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Tuesday, February 28, 2012

Afternoon Prognosis

Hodograph: A device measuring distance from its starting point as the device ascends through the atmosphere on a weather balloon. Circular shapes can indicate rotation in the atmosphere.
EHI: A combination of spinning and helicity to make for, in my eyes, the best attempt to forecast where apt tornado conditions may be.
CAPE: Instability. Values above 1000 j/kg can be conductive for thunderstorms.

SPC has lowered tornado threat and eliminated the 10% tornado risk. At the same time, the 5% tornado risk area has expanded north.
The main issue is that this tornado threat, in my eyes, may still be needed.

Latest 18z NAM is indicating high values of EHI (spinning+instability equation that can help determine a tornadic atmosphere) present on the Oklahoma Arkansas border. At that same time, storms should be just beginning to enter that area, with the 18z NAM projecting these storms to be fairly weak. However, this weakness is likely understated- At the point in time in question, the squall line will be not be a line yet, but it will be a weakly put together line of separate storm cells. Judging by forecast high EHI values and CAPE above 1000 j/kg, this should be an interesting night.

Here's a comparison for forecast hodographs of the 18z NAM (left) and the 12z GFS (right). While the models are on different runs, I have affixed them to be at the same forecast time.
The 18z NAM is honestly pretty concerning, with two roughly circular shapes present in the forecast. To start off with the whole hodograph is concerning, as the hodograph shows a half circle shape before suddenly turning into two tight bands of circles.  The sudden changeover to the circles makes me think that there may be too strong winds in the atmosphere to create a robust tornado, but that is dashed after the hodograph reveals the circular formations, both of which are likely indicative of a rotational motion in the atmosphere.
The 12z GFS is much more modest, having the hodograph take a similar half-circle-sudden-change path, but where the NAM shows the circles, the GFS shows it's attempt to make a circle, but believes that winds will be too tight for rotation to be shown. After that, the GFS hodograph shows it going off the screen but likely making a near full circle shape. Again, that is one of the more concerning features of this hodograph.

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