Magic Numbers Summary
This is a summary of the magic numbers used
in this guide for thresholds of storm and severe thunderstorm development.
Please note that the worst thing you could
do is continually refer to this page and
look for the thresholds and apply them to the current situation that you
are currently experiencing!!! The atmosphere is extremely
dynamic and these thresholds can only
be considered as a guide and not law.
Just because something is outside a threshold,
or in another does not in anyway mean that is what will happen. I
strongly suggest that you read all sections of this guide
in order to obtain a better idea on what these thresholds actually mean
and how they can best be used. For example, just because a CAPE of
950 falls in one category and a CAPE of 1050 falls in another does not
mean that the latter is significantly better than the former!
An Excel version of this page has been produced
by Steve (Feral-Wx) from Weatherzone - it can be found here,
and it may assist people in their method of forecasting!
Instability Figures
Lifted Index
|
LI Value
|
Result
|
|
+2 or higher
|
The 500mb level looks relatively
stable, might get some showers if the lower levels are cool enough though.
Storms unlikely.
|
|
0 to +2
|
Possible showers, low risk
of storms (but storms in more unstable areas might move into this region
and survive).
|
|
-2 to 0
|
Weak instability, potential
for some showers and storms.
|
|
-4 to -2
|
Moderate instability, ample
potential for storms - starting to become favourable for severe storms
if other conditions are right.
|
|
-4 to -6
|
Strong instability, more
than ample potential for storms and severe storms.
|
|
-6 and below
|
Very strong instability,
same as above.
|
CAPE & LIs
|
CAPE
(LIs)
|
Description
|
|
<
500
(-1 to 2)
|
Very
weak instability, showers likely with some isolated storms. If shear
is absolutely fantastic, then there is the chance of severe storms.
|
|
500
-1000
(0 to -3)
|
Weak
instability, showers and storms likely but generally weak unless shear
is good.
|
|
1000-1750
(-2
to -5)
|
Moderate
instability, storms (possibly severe with pulses), becoming quite severe
if shear is very good, updrafts may be strong enough to sustain large hail
(2cm+).
|
|
1750-2500
(-4
to -8)
|
Strong
instability, possible severe pulse storms in weak shear - probable severe
storms in good shear, large enough to sustain large (2cm+) to very large
hail (5cm).
|
|
2500-4000
(-6
to -12)
|
Very
strong instability, severe pulse storms likely in weak shear. Good
shear will result in severe to very severe storms with updrafts strong
enough to sustain very large (5cm+) to extreme (8cm+) hail.
|
|
4000
>
(-10 to -16)
|
Extreme
instability, severe pulse storms likely in weak shear. If you have
good shear - watch out! Updrafts strong enough to sustain hail in
excess of 10cm.
|
Shear Figures
300mb
Winds
|
Wind
Strength
|
Effects
|
|
<
20 knots
|
It is
unlikely that there will be enough wind shear at this level to help blow
away the cirrus and other high cloud that is produced from storms.
Storms would probably collapse on themselves unless the mid level shear
is relatively strong.
|
|
20 -
30 knots
|
This
is marginal, it should allow enough shear for thunderstorms, and the risk
of some severe pulses but you will need some strong instability to offset
this, or at least some good shear in the mid levels.
|
|
30 -
45 knots
|
Adequete
but not good, this should allow enough shear
for thunderstorms and even severe thunderstorms providing there's some
moderate instability too.
|
|
45-70
knots
|
Good
shear, allows reasonable outflow for thunderstorms at the 300mb level,
including supercells and severe storms.
|
|
70-100
knots
|
Very
good shear, ample outflow for all storms.
|
|
100 knots
>
|
Very
strong shear, perhaps too strong for weak storms, but fantastic for other
storms!
|
Surface
Winds
|
Wind
Strength
|
Effects
|
|
<
5 knots
|
Negligible
|
|
5
- 10 knots
|
Light
inflow, helps storms a little but not really ideal
|
|
10
- 15 knots
|
Moderate
inflow, helps storms organise themselves
near the surface, in Australia we lack a low level jet a lot of the times
and if I had a 10-15 knot surface flow I'd be very happy!
|
|
15-25
knots
|
Strong
inflow, probaby only experienced around
frontal systems in Australia,
or near the coast from seabreeze fronts
- great for severe storms and supercells!
|
|
25
knots >
|
Very
strong inflow!
|
Wind
Strength Guide
|
|
Poor
|
Marginal
|
Adequate
|
Good
|
Verygood
|
|
1000mb
|
<
5 knots
|
5
- 10 knots
|
10
- 15 knots
|
15
- 25 knots
|
25
knots >
|
|
850mb
|
<
7 knots
|
7
- 10 knots
|
10
- 17 knots
|
17
- 30 knots
|
30
knots >
|
|
700mb
|
<
10 knots
|
10
- 15 knots
|
15
- 20 knots
|
20
- 40 knots
|
40
knots >
|
|
500mb
|
<
15 knots
|
15
- 20 knots
|
20
- 30 knots
|
30
- 50 knots
|
50
knots >
|
|
300mb
|
<
20 knots
|
20
- 30 knots
|
30
- 45 knots
|
45
- 70 knots
|
70
knots >
|
Assorted Figures
Cap (summer)
|
Temperature
|
Effect
|
|
<
15C
|
Weak
cap, development likely early.
|
|
15
- 17C
|
Moderate
cap, not really ideal but should hold convection off until midday
or early afternoon - later if the trigger is weak.
|
|
17
- 19C
|
Good cap,
should hold convection off until the afternoon but will require a reasonable
trigger to break.
|
|
19
- 21C
|
Strong cap,
will need a good trigger to break.
|
|
21
- 23C
|
Marginal
- the trigger will need to be very strong or it's going to need to get
very hot to break the cap!
|
|
23C
>
|
22-23C
approaches the limit of thunderstorm development in most situations.
|
Specific Humidty to Dewpoint Conversion
|
Specific
Humid.
|
Dew Point
|
Specific
Humid.
|
Dew Point
|
Specific
Humid.
|
Dew Point
|
|
2
|
-9
|
9
|
12
|
16
|
21
|
|
3
|
-3
|
10
|
14
|
17
|
22
|
|
4
|
0
|
11
|
15.5
|
18
|
23
|
|
5
|
4
|
12
|
17
|
19
|
24
|
|
6
|
6
|
13
|
18
|
20
|
25
|
|
7
|
8
|
14
|
19
|
21
|
26
|
|
8
|
10
|
15
|
20
|
|
|
NB: The above table is a conversion chart and
not a guide like the others. |