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Tropical Depression Imelda, currently spinning out over southeast Texas near Houston, has already become the 5th wettest U.S. tropical cyclone on record. As of Thursday evening, it had dropped 41.81 inches of rain in an area near Beaumont, TX.
And now some of this moisture is headed our way.
Over the next two days as Imelda dissipates, the remaining moisture in the atmosphere will work its way northward from Texas into our area. This will help fuel on and off showers through Friday. By Saturday, the amount of water held in the atmosphere above you at any point (what meteorologists refer to as precipitable water or PWAT) will be at or greater than two inches. This is a lot - especially compared to the usual average PWAT value of 0.76 inches for the area on September 21st.
Also moving into the picture on Saturday will be a strong cold front. A cold front this strong would normally produce storms on its own. However, with a boatload of extra moisture from Imelda on Saturday, the cold front will have some jet fuel likely fire off some truly strong storms.
There exists some uncertainty on where and when exactly these storms will take place. As of now, any severe storms look to be an afternoon event. As for rainfall, the Weather Prediction Center has placed all of Iowa in an area at marginal risk for excessive rainfall on Saturday, with some parts further south at a much greater risk. Right now for our area, counties in northeast Iowa are at the greatest risk for excessive rain. However, folks everywhere should be watching given how moist the soil is from the past two weeks of storms.
While the Midwest does not receive tropical systems such as hurricanes and tropical storms, these systems do occasionally effect our weather in sometimes drastic ways. In a similar setup in 2007, moisture from Tropical Storm Erin moved into our area when an active weather pattern was already in place. The moisture boosted the system, and 10-20 inches of rain ended up falling in SE Minnesota within the two days of August 18-20 - breaking the all time precipitation record for the state of Minnesota. Saturday's event will not be nearly as drastic.
We are tracking this system. The Storm Prediction Center as of Thursday has most of Iowa and Minnesota under an area at a marginal risk of severe weather.
* A tropical depression is a tropical cyclone with winds less than 39mph. Above this, it is considered a tropical storm. Once a tropical storm attains winds above 74mph, it becomes a hurricane. All three storms can drop torrential rain.