DISCUSSION: As April ends, tornado season approaches its peak. However, this year so far, there has been fewer tornadoes than in the past 3 years. This pattern is expected to continue as the Storm Prediction Center’s (SPC) latest outlook has only slight chance for severe thunderstorms for the upcoming week which includes the beginning of May. April has generally been thought as the main start of the tornado season in the United States with May being the main peak. This April has been below average as the SPC has only reported a preliminary total of 47 tornadoes for the month alone which is far from the 3-year average of 174 tornadoes.
One of the biggest reasons there has been a low number of tornadoes so far in April has been a combination of upper-level and low-level winds. The southern plains have been receiving dry warm winds from the southwest which with help from multiple low pressure systems from Canada become westerly. In addition, there has been a dominant cold dry northwesterly flow across the northern plains. This lack of a moist southerly flow especially in the upper levels of the atmosphere removes one of the crucial ingredients of a tornado since tornadoes are typically formed because of a collision of the dry cold northerly winds and a warm moist southerly from the Gulf of Mexico. The combination of these two dry winds especially at the surface makes the surface a bit drier which increases the dewpoint depression.
The dewpoint depression is important in severe weather as it is used to determine the level of condensation (LCL). The LCL is the level of the atmosphere where if you take a parcel of air from the surface that has the moisture condensing until the moisture in the parcel is fully condensed. After you take a parcel of air from the surface to the LCL, the parcel follows the moist adiabat, which is the temperature change of a parcel full of condensation, all the way up the atmosphere to about 53,000 feet (100 mb). If the parcel crosses the temperature of the surrounding environment, there is Convective Available Potential Energy (CAPE). CAPE is the measure of buoyancy in the upper atmosphere and is related to the vertical velocity. However, an increase in dewpoint depression leads to a higher LCL and a lower moist adiabat which decreases CAPE. Also, a temperature inversion creates difficulties for CAPE due to the increase in temperature.
The combination of the temperature inversion and the higher dewpoint depression is one of the reasons that there is not such a large number of tornadoes. Another major reason there has been few tornadoes is a result of little directional shear in the winds as they reach higher in the atmosphere across much of the Central U.S. Directional shear is necessary to jumpstart rotation in order to form a tornado. To sum it up, the lack of tornadoes this year so far is due to the directional multi-level winds.
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© 2017 Meteorologist JP Kalb