Prof. Paul SirvatkaESAS 1115 Severe and Unusual

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Spotter Training and Spotter Training and Radar MeteorologyRadar MeteorologyPart 4 – Multicell Part 4 – Multicell ThunderstormsThunderstorms

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Multicell ThunderstormsMulticell ThunderstormsMulticell ThunderstormsMulticell Thunderstorms

Several distinct updraft/downdraft couplets existSeveral distinct updraft/downdraft couplets exist Can form as a cluster or a squall lineCan form as a cluster or a squall line A vertically sheared environment results in A vertically sheared environment results in

tilting of the updrafts and a longer lasting tilting of the updrafts and a longer lasting systemsystem

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Multicell StormsMulticell StormsHow Many Cells are There?How Many Cells are There?

Multicell StormsMulticell StormsHow Many Cells are There?How Many Cells are There?

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Multicell ClusterMulticell ClusterMulticell ClusterMulticell Cluster Series of evolving cells Series of evolving cells

where new updraft where new updraft growth is on a preferred growth is on a preferred flankflank

Child cells kill their Child cells kill their parents with cold parents with cold outflow airoutflow air

New updraft growth is New updraft growth is generated along outflow generated along outflow from the parent cells from the parent cells preferentially based preferentially based upon low-level upon low-level convergence and in convergence and in conjunction with low-conjunction with low-level wind shearlevel wind shear

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Multicell EvolutionMulticell EvolutionMulticell EvolutionMulticell Evolution

Southwest side of multicell cluster (looking to the northeast)Southwest side of multicell cluster (looking to the northeast)

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Overshooting TopsOvershooting TopsOvershooting TopsOvershooting Tops

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Multiple Updraft Growth in Multiple Updraft Growth in Close Proximity to Each Close Proximity to Each

OtherOther

Multiple Updraft Growth in Multiple Updraft Growth in Close Proximity to Each Close Proximity to Each

OtherOther

In moderate wind shear environments, multicell storms may take a few hours to evolve into a supercell storm

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Multicell (Squall) LineMulticell (Squall) LineMulticell (Squall) LineMulticell (Squall) Line A squall line is a line A squall line is a line

of thunderstorms with of thunderstorms with a continuous, well a continuous, well developed gust front developed gust front at its leading edge at its leading edge

Discrete cells are Discrete cells are difficult to distinguishdifficult to distinguish

New cell growth is New cell growth is generally along the generally along the whole line rather than whole line rather than on just one preferred on just one preferred flankflank

Large hail is possible Large hail is possible in the highest in the highest reflectivity coresreflectivity cores

Damaging winds are Damaging winds are possible at the possible at the leading edge but also leading edge but also further behind the further behind the front edgefront edge

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Line Echo Wave Pattern Line Echo Wave Pattern (LEWP)(LEWP)

Line Echo Wave Pattern Line Echo Wave Pattern (LEWP)(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

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

CrestCrest

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Squall Line Cross SectionSquall Line Cross SectionSquall Line Cross SectionSquall Line Cross Section Compare with supercell schematicCompare with supercell schematic Updrafts are on the leading edge of the cold Updrafts are on the leading edge of the cold

pool, hence tornado threat is small after passage pool, hence tornado threat is small after passage of gust front although hail will now occurof gust front although hail will now occur

Squall lines have well-developed cold poolsSquall lines have well-developed cold pools

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Cross Section of MulticellCross Section of MulticellCross Section of MulticellCross Section of Multicell

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Outflow Boundary (OFB)Outflow Boundary (OFB)Outflow Boundary (OFB)Outflow Boundary (OFB)

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OFB on Composite RadarOFB on Composite RadarOFB on Composite RadarOFB on Composite Radar

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Shelf Cloud Along the Shelf Cloud Along the OFBOFB

Shelf Cloud Along the Shelf Cloud Along the OFBOFB

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Shelf CloudShelf CloudShelf CloudShelf Cloud

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Underneath the ShelfUnderneath the ShelfUnderneath the ShelfUnderneath the Shelf

The texture The texture underneath the underneath the shelf cloud is shelf cloud is considered boilingconsidered boiling

Although turbulent Although turbulent and swirling, the and swirling, the underside of a shelf underside of a shelf cloud is a region of cloud is a region of warm and cold air warm and cold air mixing and thus not mixing and thus not tornadictornadic

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Boiling TextureBoiling TextureBoiling TextureBoiling Texture

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Typical LookTypical LookTypical LookTypical Look Look for ragged Look for ragged

appearance, even if it is appearance, even if it is not extremely turbulentnot extremely turbulent

Think about where the Think about where the rain is falling and rain is falling and whether or not cool air whether or not cool air is pushing away from is pushing away from the stormthe storm

Be aware that sheared Be aware that sheared motion along the motion along the outflow is often outflow is often mistaken for rotationmistaken for rotation

Sometimes, the only Sometimes, the only significant feature significant feature readily identifiable is readily identifiable is the shelf cloudthe shelf cloud

MovieMovie

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Intense SkiesIntense SkiesIntense SkiesIntense Skies

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On Top of the ShelfOn Top of the ShelfOn Top of the ShelfOn Top of the Shelf The top of a shelf is usually quite laminar (smooth)The top of a shelf is usually quite laminar (smooth) Laminar cloud texture is usually the result of Laminar cloud texture is usually the result of

forced ascent, not convective in natureforced ascent, not convective in nature Laminar banding is not uncommon in a stacked Laminar banding is not uncommon in a stacked

shelf cloudshelf cloud

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Laminar Bandings in a Stacked Shelf Laminar Bandings in a Stacked Shelf CloudCloud

Laminar Bandings in a Stacked Shelf Laminar Bandings in a Stacked Shelf CloudCloud

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Forced Ascent along Forced Ascent along Squall Line Cold PoolSquall Line Cold PoolForced Ascent along Forced Ascent along Squall Line Cold PoolSquall Line Cold Pool

Definitely dark and scary!Definitely dark and scary!

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Outflow Dominant Line of Outflow Dominant Line of StormsStorms

Outflow Dominant Line of Outflow Dominant Line of StormsStorms

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Outflow Features of a Outflow Features of a Squall LineSquall Line

Outflow Features of a Outflow Features of a Squall LineSquall Line

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Haboob (Dust Storm)Haboob (Dust Storm)Haboob (Dust Storm)Haboob (Dust Storm)

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GustnadoesGustnadoesGustnadoesGustnadoes

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GustnadoesGustnadoesGustnadoesGustnadoes Gustnadoes are not Gustnadoes are not

really tornadoes as really tornadoes as they are not they are not pendant to a Cbpendant to a Cb

Although these Although these vortices are vortices are relatively weak, relatively weak, they still pose some they still pose some threat to those in threat to those in their pathstheir paths

Note the boiling Note the boiling texture underneath texture underneath the shelf cloudthe shelf cloud

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Broken Squall Line Broken Squall Line ThreatsThreats

Broken Squall Line Broken Squall Line ThreatsThreats

Tornadoes are most Tornadoes are most likely within supercells likely within supercells and can form where and can form where storms are more storms are more isolatedisolated

These storms are most These storms are most likely at the southern likely at the southern most end (“Tail End most end (“Tail End Charlie”) or just north Charlie”) or just north of a breakof a break

Isolated storms ahead Isolated storms ahead of a squall line also of a squall line also pose significant risk for pose significant risk for severe events and/or severe events and/or tornadoestornadoes

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Bow EchoesBow EchoesBow EchoesBow Echoes

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Development of a Bow Development of a Bow EchoEcho

Development of a Bow Development of a Bow EchoEcho MovieMovie

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Bow Echo with Trailing Bow Echo with Trailing Stratiform RainStratiform Rain

Bow Echo with Trailing Bow Echo with Trailing Stratiform RainStratiform Rain

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Underneath a BowUnderneath a BowUnderneath a BowUnderneath a Bow

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MCS and MCCMCS and MCCMCS and MCCMCS and MCC A large organization of A large organization of

thunderstorms is called thunderstorms is called an MCS (Mesoscale an MCS (Mesoscale Convective System)Convective System)

A squall line is A squall line is considered a linear MCSconsidered a linear MCS

When the IR satellite When the IR satellite presents a more circular presents a more circular perspective of a large perspective of a large cluster of thunderstorms cluster of thunderstorms and it persists for more and it persists for more than 6 hours, we refer to than 6 hours, we refer to this MCS as an MCC this MCS as an MCC (Mesoscale Convective (Mesoscale Convective Complex)Complex)

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Linear vs. CircularLinear vs. CircularLinear vs. CircularLinear vs. CircularEccentricity: Ratio of the minor axis to the major axisEccentricity: Ratio of the minor axis to the major axis

Eccentricity close to 1Eccentricity close to 1Eccentricity much less than 1Eccentricity much less than 1

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Mesoscale Convective Mesoscale Convective Vortex (MCV) Vortex (MCV)

Mesoscale Convective Mesoscale Convective Vortex (MCV) Vortex (MCV)

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Multicell ThunderstormsMulticell ThunderstormsMulticell ThunderstormsMulticell Thunderstorms Gust front processes dominate multicell processesGust front processes dominate multicell processes The addition of vertical wind shear allows for new updrafts to The addition of vertical wind shear allows for new updrafts to

reach maturity in some organized fashionreach maturity in some organized fashion Even though each cell moves with the mean wind of the cloud-Even though each cell moves with the mean wind of the cloud-

bearing layer, new updrafts form where low-level convergence is bearing layer, new updrafts form where low-level convergence is strongest and is enhanced by the vertical wind shearstrongest and is enhanced by the vertical wind shear

When updrafts initiate at a preferred location, multicell clusters When updrafts initiate at a preferred location, multicell clusters formform

Updrafts that initiate along the leading edge of the cold pool will Updrafts that initiate along the leading edge of the cold pool will result in a squall lineresult in a squall line

The severe threat increases as instability and shear increase and The severe threat increases as instability and shear increase and where strong cold pools do not overwhelm new storm growthwhere strong cold pools do not overwhelm new storm growth

Severe weather events are more predictable ahead of strong Severe weather events are more predictable ahead of strong cells and with features that are easily identifiable on radar (e.g. cells and with features that are easily identifiable on radar (e.g. bow echoes)bow echoes)

Tornadoes are possible with storms that are supercellular or Tornadoes are possible with storms that are supercellular or isolated (such as a head of a line) and with storms north of a bow isolated (such as a head of a line) and with storms north of a bow echoecho