Wednesday, May 30, 2012

Slight risk for severe weather again on Thursday (updated)

Wednesday, 8:15pm update:
It appears computer models are holding off on any early morning precipitation for the Mid-South. Given the information provided in the blog post immediately below, our confidence is increasing we will see some severe thunderstorms in and around the area beginning anytime after late morning.  The most likely time will be during the afternoon and early evening hours.  Though a tornado threat can't be completely ruled out, the primary threats with tomorrow's storms will be large hail and high wind.  Storms should form into clusters or lines, which could heighten the straight-line wind threat.

MWN will provide nowcasting via social media throughout the event tomorrow.  Now is a good time to arm yourself for the potential of severe weather with the app with StormWatch+.  StormWatch+ is our personalized severe weather notification tool that will warn the locations you input when severe weather threatens.  Check it out now on iTunes/App Store or Google Play for Android.

Previous blog entry:

The Mid-South is under a Slight Risk of severe weather for Thursday. How the scenario plays out is the subject of this blog.
Another day has brought little clarification in exactly how tomorrow's weather scenario will unfold as the biggest factor in possible severe weather appears to be how overnight convection (storms) plays out to our west.  What appears certain is that at least part of Arkansas will experience one or more mesoscale convective systems (MCS's) overnight that will move generally east and weaken as they approach the metro around dawn.  These types of systems can have several effects on the weather downstream, both positive and negative, depending on their ultimate evolution.  Some of this was hashed out in last evening's blog on the topic.

Multiple mesoscale convective systems (MCSs) traverse the Plains and Midwest (June 27, 2011)
Upstream MCS activity can bring extensive cloud cover or rain to an area as they fall apart during the early morning hours, thus providing a stabilizing influence and limiting renewed thunderstorms later in the day. The atmospheric disturbances (mesoscale convective vortices - MCV's) that linger following the demise of these systems also can bring a source of lift to a region that can reinvigorate storms when combined with sufficient heating and instability.  Generally areas under the "fallout" of one of these MCS's as it weakens ends up with a greatly reduced chance of storms later in the day, while areas downstream of the "fallout zone" get the benefit of daytime heating (providing instability) and lift from the disturbance left behind by the decaying storm system upstream.

Satellite imagery at time of above radar image. Extensive cloud cover accompanies MCSs. After the MCS falls apart, the lingering cloud cover can hinder further convection later in the day.
We've seen both cases play out in the area, as well as the unmentioned case where the overnight convective system never weakens and comes through in a strong to severe state in the early morning hours.  Right now, I don't really see that happening.

So, will the overnight storms die off over AR, leaving us with an unaffected "juicy" airmass capable of generating afternoon/evening severe thunderstorms?  Or do we get the "fallout zone" - with morning clouds/showers and afternoon storms that fire off to our east?  Time will tell...  Either way, I think this will be our best chance for meaningful rain in some time.

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