Sunday, March 18, 2012

Topography influences the weather - but in the Mid-South?

It is fairly well known that topography, such as mountain ranges, has significant effect on weather patterns.  For example, air flowing up the Front Range of the Rocky Mountains lifts and condenses and can bring feet of snow to places like Denver, CO.  It's what is called an "orographic influence" on the weather.  Closer to home, it's also widely-known that the Ozark Mountains, though not to the scale of the Rockies, also can have an effect on the local and downwind weather.  But did you know that very small topographical features can also have an effect on the weather? I'm not talking about the bluffs protecting Memphis from tornadoes - that is a weather myth that never seems to die. However, there is a topographical feature in the Mid-South that had an effect on the weather just this afternoon!

Crowley's Ridge, an elevated area  (like a series of narrow hills really) that runs north-south through eastern Arkansas, is about 300-500' higher than the surrounding Mississippi River Delta, or Embayment (see image to right).  It's clearly visible on topographic maps, sticking up like a crooked spine in the middle of east AR. On days with warm, humid air in place and general instability (i.e., rising air) in the atmosphere (like many Mid-South summer days), sometimes all it takes is a little extra "oomph" to create showers and thunderstorms, absent a frontal system or upper-level impulse. Thanks to the early summerlike weather we have been experiencing, today was one such day!

Take a look at the radar image of east AR early this afternoon, shown below.  The topography is underlaid on the radar data and Crowley's Ridge is indicated by the dotted orange line. What do you know? Showers formed along Crowley's Ridge! If you were to look at a loop of the radar data, you would see them form over the crest of the ridge, then move north-northeast in the direction of the winds at 10,000 feet, which is why it rained downwind of the ridge!  That extra 400' or so of "lift" created by the ridge induced small showers to form! In fact, later this afternoon, cumulus clouds had formed once again right down the spine of the ridge (see second image below).

Showers formed over Crowley's Ridge, then moved NNE - the direction the wind was blowing at about 10,000'

Cumulus clouds, caused by warm temperatures, sufficient moisture, and a little "lift" from Crowley's Ridge, are visible in this satellite image over east AR.
Bottom line: it doesn't take a mountain to have an orographic influence on the weather, and it can happen in our own backyard!

Have you experienced weather phenomena that might have been influenced by the lay of the land below?

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6 comments:

Emily D. Aston said...

I grew up in Obion co, tn. We always believed the Miss River affected our weather b/c it seemed strong storms from mo would die down just a little when they crossed the river, then reform and produce severe storms/tornados by the time they got to weakley co...could this be true?

Vonavie said...

I live on the ridge between Paragould and Jonesboro. We feel like we live at a point that weather tends to split and go north and/or south of us. This is handy for severe weather but disappointing when it's snow time.

MWN/Erik said...

Emily - That is a perception that people from many places hold. There are many in Memphis that think that your area gets all the storms! It's all really a matter of perception, as one tends to focus on their little part of the world versus the big picture. This "myth," I'll call it, is somewhat reinforced by the fact that there do tend to be more overnight storms (especially in the summer with the right pattern) that form and move out of the Ozarks towards this region and then weaken as they arrive in the early morning hours in the Mississippi River Valley. However, on the grand scale, you're no more or less susceptible to severe weather than anyone else in the immediate area, especially 1-2 counties away. Thanks for your comment!

MWN/Erik said...

Vonavie - See my comment to Emily just posted. Pretty much the same applies to you. Most of the time it is a perception issue. In fact, I would argue from a "big picture" that northeast AR tends to get worse weather as a region than areas closer to Memphis. On average, you've got it pretty good up there for severe storms and for snow! Thanks for the comment

Beau Dodson said...

Great article and observations on the ridge. There is little doubt that terrain does impact local weather - the question is how much does it influence the weather. Great observations - I sent your article to the PAH Office

MWN/Erik said...

Beau - Thanks for reading, commenting, and passing it on! I checked out your site, blog, and social media feeds. Fantastic job! Very detailed on all of it. Good to "meet you"!
--Erik