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 15-27h - Valid 15Z/d   to 03Z/d+1

- Périod 18-30h - Valid 18Z/d   to 06Z/d+1

- Périod 27-36h - Valid 03Z/d+1 to 12Z/d+1

- Périod 12-30h - Valid 12Z/d   to 06Z/d+1

- Périod 12-36h - Valid 12Z/d   to 12Z/d+1

- Périod 36-48h - Valid 12Z/d+1 to 00Z/d+2

Charts issued at 1600Z - based on 12Z GEM regional

- Périod 03-15h - Valid 15Z/d   to 03Z/d+1

- Périod 06-18h - Valid 18Z/d   to 06Z/d+1

- Périod 15-24h - Valid 03Z/d+1 to 12Z/d+1

- Périod 27-39h - Valid 15Z/d+1 to 03Z/d+2

- Périod 30-42h - Valid 18Z/d+1 to 06Z/d+2

- Périod 24-48h - 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 3-hour intervals during the period under consideration.  For example, for the period 12-36h, 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 grid-point, 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-         UV-500  >= 20 kts

SWEAT >=  300

AND

[BE  >=  1000 J/kg   AND  LI (567 + syno) <= 0]  OR    LI (567 + syno)  <= -4

OR

BE-579 >= 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

SSI-579   >=  100   AND    LI (579b + syno)  <=  0

4-         Petits cb’s

UV-500  >= 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

BE-579  >=  500 J/kg    OR    (BE-579 >=  300 J/kg  AND  dThetaE  >= 12C)

5-         Masse d’air

EP  >= 40 mm

PC (T-3 à T+3)  >=  15 mm

AND

BE        >=  1000 J/kg    OR    LI (567 + syno)  <=  -4

OR

BE-579 >=  1000 J/kg    OR    LI (579 + syno)  <=  -4

6-         Schème de convection

PC (T-3 à T+3)  >=  25 mm

Brief explanation - Severe Thunderstorm Criteria

Criteria #1-2-3

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 short-wave.  This type of severe weather often manifests itself by strong thunderstorm gusts due to downward deviation of mid-level 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 flash-floods 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-         UV-500  >= 20 kts

SWEAT >=  250

AND

{BE  >=  900 J/kg   AND  [LI (567 + syno) <= 0 OR  DZ5 <= 0 ]}

OR

LI (567 + syno)  <=  -4

OR

BE-579 >=  900 J/kg    OR    LI (579 + syno)  <=  -4

3-         Small cb’s

UV-500  >= 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

BE-579  >=  400 J/kg    OR    (BE-579 >=  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

BE-579 >=  900 J/kg    AND    LI (579 + syno)  <=  0

6-         convection scheme

PC (T-3 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 900-850-800 mb

LI (579b + syno)  <=  0

5-       Instability based at 750-700-650-600 mb  (ACC / ACB)

LI (579h + syno)  <=  0

6-  Potentially severe thunderstorm but occurrence unlikely

UV-500  >=  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 (LI-567 + 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, (LI-579b + syno), also explained further on, is linked to a particle ascent from one of the 900-850-800 mb levels and is used as the fourth criterion.

And (LI-579h + syno), explained further on, is linked to a particle ascent from one of the 750-700-650-600 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

- UV-500... 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 SFC-925 mb.

- BE-579 ... Hydrostatic energy for a parcel ascent from one of the following levels: 900-850-800-750-700-650-600 mb.

- EHI....... "Energy Helicity Index" computed with BE

- EHI-579... "Energy Helicity Index" computed with BE-579

- SSI....... "Severe Storm Index" computed with BE

- SSI-579... "Severe Storm Index" computed with BE-579

- SWEAT... "Severe Weather Threat Index"

- dThetaE = DeltaThetaE... Thunderstorm microburst index

DEFINITION OF THE VARIABLES (continued)

- LI reg. ... Standard Lifted Index

- LI-567... Lifted Index considering lifting up to

700, 600 and 500 mb. (See below)

- LI-579... Lifted Index computed from an ascent based at one of levels 900-850-800-750-700-650-600 mb and considering that the parcel must keep to the right of environment curve through its ascent up to 500 mb level. (See below)

- LI-579b... LI-579 "b-bas(low)", i.e. for parcels lifted from 900-850-800 mb.

- Li-579h... Li-579 "h-haut(high)", i.e. for parcels lifted from 750-700-650-600 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)... "LI-567" satisfying the

synoptic conditions”

- LI(5678+syno)... "LI-5678" satisfying the

synoptic conditions”

- LI(579+syno)... "LI-579" satisfying the

synoptic conditions”

- LI(579b+syno)... "LI-579b" satisfying the

synoptic conditions”

- LI(579h+syno)... "LI-579h" satisfying the

synoptic conditions”

LIFTED INDEX

1- "LI-reg"... 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, Tenv - Tpart.

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- "LI-567"... Lifted Index (500-600-700 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 “home-made” 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 “LI-567”, was concocted in the following manner.

The "LI-567" starts with the same value as the standard LI. Then the “LI-600” and “LI-700” are examined for all points where the standard LI is <= 0. For any of these points, when LI-600 or LI-700 is positive, then an arbitrary value of +4 is assigned to LI-567. Otherwise, the value (negative or null) of the original standard LI is copied into LI-567. The result then is that LI-567 is <= 0 only if LI-600 and LI-700, as well as "LI reg (LI-500)" are <= 0 at a given point.

In other words, a negative LI-567 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- "LI-5678"... Lifted Index (500-600-700-800 mb)

LI-5678 is similar to LI-567 defined above, but with a supplementary condition with regard to the value of "LI-800" (lifted index at 800 mb).

Thus, the "LI-5678" starts with the same value as the standard LI. Then the “LI-600”, “LI-700” and "LI-800" are examined for all points where the standard LI is <= 0. For any of these points, when LI-600 or LI-700 or LI-800 is positive, then an arbitrary value of +4 is assigned to LI-5678. Otherwise, the value (negative or null) of the original standard LI is copied into LI-5678. The result then is that LI-5678 is <= 0 only if LI-600 and LI-700 and LI-800, as well as "LI reg (LI-500)" are <= 0 at a given point.

In other words, a negative LI-5678 implies that the parcel was warmer than the environment at each of the 800, 700, 600 and 500 mb levels during its ascent.

The LI-5678 is thus more restrictive than the LI-567 and removes the negative standard LI in view of lower level inhibition.

It is to be noted that we mainly use the LI-567 for the determination of regular or severe thunderstorms areas. Indeed the LI-5678 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- "LI-579"... "Lifted Index" for ascents from 900-600 mb

LI-579 is similar to LI-567 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 900-850-800-750-700-650-600 mb. Indeed, an ascent is done if

T-Td <= 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 LI-579 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 LI-567/5678/579 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 LI-567 values can occur in the middle of ridges where there is subsidence aloft and very dry mid-level air.

To further refine potential thunderstorm areas, conditions dubbed “synoptic” were added to the LI-reg or LI-567/5678/579, 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" (LI-rég, LI-567, LI-5678, LI-579, LI-579b, LI-579h) are identified below by "LI", while the resulting indices are identified by "LI+syno".

Hence,

if LI = LI-reg, then LI+syno = LI(reg.+syno)

if LI = LI-567, then LI+syno = LI(567+syno)

if LI = LI-5678, then LI+syno = LI(5678+syno)

if LI = LI-579, then LI+syno = LI(579+syno)

if LI = LI-579b, then LI+syno = LI(579b+syno)

if LI = LI-579h, 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: sfc-925 + 900-850-800 mb

(LI-579b)

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 HR-700 <= 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: 750-700-650-600 mb (ACB)

(LI-579h)

1) If LI <= 0

If EP < 10.0 mm

If (T-Td)600 >=  15 C  when bases = 750-700 mb

If (T-Td)500 >=  15 C  when bases = 650-600 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 (A-1) 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 (A-2) eliminate weakly negative initial LI values (-1 <= LI <= 0) in cases where it is very dry in the mid-levels (HR-700 <= 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 mid-levels and subsidence. The conditions are stringent because the more negative initial LI values imply greater instability, while the dry mid-level air is an indicator of strong convection.  However, in the presence of subsidence (DZ5<=0), then this potential goes to near zero.

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

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