Hurricane Hunters flew through Sunday’s severe storms

Welcome to nocturnal thunderstorm season.

Mid-July through August marks the time of year when the frequency of overnight thunderstorms increases in the Plains. Longer nights cause the upper atmosphere to cool more than a month ago.

The relatively unstable air mass with high heat and humidity near the surface begins to rise into the increasingly cool layer aloft, and thunderstorms are more likely to go boom late at night this time of year.

The National Oceanic and Atmospheric Administration has been watching nighttime storms for the first half of summer to try and understand why these storms often form in the heat of the night. The project called PECAN wraps up today.

This summer, more than 20 NOAA scientists will stay up late to learn why some thunderstorms form and grow at night, without the energy from the sun's heat. They will be participating in the Plains Elevated Convection At Night (PECAN), a large, intensive field campaign to collect data before and during nighttime thunderstorms in the western Great Plains from June 1 to July 15.

PECAN researchers will deploy instrumented aircraft, ground-based instruments, mobile radars, and weather balloons to learn what triggers these storms, how the atmosphere supports their lifecycle, and how they impact lives, property, agriculture and the water budget in the region. Meteorologists believe these targeted observations will build understanding and ultimately improve forecasts of these sometimes damaging storms.

"Large nighttime thunderstorms are an essential source of summer rain for crops, but also produce widespread and potentially hazardous severe weather, excessive rainfall, flash flooding, and unusually frequent cloud-to-ground lightning," said Conrad Ziegler, a research meteorologist at the NOAA National Severe Storms Laboratory and principal scientist for PECAN.  "Weather forecast models often struggle to accurately account for these. The PECAN field campaign will provide us with valuable insights—and improve our ability to save lives and property through more accurate forecasts.”

Sunday evening's severe storms that tore through the Brainerd Lakes, Minn., area did considerable damage.

Sunday's storms were not true nocturnal storms, because they developed in the heat of the afternoon. But as I reported Monday they attracted the attention of the PECAN project as NOAA flew the hurricane hunter aircraft into the storms from the PECAN base in Hays, Kan.

The specially equipped aircraft are part of the armada of sensors that will hopefully help produce a better understanding of the triggers for nocturnal thunderstorms.

Here's more from NOAA.

Nighttime storm triggers

Once the sun goes down, the Earth and its lower atmosphere usually loses heat and becomes more stable, an environment not so favorable for supporting thunderstorms.  In the Great Plains, however, many summer storms form after sunset, and sometimes without an obvious trigger.

PECAN scientists are interested in large complexes of thunderstorms called Mesoscale Convective Systems that can grow overnight, last for hours and often produce severe and hazardous weather. They will investigate how a low-level river of air triggers thunderstorms and supports storm evolution, what causes storms to grow into MCSs, and how MCSs respond to the surrounding environment.

In addition, PECAN researchers will test their hypotheses about how deep waves in the atmosphere form and ripple across the plains, like what happens with water when a stone is thrown in a pond, causing new storms to form after sunset. One type of atmospheric ripple is called a “bore.” Thunderstorms can create bores, but bores can also cause a thunderstorm to suddenly intensify. PECAN is the first modern campaign to study the role of bores and how they trigger and support Mesoscale Convective Systems.

Armada of instruments

More than 20 NOAA researchers and students will be responsible for gathering data with multiple instruments including the NOAA-X-Pol, a dual-pol mobile radar, two mobile balloon launch vehicles, and two “mobile mesonet” vehicles equipped with weather instruments. New to the fleet is the Collaborative Lower Atmosphere Mobile Profiling System (CLAMPS) designed by NSSL researchers to meet many of NOAA’s and its National Weather Service’s needs for lower atmosphere temperature, humidity and wind profiles. Additionally, one of the three aircraft participating in PECAN will be a NOAA Lockheed WP-3D Orion aircraft, best known for its hurricane hunting missions.

More nighttime storms tonight

This morning's respite from high dew points is a brief one. Dew points in the 50s this morning in the metro give way to southerly return flow, and rise back into the muggy 60s this afternoon.

The next low pressure wave lifts north tonight. A cluster of thunderstorms moves into southern Minnesota overnight and locally heavy downpours once again roam southern Minnesota.

The timing on rainfall potential today? Spotty showers today, with a more organized convection zone moving north from Iowa into southern Minnesota overnight.

Thursday morning's rainfall looks locally heavy from the Twin Cities south along Interstate 35 through Owatonna, Minn., and Albert Lea, Minn. NOAA's Global Forecast System and North American Mesoscale Forecast System runs have been cranking out 1- to possibly 2-inch totals that could come in by midday Thursday.

The highest chance of 2 inches in your rain gauge tomorrow morning is probably just south of the Twin Cities from Lakeville through the Minnesota towns of New Prague, Northfield and Owatonna. Another swath of heavy rainfall favors northwest Minnesota.

Heat returns this weekend

Temps and dew points climb once again starting Friday as another sultry air mass oozes north. Overnight and early morning storms are the weather flavor of choice again this weekend. Dew points rise into the unbearable 75-plus degree range again Saturday.

Saturday afternoon looks downright brutal, likely the hottest feeling day in the metro so far this year. Temps in the low 90s and dew points in the mid 70s should combine to boost the heat index over the 100 degree mark.

Enjoy today's realtive comfort and keep an eye out for local downpours by tomorrow morning.