April 27, 2011 Part 2: Forecast Accuracy

As we reflect on such a historic day we should thank the people from the Storm Prediction Center, National Weather Service, and local TV station meteorologists for forecasting this event days ahead.  This event was forecast very well from all these people and myself included.  The Storm Prediction Center highlighted the areas that were heavily impacted by this event 5 days prior to the event.  

The purple area remained highlighted with a Moderate Risk on Day 3 with an exceptionally high percentage of severe weather risk for a Day 3 Moderate Risk outlook.  The 45% purple shaded area is the area where the Moderate Risk was highlighted.

On the day of the event, a High Risk was issued as of 1:00 AM CDT (0600Z) with a high 30% chance of tornadoes over northern Alabama.  This time is close to the first round of storms that hit the area, but these were not the worst.  As the day progressed the atmosphere became highly unstable and favorable for a major outbreak as already highlighted that morning.  At 11:30 AM CDT (1630Z), Storm Prediction Center had a High Risk with a 30% chance of tornadoes and embedded in the High Risk was a small area of 45% chance of tornadoes.  Shortly before Storm Prediction Center updated the risk they issued a Particularly Dangerous Situation (PDS) Tornado Watch for all of Mississippi except for extreme southern counties.  This will be compared to my forecast for 1:00 PM CDT (1800Z) that I made on that day before the event began to unfold and what verified.

At the time I was forecasting the areas that will likely be hit the hardest, Storm Prediction Center issued a PDS for most of Alabama except for southeastern portions and extreme southwestern portions of the state.  What was most shocking for me is that when the Storm Prediction Center issued this I just finished highlighting a High Risk with the likely hood of a PDS Tornado Watch for central and northern Alabama and central Tennessee.  After I have posted my forecast, the Storm Prediction Center updated their severe weather outlook at 3:00 PM CDT (2000Z).  The High Risk with the high probability of tornadoes stayed over the same location.  Following the update, they issued a PDS Tornado Watch for central and eastern Tennessee.  I will compare the Storm Prediction Center's 2000Z severe weather outlook with my 4:00 PM CDT (2100Z) outlook and what verified.  

The last severe weather outlook for April 27, 2011 that Storm Prediction Center issued was at 8:00 PM CDT (0100Z).  The High Risk area has now been shifted over to northeastern Alabama and extreme northwest Georgia with a 30% chance of tornadoes.  My forecast was for 7:00 PM CDT (0000Z) with a High Risk bigger than the Storm Prediction Center had and over the same general areas, but included western portions of South Carolina and North Carolina and a small portion of southeast Tennessee where Chattanooga is.  Here is a comparison between the two with a verification.  

When I highlighted the High Risk area for my forecast I anticipated another PDS Tornado Watch to be issued.  Scary enough, about 6 hours and 10 minutes after I have posted my forecast, the Storm Prediction Center issued a PDS Tornado Watch at 6:50 PM CDT which is just 10 minutes before the time I forecast it would show up.

After the 27th, tornadoes kept on developing in The Carolinas up into the New England area, but were much weaker than what was experienced in Mississippi, Alabama, and Georgia.

It was horrifying to see how much the forecast models agreed that this day, one year ago, all the ingredients were going to come together for a tremendous outbreak.  It was scary to me when forecasting for this event and realizing how bad it will be, but I am proud of how well I was able to forecast for this event.  We should all be proud with such great lead time there was to warn the public because it could have been worse.  God bless all of those who were affected by this event.