April Showers & May Thunderstorms
April 30, 2012 11:01 AM
The last weekend in April was a cool, cloudy and rainy weekend as many areas saw totals above an inch of rain, ending a somewhat rainy April. Thanks to cloud cover and rain, temperatures dropped 10 to 15 degree's below average a stark contrast to the record warm, dry March.Rain started Thursday evening in many areas and moved east however dry air just above the surface in the east caused rain drops to evaporate almost entirely before making it to the ground causing many areas to stay mostly dry for almost the entire daytime hours of Friday. As the atmosphere moistened Friday evening in the east it allowed heavy rainfall to move out of Nebraska and into the Sioux Empire. Heavy rain continued Friday overnight, which is where the majority of the rainfall totals added up. Saturday and Sunday, light scattered showers added a few hundredths of an inch to many places. When the sun finally emerged on Monday afternoon rainfall totals added up from over an inch to only a few hundredths of an inch.
(Rainfall Totals from April 26-30)April has been a stark contrast to an abnormally dry and warm March. Many areas saw great improvement to their drought conditions and added much needed soil moisture just in time for spring planting.
(April Monthly Rainfall Totals)We start off the new work week and the month of May warm, with temperatures between five and 15 degree's above average. With warmer weather moving in this week and a passing shortwave, showers and thunderstorms will be possible this week. The Storm Prediction Center has issued a slight risk for severe weather on Tuesday.
(SPC Convective Day 2 Outlook)Tuesday's slight risk encompasses the Interstate 29/90 corridor, northern Nebraska, northwest Iowa and southwest Minnesota. The primary threats for severe weather will be damaging winds and large hail. Isolated tornadoes are still a possibility but this particular event is not as conducive to tornado genesis due to high LCL height's and low unidirectional shear values.
As always, straight line winds can be just as dangerous and damaging as tornadoes. Add large hail to strong winds and the threat can be even more hazardous. This weekend severe storms in Texas and St. Louis, Missouri caused major damage with baseball size hail and strong winds
(Damage caused by baseball size hail and high winds in St. Louis, Missouri. Image courtesy: Aaron Brown)Tuesday's threat for severe weather will start in late afternoon and early evening as CAPE values will climb into the two to three thousand range. CAPE (Convective Available Potential Energy) is the amount of energy available for a storm to tap into. The larger the CAPE value the more energy a storm can use to become strong to severe. This time of the year CAPE values around one to two thousand can produce a good sized severe storm.
(NAM CAPE forecast valid 18z Tuesday May 1st)Another indication of favorable conditions for severe development is: LI's or Lifted Index. This index is an indication on the tendency of the atmosphere to either allow air parcels to rise or sink. Positive LI values indicate a tendency for a stable atmosphere (sinking air), while negative numbers indicate an unstable environment (rising air). Rapidly rising air causes strong updrafts/downdrafts to form within a storm. The more intense the updrafts the better chance for large hail and strong winds. LI's on Tuesday are forecast to be very negative.
(Nam model Lifted Index Forecast valid 18z Tuesday May 1st)Another tool used to forecast severe weather is called: CIN or Convective Inhibition. Commonly referred to as a "Cap" CIN indicates how strong of a "cap" or "plug" is on the atmosphere not allowing the storm to tap into the available energy (CAPE). The smaller the number the more favorable the atmosphere will be for severe weather development. Tuesday cin is forecasted to drop very close to if not at zero in the afternoon hours.
(Nam CIN forecast valid Tuesday 18z)Forecasts will change so it is important to stay on top of changing weather conditions. When away from your television, you can check up to the second weather conditions on the KDLT APP for Android and iPhones and as always KDLT.COM