Numerous Roll Clouds Seen Early Wednesday Morning

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    A decaying area of showers and storms moved east across northern Illinois Wednesday morning between 4:30 and 8 am.  This area of storms had persisted since late Tuesday evening, having developed across northern Iowa.  This was responsible for scattered severe wind, along with very heavy rain across northeast Iowa.  Because of the long duration of the storms and overall favorable environmental wind shear, the rain-cooled air that built underneath the storm complex became large and spread out ahead of the storms.  This is common underneath organized areas of storms, with the front edge being detectable by radar and visible as a roll cloud. Note thesimilarities, yet differences between a roll cloud and shelf cloud. However, what made Wednesday morning's outflow more unique was that there were numerous outflow edges on radar and associated roll clouds.  See the below imagery and photo for observations of this.

Radar Imagery From 4 am - 7 am

Radar Loop

The outflow boundaries are visible as fine lines of light reflectivity ahead of the storms, oriented in the same fasion as the leading edge of the storm.  This defines the outer boundary of the more dense, rain-cooled air sweeping out from the storms.  For a still image showing at least seven of these outflow boundaries across the Chicago area, click here.

Shelf Clouds

Shelf Clouds

This photo was taken around 6:15 am, looking westward from NWS Chicago in Romeoville and depicts what appears to be multiple roll clouds.  Already by this time, two roll clouds had passed the office.  There were other reports of multiple such clouds and associated gust fronts.

Photo courtesy of Adam Lucio.

   It is a bit challenging to say why the outflow from Wednesday morning's storm complex behaved this way, although there is one decent possibility.  Late Tuesday evening, another stronger storm complex had passed through northern Illinois, bringing scattered severe winds and widespread rainfall.  The rain-cooled air from that, along with nocturnal cooling overnight had allowed for a strong stable layer to develop, as seen on the below early morning aircraft sounding from near Rockford.

AMDAR sounding

The purple shaded area indicates a stable layer, as defined by a part of the atmosphere under a temperature layer that warms with height.

   As rain-cooled air, or outflow, from the storms moved eastward under this strong stable layer, the outflow may have experienced atmospheric ducting.  This is propagation where the entity moving through, in this case the rain-cooled air moving through, is undulated by an atmospheric difference, in this case temperature rates of change (stability).  The undulation can be almost wave-like, and such could explain the nearly equidistant spacing between the outflow edges observed on radar.  Atmospheric ducting can even be noted on radar at times without storms.

    Below is another aircraft AMDAR sounding from Chicago from just after the outflow moved through.  Note the northwest winds associated with the outflow, highlighted by an orange box.  There are three warming layers as opposed to one strong one, which may have caused multiple ducting areas.  This may also have been an atmospheric result of multiple outflows originating at different times from the storms, since this is sampled within the rain-cooled air.

AMDAR sounding

The purple outlined portions of the temperature trace indicated warming layers, which may have led to ducting at different levels of the lower atmosphere.


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