Data Disparity

The Quad Cities branch of the National Weather Service has gone ahead and issued a FLASH FLOOD WATCH for its Illinois counties through Friday. The other offices that cover our area (Milwaukee/Chicago) haven’t jumped on the bandwagon just yet. The amount of rain we receive is still very much up in the air. From the data I have scoured through, some parts of our viewing area could receive less than 0.25″, while others could see a couple of inches of liquid.

I have posted the 6 hour precipitation totals for Friday morning as expected by the NAM and GFS models. Typically when we are analyzing rainfall amounts within the first 48 hours of our outlook, the models tend to look alike. This helps to build confidence in a solution. As you can see, these two models disagree completely upon what is going to happen in the next 24 hours. The NAM model on the left is cranking out an MCS with heavy rain tomorrow morning, while the GFS model is posting merely isolated storms.

Another reason for a lack of timing on this activity is that the models are initializing very poorly. This means that they are not gaging correctly where the current storms are. For example, right now we’ve got thunderstorms in central Iowa. Unfortunately, one model missed them completely and one had them up on the Minnesota/Iowa border. If they can’t get the first 6 hours of the forecast correct, you have to take the rest of the data with a grain of salt.

Here’s my take on the rest of this Thursday: the activity out in central Iowa should reach the Stateline late this afternoon and into the evening hours tonight. I don’t expect much in the way of severe weather, but we could get a few heavy downpours. -ADAM

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This post was written by qni_it on June 26, 2008

2 Comments so far

  1. Justin June 26, 2008 8:26 PM

    Do you still think we’re going to see rain this evening or will it hold off until tonight?

  2. Renee June 26, 2008 9:15 PM

    Why would a model do such a bad job of forecasting just 6 hours ahead? What factors are causing it to be so difficult to forecast this particular set of storms?

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