Update: april 2008
CMC
SUMMER SEVERE WEATHER
DESCRIPTION OF CHARTS ON
THE WEB
OBJECTIVE
CONVECTIVE CHARTS
ISSUE
TIMES
Charts issued at 0400Z  based on 00Z GEM
regional
 Périod 1527h 
Valid 15Z/d to 03Z/d+1
 Périod 1830h 
Valid 18Z/d to 06Z/d+1
 Périod 2736h 
Valid 03Z/d+1 to 12Z/d+1
 Périod 1230h 
Valid 12Z/d to 06Z/d+1
 Périod 1236h 
Valid 12Z/d to 12Z/d+1
 Périod 3648h 
Valid 12Z/d+1 to 00Z/d+2
Charts issued at 1600Z  based on 12Z GEM
regional
 Périod 0315h 
Valid 15Z/d to 03Z/d+1
 Périod 0618h 
Valid 18Z/d to 06Z/d+1
 Périod 1524h 
Valid 03Z/d+1 to 12Z/d+1
 Périod 2739h 
Valid 15Z/d+1 to 03Z/d+2
 Périod 3042h 
Valid 18Z/d+1 to 06Z/d+2
 Périod 2448h 
Valid 12Z/d+1 to 12Z/d+2
DESCRIPTION
^{GENERAL THUNDERSTORMS}
^{MARGINALLY SEVERE THUNDERSTORMS}
^{SEVERE THUNDERSTORMS}
^{ }^{DETERMINATION OF THE AREAS}
^{ An area of thunderstorms (general, marginally severe or severe) for a given period is determined from an evaluation of the criteria, discussed further on, at 3hour intervals during the period under consideration. For example, for the period 1236h, the criteria are evaluated for T0+12, T0+15, T0+18, T0+21, T0+24, T0+27, T0+30 T0+33 and T0+36.}
^{ }
^{ }^{For a given gridpoint, it is sufficient that one of the criteria, for example for a severe thunderstorm, be met at one of these 9 forecast times, for that point to be considered part of a red potential severe thunderstorm area for that period. }
^{ }
^{ On the other hand, whether it be for severe, marginally severe or general thunderstorms, the criteria are examined in the indicated order, and as soon as any one of the criteria is met, the evaluation stops. Thus, if the first of the criteria examined is satisfied, a potential has been identified, and so further examination is halted. If it were not satisfied, then the second is examined and so on until the last of the criteria. }
^{ }
^{ The
examination is done first for severe thunderstorms. Then, if the criteria are not met, the
criteria for “marginally severe” are examined to see if they can be
satisfied. Of course, if one of the
criteria for severe or marginally severe thunderstorms is met for a given
point, then there is automatically a potential for a general thunderstorm. If criteria for severe or marginally severe
thunderstorms are not met, then the examination of general thunderstorms criteria
proceeds. }
CRITERIA (See definition of variables below)
SEVERE THUNDERSTORMS
1 UV500 >= 20 kts
SWEAT >= 300
AND
[BE >= 1000
J/kg AND LI (567 + syno) <= 0] OR LI (567 + syno) <= 4
OR
BE579 >= 1000 J/kg OR LI (579 + syno) <= 4
2
EHI >= 1
AND LI (567 + syno) <= 0
OR
EHI 579 >=
1 AND LI (579b + syno) <= 0
3
SSI >= 100 AND LI (567 + syno) <=
0
OR
SSI579 >= 100
AND LI (579b + syno) <=
0
4 Petits cb’s
UV500 >= 50 kts AND SWEAT >= 200 AND DZ5 <= 0
AND
LI (567 + syno)
<= 0
BE >=
500 J/kg OR (BE >=
300 J/kg AND dThetaE >= 12C)
OR
LI (579b + syno)
<= 0
BE579 >=
500 J/kg OR (BE579 >= 300 J/kg AND
dThetaE >= 12C)
5 Masse d’air
EP >= 40
mm
PC
(T3 à T+3) >= 15 mm
AND
BE
>= 1000
J/kg OR LI (567 + syno) <= 4
OR
BE579 >= 1000 J/kg OR LI (579 + syno) <=
4
6 Schème de convection
PC
(T3 à T+3) >= 25 mm
Brief
explanation  Severe Thunderstorm Criteria
Criteria
#123
The
three first criteria are based on the principal elements observed in the
structure of severe thunderstorms and their environment, i.e. strong shear and
strong hydrostatic energy / instability.
Criterion
#4
The
fourth criterion, called “Small CB’s”, refers to cases where the thermodynamic
potential (humidity / instability) is less pronounced, but where the dynamics
and strong winds aloft play an important role.
In these cases the lowering of 500 mb heights
(DZ5 <= 0) in the last 3 hours is required, as this can be indicative of an
approaching shortwave. This type of
severe weather often manifests itself by strong thunderstorm gusts due to
downward deviation of midlevel winds, and hence the criterion of 50 knots or
greater winds at 500 mb. As well, it is well understood the use of dThetaE, an
index developed to indicate wet microburst potential.
Criterion
#5
The
fifth criterion, called “Air mass”, refers mainly to the potential for strong
thunderstorm related flashfloods in situations where the instability and
humidity are important, but the dynamics are weak. Here it is required that the model convection
scheme gives at least 15 mm in 6 hours
(PC >= 15). So this criterion will mainly be detected for "marginally
severe" areas where no PC is requested. It is expected that this will be
more representive for these humid and unstable zones which are
sometimes very extensive but for which triggering is quite uncertain.
Criterion
#6
This
last criterion gives free reign to the model convection scheme. If none of the first five criteria are met,
as a last resort the model convection scheme is used to see if it has detected
important convection elsewhere.
CRITERIA (See definition of variables below)
MARGINALLY SEVERE
THUNDERSTORMS
1 SEVERE THUNDERSTORM
2 UV500 >= 20 kts
SWEAT >= 250
AND
{BE >= 900
J/kg AND [LI (567 + syno) <= 0 OR DZ5 <= 0 ]}
OR
LI (567 + syno) <= 4
OR
BE579 >= 900 J/kg OR LI (579 + syno) <=
4
3 Small
cb’s
UV500 >= 40 kts AND SWEAT >= 150 AND DZ5 <= 0
AND
LI (567 + syno)
<= 0
BE >=
400 J/kg OR (BE >=
200 J/kg AND dThetaE >= 12C)
OR
LI (579b + syno)
<= 0
BE579 >=
400 J/kg OR (BE579 >= 200 J/kg AND
dThetaE >= 12C)
4 Small
cb's – FZLVL
SWEAT
>= 150 AND BE >= 300 J/kg AND LI (5678 + syno) <=
3 AND DZ5 <= 0
AND
6000 <= FZLVL <= 9000
5 Air
mass
EP >=
30 mm
AND
LI (567 + syno) <=
4 OR LI (579 + syno) <= 4
OR
BE
>= 900
J/kg AND LI (567 + syno) <= 0
OR
BE579 >= 900 J/kg AND LI (579 + syno) <=
0
6 convection scheme
PC
(T3 à T+3) >= 15 mm
Brief
explanation  marginally severe thunderstorms
As
mentioned above, the criteria for “marginally severe” are examined only if the
criteria for severe thunderstorms are not met.
These criteria are the same, but slightly relaxed compared with the
preceding.
The
goal is to permit a certain margin of error or of maneuverability, given that
it consists of an objective evaluation of convection. For example, it would be unfortunate to
indicate a severe weather potential with a hydrostatic energy of 1000 J/kg, but
not for 999 J/kg.
On
the other hand, the criteria for EHI and SSI are not considered for the
“marginal” cases, given that these indices are fairly categorical as to the
thresholds used to determine tornadic or other severe
thunderstorm potential, and so don’t lend themselves to a relaxation.
It
is also to be noted that the criteria "Small cb's
– FZLVL" is a particular case as it is not showing up in the severe
thunderstorms criteria. This criteria is for detection of thunderstorms giving
hail in a relatively cold air mass and this is the reason for requesting low
freezing levels, namely between 6000 feet and 9000 feet. There is however no restriction
on 500 mb wind speed as for the more classical
"Small cb's" case which is considered just
before the "Small cb's – FZLVL" in the
program loop. It results that, if no classical "Small cb's"
are detected, then the program looks for the "Small cb's
– FZLVL" criteria which mainly refers to small hail in a cold air mass,
for example within a cold low area. And as these situations indeed mainly allow
for generally small hail, that is the reason why this criteria is only
considered for "marginally severe" thunderstorms.
CRITERIA (See definition of variables below)
GENERAL THUNDERSTORMS
1 SEVERE THUNDERSTORM
2 MARGINALLY SEVERE THUNDERSTORM
3 Surface
based instability
LI (567 + syno) <= 0
4 Instability
based at 900850800 mb
LI (579b + syno) <= 0
5 Instability
based at 750700650600 mb (ACC / ACB)
LI (579h + syno) <= 0
6 Potentially
severe thunderstorm but occurrence unlikely
UV500 >=
20 kts
SWEAT >= 250
AND
BE >= 900 J/kg
LI
(567 +syno) > 0
AND DZ5 >
0
Brief
Explanation  general thunderstorms
As
mentioned above, if the criteria for severe or marginally severe thunderstorms
are met, then there is automatically a potential for a general thunderstorm,
hence the first two criteria.
If
criteria 1 or 2 are not met, then criteria 3 to 5 are examined systematically
in order. We start with the lower
atmosphere with the lifted index, specifically the (LI567 + syno), which is explained further on. If this criterion is not met, then we examine
possible ascents at 50 mb intervals between 900 and
600 mb.
Hence,
(LI579b + syno), also explained further on, is
linked to a particle ascent from one of the 900850800 mb
levels and is used as the fourth criterion.
And
(LI579h + syno), explained further on, is linked to
a particle ascent from one of the 750700650600 mb
levels and is used as the fifth criterion for ACC/ACB detection.
A
sixth criterion has been added for situations where hydrostatic energy and wind
shear are sufficient to allow for a severe or marginally severe thunderstorm
but dynamics is too weak (DZ5 > 0) and low level inhibition is quite high
(LI(567+syno) > 0), so that it is unlikely that a thunderstorm will even be
triggered. However, considering that if a thunderstorm is nevertheless showing
up in these conditions, it would likely be strong, even possibly severe, we then at least leave it as a regular thunderstorm.
DEFINITION
OF THE VARIABLES
 UV500...
wind speed at 500 mb
 DZ5... 500 mb height change in the last
3 hours
 EP...
amount of precipitable water from the surface
to 400 mb
 PC...
amount of convective precipitation
(model
convective scheme) for a 6 hour period
centered on
the diagnostic hour.
 BE... Hydrostatic
energy for a parcel ascent from layer SFC925 mb.
 BE579 ... Hydrostatic energy for a
parcel ascent from one of the following levels: 900850800750700650600 mb.
 EHI.......
"Energy Helicity Index" computed with BE

EHI579... "Energy Helicity Index" computed
with BE579
 SSI.......
"Severe Storm Index" computed with BE
 SSI579...
"Severe Storm Index" computed with BE579
 SWEAT...
"Severe Weather Threat Index"
 dThetaE = DeltaThetaE... Thunderstorm
microburst index
DEFINITION
OF THE VARIABLES
(continued)

LI reg. ... Standard Lifted Index

LI567... Lifted Index considering lifting up to
700, 600 and 500 mb. (See below)
 LI579... Lifted Index computed from
an ascent based at one of levels 900850800750700650600 mb and considering that the parcel must keep to the right
of environment curve through its ascent up to 500 mb
level. (See below)
 LI579b... LI579 "bbas(low)", i.e. for parcels lifted from 900850800 mb.
 Li579h... Li579 "hhaut(high)", i.e. for parcels lifted from
750700650600 mb.

LI+syno... Lifted Index considering certain
“synoptic conditions”
(See below)

LI(reg.+syno)... Standard LI
("LI reg"), satisfying the
“synoptic
conditions”

LI(567+syno)... "LI567" satisfying the
“synoptic
conditions”

LI(5678+syno)... "LI5678" satisfying the
“synoptic
conditions”

LI(579+syno)... "LI579" satisfying the
“synoptic conditions”

LI(579b+syno)... "LI579b" satisfying the
“synoptic conditions”
 LI(579h+syno)... "LI579h"
satisfying the
“synoptic conditions”
LIFTED
INDEX
1 "LIreg"...
Standard Lifted Index
The
standard lifted index is calculated by lifting a parcel from the surface to 500
mb. The value
of the index is thus determined by the temperature difference between the
raised parcel and the environment, T_{env}  T_{part}.
In
actual fact, the lifted index calculated by the model considers lifting parcels
from the 5 or 6 lowest levels of the model. The index is calculated for each of
these levels, and the model retains the minimum value for that point, i.e. the
most unstable.
2a "LI567"... Lifted
Index (500600700 mb)
Usage
of the standard lifted index as is entails certain difficulties in that the
lifting is forced. It can thus occur
that the parcel remains colder than the environment for a large part of its
ascent before ending up warmer at 500 mb, and hence
giving a negative value to the index, but with a doubtful thunderstorm
potential.
To
get around this somewhat, various “homemade” lifted indices were developed
here at CMC by considering the temperature of the parcel compared to that of
the environment at different levels in its forced ascent.
In
this way the lifting of parcels to 600 and 700 mb was
programmed, creating lifted indices at those levels. Hence, a negative lifted
index at 600 mb indicates that the parcel is warmer
than the environment at that level, and similarly for a negative lifted index
at 700 mb.
These
indices are calculated in the same way as for the standard lifted index, i.e.
if we continue the ascent to 500 mb, we obtain the
standard lifted index.
Armed
with the information obtained from these multiple lifted indices, a lifted
index, baptized “LI567”, was concocted in the following manner.
The
"LI567" starts with the same value as the standard LI. Then the
“LI600” and “LI700” are examined for all points where the standard LI is <=
0. For any of these points, when LI600 or
LI700 is positive, then an arbitrary value of +4 is assigned to LI567.
Otherwise, the value (negative or null) of the original standard LI is copied
into LI567. The result then is that LI567 is <= 0 only if LI600 and LI700, as well as "LI reg (LI500)" are <= 0 at a given point.
In
other words, a negative LI567 implies that the parcel was warmer than the
environment at each of the 700, 600 and 500 mb levels
during its ascent. In this way, negative
standard LI values are eliminated for profiles where there are important
inhibitions to lifting.
2b "LI5678"... Lifted
Index (500600700800 mb)
LI5678
is similar to LI567 defined above, but with a supplementary condition with
regard to the value of "LI800" (lifted index at 800 mb).
Thus,
the "LI5678" starts with the same value as the standard LI. Then the
“LI600”, “LI700” and "LI800" are examined for all points where the
standard LI is <= 0. For any of these points, when LI600 or LI700 or LI800 is positive, then an arbitrary value of +4 is
assigned to LI5678. Otherwise, the value (negative or null) of the original
standard LI is copied into LI5678. The result then is that LI5678 is <= 0
only if LI600 and LI700 and LI800, as well as "LI reg (LI500)" are <= 0 at a given point.
In
other words, a negative LI5678 implies that the parcel was warmer than the
environment at each of the 800, 700, 600 and 500 mb
levels during its ascent.
The
LI5678 is thus more restrictive than the LI567 and removes the negative
standard LI in view of lower level inhibition.
It
is to be noted that we mainly use the LI567 for the determination of regular
or severe thunderstorms areas. Indeed the LI5678 being more restrictive at low
level, its use at large could prevent detection of thunderstorms that could
even be severe in situations of capping inversions that allow for increasing
potential energy.
2c "LI579"...
"Lifted Index" for ascents from 900600 mb
LI579
is similar to LI567 defined above, except that the former is related to
parcels lifted from above de surface, that is at each
50 mb between 900 and 600 mb.
For
a given grid point and at a given forecast time, tentative ascents are made at
each of the levels 900850800750700650600 mb.
Indeed, an ascent is done if
TTd
<= 1°C at the considered level. If this
condition is satisfied, then ascent is done as if the parcel were completely
saturated, i.e. along moist adiabat up to 500 mb level where LI is computed. Between all ascents that
could thus be done from above mentioned levels, the minimum LI is retained.
Also,
a negative LI579 will keep its negative value only if the parcel stayed to the
right of the environment temperature curve all through its ascent from the base
level up to 500 mb, this condition being verified at
50 mb intervals.
3 "LI+syno"...
"lifted index" + "synoptic"
conditions
The
LI567/5678/579 has proved very useful, but its evaluation in the operational
context has shown that it also can be negative in situations where thunderstorm
potential is doubtful. For example,
negative LI567 values can occur in the middle of ridges where there is
subsidence aloft and very dry midlevel air.
To
further refine potential thunderstorm areas, conditions dubbed “synoptic” were
added to the LIreg or LI567/5678/579, thus
obtaining "LI(reg.+syno)",
"LI(567+syno)", "LI(5678+syno)" and the
"LI(579+syno)".
Here
are the “synoptic” conditions and the manner in which they are handled.
"SYNOPTIC"
CONDITIONS
The initial "lifted indices" (LIrég, LI567, LI5678, LI579, LI579b, LI579h) are
identified below by "LI", while the resulting indices are identified
by "LI+syno".
Hence,
if LI = LIreg, then LI+syno = LI(reg.+syno)
if LI = LI567, then LI+syno =
LI(567+syno)
if LI = LI5678, then
LI+syno = LI(5678+syno)
if LI = LI579, then LI+syno =
LI(579+syno)
if LI = LI579b, then LI+syno =
LI(579b+syno)
if LI = LI579h, then LI+syno =
LI(579h+syno)
The
conditions are applied only on those points where the initial LI is <= 0. If
it is decided to keep the resultant negative or zero lifted index (LI+syno), then it is simply the initial value that is
kept. Otherwise, a value of +4 is forced
in the resultant index (LI+syno).
"SYNOPTIC" CONDITIONS
A LI : ascent base level: sfc925 + 900850800 mb
(LI579b)
1)
If LI <= 0
If
EP < 5.0 mm
then LI+syno
= +4
If 5.0
mm <= EP < 10 mm
then LI+syno = +4
except if
LI
<= 2
2) If 1 <= LI
<= 0
. If BE < 100 J/Kg and DZ5 > 0 then LI+syno = +4
. If HR700 <= 20% then
LI+syno = +4
. 3)
If LI < 1
. IF HR_700 <= 20% and DZ5 > 0 then
LI+syno = +4
B LI : ascent base level: 750700650600 mb (ACB)
(LI579h)
1)
If LI <= 0
If
EP < 10.0 mm
If
(TTd)600 >=
15 C when bases = 750700 mb
If
(TTd)500 >=
15 C when bases = 650600 mb
Then,
if one of these 3 conditions is encountered, we
force LI+syno = +4
2)
If 1 <= LI
<= 0
If
BE < 100 J/Kg, then LI+syno = +4
Brief
explanation of the “synoptic” conditions
Condition
(A1) eliminates negative initial LI values when the air is too dry. However,
approaching EP=10 mm, LI values that are sufficiently unstable (<= 2) are
conserved and will lead to thunderstorm potential.
The
conditions in (A2) eliminate weakly negative initial LI values (1 <= LI <=
0) in cases where it is very dry in the midlevels (HR700 <= 20%) and in
cases where subsidence is favoured (DZ5 > 0) and the instability is marginal (BE < 100 J/Kg).
The
last condition eliminates more strongly negative LI values (LI < 1) if there is both dry air in the midlevels and subsidence. The conditions are
stringent because the more negative initial LI values imply greater
instability, while the dry midlevel air is an indicator of strong convection. However, in the presence of subsidence (DZ5<=0),
then this potential goes to near zero.

Conditions
in (B) are similar to those in (A) except sligthly
modified for convection aloft.
