Transcript Common Severe Weather Weather Soundings

Severe Weather Soundings
and
Wind Shear Environments
Typical Synoptic Severe Weather Pattern
Severe Weather Soundings
• Type “A” or Inverted “V” Sounding
– Most commonly found in the High Plains, Great
Basin, and Desert SW
– Low Humidity at most levels, but especially at
lower levels
– Produces Storms with very high cloud bases
• Mostly Virga
– Largest Severe Weather Threat:
• Severe Straight-Line Winds
• Dry Microbursts
• Due to evaporational cooling of precipitation falling out
of the cloud base
Severe Weather Soundings
• Type “A” Sounding Cont.
– Downdrafts are normally much colder than the
environment and therefore more dense through a
deep layer of the atmosphere
– Rate of cold air production is proportional to the
amount of evaporation taking place
– Ratio of evaporated rain (Virga) to rain reaching
the surface increases with the increasing height
of the cloud bases
Inverted “V” Sounding
Inverted “V” Sounding
Inverted “V” Sounding
Severe Weather Soundings
• Type “B” or Loaded Gun / Goalpost
Sounding
– Moist air in the boundary layer with dry air
aloft
– Typically found in the Central & Southern
Plains
– Large supply of moisture in the boundary
layer provided by a low-level southerly flow
(mT air)
– Low-level moist convergence on the nose
of a low-level jet (LLJ) (850mb)
Severe Weather Soundings
– Very warm and dry air at mid-levels (cT air)
off of the Mexican Plateau from an elevated
region known as the EML or Elevated
Mixed Layer
– Provides a strong capping inversion which
will inhibit the premature release of the
convective instability
– Most commonly associated with the warm
sector of Spring & Fall mid-latitude
cyclones
Loaded Gun Sounding
Loaded Gun Sounding
Loaded Gun Sounding
A severe weather environment that
includes an EML usually results in highend convection. Why?
• The EML prevents deep,
moist convection until high
potential instability is
achieved (bottom of EML
acts as a lid or cap).
• In the absence of deep,
moist convection, warm,
moist low level air can flow
northward in an unimpeded
manner (underrunning).
• Tendency to keep storms
from becoming overly
widespread (the exception is
for severe MCSs).
A severe weather environment that
includes an EML usually results in highend convection. Why?
• Prevention of deep vertical
mixing. Generally does not
allow SFC dewpoints to mix
out.
• Very steep lapse rates in
mid levels enhances CAPE
= fast updraft accelerations.
• DCAPE (Downdraft CAPE)
enhancement
Elevated Heating = Steeper Lapse Rates
Cold
Cool
Cold
Warm
In the absence of widespread diabatic
processes, EMLs are advected downstream
without changing much character at all:
Confirming the sounding does contain
an EML – trace back to source region
SSM
ALB
Backward
trajectory
analysis
LBF
ELP
ELP 12z 25Aug73
LBF 12z 26Aug73
SSM 12z 27Aug73
ALB 12z 28Aug73
Severe Weather Soundings
• Type “C” or Humid/SE Sounding
– Found in Midwest & SE U.S. in the late Spring
and Summer
– Associated with a barotropic environment
• Non-advection environment in which isotherms parallel
isoheights/isobars
– Very deep layer of moist air (mT), generally
extends from sfc to at least 700mb
– Very small amount of evaporation, so generally
light to only moderate downdrafts
– Greatest Threat: Heavy/Flooding Rains
– Convection is initiated from differential surface
heating and a lack of a capping inversion
Humid / Rain Sounding
Humid / Rain Sounding
Humid / Rain Sounding
Launched into Convection
Launched into Convection
Severe Weather Soundings
• Wet Microburst Sounding
– Similar to Type “C” sounding in that there is a deep layer
of moist air
– However, there is significant drying aloft
– Most common in the Southern Plains, Midwest, and
Southeastern U.S.
– Deep layer of moisture begins at sfc and extends to
approximately 700mb
– Moist air is capped by a dry layer that begins at 700mb –
600mb
– Dry air provides evaporative power
• Get the production of negative CAPE (B-)or Downdraft CAPE
(DCAPE)
• This leads to intense downdrafts and downbursts
Wet Microburst Sounding
Wet Microburst Sounding
Severe Weather Soundings
• Low-Level Jet Sounding
– The low level jet is a high speed return of warm and moist
air from the south or southeast; moisture source is the
Gulf of Mexico
– Most common and intense over the Plains states and
Southeast states
– The low level jet occurs in the warm sector of a
developing mid-latitude cyclone in the Central and
Eastern U.S.; occurs generally ahead of the cold front
boundary
– Intensity of low level jet is increased due to temperature
gradient between cooler high elevations in the high plains
compared to warmer East Great Plains at night. Can also
intensify by the warm sector of a mid-latitude cyclone
being east of the cold sector.
Severe Weather Soundings
• Low-Level Jet Sounding Cont.
– Low level jet adds heat, mass and momentum to
developing thunderstorm and produces low level
speed and directional shear (results in very high
Helicity values)
– Produces abundant WAA (warm air advection)
that may break a weak to moderate cap. WAA
produces broad synoptic scale uplift
– Strongest low level jet winds are generally at the
top of Planetary Boundary Layer due to less
friction than at the surface
– Advection may well be over 65 miles per hour
Low-Level Jet Sounding
Severe Weather Soundings
• Elevated Convection Sounding
– Most common in the cool season on the north
side of a frontal boundary (in the cool air)
– Parcels do not rise from surface during elevated
convection. Parcel lapse rate on skew-T from
surface is useless when the boundary layer is
very stable.
– Parcel will generally rise from top of temperature
inversion during elevated convection. On the
sounding below, a parcel rising from the 700-mb
level will be much less stable than a parcel rising
from the surface.
Elevated Convection Sounding
Elevated Convection Sounding
Elevated Convection Sounding
Wind Shear Environments
• What is shear?
– The rate of change of the wind in both the
horizontal and the vertical
• Organizational capacity of the wind
• Two parts: Speed and Direction
– Strong speed shear is detrimental to the growth
of small or weak storms
– Large cells are typically enhanced by wind shear
Wind Shear Environments
Wind Shear Environments
• Speed Shear – Change in speed with height
Wind Shear Environments
• Directional Shear – Change in wind direction
with height
Wind Shear Environments
• No Shear
– Little cell movement
– Downdraft will pool equally in all directions
– Convergence along the outflow may initiate new
and weaker cells if uplift and B+ are great enough
– New cells will die quickly as a result of stable air
behind the gust front
Wind Shear Environments
• Moderate Shear
– New cells growing along outflow will move
downshear
– Therefore having a better chance at long life
– Increase in storm relative inflow
– Magnitude of the inflow is better matched to the
magnitude of the updraft
– Good match between inflow and updraft strength
leads to redevelopment of the updraft and may
force new cells to form on the right flank of
original cells due to enhanced convergence
– New cells on right flank is known as Discrete
Propagation
Wind Shear Environments
• Strong Shear
– Production of updraft rotation
– Takes place through the tilting of horizontal
vorticity
– Rotation produces a pressure gradient
– HPG produces a very strong vertical jet
– Horizontal vorticity tube can be stretched in the
updraft and produce a rotating updraft
– Possible tornado production
– Cell rotates and propagates to the right of the
mean flow – called Continuous Propagation
Wind Shear Environments
• Unidirectional Shear
– Weak:
• Short-lived cells with a gust front that may produce
short-lived secondary convection directly downshear
– Strong:
• Splitting cells
– Caused by forcing that splits the updraft into two separate
storms
– Anticyclonically rotating storm will continue to the left of the
environmental winds
» Dissipates after a short period of time
– Cyclonically rotating storm will continue either along or to the
right of the mean environmental flow
Wind Shear Environments
• Curved Shear
– Veering of the winds on a sounding
• Clockwise with height
– Weak:
• Weak, short-lived cells with short-lived regeneration of
new storms along the gust front
– Strong:
• Hydrodynamic Forcing only on the right flank of the
parent cell (storm)
• Produces a single quasi-steady cyclonically rotating
updraft
• Supercell thunderstorms
– If it becomes a right mover, the storm can sustain itself for
long periods of time due to enhanced inflow