Notes on Advanced Severe Weather Spotter Training webinar #skywarn.

1. On the Agenda

  1. Reporting refresher
  2. Severe weather meteorology
  3. Storm structure and evolution (deep dive)

At the NWS office:

  • They know theres a storm

  • They think theres severe weather

  • They need groundobservations to verify

  • They need a prepoderance of evidence

    • Mainly radar data and spotter reports
    • Conceptual models
    • Storm history
    • Forecaster intuition
    • Forecaster experience
    • Potential storm impacts
    • Potential warning impacts

Deciding on a type of warning:

  • spotters are very important for this component

NWS Warnings

  • Paradigm:
    • Severe Thunderstorm worning
      • elevated wording
      • tornado possible tornado
      • radar
      • observed
      • considerable
      • catastrophic
      • Spotter reports drive the message

2. Reporting refresher

What makes a good report?

  • Timely
  • Concise
  • Calm
  • Clear
  • Accurate
  • Objective

Components

  • who are you
    • name
    • im a trained spotter in NWS norman
    • if with spotter group
  • where are you
  • what do you see
  • where did you see it
  • when did it happen

Where to report

  • 405-325-3816
  • spotternetwork.org
  • amateur radio: WX5OUN
  • twitter: @NWSNorman/@NWSTulsa
  • mPING: mping.nssl.noaa.gov
  • after the fact: sr-oun.spotter@noaa.gov

If reporting a tornado

  • distinguish between tornado, funnel cloud, wall cloud
  • are you sure its a tornado?
  • if you can see ground or cloud base under it, consider funnel cloud or wall cloud
  • is it connected to the cloud base
  • is it in contact with ground
  • is it in the part of the storm where tornadoes develop
    • distinct from gustnados and landspouts
  • is it rotating
    • or just a big downdraft
  • can you see any debris

Describing a tornado

  • report exactly what you see
  • choose words wisely

Severe Thunderstorm Definition

  • 1"+ hail & 58mph wind and tornado
  • wind damage and hail damage very useful in reports

Wind Damage

  • underrated, but important to report
  • related to outbuildings, trees, powerlines, etc.

Reporting Damage

  • Location
  • Time it happened (if known)
  • Description/images/video
  • If not sure what caused it, dont assume it was a tornado

Power Flashes

  • Arc from shorted out powerline
  • Wind causing lines to touch
  • Object resting across live wires
  • Report power flashes
  • Dont assume that a power flash means a tornado
  • Can help confirm tornado if other clues are present

In General

3. Severe Weather Meteorology

Mostly mesoscale scale features rather than storm scale

Where to monitor data:

  • www.spc.noaa.gov
    • Forecast tools page
      • Upper air maps
      • Skew-T charts
      • Mesoanalysis maps with derived severe weather parameters

Ingredients for supercells:

  • moisture
  • instability
  • lift
  • windshear

Moisture

Oklahoma

  • main moisture source is Gulf of Mexico
  • Measurement: dewpoint around 50-55* for supercells, higher for tornadic supercells
  • When air feels real most, basically
  • Example. May 9th 2010, dewpoints where low to mid sixties
    • 90 * fahrenheit air/ 68*F dew

Tornado Likelihood:

  • low level moisture
  • causes higher instability in low levels
  • supports stronger and persistent updrafts

Southern/southeasterly winds across plains transport low level moisture

  • difference between dewpoint and low level related to likelihood

Monitoring Moisture

  • oklahoma mesonet website has great observations on moisture
  • SPC mesonalaysis graph page
  • Raw Skew-T plot moisture from surface to top of atmosphere

Instability

Stable vs unstable:

  • balloon example. Cool air at surface, balloon will want to rise less
  • cool lower, warm upper layer is stable
  • inverse is unstable

The cap

  • theres a sliver of warm air that is keeping air parcels from rising any further upward
  • inhibits severe thunderstorm development
  • AKA convective inhibition (CIN)

Skew-Ts

  • Red line is environment temperature
  • Temperature on x axis, pressure on Y
  • How to read:
    • temperature at 500 millibars, follow y line to the red line at 500 millibars
    • yellow is the temperature of bubble of air as it rises
      • it rises and cools
    • green is dewpoint
    • area between redline and yellow line
      • greater area is greater convective available energy (CAPE)
    • A cap on a skew t looks like an area on the bottom where parcel temperature is lower than environment temperature at the altitude

Monitoring instability

  • greater than 1000 joules per kg is considered unstable
  • SPC mesoscale analysis
    • thermodynamic data
      • most often use CAPE and Mixed layer CAPE
    • Lifted index used to determine instability of an airmass
      • comparing temperature at two different levels
    • Lapse rates
      • mostly used for hail potential estimation
      • rate that temperature is changing as you gain altitude
      • closer to 7.5 or greater indicate severe hail threat
    • LCL Height
      • Lower the cloud base the more potential for tornado development
  • instability in NWS discussions

Lift

Multiple kinds. Means “something to give a nudge upward”

May include:

  • Fronts (cold & warm)
  • Dry lines
    • can be important in our part of the country
    • seperation of humid and dry air
  • Outflow boundaries
    • thunderstorms can generate lift themselves
  • Sufficient solar heating discrepencies in ares

Frontal Lift:

  • lift along a warm front is more gradual than a cold front
    • lighter percipition, percipitation well ahead of a front
  • Cold front
    • air rises more abruptly, rain right ahead of the front

Convergance

  • Surface heating
    • some land areas heating up more than others
    • locally warm air will rise and surrounding air will move in from the sides to fill its place
    • no where to go but up
    • enough of it, clouds are generated. if sustained, can produce thunderstorms

Dry Lines

  • big discrepencies in moisture,
  • watch for intersecting air currents pushing those areas together
  • storms often form right in front of the dry line, on the moist side

Outflow boundaries

  • could outflow pushes away from a storm
  • the area between the boundary and the storm has potential for new storms.
  • that area is a zone of lift

Monitoring Boundaries

  • SPC mesoanalysis page

4. Wind Shear

  • increase of wind speed or direction with height
  • important for forming supercells, without it the storm kills itself by downdrafting on itself
  • Deep sehar needed for supercells

Multiple different types

  • simple change with windspeed with height
  • change in direction with height
  • change in wind speed AND direction with height
    • biggest potential for instability

Skew-T charts

  • Windws plotted at right

  • Wind speed

    • Add the lines
      • long line is ten, short line is 5
      • triangle is 50

  • Shear for supercells

    • Do winds change in direction and speed as you move from bottom of sounding to 500 mb level

  • Shear for tornados

  • Change in wind speed and direction causes vortex tubes

    • An updraft can tilt this vortex tube and caued vertical rotation

  • Review

    • Supercell has a persistent rotating updraft (aka mesocyclone)
    • Low level wind shear is the focus for tornado likelihood

Likelihood scenario

  • high low level windshear in the lowest levels (esp. lowest 1 km)
  • surface winds become more easterly (backing of winds)
  • Increases along boundaries such as outflow
  • Tornadogenesis more likely if a supercell travels parallel along the boundary
  • Sometimes storm-scale environment may compensate for a lack of larger scale environmental conditions

In this part of the country, the low level jet enhances low level shear/helicity

Monitoring Wind Shear:

  • spc page has wind shear tab:
    • Effective bulk shear - deep layer wind shear
      • greater than 30/35 knots is good enough for supercell dev
    • Low level windshear
      • surface to 1 km
        • higher values suggest better potential
        • greater than 20 knots is ideal for tornadogenesis

5. Storm Structure and Evolution

Build upon the mesoscale features

Anatomy of a supercell: side view

  • Storms start with an updraft
    • strong updrafts more likely to produce severe weather
    • can be above 100 mph
    • what goes up must come down. Air cools and causes rain/downdrafts
    • Rotation is what makes the supercell persistent

Types of supercells

  • low precipitaiton supercell
    • less dangerous
  • Classic supercell
    • medium dangerous
  • High precipitation supercell
    • most dangeorus, mostly because of visibility

Side view vs top view: LP supercell:

  • weaker reflectivity
  • smaller flanking line Classic supercell
  • more defined flanking line, greater reflectivity

HP supercell

  • Large, high reflectivity hook

Types of downdrafts

  • Forward Flank Downdraft (FFD)
    • Usually to the east or northeast of supercell
    • Do not want to be near - low visibility an
  • Rear Flank Downdraft
    • very strong winds
    • can be more damaging than the tornado itself
    • can possibly by clear slots
    • can be wet dry or both
    • warm is buoyant and unstable
    • RFDs can be wet or dry (precip)
    • Warmer RFDs are more unstable

Mesocyclone

  • defining feature of a supercell
  • rotating thunderstorm updraft

MS formation

  • ambient shear creates a horizontal vortex tube
  • updraft tilts the vortex up and streches it, incresing windspeed

Tornadoes

  • an intense rotating column of air
  • that is in contact with the ground
  • that extends from a thunderstorm cloud

Tornadogenesis stages

development:

  • can develop from ground upward or the middle or low levels downward
  • RFD and precipitatio southwest may signal higher likelihood

mature:

  • strongest, most dangerous
  • near vertical orientation, may not extend to ground or is hidden
  • if there is debris on the ground, there is a tornado

dissipating

  • RFD wraps around the tornado
  • RFD cuts off inflow
  • tilts/becomes more ropelike or snakey

Cyclic supercells

  • inflow can be refocused and cause a new tornado to form

Current Theory on Supercell tornadogenesis

QLCS

  • Quasi Linear Convective System
    • squall lines and bow echoes
  • Can be very dangerous
  • Supercells have mesocylones, QLCS have mesovortices
  • Can be just as strong but are very different
  • Dont expect updraft striations
  • Often rainwrapper
  • Not all mesovortices produce tornadoes
  • 90% of these mesovortices ef1 or less
  • can still be very dangerous
  • May 25th 2019 El Reno tornado
    • Short lives, brutal tornado
  • Much more dangerous to stormspot
    • quick moving
    • rain wrapped
    • short lived

Scales of vortices

  • Extratropical cyclones
  • Mesoscale convective vortices
  • mesocyclones
  • tornadoes
  • suction vortices
  • hurricanes
  • mesolows
  • mesovortices
  • gustnados
  • landspouts
  • dustdevils

Landsputs

  • higher base

Gustnado

  • along outflow of thunderstorm
  • detached form cumulonimbus clouds
  • any sort of wind shear can cause rotation
  • can have damaging wind gusts

Safe Positioning

  • Storm motion

  • Road network

  • Position of vehicle

  • Be aware of splitting supercells

    • Left split - more likely hail
    • Right split - more likely tornadoes
      • can even turn south/southest

Most important point

  • Dont get killed