There are three types of supercell thunderstorms: Classic supercells, HP (high precipitation) supercells, and LP (low precipitation supercells). Low level moisture and the value of precipitable water (PW) strongly determine which type of supercell a severe thunderstorm will be. LP supercells in the U.S. primarily occur in the Lee of the Rockies while HP supercells are common in the eastern Great Plains and eastward. Classic supercells are common in the Great Plains but can occur in regions where LP and HP supercells are common. The type of severe weather to expect from each of these supercell types and the radar reflectivity signature is given below: LP SUPERCELLS: STRONG STRAIGHT-LINE WINDS (due to a high amount of evaporative cooling as precipitation evaporates into surrounding low and mid-level dry air). LARGE HAIL (low amount of moisture results in a small water loading of the updraft, low PW values (less than 1.00 inch) allows the updraft to be stronger than it otherwise would have been if PW values were high). I have seen cases were an LP supercell produced baseball size hail and no other precipitation. TORNADOES (tornadoes tend to be weaker in association with LP supercells as compared to other supercell types because the forward flank downdraft and rear flank downdrafts are not as well defined and the storm is higher based). Tornadoes, if they do occur, are very visible in person due to the light amount of precipitation and a higher based wall cloud. LP supercells tend to be higher based than the other supercell types because air must rise to a higher level in the troposphere in order for condensation to occur. LP supercells are extremely difficult to locate on radar reflectivity since there is a lack of precipitation wrapping around the wall cloud. CLASSIC SUPERCELLS: Classic supercells give the best indication of a hook echo on Doppler radar. Many classic supercells are isolated from other storms. Classic supercells have varying degrees of hail size, tornado strength, and strength of straight-line wind. CAPE (how unstable the troposphere is) along with PW, freezing level and elevation determines how large hail will grow. Dry air in the mid-levels (large dewpoint depression) along with moisture in the PBL (high dewpoints) result in strong straight-line wind and convective instability. Strong low level speed and directional shear along with high CAPE form a favorable environment for tornadoes. HP SUPERCELLS: HP supercells have a kidney bean shaped appearance on Doppler radar. Many HP supercells occur in multicell supercell clusters or when a classic supercell draws increasing amounts of moisture into the circulation. These are often termed rain-wrapped supercells. Tornadoes are generally more deadly when they are rain-wrapped since their approach is unknown until they are right on top of you. Hail tends to be smaller in association with HP supercells than in association with the other supercell types because: 1) CAPE is reduced due to intense water loading 2) Freezing levels tend to be higher and elevations lower in association with HP's (compare high plains to Southeast U.S.). Strong straight-line winds are not as common with HP's since moisture extends through a higher depth in the troposphere. Evaporative cooling and negative buoyancy are lower because the dewpoint depression in the lower and middle troposphere tend to be lower (dewpoints higher). |