May 20, 2013

May 20, 2013: S-Central Oklahoma

More Tornadic Supercells Today in Oklahoma:  Preliminary Target for Storm Intiation---Lawton, OK

(More complete discussion with illustrations later.  Also, still have not had time to fill in log with pictures.   Looks like we may have a down day soon, on which I can catch up).

We overnighted in Ponca City, Oklahoma.   Just returned from a nice run around Lake Ponca.

The morning weather map shows a surface low in sw Oklahoma, with a dry line extending southwestward into north Texas.  A cold front extends from that dry line, with the intersection near Altus OK, back northwestward.

A warm front extends from that intersection across Oklahoma to se KS.  That three way intersection is often referred to as a "triple point" by severe weather meteorologists.

Ahead of the dry line, an air mass rich in moisture is found to the warm front.  That air mass is projected to have surface based CAPE values (a measure of how warm ascending air parcels will be relative to their surroundings through the deep troposphere) of 4000-5000 J/kg (see graphic to right).   These really are values at the upper end of the sort we usually see with severe weather, and indicate (Metr 201 and 415/715 students will know this) updrafts that should support extremely large hail.  On the map above, the light green represents areas with 60F+ dew point, and the darker green 70+.

Once again a very strong jet stream  (see graphic at upper right) will produce wind shear values favorable for supercell convection and, in parts of Oklahoma, low level wind shear favorable for these supercells to develop tornadoes.

The issues center on two aspects of  the forecast storm motions.  First, the storms will be moving at an angle that's too close to the dry line, instead of at right angles.   This could mean that each storm will be running over a previous storm's cold pool....and we can end up with a big line of storms, as we did yesterday in northern Oklahoma.  If that happens, the time window for tornado formation will be small...before each storm runs into that cold pool.

Second, unless the surface winds cooperate, storm motions could be 40-50 mph, as they were yesterday, making for exceedingly difficult's almost impossible to set in one location and take pictures, for example, when the storm rushes by like that.  Fortunately, the surface winds are forecast to be more southerly or south-southeasterly, rather than hard southeasterly.  In that case, we're likely to have storm motions of around 30 mph.

In any case, we are off to our target area, where the visible image above shows a low level cumulus field already bubbling.