Monday, May 23, 2011

After a terrible tornado in Joplin, another risk tomorrow...

As most people are well aware by now, a horribly strong tornado ripped through the heart of Joplin, Missouri on Sunday evening.  With 116 people confirmed as killed so far, this is the deadliest tornado to strike the US since the 1940s.  The photos and videos of the aftermath of this storm are everywhere.

I had a very slight acquaintance with Joplin.  Every time I would drive between my home in northern Illinois and the University of Oklahoma in Norman, I would stop in Joplin, often for lunch and always to get gas.  The gas station at which I always stopped no longer exists.  Their delightful main street with its Route 66 nostalgic decorations is decimated.  I just can't believe the scenes I'm seeing.

And, unfortunately the weather is gearing up for yet another round of severe storms on the southern plains.  The SPC already has a moderate risk out for Tuesday.
Fig 1 -- SPC Day 2 convective outlook as of 1730Z, April 23, 2011.  From the SPC.
Joplin is on the edge of the moderate risk area, but most of Oklahoma and eastern Kansas fill out that risk area.  The SPC is calling tomorrow a "classic" severe weather outbreak scenario.  Since I don't have much time tonight, I'm just going to show the NAM model graphics for 00Z, Wednesday (Tuesday night).

First, the 500mb forecast shows a compact shortwave moving out into the central plains with a small, but still powerful jet streak on the southern side of the trough:
Fig 2 -- NAM 30-hour forecast of 500mb height (contours) and winds (colors) at 00Z, Wednesday (Tuesday night), April 25, 2011. From the HOOT website.
Interestingly enough, it's these very compact shortwaves that can often pack the biggest punch.  We can imagine with this scenario that a surface low with trailing cold front would be located over northern Oklahoma.  Dropping down to 850mb, we can see the beginnings of the nocturnal increase of the low-level jet:
Fig 3 -- NAM 30-hour forecast of 850mb height (contours) and winds (colors) at 00Z, Wednesday (Tuesday night), April 25, 2011. From the HOOT website.
Winds in the 850mb jet are out of the south over Oklahoma, whereas winds at 500mb are out of the west--a fair amount of directional wind shear, favoring strong supercells.  A quick glance at surface moisture (in terms of the dewpoint) shows that there is indeed a lot of energy to work with at the surface:
Fig 4 -- NAM 30-hour forecast of dewpoint temperatures (colors) and winds (barbs) at 00Z, Wednesday (Tuesday night), April 25, 2011. From the HOOT website.
Those are 70 degree dewpoints being forecast for eastern Oklahoma, southeastern Kansas and into Arkansas.  A sharp dryline is visible across western Oklahoma and down into Texas.  With rich moisture and surface convergence along these boundaries, storms are bound to fire.

For another look at all of these ingredients being condensed together in the models, here's a forecast sounding for 21Z on Tuesday afternoon at Norman, OK:
Fig 5 -- NAM 27-hour forecast sounding for Norman, OK, at 21Z, Tuesday, April 24, 2011.  From the HOOT website.
Very steep lapse rates in a virtually uncapped profile. The dewpoint temperature is up in the 70s, but not nearly as high as the forecast surface temperature of around 90.  This implies some lift is going to be needed to get the surface air up to its level of free convection to tap into the convective available potential energy.   But, in the larger-scale environment we saw with divergence aloft and convergence near boundaries at the surface, so this should not be a problem.  Speaking of convective available potential energy, this forecast sounding is suggesting a surface-based CAPE value of 4567 J/kg.  Lots of potential instability.  Notice in the winds that there's also a fair amount of directional shear (like we saw in the 500mb and 850mb maps).  Winds at the surface are out of the south, and they rotate around, becoming more westerly by around 400mb.  The speed shear isn't as strong as I've seen recently, but remember this is at 21Z, which is the middle of the afternoon.  The speed of the low-level winds really picks up right around and after sunset.  That will increase the low-level wind shear and make any storms that exist around that time particularly dangerous.

Even some medium-scale models are explicitly forecasting some major storms.  Here's the OWL/WRF model at the University of Oklahoma and its forecast for simulated composite radar reflectivity at 00Z on Tuesday evening:
Fig 6 -- 48-hour OWL/WRF forecast of simulated composite reflectivity valid 00Z, Wednesday (Tuesday evening),  May 25, 2011.  From the HOOT website.
This is showing storms at around 7PM CDT (just before sunset) in northern Kansas and also in northern Oklahoma.  I should note, this is a 48-hour simulation--this does NOT mean that there will be storms in these places.  What these kinds of forecasts can hint at, though, is the timing of when storms will start going up and also what kind of mode they will be in.  Individual cells?  A line of storms along the front?  Here we still are seeing individual cells.

We can also simulate an infrared satellite image from the model output.  Here's the simulated infrared satellite image from the same model at the same time:
Fig 9 -- 48-hour OWL/WRF forecast of simulated infrared temperature at top of atmosphere, valid 00Z, Wednesday (Tuesday evening),  May 25, 2011.  From the HOOT website.
I think this sort of image does a better job at showing those modes of convection as opposed to the simulated composite reflectivity.  Here you can see some explosions of clouds that are developing storms down into southern Oklahoma and the Dallas/Fort Worth area.  This shows developing storms that haven't shown up on the composite reflectivity yet. So...even at 48 hours out models are hinting that we're could see discrete storms on Tuesday.  Discrete storms in a high shear environment with surface dewpoints in the 70s--a recipe for supercells, large hail and tornadoes.  Unfortunately, Joplin is not out of the woods yet.

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