DanH

DanH

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Question for the more experienced; Is there a practical reference for estimating how fast a thunderstorm is moving across the ground? I assume storm travel speed is a function of upper level winds, but which upper level?
 
I can tell you from personal experience that it's too bloody fast in any case.

Seriously, the only way I know to check a storm's groundspeed is to use the surface windspeed. This is probably wrong, though; best thing is to run as fast as possible in the other direction.
 
T-storm speed

A building T-storm can 'reach out and touch you' from further distance than you would imagine. It's not the drift or speed of a storm across the ground that is so dangerous.

When they reach a certain stage of development, a huge downrush of wind can produce a 'first gust' phenomena that you cannot fly thru in any aircraft, much less an RV. In fact, in my airline days, simulator periods were dedicated to flying the profiles flown by accident aircraft. Seasoned pilots knowing they were about to encounter downbursts and gusts would normally crash the simulator at least 90% of the time. That's in a powerful jet at max power flown at the stall warning speeds (stick shakers).

Once while flying VFR a few hundred feet across the Arizona desert in a C-130H at >250 knots, a downburst from a T-storm more than 20 miles behind and to one side caused the C-130 to be blown at about a 40 degree sideways angle and the resulting surge quickly overpowered the parameters of the installed dual INS systems and tumbled them.

Be especially careful in your RV during takeoffs and landings when developing T-storms are within 10 miles. Watch the windsocks, flags,dust, tree leaves, bushes,etc.for possible gusts.
 
The local news always gets on and tells how fast a t-storm is moving in. I know that doesn't do you any good in your plane but its never too fast to out-run in an RV. Hurricanes don't even move all that fast. Felix is currently moving at about 15 mph.

I know I didn't answer your question but I'm not among the more experienced. Just thought I'd throw this out there.
 
396/496

Great Post Vern....

Just thought I would mention that the 396/496 has data blocks for fully developed T-storms. The data blocks show direction, speed, tops and DB level.

-Rob
 
In FAA weather school they told us that the guiding winds for thunderstorms are considered to be the winds at 500 MB (about 18,000 feet). I think that works well for direction but have never paid much attention to speed. Surface winds mean nothing at all in this context. In the building stage surface winds would tend to be toward the storm and they would tend to be away from the storm in the dissipating stage or certainly during any downburst event.

Looking at the Garmin 496, it is interesting how much variation in individual cell movement there can be, in the same general area.
 
Concur with Vern

If I can see a thunderstorm I don't fly. The gust fronts associated with some of these are horrendous.
 
Thunderstorms do not simply travel with the air mass that they are in - they are a wave-like phenomenon. In other words, they are developing on one side and dissipating on the other. They commonly move, at least in the midwest, somewhere between 15 and 50 mph, but that doesn't mean that the wind is that high anywhere ahead of them.

One place to find the speed is to look at some of the weather sites. The duats site shows tops and speed/direction, I don't remember which others also do. But, your question is related to an airborne estimation, and that seems difficult without the onboard weather.

BTW, I would not fly a third of the summer days if I stayed on the ground when I can see a TS. No airplane would ever fly in FL! It is like every other decision we make regarding weather - it depends.
 
I can tell you they are;

ALWAYS FASTER THEN YOU THINK! :eek:
ALWAYS HIGHER THEN YOU THINK!
ALWAYS CLOSER THEN YOU THINK!
 
The question is a matter of airborne storm speed estimation. Down here in the SE we get big cells and lines of course, but we also get lots of isolated smaller ones and as Alex said, you learn to deal with them or you don't fly much. I'd like to think that everyone here understands storm downflow, outflow, and sensible minimum distance from t-storms of various sizes. If not, you'll get a memorable lesson sooner or later <g>.

Didn't know about the data blocks on a 396-496. I used a borrowed 396 on Monday for the first time and pretty much left it on the map screen with a weather overlay. More learning required. Nice tool and I will have XM in the new airplane.

With an RV you generally have speed and range as the ace up your sleeve, assuming you always maintain an "out" course. Some of the lighter, slower airplanes are more critical; no speed and sometimes as little at 10 gallons usable when full. Best if you make better decisions earlier. Consider this simple example. You're humping along in a J-3 at 70 mph groundspeed. You have a moderate size isolated t-storm about 20 miles off to the right of your course line, and it appears it will intersect your course line at a right angle about 20 miles ahead of your current position. If it is moving at 35 mph you should arrive at the point of intersection with 10 miles of leeway. You may or may not consider that a reasonable minimum, but at least you have a considered number. Clearly anything more than a 35 mph storm groundspeed is gonna make things iffy. It is an example of the old axiom about not flying your airplane anyplace your mind hasn't already gone.

The note about 18,000 ft winds is interesting. If they are a reliable indicator of storm movement, it means upper level winds are useful flight planning info even if you'll be flying low in a basic eggbeater.

Amusing side note; I cheerfully admit the first thing to pop into my head when I see a t-storm is the line from Monty Python's Holy Grail; "Run away, run away!". On first impression, XM weather reminds me of the original PacMan game from the 70's <g>
 
In addition to all the good points already made, I'd add that you want to look at the causal factors for the thunderstorms to figure out what they are going to do. If they are frontal (caused by a front), then figure out the speed of the front - they can be predictable (at least until they hit a coast or mountain range). Down here in Texas and the gulf coast, we get a lot of non-frontal "air mass": storms that just grow up and dissipate without moving. I just flew through that stuff this afternoon on my way back from D.C.....I like that kind, as they are pretty much standing still for you to work around them. Yet another day when XM on the 396 made the trip a piece of cake!

Paul
 
It depends...

A single cell storm will "generally" move along with the 15-20k (altitude) winds. BUT - the multi cell storms are more common - not always moving from their start point - just "extending" into a line. Their formation depends on the local area conditions... in the bigger picture air mass :confused:

Remember: weather is an exact science :)

WildThing
 
Outflow

Not exactly on topic, but pertinent. I find that one of the most significant factors affecting the ride "near" t-storms is the relative humidity of the air mass below the base. On a humid day, there may be scarely a bump if you happen to wander into some precip. On dry days, the dreaded downburst is pronounced and enhanced by evpaorative cooling as the precip falls beneath the cloud. Feels like an elevator ride to the ground.

No flames - I'm not talking about flying through precip beneath fully developed, lightning producing cells - just the garden variety, small cumulous buildups trailing some precip. If air is wet, diversion to avoid downdrafts can be less. And I don't mind a free car wash occasionally.
 
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