Download Terminal Objective 6

T.O.6.4.4
A) Describe the characteristics of heavy rain events
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Severe Rain events: >=50 mm/1 hour, or >= 75 mm/3 hours, or causing significant damage
due to flooding
Heavy rain events occur with intense storms with strong instability, moisture, and shear. Long
duration caused by slow-moving or continuously propagating new cells along the same track.
They can be ordinary cells, supercells, or multiple cell storms such as MCSs.
Ingredients for Heavy Convective Rainfall:
1) strong and sustained updrafts
2) high moisture content with high surface dewpoints
3) high relative humidity through a deep tropospheric layer
4) slow moving system or system repeatedly move over the same area
5) weak to moderate vertical wind shear is present through the cloud layer
6) stationary re-development along the cold pool due to interaction with either low-level
shear, low-level boundaries, or orography
Four major weather patterns supporting heavy rain events:
1) Synoptic type associated with a relatively intense synoptic-scale cyclone or frontal
system. 500 mb: a major trough moving slowly eastward or northeastward, and the
surface front is quasi-stationary. Convective cells repeatedly form and move rapidly
over the same general area. Heavy rain occurs in the warm air.
2)
Frontal Type:
 A surface quasi-stationary synoptic-scale front (usually oriented east-west) that
triggers and focuses the convection. Heavy rain occurs on the cold side of the surface
front as warm, unstable air overruns the frontal zone
 A southerly low-level jet draws moisture northward and enhances convergence along
the front
 Winds above the frontal surface nearly parallel the front and convective storms
repeatedly form over a small region and move over the same terrain downwind
 Events are primarily nocturnal and other severe weather elements are rare
 In most, a detectable short-wave trough is present just upstream. Cells moving along
the frontal zone have an unimpeded source of heat and moisture flowing in at low
levels along their right flank.
 The location of heaviest rain is frequently near and just upstream of the long-wave
ridge position. A weak surface mesolow may be present along the frontal boundary,
increasing the convergence and inflow into the storm area
3)
Mesohigh type: associated with a quasi-stationary outflow boundary generated by
previous convection. The heaviest rain occurs on the cool side of the boundary, to the
south or southwest of the mesohigh pressure centre. The presence of a front nearby is
not a requirement for this type. Winds nearly parallel the boundary (as with the frontal
type ) and storms repeatedly form and move over the same area.
4)
Circulation System Type is associated with slow moving warm-top convection.
 The convection is organized around a significant middle-level vorticity centre into a
comma-shaped pattern.
 A weak surface wave can also form.
 The upper winds are frequently light in the vicinity of the heaviest rainfall and the
resulting speed of the upper trough is small.
 The precipitation is enhanced if, given sufficient moisture, an upper-level jet streak
or speed maximum rounds the base of the trough. This increases the magnitude of the
vorticity centre and accelerates the low-level jet entering the system from the
southwest, thereby enhancing both the flux of moisture into the system and the
strength of the overrunning
B) Describe briefly the characteristics of the following convective weather
phenomena: tornado, cold air funnel, land spout, waterspout, and dust devil
Tornado:
1) The tornado is a violently rotating, tall, narrow column of air, measuring 10-1000m in
diameter that extends to the ground (generally) from a cumuliform cloud. It is visualized by
a funnel shaped cloud pendant from the cloud base and/or a swirling cloud of dust and
debris rising from the ground.
2) Tornadoes are typically classified into two groups; supercell tornadoes, and non-supercell
tornadoes
3) Supercell tornado (but only 15-20% of supercells are tornadic)
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require a pre-existence of a parent supercell
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develops at the base of a supercell thunderstorm
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Associated with a deep, persistent rotating mesocyclone within supercells
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larger and longer-lived than non-supercell tornadoes
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Accounts for the vast majority of tornado-related deaths, injuries, and property damage.
4) Non-supercell tornadoe
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include such vortexes as gustnadoes, landspouts, and waterspouts
Typically form along outflow boundaries or gust fronts or beneath rapidly ascending
cumulus towers
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They are generally short-lived, weak, and essentially impossible to forecast or detect
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Nevertheless, they are more common than supercell tornadoes
Tornadic supercells occurs when:
1) presence of surface or low-level boundaries (outflow boundaries, drylines, etc.) are
key components in the production of tornadic supercells
2) environments with extreme CAPE may favour tornadic supercells
3) Processes that enhance the development of low-level rotation (inflow highly
streamwise, increased RFD strength) and lower cloud base would help to increase
the potential for supercells to become tornadic
4) high relative humidity near the ground (>70%) - resulting in lower cloud base
5) inflow highly streamwise - supercell track along pre-existing low-level boundary or
convergence axis (e.g., outflow, front, dryline)
6) baroclinic generation of horizontal vorticity along forward flank downdraft
7) some dry air at mid-levels to increase RFD strength
8) strongly curved hodograph to enhance SRH
Supercell thunderstorm:
• Main defining characteristic is a quasi-steady rotating updraft
• Persist for several hours
• Diameters of ~20-50km
• Are rare but account for large proportion of severe weather events (large hail,
tornadoes, heavy rain, winds)
• Develop in moderate to high CAPE environments (typically > 500 J/kg) with
strong, deep vertical shear (0-6km > 30kts)
Cold air funnel:
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funnel cloud spawned by large cumulus clouds or weak thunderstorms
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usually short-lived and do not have the energy to reach the ground
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But some may touch down briefly and become destructive over a very small area
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In general, they are less violent than most other types of tornadoes.
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Typically, the days when they occur are a bit cooler than normal with large puffy cumulus
clouds, showers or weak thunderstorms developing in the late morning or early afternoon
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These funnel clouds will normally appear with little or no warning. A special tornado
watch will be issued when they are predicted or sighted
Land spout:
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Landspouts usually have a narrow, rope-like condensation funnel extending from the
cloud base to the ground
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Are seen under small storms or large, growing cumulus clouds
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They form during the growth stage of convective clouds by the ingestion and tightening
of boundary layer vorticity by the cumuliform tower's updraft
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Most often occur in drier areas with high-based storms and considerable low-level
instability
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Generally are smaller and weaker than supercellular tornadoes, though many persist in
excess of 15 minutes and some have produced F3 damage.
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Specific tornado warnings are issued when landspouts are predicted or reported.
Waterspout:
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A waterspout is an intense columnar vortex (usually appearing as a funnel-shaped cloud)
that occurs over a body of water and is connected to a cumuliform cloud.
I is a nonsupercell tornado over water and is weaker than most of its land counterparts
Cool, unstable air masses passing over warmer waters allow vigorous updrafts to form,
which can tighten into a spinning column when captured by a passing thundershower
Waterspouts are just as dangerous on water and shoreline areas, often collapse after
moving a few hundred metres inland
If conditions are cool and cloudy, with showers but no organized storms, then the
appearance of a tornado-like funnel over water can be identified as a waterspout. If a
severe storm with a tornado happens to pass over a stretch of water, the tornado is
sometimes called a tornadic waterspout and would be just as dangerous as a tornado is
over land
Dust devil:
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A dust devil or whirlwind is a rotating updraft, ranging from small (half a meter wide and
a few meters tall) to large (over 10 meters wide and over 1000 meters tall).
On sunny, dry days, heated air near the ground can rise in small, spinning columns. If
these columns occur over dusty ground, a dust devil may be observable. The stronger
ones become visible when loose grass, hay or dust gathers into the whirl and rises up the
column.
look like a weak tornado at the bottom but rarely extend higher than 100 metres.
They are only seen in fair weather - sometimes without a cloud in the sky. Larger dust
devils can extend hundreds of metres high, toss lawn furniture and lift objects weighing a
hundred kilograms, but are generally not a threat otherwise. Dust devils near a highway
deserve caution as vehicles passing through them can be difficult to control.
Like other weak circulations, meteorologists can tell which days and general areas are
most likely to have dust devils, but they cannot be forecast and are not observable on
Doppler radar. Severe Weather Warnings are not usually issued for dust devils
C) Describe the characteristics of hail producing thunderstorms
Favorable ingredients for large hail:
strong updraft (to support hailstones aloft long enough to become very large)
large and ‘fat’ CAPE to support vertical acceleration
Vertical perturbation pressure gradients aloft along storm flanks strengthen updraft speeds.
Deep vertical wind shear to increase storm organization and prolong updraft, - to lengthen
transit time - supercell and strong multicell structures allow prolonged hail growth. Also
increases updraft acceleration from perturbation pressure gradients due to dynamic effects.
5) Low wet-bulb freezing level - reduces thickness of melting layer during descent
6) Low mean temperature of downdraft air - lower the melting rate
7) Large hail size aloft - smaller stones fall more slowly resulting in prolonged melting
8) Cool environments with steep temperature lapse rates in the mid-levels and moderate to high
relative humidity favor the development of hail. Cold lows typically assemble these
ingredients.
9) Since supercell storms typically have very strong and persistent updrafts, they often yield
large hail. Hailstones golf-ball size and larger almost exclusively result from supercells.
Characteristics of hail storm on Radar display:
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Reflectivity over 45 dBZ at mid-levels.
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At least a 6 km overhang over the low-level reflectivity gradient
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The presence of a WER or BWER with the storm top over the strong low-level reflectivity
gradient.
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The formation of the hook echo at low levels.
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3)
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The TVS appearing as an anomaly in a Doppler display.
D) Describe the characteristics of straight line wind events such as dry and wet
microburst
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Downburst: strong downdraft which induces an outburst of damaging winds on/near the ground
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Downbursts are divided into two spatial and temporal scales.
Macroburst
1)
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large downburst with outburst winds extending > 4km in horizontal dimension
An intense macroburst often causes widespread, tornado-like damage
Damaging winds, lasting 5 to 30 minutes, could reach 60 m/s (134 mph).
Microburst
2)
small downburst with outburst, damaging winds not extending 4 km
peak winds lasting 2 to 5 minutes.
 In spite of its small horizontal scale, an intense microburst could induce damaging winds as
high as 75 m/s (168 mph).
Forcing mechanisms:
1) Evaporation cooling  negative buoyancy, enhanced by
 Small drop size (evaporate quickly)
 Large liquid water content (keep evaporation process going)
 A dry adiabatic layer (offset adiabatic warming of descending air)
2) Precipitation loading (large liquid water content enhances both evaporation cooling and
water loading)
3) Change in vertical pressure gradient (contributing to vertical acceleration, esp. in severe TS)
4) Downward momentum transfer of stronger winds aloft
Vortex ring
Downward
circulation,
reinforce
“downdraft”
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Return
updraft,
Causing a
ring of low
pressure ring
“mesohigh”
“gustfront”
Dry microburst
Main features:
1) Associated with little or no precipitation and high-based clouds
2) Shallow moisture at mid-levels + instability
3) Deep, well-mixed, dry BL (inverted V)
4) Driven by evaporative cooling and negative buoyancy
5) Peak winds last 2-5 minutes and only spread 4km or less
6) Extremely hazardous to aircraft: difficult to detect on radar and have a benign appearance
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Difficult to pinpoint but can come up with a general threat area
Tend to occur in families (i.e., see one and expect more).
Daytime phenomena, most often occur in late afternoon
unstable with shallow
moisture
Hot & Dry
Forecasting dry microburst:
1) Assess the thermodynamic potential for dry microbursts.
 moist mid levels ~ 500 mb; T-Td < 8 oC, RH > 55%
 dry low levels ~ 700 mb; T-Td > 8 oC, RH < 35-45%
 convectively unstable
700-500 mb lapse rate >= 8 oC/km
T850 - T500 >= 33 oC
T700 - T500 >= 24 oC
LI < 0 (but may be slightly positive)
(LI = T500env – T500parcel)
Dry Microburst Potential Index (dmpi): > 8
WINDEX (wi): gust speed in knots
2) Determine the most likely location for microburst occurrence considering factors such as
dynamics and local terrain effects
3) Modify the sounding to the afternoon conditions of temperature and moisture and estimate the
wind speed (from CAPE, DCAPE)
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Wet Microburst
Main features:
1) accompanied by significant precipitation between the onset and end of the high winds
2) warm moist low-levels and dry unstable mid-level (RH<35%)
3) driven by water-loading, evaporative cooling and negative buoyancy
4) significant CAPE (updraft) and DCAPE (downdraft)
5) Strong winds aloft and fast storm motion
Forecasting wet microburst:
1) Assess the thermodynamic potential for dry microbursts.
 dry mid levels ~ 500 mb; RH < 35%
 moist low levels: RH > 55%
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convectively unstable
700-500 mb lapse rate >= 8 oC/km
T850 - T500 >= 33oC
T700 - T500 >= 21oC (high risk of server wx); 18oC (moderate; 16oC (low)
LI < -1 (LI = T500env – T500parcel)
UVV > 0
e (sfc to mid-level) > 20
2)
Assess the dynamic potential (severe weather composite) and consider the most likely location
for wet microburst occurrence.
3)
Modify the sounding to the afternoon conditions of temperature and moisture and estimate the
wind speed (from CAPE and DCAPE, also winds aloft)
E) Describe the characteristics and climatology of lightning
Characteristics
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Lightning is an electrical discharge caused by the separation of positive and negative charge in
cloud. Most clouds are to some degree electrified and thus may break down the air molecules
into ions, and separate them internally into positively and negatively charged regions within
the cloud.
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Cumulonimbus cloud has a great ability to separate and build up charged regions internally
and is the only cloud type which can build up enough charge to generate an electrical
discharge, which may be seen as lighting and heard later as thunder
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Lightning poses an extreme hazard to life and property (human and animal death, forest fires,
aircraft damage etc)
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The two most prevalent lightning types are intracloud or cloud-to-cloud (CC) lightning and
cloud-to-ground (CG) lightning
Climatology (in Canada)
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Lightning was detected on approximately 10 to 30 days per year over most of interior southern
Canada. The greatest number of lightning events in Canada, about 45 days per year, was seen
in the western Alberta foothills. The marine area southeast of Nova Scotia reported more than
40 days per year while Southwestern Ontario saw up to about 35 days per year. The least
activity (0 to 10 days/year) occurred over Western British Columbia, along the continental
divide, the region east of New Brunswick and the far north. The northern lightning boundary
lies near the 15 degree mean daily temperature isotherm for July.
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The vast majority of lightning over land occurred in the warm months of May to October. In
the west, 98.9 % of all lightning observations were made in these months while the number in
the east was 93.0%. July was the predominant month in the west while June and July were
predominant in Southern Ontario. Lightning was observed year round over the ocean south of
Nova Scotia.
T.O.6.4.5
a) Describe how an operational convective assessment is performed, what factors
are important to look for, and why they are important: instability, moisture, vertical
motion, and wind shear
Moisture
 Fuels thunderstorms through latent heat release (needs to be present at base of unstable layer
to achieve free convection)
 The buoyancy of a parcel is directly affected by moisture
 Form precipitation (possible heavy, even flooding); evaporation cooling and precipitation
loading enhance downdraft and downbursts
 Operational assessment:
1) 700mb chart: (T-Td), moist area (mid cloud) and dry area
 High mid level cloud cover hampers insolation for SFC based convection
 Dry mid level enhances downdrafts/microbursts
2) 850mb chart:
 Moisture axis: if over SFC moist axis  deep layer moisture; important for elevated
or nocturnal convection
 Moist Tongue: see area of strong moisture advection (Td > 10C, 12C …)
 Adding moisture increases CAPE (updraft)
3) sfc map:
 Moisture axis: SFC Td not always representative of moisture in BL
 Moist Tongue: 12˚C or so … case dependent, look for collocation with convergence
(mfc)
 highlights moisture gradients
4) Tephi: (mix-ratio, depth of moisture); can parcel reach LFC?
5) Sfc obs (metar): moisture in BL
6) Satellite (morning ST/FG): evidence of significant moisture in PBL
Instability
 Convective storm need instability to induce updraft and release latent energy
 It characterizes updraft strength and modulates storm intensity.
Operational assessment:
1) Tephi/Prog Tephi:
 lift parcel (modify tephi)  CAPE or static instability
 Tw lapse rate >= pseudoadiabat rate  PI
 Tw > LVWA  LI
2) 500mb chart:
 Thermal Trough: leading edge cold advection; cold advection destabilize the column
3) 700mb chart:
 Thermal Ridge: cold advection behind erodes capping lid; ahead destabilizes wrt
elevated convection (e.g., Acc along TROWAL)
 Thermal Trough: trailing edge cold advection  beginning stabilization (SFC based)
 Shade areas of warm/cold advection
 Differential temp. adv. 700-500mb (T75 = T700 – T500)
 Dry intrusion: Steeper lapse rates; enhance microbursts and RFD in supercells through
entrainment in downdraft
4) 850mb chart:
 LL moist advection increases CAPE and updraft strength (latent heat release)
 LL warm advection destabilizes profile (veering) but can strengthen cap
 Thermal Ridge: capping lid and temperature advection; elevated convection ahead of
ridge
5)
Surface chart:
Thermal Ridge: less important than moisture but highlights warm tongues
Satellite/radar/lightning: evidence of Acc / TCu
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6)
Wind shear
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Low-level shear important for cold pool interactions and new cell generation (multicells)
Deep shear important for tilting updraft prolonging storm longevity
For supercells deep shear necessary for generation of mid-level rotation and vertical
perturbation pressure gradients
Operational assessment:
1) Plot a hodograph using winds at sfc, 850, 700, 500 and 250mb
2) Estimate mean wind and shear at sfc-500mb (0-6km) and sfc-700mb (0-3km) and SRH
3) Jet in 500mb (80kt):
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indicate deep shear (0-6km), For severe TS (esp. supercells) look for > 30kt
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updraft tilt
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storm motion
4) Jet in 700 mb (30kt):
 indicate low level shear (0-3km)
 updraft tit
 storm motion
 increase Storm Relative Helicity (SRH), if the winds veer through the inflow depth
5) Jet in 850mb:
 Increase low-level shear (e.g., cold pool interactions, tornadoes)
 Increase SRH if winds veer in BL
 cyclonic side favors MCCs
Vertical motion (trigger):
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For convection to occur, air must be lifted to LFC
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Vertical motion aids in destabilize, erode the capping lid through ascent, enhance updraft
strength
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Mechanisms: isolation, topographic lift, low level convergence, vorticity advection aloft,
along frontal boundary and drylines, gravity waves etc.
Operational assessment:
1) 250 mb:
 ageostrophic circulations (right entrance - left exit)
 cyclonic side of jet
 quantitatively use divergence at 250mb or PVA at 250mb on GRIB viewers
2) 500 mb:
 PVA  associated with divergence aloft
 can trigger convection or intensify existing convection
3) 700 mb:
 vertical velocity chart; descent can inhibit convection
4) 850mb:
 deep layer of convergence favorable for both SFC and elevated (nocturnal) convection
 trough on LL Max. Wind chart from CMC
5) Surface chart (sfc convergence):
 sfc trofs, wind shift line, front, dry lines
 isallobaric field: increase wind convergence
6) Satellite/Radar: look Jet stream and vertical motion (intensity and propagation)
7) Topographic lift
B) Describe how to locate a convective threat area
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The MIST principle
A thorough assessment of M.I.S.T conditions as described in TO (A)
Assess where requirements for thunderstorms overlap…threat area
shear can be difference between weak single-cells and severe storms
consider shallow wind shear and deep wind shear
Use the Miller Technique:
Composite chart depicting significant processes or features relevant to convective initiation
or persistence
Using specific symbology representing features at isobaric levels (including SFC &
convective parameters)
Intersection of lines does not necessarily equal convection
Define threat area using features/processes
Incorporating Miller features and a few convective parameters from model progs.
** Stability/moisture parameters:
• CAPE (Jkg-1) (Convective Available Potential Energy), quantifies buoyant energy available
to thunderstorm updrafts based on difference between lifted parcel and ambient temperatures.
• Lifted Index (LI < 3°C), based on difference between lifted parcel (from the surface) and
environmental temperatures at 500mb. More negative, more unstable.
• Showalter Index (SI < 4°C), similar to LI but parcel is lifted from 850mb
• Total totals Index (TT > 40): examine the depth of boundary layer moisture.
TT  T850 Td 850  2T500
• K Index: Similar to TT but includes mid-level dewpoint depression
K  T850 T500  Td 850  T700 Td 700 
• Lapse Rate: Quantifies rate of temperature change with height
• CIN: Characterizes negative buoyancy below the LFC.
p
CIN  R  T p Ta  d ln p
• PW Integrated waterpin a 1m2 column over the surface
• MFC: moisture flux convergence, combines mass convergence and moisture advection to
identify favourable axes for convective initiation
• LFC Height: Supercell (and other) tornadoes more likely if LFC < ~6500ft
• Thunderstorms in general more likely if LFC < 10000ft
• LFL-LCL Height, Similar to CIN as large difference will inhibit convection
• LFC-LCL RH: Near saturation suggest little entrainment as parcel ascends from LCL to LFC
increasing likelihood of convection
LFC
SFC
** Wind and Wind Shear Parameters
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0-6km Shear Magnitude: The length of the hodograph from the SFC to 6km, Estimates if
sufficient shear to tilt updrafts and generate mid-level rotation, 30kts typically used as lower
limit for supercells
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0-6km Mean Shear: Length of the hodograph divided by the depth of integration
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SRH: Characterizes streamwise vorticity in storm relative inflow that can increase tendency
for rotating updrafts
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0-1km Shear Vector: >15kt favours tornadoes
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0-2km SR (storm relative) Winds: >15kts favourable for persistent supercells
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Anvil Level SR Winds: May discriminate between supercell types
** Combined Stability / Shear Parameters
 BRN (bulk Richardson Number): Ratio of CAPE and shear, BRN=2CAPE/ (u2+v2)
 SWEAT: capture most of MIST. SWEAT  12Td 850  20(TT  49)  2f 850  f 500  125(S 500850  0.2)
 EHI: energy helicity index. uses 0-3km SRH but 0-1km EHI has shown better discrimination
(CAPE )(SRH )
between tornadic and non-tornadic supercells
EHI 
5
1.6  10
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SCP: supercell composite parameter, combine instability, shear and SRH
STP: significant tornado parameter.
Craven SigSvr (significant severe weather parameter)
0-3km VGP (vorticity generation parameter)
T.O.6.4.6
A) Describe elevated convection patterns i.e. associated with upper troughs; dry
lines, etc.
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Features:
Usually initiated above boundary layer, so SFC heating not a factor
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Embedded elevated convection is an important part of wintertime snowfalls.
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supported by steep mid-level lapse rates, moisture at the base of an unstable layer and
elevated trigger mechanisms (often above a thick layer of ST)
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Generally associated with large scale dynamic lift of synoptic scale patterns and
circulations, which destabilizes the atmosphere (the moist air in the lower portion
saturates, latent heat release  slower T decreasing related to upper portion  increased
instability)
Trigger mechanisms:
1) ascent associated with warm advection (eastward and poleward of lows)
2) positive vorticity advection (downstream of shortwave troughs)
3) cooling aloft associated with upper trough
4) zones of rising air in frontal circulations
5) upward motions along drylines
6) upward motion in the left-front and right rear quadrants of jet streaks
7) upward motions induced by gravity waves
8) low level jet
9) enhanced lift due to intersecting boundaries
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Dry Lines/Dry Troughs
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The dryline is one of triggers for elevated convection
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It provides a convergence boundary (i.e., a lifting mechanism) in the vicinity of which
convective cloud such as ACC and snow flurries may develop in winter.
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This is also important is summer for thunderstorm development. Typically moist air floods
into the region ahead of the dry line with the dry line/dry trough providing the trigger for the
convection
Upper Troughs/Weak Thermal Ridge Couplet
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A fairly weak thermal ridge in 500, 700, and 850 mb followed by a fairly weak upper trough.
There is weak PVA and cooling aloft associated with the trough, which promote vertical
motion, embedded ACC forms near the thermal ridge. Frequently forecasters in Canada
analyze a trowal to describe this process.
B) Describe symmetric instability and Conditional Symmetric Instability (CSI)
Symmetric instability is a combination of gravitational or static stability and inertial stability
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Static stability measures the resistance of the atmosphere to vertical motion.
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Inertial stability measures the resistance of the atmosphere to horizontal displacements of
air parcels; exists when the absolute vorticity of the mean flow is less than zero.
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A parcel can be stable to purely horizontal or vertical displacements (inertially and statically
stable), but may become buoyant when a combination of horizontal and vertical perturbations
produces a slantwise displacement of the parcel. This is known as symmetric instability. In
order for SI to be realized the atmosphere must be at or near saturation. Saturation is the
element that is conditional when referring to CSI. The release of Conditional Symmetric
Instability produces slantwise convection.
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Characteristics:
1) CSI is a synoptic feature but resulting in mesoscale (narrow and banded) convective wx;
producing areas of locally heavy precipitation, several hundreds km long, 5-40 km wide
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2)
3)
4)
5)
CSI is usually a winter time or cool season phenomenon. During the summertime, daytime
heating produces such strong vertical mixing that gravitational or static stability processes
dominate.
Usually found embedded in larger areas of upward vertical velocity
Slantwise convection can take several hours to develop based at mid levels in a layer 100300 mb thick, typical CAPE values 50-300 j/kg
Strong thermal gradient baroclinic atmosphere with cold air to the west
Favorable condition for CSI:
1) A moist or nearly saturated atmosphere in the layer (Moist)
2) Near neutral or slightly stable layer (instability)
3) speed shear (wind speed increasing with height), direction may veer slightly (shear)
4) strong thermal gradient (moderate to strong baroclinic zone)
5) area of small absolute vorticity, which suggest weak inertial instability
6) weak large scale vertical motion
7) EPV < 0 at least 100 mb thick
8) Multiple or isolated convective bands oriented parallel to the thermal wind
9) Absence of gravitational and/or upright instability (if these are present they will dominate)
Diagnosis:
1) WADS: examine cross-sections of geostrophic angular momentum (Mg) and equivalent
potential temperature (e). Cross sections are constructed perpendicular to the thermal wind
field; CSI exists in areas where the slope of Mg surfaces is less than that of e surfaces.
2) CMC EPV product: Cross-sections of EPV can be constructed in the area of interest, CSI
exists in environments where EPV is negative (i.e., EPV < 0)
c) Describe nocturnal thunderstorm processes
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Nocturnal thunderstorm process is elevated convection occurring at night, usually initiated
above PBL, SFC heating is not a factor.
Often supported by steep mid-level lapse rates, moisture at the base of an unstable layer and
elevated trigger mechanisms such as the nocturnal low-level jet
As radiation inversion develops at night, SFC winds become decoupled from those aloft,
winds aloft become supergeostrophic, forming LLJ. Moisture can be advected by LLJ above
the PBL, either feed into developing MCSs or provide trigger for new nocturnal TS
development (enhance wind convergence)
The latent heat release by MCS creates an anticyclonic flow at upper levels and a cyclonic
circulation in the low-middle levels, usually above a shallow layer of cool air at the surface.
The enhanced low-level circulation can augment the low-level jet with a concurrent increase
in the amount of moisture feeding the system. The anticyclone in upper levels often enhances
the upper jet to the north and west of the MCS.
Nocturnal radiation is a common cause of modification. Radiation cooling above the cloud
layer may create an unstable condition of cold air above the warm air that lies below the
clouds. This further favors nocturnal thunderstorm development.
A method for anticipating nocturnal MSCs:
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Two key elements to examine (using late afternoon data)
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the position of the low-level jet
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the strength of the low-level frontogenesis
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MCS or MCC will normally form close to intersection area between the low-level jet and the
front. MCS’s formed when the low-level front was intensifying, Frontogenesis implies a
thermally direct circulation (ascent in the warm air and subsidence in the cold air), the
ascending branch triggering the convection