Friday, October 12, 2012

Severe Weather Outbreak Likely Saturday


Water vapor imagery indicates the disturbance of interest is in the southern Rockies, where elevated water vapor values are present. Thunderstorms look to be attempting to fire in portions of New Mexico and Arizona, indicating that the threat for thunderstorms in the Plains and Midwest tomorrow remains.

It should be noted that severe weather was observed upon the system's landfall last night in south California. A rule of thumb is that the stronger weather the system brings once it is onshore, the worse weather can be expected in areas that will eventually be affected. Applying that rule to our current situation means that the Plains and Midwest will be in for a bumpy Saturday evening into the overnight hours.

Model guidance fairly messy at this hour with the system. Because of limitations of the ECMWF, I will go to my own devices with the NAM and GFS. GFS prefers a more organized squall, while NAM essentially drenches the lower Great Lakes in a few mesoscale squalls over the course of 24-36 hours beginning Saturday afternoon. Upon trying to find the root cause of such model hysterics, initialization proves little- both the GFS and NAM had the system at the same spot as of 12z initialization. Initial strength did differ, but that is not of significance to me.

Bringing the WRF-ARW model into the picture for assistance to model troubles reveals a solution in between both models. The 12z WRF takes a fairly strong squall line through the Midwest and Plains, yet persists with precipitation through the Lower Great Lakes much like the NAM. In this case, the WRF seems like a good compromising point for the NAM/GFS.

Attention turns to severe threat and just what we are looking at. I believe the main threat will definitely revolve around tornadoes and damaging winds, as an enhanced jet stream and lower level winds have potential to mix down to the surface. In the midst of lacking instability (an idea thrown out by the latest NAM model), strong surface winds and enhanced shearing will get any developing cells on their feet with an initial tornado threat before the storms become more linear, favoring a squall line position.

Not concerned about timing issues within the models- storms should initiate during the day in the Plains and western Midwest before entering the interior Midwest and Lower Great Lakes overnight, likely reaching east Michigan towards daybreak.

This event looks to be the last significant one of the season, unless long range ensembles turn out to be correct in a few unusually strong systems coming through.


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