Sunday, July 21, 2013

"Northwest flow" contributes to rounds of showers and t'storms this week

If you were hoping for better rain chances this week, then you're in luck!

A weather pattern that emerges maybe a couple of times a summer will take hold this week, in the wake of an unusual upper-level pattern in mid-July that brought east-to-west moving storms. Until a couple of days ago, the Bermuda high pressure system - typically stationed over the western Atlantic and southeast U.S. in the summer - relocated to the Ohio Valley and Midwest, bringing record high pressure to Ohio and Pennsylvania (see "'Drunken' Weather Pattern Brings Deadly Heat" by Climate Central).

Anti-cyclonic (clockwise) flow around the high meant that precipitation that affected the Mid-South moved from east to west - exactly opposite of what we usually see, and what our friend @NashSevereWx on Twitter refers to as "Crazy Ivan" storms, referencing sharp turns made by Russian submarines in The Hunt for Red October (listen to the use of the term in this YouTube video if you're interested).

Now that the rare "Red October" pattern has straightened itself out, another large eastern U.S. trough, like the setup we had early in July, puts the Mid-South squarely in a "northwest flow" regime, in which our mid-level wind (at ~20,000') will be blowing from the northwest (see graphic below).  Summertime northwest flow is notorious for producing rounds of showers and thunderstorms, some of which become mesoscale convective systems (MCS's) that bring very heavy rain, thunder, gusty wind, and occasionally severe weather in the form of damaging wind and hail.  In fact, "Hurricane Elvis" (which happened ten years ago tomorrow) was exactly one of these types of events - a northwest flow MCS that became a derecho with extreme wind and sheets of rain.  We're not predicting anything close to that this week, as many factors have to come together perfectly to produce an event of this magnitude, but we'll be on the lookout for anything that might bring the chance of severe weather.

A northwest flow regime will setup for at least the first half of this week, bringing potential rounds of precipitation
Timing of these rounds of storms can be difficult as the upper-level impulses that drive them can be somewhat unpredictable more than a couple of days out, however current thinking is that the Mid-South may be in line for these disturbances on Monday morning, Tuesday morning, and Wednesday afternoon.  Obviously this is all subject to change, and no two disturbances, nor their effects, are alike. For now, we expect heightened chances of showers and thunderstorms as they approach the area. Effects of any morning convection will have major implications on the forecast for the remainder of the day - including temperatures and instability that would normally fire off afternoon summertime storms. Suffice it to say, the forecast from one day to the next could change fairly drastically once we know more about the timing and effects of these northwest flow disturbances!  For now, we expect an unsettled week with everyone having a decent chance of a couple of inches of rain by Thursday, locally higher.

The GFS model forecast of total precipitation through Thursday AM. Memphis (black dot w/ "M") would receive ~2" of rain from this modeled scenario.
The Weather Service forecast of total precipitation through Friday AM. Memphis (black dot w/ "M") would receive just under 2" of rain in this scenario.
We'll always have the latest on our social media channels and will update this blog as conditions warrant.  Download our mobile app for current loops of StormView Radar, our latest forecast, Twitter feed, and of course StormWatch+ severe weather alerts. Links to the app and social media feeds are listed below.

 Erik Proseus
MWN Meteorologist

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