Forecasting Damaging Winds

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Forecasting Damaging Winds

• Jeff Evans
• SPC

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Total U.S. Severe (1950-2000)(svrplot)

• Total wind
• 186,139
• 454 deaths
• 6,746 injuries
• Total hail
• 129,923
• 11 deaths
• 760 injuries
• Total tornado
• 41,550
• 4499 deaths
• 77,223 injuries

• Severe Thunderstorm Winds account for half all
  severe reported in the lower 48 states!

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Ashley and Mote (BAMS 2005)
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Ashley and Mote (BAMS 2005)

• 69
• Outdoors

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Damaging Winds

• Severe wind most difficult threat to forecast
  without high FAR according to majority of SPC
  forecasters.
• Environments can appear very similar.
• Wider range of environments than supercells,
  tornadoes or large hail.
• Events can develop with little advanced warning.
• People outside may not have access to warning
  information.
• Boaters/Campers very susceptible!

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Damaging Convective Winds

• Unorganized
• Microbursts (short-lived)
• Wet
• Dry
• Organized lines
• Bow echoes
• Squall line
• Low-topped, strongly forced lines
• High-based, organized outflow (Haboob)
• Supercells
• RFD
• Inflow

• Klimowski et al. (2003) found 2/3 of damaging
  winds in Northern High Plains associated with
  organized convection.

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Downdrafts in principle

• Entrainment of potentially dry air important in
  initiating downdraft (usually at mid levels).
• Entrainment below this level, however, may be
  detrimental to downdraft intensity.
• For maximum downdraft at sfc
• Dry air near the melting level
• High RH at low levels.
• Downdraft speed determined by Tv difference
  between parcel and ambient air, which is greater
  when low level RH is high.
• Parcel Tv determined by its initial condition w/o
  further mixing.
• Srivastava(1985)

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Downdrafts

• Most downdrafts driven by cooling from phase
  changes.
• Condensate loading and entrainment both can be
  important in initiating downdrafts.

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Dry Microbursts

• Low reflectivity
• lt 0.25 mm rain or radar echo lt 35 dBZ.
• Intensity related to
• Drop size
• Rain intensity
• Sub-cloud lapse rate
• Cooling from phase changes primary focusing
  mechanism for low reflectivity microburst.

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Freezing line
Deep layer / area for acceleration
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Forecasting implications?

• Most forecasting guidelines continue to focus on
  collocation of dry boundary layer/steep low level
  lapse rates and moistening mid levels.
• Inverted-V profile
• Very common in warm season across intermountain
  West or along high plains (high FAR).
• At SPC, additional focus on organizational
  potential and/or thunderstorm coverage
• Sufficient steering flow off higher terrain?
• Approaching shortwave trough?
• Can forecast of hydrometeor type improve
  forecast-ability of dry microbursts?

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Wet Microburst

• In absence of steep sub-cloud lapse rates, high
  water content needed.
• Unlike dry microburst, much of the initial
  downdraft still buoyant (warmer than ambient
  air).
• Indicates importance of precipitation loading in
  driving wet microburst.

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Wet Microbursts

• Precipitation loading primary mechanism driving
  downdraft.
• Drag of precipitation needed for downward
  acceleration
• Higher mixing ratios are necessary for these
  downbursts to form
• Melting of ice (hail) important in downburst
  formation (Wakimoto and Bringi, 1988)

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Atkins and Wakimato (1991)

• Examined wet microbursts in nrn AL
• Warm, moist boundary layer overspread by EML.
• D Qe between surface and min aloft
• gt 20 K on soundings favored microbursts.
• lt 13 K favored non-micoburst Tstms.
• Min. Qe level not always equal level of driest
  air.
• Precip. core composed of ice.
• Core exceeds level of min. Qe, with convergence
  into downdraft at that level.

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Microburst overview

• Dry microbursts
• Evaporation and negative bouyancy
• Wet microbursts
• Evaporation starts, but precipitation loading and
  melting of frozen precip. drives downdraft.

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Each point represents a microburst.From
Srivastava (1985)
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Microburst overview

• Dry microbursts
• Evaporation and negative bouyancy
• Wet microbursts
• Evaporation starts, but precipitation loading and
  melting of frozen precip. drives downdraft.
• Most microbursts east of the Rockies are more of
  a mixture of both.
• Moderate to large CAPE
• Steep sub-cloud LR large sfc T-Td spreads
• Plentiful moisture heavy rain potential

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Organized Severe Windstorms

• In organized severe windstorms, severe surface
  winds can be caused by a multitude of processes.
• Traditional downdraft physics, plus
• Storm- and even Meso- scale processes.
• Bow echoes and organized lines of storms
• Occur in a wide variety of environments
• Evans and Doswell (2001) found the range of CAPE
  and Shear for derechos exceeded that of
  supercells!
• Come in many sizes

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Storm Scale
Outflow bndry
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Mesoscale
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Synoptic scale
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Ideally?

• Numerical simulations (Weisman) define strict
  range of CAPE/shear for long-lived bow echoes.
• High CAPE
• Strong shear in lowest 0-3 km, with little shear
  above
• Operationally we see a much larger range of
  parameters (ED01).
• Weisman may show only a subset of a larger range.

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Bow Echo evolutions

• Bow echoes commonly evolve from
• HP supercells which become dominated by outflow.
• Cell mergers within multicell cluster.
• Rapidly moving, forced large scale ascent.
• Small accelerations within larger squall line.

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HP supercell evolution

• Moller one of first to publish this evolution.
• Generally occur in very unstable environments,
  with abundant moisture.
• Shear is strong, though fast motion and/or weak
  mid/upper level winds lead to insufficient SRW
  and precipitation wraps around meso.
• Also can evolve after a merger when dense/cold
  outflow disrupts low level balance under meso.
• Recall the HP supercell structure

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Heavy Precip. supercell
IF THE STORM RELATIVE WINDS AT MID-UPPER LEVES
ARE WEAK A LARGE AMOUNT OF PRECIPITATION WILL
FORM NEAR/IN THE UPDRAFT AND WILL WRAP AROUND THE
MESOCYCLONE- HP SUPERCELL
HP SUPERCELLS TEND TO BE OUTFLOW DOMINATED.
RAIN-COOLED OUTFLOW UNDERCUTS THE MESOCYCLONE
LIMITING THE POTENTIAL FOR LONG-LIVED
TORNADOES. VERY LARGE HAIL AND BOW ECHO EVOLUTION
COMMON.
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Brooks-Doswell-Cooper
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Brooks-Doswell-Cooper
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HP supercell evolution

• Once heavy, rain-cooled air mass wraps around
  mesocyclone, it undercuts initial updraft and bow
  echo can evolve if
• Downstream air mass is unstable and weakly
  capped.
• Allowing outflow along leading edge of dense cold
  pool to initiate storms along its forward edge.
• Surface boundary paralleling bow echo path.
• Increases convergence and potential for continued
  propagation
• Continued feed of moist, unstable air.
• At this point, mesoscale structures
  enhance/sustain larger scale bow echo/damaging
  surface winds.
• Rear-Inflow Jet
• Bookend vortices
• Enhanced pressure gradient at surface.

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HP-supercell can sustain both strong low level
meso and a trailing bow echo.
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Cell Merger Evolution

• Brian Klimowski, et al. (2003)
• 41 of bow echoes in NHP result from cell
  mergers.
• Aggressor storm moves faster than 0-6 km mean
  wind.
• Postulates that collision increases the
  precipitation rate (and associated outflow)
  leading to a bowing or accelerating segment.
• Increase in breadth and magnitude of the radar
  reflectivity, and the production of an arc of
  reflectivity (bow) within 1020 min.

No data examined about FAR with mergers. How many
mergers occur and dont evolve into bow echoes?
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Squall lines

• Damaging squall lines can occur year round in all
  parts of the country.
• Can be driven by strong large scale forcing
• Or internal mechanics
• Cold Pool, RIJ, etc
• Been observed in very low CAPE and extreme CAPE
  environments.
• Can be low reflectivity (sometimes with little
  lightning).
• Tough to anticipate w/o large FAR.

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Cool season squall lines

• Commonly occur ahead strong, progressive mid
  level troughs with deep lift along surface cold
  front, and strong subsidence in its wake.
• Large pressure rises behind the front
• Threat of strong to severe wind if any CAPE
  (lightning) and either or both the following
• Strong wind just off the surface with a sizeable
  portion of the vector perpendicular to the line,
• Steep lapse rates
• Especially in the sub-cloud layer which increases
  mixing and downward transport of higher momentum
  air.
• Derechos more common in cool season from the Srn
  Appalachians to the Piedmont region.

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Cool season derechos (Sept.-Apr.) 1980-2001
Coniglio
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Shear vector parallel to initiating
boundarystorm interactions lead to line
formation, with end supercells possible.Or,
Recall what Controls Convective Mode?
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Initiating boundary moving near speed of
stormsCells, even if initially discrete, fail
to move away from front and forced squall line
ensues.
What Controls Convective Mode?
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Case in pointMarch 8th, 2005
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Warm season squall lines

• Less common into the Carolinas than cool season,
  but still may see a couple derechos each year
  persist to the Atlantic coast.
• For this region, they usually initiate over
  Southern Great Lakes to the Mid MS river valley.

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June 4th, 1993
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Aug. 9th, 2000
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Warm season derechos

• Evans and Doswell (2001) validated earlier
  studies and found warm season derechos dominated
  by propagation/cold pools.
• Greatest difference between derecho and
  non-derecho MCSs with system speed/0-6 km mean
  wind.
• Focus on four terms for forecasting warm season
  derechos
• Instability
• Organization
• Forward motion
• Cold pool
• Like other phenomena, no single parameter
  effective at forecasting.
• Composite parameters show more skill.

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Derecho Composite Parameter

• DCP uses normalized values based off ED01
• Downdraft DCAPE / 980 J kg-1
• Organization 0-6 km Shear / 20 kt
• Forward momentum 0-6 km Mean Wind / 16 kt
• Updraft MUCAPE / 2000 J kg-1
• DCAPE/980MUCAPE/20000-6MnWd/16kt0-6shear/20kt

Hope to get DCP on SPC Hourly Mesoscale Analysis
Page this convective season
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Downdraft CAPE

• Gilmore and Wicker, 1998 The influence of
  mid-tropospheric dryness on supercell morphology
  and evolution.
• Min theta-e within 100 mb thick layer
• Descend this theta-e min down moist adiabatic
• Area between this trace and T profile DCAPE
• Continue this for every obs. layer up to 400 mb
• DCAPE largest area

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Summary

• In weakly organized environments
• Microphysics important in downburst intensity.
• Dry microbursts initiated by evaporation or
  sublimation, but maintained by negative
  buoyancy.
• Wet microbursts initiated by precip. loading and
  evaporation, and maintained by evaporation.

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Summary (cont.)

• Organized convective windstorms
• Evolve from complex interaction within storm and
  between storms.
• Occur in environments which favor downdraft
  initiation
• Mid level dry air
• Dry sub-cloud lapse rates
• Microphysics also important in favoring
  downbursts, however organized-processes
  supplement severe surface winds.
• In supercell or multicell environments,
  forecasters should look for environments
  supporting outflow dominated storms for bow echo
  development.
• Watch cell mergers closely.

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Summary (cont.)

• Forecast of steep sub-cloud layer lapse rates and
  strong cold front approaching
• Monitor for linear organization of convective
  elements along front.
• Moderate to strong winds just off the surface can
  be transported downward.
• Wind damage increasing possibility.

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Enough Already!Questions?