Download Severe and Unusual Weather ESAS 1115

Severe and Unusual Weather
ESAS 1115
Spotter Training and
Radar Meteorology
Part 4 – Multicell Thunderstorms
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Multicell Thunderstorms
 Several distinct updraft/downdraft couplets exist
 Can form as a cluster or a squall line
 A vertically sheared environment results in tilting of
the updrafts and a longer lasting system
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Multicell Storms
How Many Cells are There?
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Multicell Cluster
 Series of evolving cells
where new updraft growth
is on a preferred flank
 Child cells kill their parents
with cold outflow air
 New updraft growth is
generated along outflow
from the parent cells
preferentially based upon
low-level convergence and
in conjunction with low-level
wind shear
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Multicell Evolution
Southwest side of multicell cluster (looking to the northeast)
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Overshooting Tops
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Multiple Updraft Growth in Close
Proximity to Each Other
In moderate wind shear environments, multicell storms may
take a few hours to evolve into a supercell storm
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Multicell (Squall) Line
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A squall line is a line of
thunderstorms with a
continuous, well
developed gust front at its
leading edge
Discrete cells are difficult
to distinguish
New cell growth is
generally along the whole
line rather than on just one
preferred flank
Large hail is possible in
the highest reflectivity
cores
Damaging winds are
possible at the leading
edge but also further
behind the front edge
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Line Echo Wave Pattern (LEWP)
Refers to a series of bow echo segments; a wavy appearing
squall line; can result in an enhanced severe threat at the
leading edge of the bow or in the “crest” of the waves
Crest
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Squall Line Cross Section
 Compare with supercell schematic
 Updrafts are on the leading edge of the cold pool,
hence tornado threat is small after passage of gust
front although hail will now occur
 Squall lines have well-developed cold pools
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Cross Section of Multicell
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Outflow Boundary (OFB)
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
OFB on Composite Radar
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Shelf Cloud Along the OFB
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Shelf Cloud
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Underneath the Shelf
 The texture underneath
the shelf cloud is
considered boiling
 Although turbulent and
swirling, the underside
of a shelf cloud is a
region of warm and
cold air mixing and thus
not tornadic
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Boiling Texture
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Typical Look
Movie
 Look for ragged
appearance, even if it is
not extremely turbulent
 Think about where the rain
is falling and whether or
not cool air is pushing away
from the storm
 Be aware that sheared
motion along the outflow is
often mistaken for rotation
 Sometimes, the only
significant feature readily
identifiable is the shelf
cloud
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Intense Skies
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
On Top of the Shelf
 The top of a shelf is usually quite laminar (smooth)
 Laminar cloud texture is usually the result of forced ascent,
not convective in nature
 Laminar banding is not uncommon in a stacked shelf cloud
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Laminar Bandings in a Stacked Shelf Cloud
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Forced Ascent along
Squall Line Cold Pool
Definitely dark and scary!
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Outflow Dominant Line of Storms
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Outflow Features of a Squall Line
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Haboob (Dust Storm)
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Gustnadoes
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Gustnadoes
 Gustnadoes are not
really tornadoes as
they are not pendant
to a Cb
 Although these vortices
are relatively weak,
they still pose some
threat to those in their
paths
 Note the boiling
texture underneath the
shelf cloud
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Broken Squall Line Threats
 Tornadoes are most likely
within supercells and can
form where storms are
more isolated
 These storms are most
likely at the southern
most end (“Tail End
Charlie”) or just north of
a break
 Isolated storms ahead of
a squall line also pose
significant risk for severe
events and/or tornadoes
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Bow Echoes
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Development of a Bow Echo
Movie
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Bow Echo with Trailing
Stratiform Rain
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Underneath a Bow
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
MCS and MCC
 A large organization of
thunderstorms is called an
MCS (Mesoscale Convective
System)
 A squall line is considered a
linear MCS
 When the IR satellite
presents a more circular
perspective of a large
cluster of thunderstorms and
it persists for more than 6
hours, we refer to this MCS
as an MCC (Mesoscale
Convective Complex)
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Linear vs. Circular
Eccentricity: Ratio of the minor axis to the major axis
Eccentricity much less than 1
Eccentricity close to 1
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Mesoscale Convective Vortex
(MCV)
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Multicell Thunderstorms
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Gust front processes dominate multicell processes
The addition of vertical wind shear allows for new updrafts to reach
maturity in some organized fashion
Even though each cell moves with the mean wind of the cloud-bearing
layer, new updrafts form where low-level convergence is strongest and is
enhanced by the vertical wind shear
When updrafts initiate at a preferred location, multicell clusters form
Updrafts that initiate along the leading edge of the cold pool will result in
a squall line
The severe threat increases as instability and shear increase and where
strong cold pools do not overwhelm new storm growth
Severe weather events are more predictable ahead of strong cells and
with features that are easily identifiable on radar (e.g. bow echoes)
Tornadoes are possible with storms that are supercellular or isolated (such
as a head of a line) and with storms north of a bow echo
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather