Tuesday, March 13, 2012

Harrisburg, IL Federal Disaster Aid Request Turned Down

In a disappointing move, FEMA has rejected a request by the Governor of Illinois to provide aid to Harrisburg, Illinois, which was devastated by tornadoes in the last several weeks. FEMA rejected the request, saying it was not bad enough to constitute aid. Harrisburg was hit by an EF-4 tornado. I am personally very displeased with this decision and wonder how an EF-4 tornado doesn't qualify any town for federal aid.

Negative Tilted Storm May Pose Severe Weather Outbreak Risk Next Week

I am watching the threat for a potential severe weather event next week, mainly from the weekend into about midweek. It will be such a long timeframe because it will take a while for the system to dig south enough to eject into the Southern Plains.

This storm has many characteristics that put me on edge. First, it will be a negatively tilted storm. This means that cold air will be overrunning the top of the system way up in the atmosphere. As we know, instability is created when warm air rises into air cooler than the warm air at the surface is. If the cold air overruns the system, it adds more cool air to the mix, thus increasing instability. This is why negatively tilted storms usually produce the worst severe storms ("usually" meaning they typically have the most potential to).

Secondly, we will have a strong ridge in the East US. This means warm temperatures for the entire area east of the Rockies. However, when one gets to a ridge of this magnitude, it means that there are strong mechanisms both keeping it in place and supporting that strength. This is when theta-e comes into play. In simplest terms, higher theta-e increases instability in the atmosphere. Seeing the strong ridge makes me worry that there will be a strong theta-e, adding to the instability.

Next, we focus on a more important piece- wind shearing. The system in question is forecasted to be digging (strengthening) strongly as it progresses eastward. This is reflected on the image above, with the lines on the images being dragged south and around the mass of colors, which indicates the presence of the storm system. Notice how these lines then shoot vertically north into the high Plains. This is because of the ridge. However, because there is such a tight gradient (difference) between the two items in play (storm system and the ridge) shown by the lines, there will be some shearing.
To me, this is a very good set-up for wind shear. A strong ridge is rubbing against a strong storm system, and this friction is one of the classic textbook scenarios for quality wind shear. Now, I take a look at the GFS forecast for the same timeframe as the image at the top of this post (hour 180), and my fears are confirmed- upwards of 70 knots of deep layer shear (DLS) is possible in the area. DLS is absolutely crucial for tornadoes, and when you get into this high of shearing wind speeds, serious things may happen.
Another way you can tell that there will be high winds are the big wind barbs that have a bunch of lines sticking out of them in the area circled in red. The more lines (or opaque triangles) on a wind barb indicates a higher wind speed. Wind barbs with one small line mean a very low wind speed.

The area I have encircled in red identifies the area I believe is at risk based on the current forecast. Of course things will change (possibly dramatically), but for now, the area circled appears to be the epicenter for the highest instability and wind shearing- the two ingredients necessary for severe thunderstorms and tornadoes.

I have a bad feeling about this after seeing all of these clues put out by the GFS. If it does verify, I am definitely concerned for those that may be affected.