Sunday, June 29, 2014

Potentially Dangerous Tornado & High Wind Discussion for Monday, June 30

This discussion will focus on the anticipated tornado and high-wind threat on Monday, June 30.

The Storm Prediction Center has issued a Slight Risk of severe weather on Monday in the 15% shading, which extends from Oklahoma into Kansas, Missouri, Iowa, Illinois, Wisconsin, and into Michigan. An enhanced risk of severe weather exists from eastern Kansas into southeast Iowa, northern Missouri, much of Illinois and portions of Indiana and Wisconsin. This enhanced area is where we are watching for potentially significant severe weather on Monday, June 30, which may include tornadoes and a swath of damaging winds.

Short range analysis from the Weather Prediction Center, valid for Monday morning, shows multiple features relevant to tomorrow's expected severe weather outbreak. We see a decaying mesoscale convective system (MCS) progressing across Illinois, moving eastward as it does so. This complex of thunderstorms will have formed today (Sunday) out to the west, and will have produced additional severe weather in those areas (see Sunday's severe weather discussion here). We also see a stationary front draped across Nebraska, Iowa, and Wisconsin, which is the focus of some early-day convection in northwest Iowa per this forecast graphic. However, the main concern here is if the morning MCS over Illinois will limit convection later in the day due to the MCS-associated cloud cover.

If we look at the forecasted relative humidity values at the 500mb level of the atmosphere from the NAM, valid in the early hours of Monday, we see only limited moisture across Illinois and Indiana, despite the expected presence of the decaying MCS. This tells me that while we can expect cloud cover in the aforementioned regions due to the thunderstorm complex moving east, the cloud deck shouldn't be thick or extensive enough to severely limit convection later in the day, and this is what makes me believe we are looking at a potentially more explosive environment on Monday afternoon.

By Monday afternoon, we see extensive instability has developed over the Midwest, focused in on the areas projected to receive severe weather. We see a swath of 4000 j/kg CAPE (convective available potential energy) stretching from Oklahoma to Illinois, with a pocket of 5000 j/kg CAPE existing in south-central Iowa and northern Missouri, likely the initialization point of Monday's convection.

There is ample concern that Monday's event could produce tornadoes. This stems from a very dynamic wind field in the upper, middle, and lower levels of the atmosphere, thanks in part to the dominant upper level low in southern Canada. These dynamic wind fields then create ample wind shear, a necessary ingredient for tornadoes. With the aforementioned extreme instability, tornadoes are very possible with the initial cells that form Monday afternoon out in Iowa and northwest Illinois, prior to coagulation into a cluster of thunderstorms. From there, a potentially dangerous damaging wind threat may be expected to evolve. You can stay tuned to our Facebook Page and Twitter account for more frequent updates on the situation.


Severe Weather Outbreak Discussion for Sunday, June 29

A severe weather outbreak is expected today, June 29.

The Storm Prediction Center has posted a Moderate Risk of severe weather for southeast Nebraska, northeast Kansas, northern Missouri, and the southern half of Iowa in advance of severe weather expected today. Current discussions indicate ongoing convection over Iowa will be clearing later on in the day, paving the way for diurnal heating to develop and destabilize the environment to provide a breeding ground for severe thunderstorms.

The NAM model indicates deep instability will develop over Iowa and Nebraska later on this evening, where convective available potential energy (CAPE) values look to exceed 3000 j/kg in spots. The development of this instability depends severely on the morning convection over the same areas right now. If cloud cover tends to linger over the area, we could see a reduced severe weather risk, as destabilization will be limited. However, if cloud cover can clear quickly enough, such CAPE values will be easy to come by.

When the storms do form, it looks like they will have decent potential to be originally tornadic. The NAM model places over 50 knots of shear near Iowa, increasing the likelihood that these initial cells that form may have a tornadic component to them. With such deep instability and shear, it is no wonder the Storm Prediction Center elected to go for a higher-level tornado threat for this Moderate Risk area. As the night progresses, the initial cells are expected to come together to form a mesoscale convective system (MCS), also known as a body of severe thunderstorms, that will be moving to the east.

To summarize, those in the Moderate Risk area today should prepare for:
• Damaging winds in eastern Iowa
• Tornadoes and very large hail in central Iowa into Missouri with the initial cells
• General poor weather conditions across the Plains and Midwest