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

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

A severe weather environment that includes an EML usually results in high- end 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 high- end 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 Warm Cold Cool Cold

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

Backward trajectory analysis ELP LBF SSM ALB

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

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

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

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

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