Convective Severe Flashcards
Thunderstorm Regions
Continental Divide to the Appalachian mountains
“Tornado Alley” is has the highest tornado activity
March - June: Most tornados
March - Sep: More severe storms
This time frame is aligned with movement of the PFJ
Heavy Rain regions
Summer - SE
Winter - W Coast
Flash Flood
Most common Apr - Sep (July)
During afternoon/evening with max heating
Arid environments can flood more easily due to lack of moisture in the soil
Buoyancy (four stages)
Stage one: parcel achieves equilibrium
Stage two: Moves past equilibrium and rises (updraft/anvil)
Stage three: Precipitation drag & dry air entrainment (3-5km) creates downdraft
Stage four: Cold pool at SFC (evap cooling/dry air entrainment), neg buoyancy
CAPE
Temperature difference between ascent path and environment (Positive buoyancy)
Often in the range of 1000-2000L/kg
Cold Pool Strength
Deeper the pool + stronger the temperature difference = Increased winds
Downbursts
Very strong concentrated downdraft at least 40kts
Occur at the edge of a gust front
Microburst (general)
Dangerous small scale down bursts (.2nm-2.4nm)
Winds equal to F3 tornado
Can change in velocity of 50kts+
Wet Microbursts
“High reflectivity” microbursts
Convection develops in a surface based moist layer with dry air aloft
Mixing from these two layers along with precipitation drag create burst
Dry Microburst
“Low reflectivity” microbursts
Forms above a deep surface based dry layer
Precipitation falls and evaporates before reaching the ground
Downward acceleration will continue as long as the descending air is cooler than the surrounding environment
Hybrid Microburst
Changes from wet to dry or dry to wet
Hail and grapple are possible with strong winds
Type 1 Air Mass
Great Plains - Loaded Gun
Unstable sounding with an inversion that acts as a lid
Low level heating, mechanical lift and evap cooling can break the inversion
Severe storms
Type 2 Air Mass
Gulf coast (SE)
>80F and winds that decrease with height
Good thunderstorm producer but marginal severe weather producer
Type 3 Air Mass
Pacific Coast (Cold core) Cool mP in low levels SFC heating or warm waters that cause warming (COW) Hail producer due to low freezing level Lower levels of 50-70F and RH >70%
Type 4 Air Mass
Inverted V (desert regions)
Downbursts and virga
High wind producer
Type A synoptic
Dry Line (convergence along dry line)
>10F difference at line, 55F isodrosotherm
Moves faster than expected
Hail, winds and tornado threat
Most severe at bulges and during max heating
Type B synoptic
Frontal
Along or ahead of a cold front
Major tornado outbreaks
Max heating
Type C synoptic
Overrunning
Density based
Low level jet intersects warm/stationary front
Max intensity during max heating
Hail very common, tornados not common unless dew point reaches 14C/50F
Type D synoptic
Cold core
Cold core aloft/ Cool mP in low levels
Well defined cold core low or cut off low
Hail common but Tornados are seldom
Threat area is in the zone of intense low level wind ahead of dry intrusion
Max threat at max heating
Type E synoptic
Squall Line
Mature occluded system and low level jet
Threat area N of warm front and most intense at max heating
Usually in connection with overrunning and frontal
Mesoscale Convective System (MCS)
> 100km
May evolve from one cell or a small group of cells
Significant precipitation in eastern US
Squall Line
Usually created from an outflow boundary
> wind shear, > magnitude of shear, > CAPE = > severity
Weaker- narrow line // Stronger- comma appearance (2-4hrs)
Capped on each side with line ends
More severe weather on S portion // stratus and precipitation on N side
H’er pressure within the N cyclonic flow
If the squall line is stronger the comma may be distorted and is referred as MCV
Propagation driven by the speed of the cold pool
Usually follows the fronts/winds, if smaller than may move at an angle (bend)
Squall lines in the tropics
Taller, slower, weaker, symmetric and moves from E to W
Bow Echos
10-65nm long
Associated with severe winds
Form into a bow shape and transition to a comma shape
The core of the bow is the “rear inflow jet”
The smaller the echo the stronger the winds
Severe: -8 LI, CAPE >2500, 700mb winds 33kts (shear in lowest 2-3km)
Stronger the RIJ, > winds, > temperature difference
Derecho
> 50kts and 250nm
3 reports of F1 damage OR 65kts/40nm with no more than 3 hrs in b/w
Progressive: single
Serial: multiple
Mesoscale Convective Complexes (MCC)
~320nm, 6-12hrs
(-32C/100,000) (-52C/50,000)
Mostly nocturnal
700-500mb determines speed of MCC
- Genesis
- Development
- Mature
- Dissipating
Cumulus stage (TS)
Base at CCL
Updraft only
Lose top
Radar echo forms
Mature stage (TS)
Up and downdrafts
Max vertical extent
Anvil top
Heavy rain (virga)
Cold pool 3000-3500ft deep
Downdraft (precipitation drag, dry air entrainment/evap cool)
Hail at freezing level and wet bulb freezing level
Dissipating stage (TS)
Only downdraft (inflow of warm, moist air is cut off)
Drizzle and light rain - recipitation ceases
Winds weaken
Lightning is still a hazard
“Orphan anvil” cloud
Convective Ingredients
Instability: heating, moisture or vertical motion
Moisture: depth or near sfc moisture layer
Lift: synoptic- cannot act alone / mesoscale- can initiate alone
Exhaust: severe TS requires an exhaust (ULD)
Holographs (segment, bulk and total)
Shear segment: magnitude between two levels
Bulk shear: magnitude between two levels w/out the in between
Total shear: net length where all segments are added together
Wind shear speeds
Ordinary: ~20kts
Multi-cells: ~35kts
Supercells: ~50kts
Single cell TS
Zig zag (disorganized)
30-60min
One updraft
Tornados rare but high winds and hail are possible
Highly dependent on instability
“Pulse” storm is common (short lived nature)
Multi-cell TS
Modestly organized made up of a succession of single cell storms
Mod low level shear (30kts in lowest 2-3km)
Each cell has cold outflow boundary where new cells develop every 5-15 min
Straight line hodograph
More severe weather threat, hail >3/4th, tornado risk and flash flooding
Supercell TS
Long-lived or persistent core with a rotating updraft
Hooked hodograph
Large hail, strong winds and tornadoes
Mesocyclone
Rotating vortex with an updraft in a super cell storm
Develop with a tilted environment and/or horizontal wind shear
1-5nm diameter
50-150kts
Weak Echo Region (WER)
At the mid levels an echo overhang to the “south” of the precipitation region
Good indication of a potentially severe storm
Bounded Weak Echo Reigon (BWER)
Even stronger updraft where the overhang hooks further over and creates a “cavity” in the mid levels
Can produce large hail >2”
Hook echo (pendant echo)
Mesocyclone eventually becomes so strong it wraps around the updraft within the storm (precipitation wraps also)
V-notch
Mid and upper levels are blocked by the intense updraft
Forward flank downdraft (FFD)
Cool and moist air that goes to the SFC and spreads (towards PBL winds)
Rear flank downdraft (RFD)
Warm and dry (adiabatic)
Behind the storm
Flanking line
Leading edge of RFD
Wall cloud
Lowering of cloud base below supercell storm
Appears as a lowering of the updraft
Tail cloud
Right of wall cloud (FFD)
Heavy rain and hail
Main updraft or hook echo
Tornados
Where RFD joint the FFD w/in the hook echo
Classic supercell
Wedge shaped, hook echo, WER/BWER and inflow notch
High precipitation supercell
Kidney bean shaped
Larger system due to moisture
Extensive hail, tornadoes, downburst and flash flooding
Low precipitation supercell
Smaller due to lack of moisture
Rain is still possible
Less chance of a tornado
Shallow supercell
Much smaller both horizontally and vertically than other supercell types
Occur with a hurricane falling inland or wintertime high shear low buoyancy situations
~2000ft tall and 3nm in diameter
Forecasting challenge due to size
Tornado potential
Supercell environment
CAPE values of 1000-2000J/kg
~50kts in lowest 20,000AGL
Either severe wind shear or high CAPE values alone can great a severe storm
Supercell formation
When the vertical wind profile is sheared, horizontal vorticity is present in the environment
The shear and buoyancy gradients across the cloud cause tilt
Stronger the shear, stronger the updraft and less tilt
Precipitation falls down the shear of the updraft
Updraft and low level horizontal vorticity create a mid level vortex couplet
Supercell dissipation
Cold pool cuts off unstable air
Unfavorable environment (moisture decrease, stability, mnts etc)
OR collides w/ other convective storms
