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 updraft. For 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. TORNADO- Strong veering of wind in boundary layer. Look for loaded gun sounding with plenty of convective instability. Strong 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 shear and instability. 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). |