Thursday, July 15, 2010

Predictability

Tough forecast day for SPC in light of the disappearance of the spring forecast experiment model suite. I know I felt the loss of NCARs model when I was making my forecast for Minnesota and Iowa.

The story starts with obscenely high values of dew points across the midwest ... 80+ dew points in Iowa and above 75 just about everywhere else. A potent little low was forecast to move through the area around the Twin Cities with a moderate risk of tornadoes given the extreme instability and large vertical shear.

What was more obvious today was that the hodograph curvature would be decreasing through the day as the cold front advanced eastward. A remnant MCS was forecast to depart the area and another was forecast to erupt in its wake. The NSSL model had a hard time with that component of the forecast, instead developing the convection along the cold front and slightly ahead.

The hard part was utilizing this model run to determine what role the remnant MCS would play in terms of any leftover boundaries, the role of the associated cloud field to the west, and the cold front even further to the west. As it turned out, the departing MCS gave way to another MCS accompanied by damaging winds and tornadoes. The cold front lit up in sections appearing to be linear but isolated enough to warrant tornado warnings before collapsing. About 5 hours later as the front passed the Twin Cities it lit up again.

All in all it was a hard forecast because the model simulation was not quite capturing the episodic nature of the convection. This is one forecast where the model was working against the forecasters despite the somewhat strong forcing for ascent. The details were absolutely lacking in the model forecast and this case will be a prime candidate for further research into why this happened.

This radar image sums it up nicely with the western most convection marking the cold front and accompanied by TOR warnings, Then the cloud field marked by the 25-30 dBz echo, then the isolated convection which exploded into the MCS over the Twin Cities, and the remnant MCS is off to the east.