer my original question.

 

[Todd Lemery]

Realistically, i’d run like hell one way or the other (you said that it’s coming directly towards you) until I A) cleared the circulation B) found a ditch or hole to drop into or C) join the flying cows....

 

[Alex Elmore]

You didn’t answer my original question.

I'd say he did. If you put yourself in a dangerous situation, such as the one you described, expect that there's a good chance bad things are probably going to happen to you and at that point, there may be nothing you can do about it. You shouldn't put yourself in front of any tornadic circulation; regardless of what other chasers do and regardless of the type of road you're on. You always run the risk of your car breaking down, flat tire, battery dying, etc., so you always have the chance of getting "stuck". If you're close enough to be able to tell that it's moving directly at you and it's a wedge, it's probably going to be too late to run from it (especially if the road is muddy). Diving into a deep ditch may help, but if the circulation is strong enough and passes directly over you, that probably won't save you. Bottom line: don't drive on muddy roads, and don't put yourself in the path of any type of tornado.

 

[MikeD]

Realistically, i’d run like hell one way or the other (you said that it’s coming directly towards you) until I A) cleared the circulation B) found a ditch or hole to drop into or C) join the flying cows....

I like this

 

[MikeD]

I asked this question because IF it happens, I want to know what to do.

 

[Randy Jennings]

Just a question: What should I do when my car gets stuck in mud on a dirt road in the middle of nowhere and a wedge is half a mile away and heading directly for me?

While this thread has gotten off topic, I think we should treat this as a serious question. Anyone who thinks it can't happen to them because they are good/careful/etc, is kidding themselves - we all need a backup plan. While avoiding the situation (by not using muddy roads and making sure your vehicle is in top shape), is the best course of action, I do have to agree that plan B is get away if you can. If there is no one around to give you a ride and you can't out run it foot, plan C is a harder choice. Conventional direction is to lie in a ditch and cover your head. Between flying derbis and flooding, I'm not sure I could bring myself to do that. On May 31, 2013 5 people where killed in OKC while trying to take cover in a drainage culvert. It would be hard for me to get out of my car, although there are plenty of people who have been killed in cars (8 people killed in cars in El Reno the same day). So for me I guess the answer is it depends - if I was in that situation and there was no other suitable shelter nearby but there was a ditch or culvert that I did not think would flood and had some protection from flying debris, I think I would use it. If all I had was a flooded culvert or a very shallow ditch with no cover, I think I would take my chances and stay in the car (with my seat belt on).

 

[Jeff House]

Helmets in the car is a good idea in case something like this happens. Regardless of which course of action, a helmet offers more head protection and one can use hands for their spinal cord back of neck. Remember that is why they had us put our hands on our necks in school.

I suppose if you have football or hockey gear for your chest and back, bring it too. We always plan to watch from the side, out of harm's way, but it could happen. Tornado turns sharply and car stalls..