Wednesday, May 11, 2011

Poor Convective Timing and Cold Pools

Many people I know were discussing the moderate risk of severe weather that the Storm Prediction Center had issued last night for today in western Oklahoma:
Fig 1 -- SPC Day 1 convective outlook as of 12Z, May 11, 2011. From the SPC.
However, by mid-morning, that moderate risk had gone away (and the slight risk had expanded considerably further north)...
Fig 2 -- SPC Day 1 convective outlook as of 1630Z, May 11, 2011.  From the SPC.
So what happened?  Why did the severe potential decrease over that area from what we were expecting?

The answer lies in the timing of convection with this surface low--and what convection in the early morning does for the next day.

Take a look at Norman, Oklahoma's sounding from 12Z this morning:
Fig 3 -- KOUN sounding from 12Z, May 11, 2011. From the SPC.
This is a pretty good sounding in terms of its thermodynamics if we're looking for severe weather.  There's high CAPE values and a capping inversion is present (the winds are another story, however).  Remember that the capping inversion is what keeps a "lid" on the convection during the day.  We see that capping inversion as the abrupt warming with height shown in the red temperature line right at 850mb in the above diagram.  Now, often we talk about the capping inversion being "bad" for convective development because it prevents storms from forming.  However, to get severe storms, particularly supercells, you want there to be some sort of capping inversion in place.  This allows two things:
  1. By having a capping inversion, it ensures that storms can't just fire off everywhere--only in unique places where there is enough lift to overcome the capping inversion can storms form.
  2. Having the capping inversion in place allows the surface to heat up (and possible more moisture to advect in) throughout the day, creating an even more unstable environment so that if that instability is released in the late afternoon, the convective development is particularly explosive.  With no capping inversion in place, as soon as the surface started heating at the beginning of the day things would quickly become unstable and storms would fire off without taking advanatage of a full-day's worth of heating.
So, for severe, supercellular storms, we do want there to be a capping inversion, particularly in the morning to mid-afternoon hours.

Also, in the above sounding, notice that I drew a yellow circle around the layer right at and above the capping inversion.  Notice how dry the air is right here--the dewpoint temperature (shown by the green line) is much less than the actual air temperature, indicating dry air here.  Now, this can be a good thing for severe thunderstorm formation as dry air aloft tends to contribute to stronger downdrafts and more damaging winds at the surface (but that's for another blog post).  But, for now, just keep this feature in mind.

This morning, as the sun came up, the visible satellite image showed this:
Fig 4 -- GOES-E visible satellite image from 1316Z, May 11, 2011.  From the HOOT website.
You can see the nice, comma-shaped curl of the low-pressure cyclone over eastern Colorado.  But notice what we have in western Kansas and western Oklahoma--lots of clouds!  It turns out that there was enough lift and enough low-level moisture early this morning to cause showers to form over western Oklahoma and western Kansas.  So what does this mean?

Mid-level winds this morning were generally out of the southwest, as shown in this GFS analysis from 12Z this morning:
Fig 5 -- GFS analysis of 700mb winds (colors) and height (contours) for 12Z, May 11, 2011.
 So what would these winds do?  They brought the clouds (and their moisture) northeastward and into the moderate-risk corridor that the SPC had outlined.  How significant was this moisture?  Here's the special sounding launced at Norman five hours later at 17Z:
Fig 6 -- KOUN sounding from 17Z, May 11, 2011.  From the SPC.
Notice how much the dewpoint has increased in the area I had circled-- the air is no longer very dry there--it's nearly saturated (i.e., the air temperature and the dewpoint temperature are about the same).  So what happened when those clouds moved in?  Remember that at 12Z this layer of air was rather dry over Norman.  So, as clouds and their liquid water droplets moved in, the liquid water in those clouds started evaporating into the much drier air.  In one of my previous blog posts, I talked about how condensation (going from water vapor to liquid water) releases a lot of energy, which in turn warms the air.  When the opposite happens--when liquid water evaporates into water vapor--it consumes a lot of energy, which in turn cools the air.  This is a phenomenon called evaporative cooling.

You can see in the sounding above that that's exactly what happened--not only has that circled layer become moister, but the temperatures have cooled down considerably.  In fact, they've cooled so much that the capping inversion (which was a layer where temperatures warmed considerably with height) is no longer visible--the cooling due to the cloud water has removed the cap!

Now remember what I said before--to get supercellular types of storms, you want the capping inversion to linger throughout the day, at least into the middle of the afternoon, to allow instability to build and to prevent storms from firing up everywhere.  But now it's late morning and the cap seems to have completely disappeared over Norman due to this cloud intrusion.  The result?  Here's the radar two hours later at 1938Z:
Fig 7 -- NEXRAD base reflectivity radar mosaic for 1938Z, May 11, 2011.  From the NWS.
Storms basically just fired up everywhere.  As a result, though there was and still is a risk of severe weather, the potential for isolated supercells and strong tornadoes has gone away.  The cap just eroded too early.  The wind shear also wasn't that great yet anyhow--we needed a strengthening of the winds aloft forecast for later in the day to get the kind of shear necessary to support strong rotation.  So, things were just timed poorly.

One interesting facet of these storms is how they have fired and propagated ahead of the convergence along the main dryline/front.  Here's the surface analysis from the SPC around the time of the radar image above:

Fig 8 -- SPC surface analysis of dewpoint temperature (color shadings), temperature (red contours), mean sea-level pressure (black contours) and wind (barbs) for 19Z, May 11, 2011.
Some features stand out.  Note the surface low (pressure is in the black contours) is analyzed in extreme southeastern Colorado near the Oklahoma panhandle.  The dryline is clearly visible as the strong gradient of moisture extending along a north-south line across far western Oklahoma.  It's the area where the dewpoint temperature (the colored shadings) drop from greens down through blues to nothing--indicating dewpoints to the west of the dryline are below 56 degrees Fahrehnheit while to the east they are in the upper 60s.  Also notice in the wind barbs how there is convergence along the dryline--winds to the west of the dryline are out of the west whereas winds to the east of the dryline are out of the south-southeast.

However, the storms at that time were more in west-central Oklahoma--slightly ahead of the dryline.  Look at the the temperature contours (the red lines) in western Oklahoma.  See how underneath the storms it's much colder than it is elsewhere?  In fact, while temperatures are in the mid-80s in eastern Oklahoma, underneath the storms it gets down into the mid-60s.  This is a result of all that cold, downdraft air falling with the rain underneath the storms.  This zone of colder temperatures underneath the storms is often referred to as the "cold pool". 

Here's a map from the Oklahoma Mesonet from one hour later.  The storms have moved further east, and you can see that there's a wide swath of colder temperatures underneath them:
Fig 9 -- Air temperature at 2m from the Oklahoma Mesonet at 5:25PM CDT, May 11, 2011.  From the Oklahoma Mesonet.
Notice that both in front of and behind the storms the temperatures are in the 70s, whereas underneath the storms, the temperatures drop to the low 60s.  This is a very well-defined cold pool.

One interesting thing about cold pools is that they can provide a lifting mechanism for storms to propagate into areas even when there is a capping inversion present.  Think about the leading edge of the storms and their cold pool--it's like a miniature cold front.  Downdraft air from the thunderstorms can blast out in front of the storms and provide convergence and lift as it runs along.  This allows storms to keep going as they track along with the leading edge of their cold pool. So, if a bunch of storms can establish a decent cold pool, they don't need to have a front or other convergence around to provide lift--the leading edge of their cold pool can provide them with their own lift.  It's a lot like the squall-line and bow-echo dynamics I discussed in a previous blog post--convergence along the leading edge of their cold pools are what keep them going.

So...even though the moderate risk got cancelled, there still have been a lot of severe storms today, including in the convergent zone along a warm frontal boundary in the upper midwest.  This slow-moving cyclone will continue to track across the country over the next day or two, bringing even more chances of severe weather.

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