Wednesday, June 4, 2014

Northwest flow MCS's result in chances of severe storms

An upper-level high pressure ridge over the southern plains extends eastward over the Mid-South and is suppressing thunderstorm activity, despite warm and humid conditions. However, this ridge will be pushed back to the west slightly the next few days, putting the Mid-South in a weak northwest flow situation. Northwest flow is when the winds at the steering level (around 15-20,000', which dictate the direction storm complexes move) blow from the northwest to the southeast over the area. This is shown pretty well below.

The steering flow for thunderstorm complexes will be from the northwest (thus "northwest flow") the next couple of days. The northwest flow is created by a ridge of high pressure to our southwest and a weak trough to our northeast.

In addition to the northwest flow aloft, a cold front will be oriented east-west just north of the metro tomorrow and just south of the metro on Friday. This front will act like the highway in the sky for complexes of thunderstorms triggered by upper-level disturbances over the Plains to move near or over the region.

A cold front sits over the Mid-South on Thursday evening, creating a "storm track" for systems moving in from the Plains.

These complexes (MCS's, or mesoscale convective systems) can bring rounds of strong to severe thunderstorms, primarily in the form of damaging wind and hail. Northwest flow MCS's are fairly predictable while in their mature stage, but can be very difficult to predict when they will fall apart. They frequently form in the Plains overnight and move east into the Mississippi Valley during the morning hours during summertime northwest flow events.

So, you guessed it, we'll be watching carefully as probably more than one MCS moves near, or into, the Mid-South the next few days. The Storm Prediction Center is also monitoring and forecasting the same thing, thus they have placed the Mid-South and areas upstream in the northwest flow in a Slight Risk zone for potential severe weather on both Thursday and Friday.

There is a Slight Risk of  damaging wind and large hail on Thursday over the Mid-South.

Another Slight Risk exists on Friday in weak northwest flow.

I believe the most likely day for strong storms from an MCS will be Friday and that the threat for Thursday could be just north of the metro, however we have chances of t'storms in the forecast tomorrow as well as Friday (and for that matter right through the weekend). By the way, don't be surprised to see one of these complexes tonight over southern Missouri and Kentucky that moves into northern TN. Although we could get a few showers early Thursday from this system, it should be in a dissipation state as it drops further south towards the metro.

We'll obviously be watching closely and provide updates on our social channels as things come into focus a little better. Just be prepared for the possibility of strong to severe storms the next couple of days as storms roll into the area from the northwest! If you're headed out to the FESJC golf tournament, heed all weather warnings from tournament officials and seek shelter if the sky looks threatening! (And also pack plenty of sunscreen and stay hydrated as heat and humidity will result in afternoon heat indices well above 90.)

One last side note: you can now monitor our severe weather risk and upstream weather systems even easier with the latest update to the MWN mobile app! We've added satellite imagery and SPC severe weather outlooks to MWN for iPhone users with Android users getting the same update within the next day or two. Download the app (or the most recent update if you already have it) by clicking here.

Erik Proseus
MWN Meteorologist

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