Day 1 Severe Storms Forecasting

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Day 1 Severe Storms Forecasting

• Jim LaDue WDTB
• 08 June, 2005

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Topics

• Use todays example to discuss 2-8 hour severe
  storms forecast strategies
• Convective forcing lines vs. isolated
• Convective mode
• If isolated, what type?
• If a line, what type?
• Hazards type?
• Timing
• When is initiation likely?

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General philosophy

1. Diagnose the current weather with real
   observations first
2. Then compare reality to what the model analyzed
3. Then use the model and your understanding of its
   errors to make a prognosis.

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Which model run to believe?
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(No Transcript)
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A simple methodology for convective initiation
Analyze regions of potential Convective
instability
After that is figuring what stormtype therell
be.
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General regions of potential convective
instability
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Forcing mechanisms

• What fits your conceptual model best?

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Short-wave forcing

• 500 mb Q-vectors

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Synoptic forcing mechanisms

• Complications of playing the jetstream game

Ascent only where the red dots are located
Cyclonic curved jet
anticyclonic curved jet
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• Playing the jet game isnt as critical with a
  bowling ball low

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Forcing contd

• Vertical motion from the RUC
• Vertical motion acts to remove the CAP
• Not the only type of CAP remover

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Differential thermal advection

• Warm advection down below or
• Cold advection up above
• Bottom line, the sounding destabilizes

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• Differential thermal advection
• 700mb cooling

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Differential thermal advection/heating

• 850mb
• Ascent cooling
• Compensated by solar heating

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• Bottom line?
• Note lifting of the CAP at FWD

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Low-level forcing mechanisms

• Low-level frontogenesis
• How deep is it?
• Dryline
• Poor at forcing CI
• Thats good
• Any other troughs?

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Deep frontogenesis

• Shows up from surface past 850 mb

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Surface boundaries
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Convective forcing summ
frontogenesis
DPVA
WAA
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Forcing, now coverage

• CI is underway. The strongest forcing areas will
  most likely generate linear modes
• But geometry of forcing, especially with
  boundaries is important

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Boundary geometry

• Things to consider
• Boundary-relative steering layer flow
• Boundary-relative anvil-layer flow
• Shear relative to lines of forcing

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Boundary-relative steering layer flow

• Promotes more initiation if this value is small
• Can be good if CIN is a problem
• Too little CIN with forcing and this can be a
  problem

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Boundary-relative Flow Parameters
Boundary-relative flow
Flow
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Effects of Boundary-relative kinematics on storm
morphology

• Steering flow

stable
unstable
Is the storm going to remain on, fall behind or
overtake a boundary? This may affect storm type
beyond CAPE and shear
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Boundary-relative anvil-layer flow

• If parallel to a line of forcing
• This can promote interstorm seeding and cold pool
  development
• If directed ahead of a the forcing line
• Limits cold pool development greater chance of
  isolated modes
• If directed behind line of forcing
• Depends but it can promote a rear inflow jet

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Effects of Boundary-relative kinematics on storm
morphology

• Shear

Shear vector
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Now given your expected coverage are likely, what
are the threats?

• Severe winds,
• Severe hail
• Tornadoes
• Heavy rain

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Ingredients for supercells and severe squall
lines/bow echoes

• Deep moist convection (CAPEgt a few hundred j/kg)
• Strong vertical wind shear
• Best represented by 0-6 km bulk shear
• Subtract the winds at 6 km from the boundary
  layer
• Can be represented by Bulk Richardson Number
  Shear or BRN shear 0.5 (Uavg)2where Uavg is
  the difference from the mean 0-6 km wind and the
  mean wind in the lowest 500 m.

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Do I have enough shear?
If I have around 18 m/s (35 kt) of shear between
500 mb and close to the ground. Just eyeballing
500mb, look for at least 30kt in the lower plains
and 20 kt in the high plains. I personally look
for that 40kt of shear
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supercell motion
4. The right (left) mover is about 8 m/s right
(left) of the mean wind along the thin line.

1. Draw the shear vector from the surface to about
   6km (in red here)
2. Plot the mean 0-6km wind if it isnt there
   already (green dot)
3. Plot a line perpendicular to the shear vector
   that passes through mean wind (thin line)

sfc
L
R
6 km
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Horizontal cross sections of supercell motion
Make sure you are aware of ordinary and supercell
motion before leaving.
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Multicell Motion

• If a multicell backbuilds, heavy rain is a
  potential threat
• Use original MBE Vector (Corfidi) Technique

Vcl 0-6 km mean wind Vllj direction of 0-1.5
km wind Vmbe multicell motion
After Corfidi et al. (1996)
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Multicell Motion

1. Boundary interactions

• Modulates/enhances development of new convection

Blue steering layer flow Greentriple pt
motion Red multicell motion (Weaver, 1979)
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Supercell tornado threat

• We dont know the ingredients and this is still
  frontier science
• But here are some parameters to look for deciding
  whether to chase or not.
• High storm relative helicity (SRH), especially in
  the lowest 1km
• A strong sustained updraft, preferably one that
  begins close to ground, strong buoyancy in low
  levels
• Warm moist rear flank downdraft, low LCL is a
  good starting proxy parameter

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Storm Relative Helicity

• Air that is spinning around on its axis in the
  direction of motion (a thrown football)
• It is storm relative, therefore one must
  anticipate storm motion prior to storms
• Best visualized on a hodograph
• Also can be represented as a number in units of
  m2/s2 and contoured
• Usually measured in the lowest 3 km but now
  measured also in the lowest 1 km.

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SRH

• Recent research shows better discrimination
  between tornadic and nontornadic supercells with
  0-1km SRH. Most sounding programs and maps use 0
  3 km SRH.
• Look at soundings for evidence of high 0 1 km
  SRH.

Edwards and Thompson, 2000
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Simple and perhaps better0-1km shear

• Look for 20 kts or more for most significant
  mesocyclonic tornadoes

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SRH contd.
SRH can be enhanced by supercells themselves,
especially supercells utilizing high CAPE and
shear.
Estimated hodograph within 20 km of the storm in
following page.
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Warm moist RFD

• This cannot be so easily anticipated and every
  storm can have different RFD temperatures
• But high RH boundary layers with low cloud bases
  (LCL) seem to have some relation

Rasmussen and Blanchard, 1998
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Estimating LCL heights

• Look at surface obs in an unstable airmass
• LCL 222 (T Td) LCL in feet, Temps in F
• LCLs should be less than 1500 m for best tornado
  threat
• LCL height also displayed from soundings

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LCL height on the SPC product
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This storm is creating its own SRH
CAPE 4800 j/kg SRH initially at zero 0-6km
shear 60kt
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Strong low-level buoyancy

• Recent research courtesy of Jon Davies, Bill
  McCaul, suggest strong low-level buoyancy is
  associated with most significant tornadoes.

http//home.kscable.com/davies1/LLbuoyprimer/LLbuo
y_background.htm
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Strong low-level buoyancy

• There also is a lower Level of Free Convection
  (LFC) with most significant tornadoes.

http//members.cox.net/jdavies1/waf796/waf796.htm
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Strong low-level buoyancy

• There also is a lower Convective Inhibition (CIN)
  with most significant tornadoes.

http//members.cox.net/jdavies1/waf796/waf796.htm
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LFC height example

• A little marginal in SE OK. Good in NC KS

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Nonmesocyclonic tornadoes

• Prefer strong low-level vertical vorticity and
  good low-level lapse rates/buoyancy

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How to forecast HP, CL, LP supercells

• Storm-relative anvil layer winds likely affect
  the storm type
• LPs more common with SR anvil winds gt 30 m/s
• Classics SR anvil winds 18 30 m/s
• HPs SR anvil winds lt 18 m/s.
• Storm-to-storm seeding
• Several storms in close proximity seed each other
  increasing rain potential and HPs
• Moisture
• This is a distant third but very dry atmospheres
  may keep storms LP

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Storm-relative anvil layer winds
SR winds in range for classics. Isolated storm
becomes long-lasting Hoisington, KS storm.
Photo by Corey Mead
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Interstorm seeding
Storm on flanking line merges with target
storm. Result was possibly a complicated storm
structure during initial stages and possible
interruption in tornadogenesis.
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Forecast methodology

• As you approach initiation time, concentrate more
  on satellite, surface, profilers, radars to
  update your analysis
• If the mesoscale models look good, use them for
  your supercell, tornado parameters.

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Summary

• Determine expected convective coverage
• Low coverage implies updraft shear dynamics
• High coverage implies organized multicells, cold
  pool/shear dynamics in addition to updraft/shear
  dyamics
• Then determine your storm type and main hazards
• Many of the parameters can be used for multicell
  and isolated cell convection
• Hodograph/Skewt analysis is important!
• Do not trust the models