|FORECASTING SEVERE WEATHER USING SKEW-T
METEOROLOGIST JEFF HABY
One of the most important times to examine soundings is during times when
severe weather is likely. Skew-T's can
give you a general idea of the character of the severe weather. Below are severe weather phenomena and how
to generally identify it's potential from the Skew-T diagram.
Strong straight-line WINDS-- Look for a
hydrolapse and large
dewpoint depressions in the mid-levels of the
troposphere. Winds will also occur in association with an
inverted-V sounding. The moist air parcels from the
storm mixes with the surrounding
dry air. This evaporative cooling produces negative buoyancy, causing
air to accelerate toward the surface. High based storms will generally have stronger winds since the downdrafts have a
longer distance above the surface to accelerate to the surface.
LARGE HAIL-- Lower values of
PW (precipitable water) preferred. Large PW values will water load the
large hail you need a large updraft and thus large
CAPE; High PW impedes this. PW less than 1.25 inches is
relatively low. PW above 1.75 will significantly water load the updraft.
LP and classic supercells have largest
hail. Large PW (i.e. greater than 2.0 inches, can reduce
upward vertical velocity of updraft by more than half)
As mentioned, the more CAPE the better.
Hail is more likely in high elevation areas since the freezing level is
closer to the surface. A low freezing level is beneficial for hail since the hailstones will not have as much
time to melt before they hit the ground. A supercell is needed to produce large hail. Look for
loaded gun sounding
and convective instability.
veering of wind in
boundary layer. Look for loaded gun sounding with plenty of convective instability.
upper level jet will tilt thunderstorm, ensuring it will be a supercell. MUST have winds in the boundary
layer averaging above 20 knots. Strong
low level jet along with veering boundary profile adds large storm
relative inflow into storm. This produces large
Helicity values. There needs to be a good balance between
HEAVY RAIN (flash flood)-- High PW value, well above climatological norm. Strong low level forcing but with
relatively weak upper level wind.
Moisture convergence into stationary low level feature (such as a stationary
front, tropical circulation).