The Effects of Isaac on the Drought

In my last column, I mentioned the possibility of tropical weather systems affecting Missouri’s weather, and bringing some relief to the drought. That happened, of course, when Isaac came through and dumped a good amount of rain over many parts of Missouri. A few places in Missouri did not get any rains from Isaac, but most of Missouri saw excellent rains, usually an inch or more. Some areas received over six inches.

But was it enough to end the drought? The Drought Monitor is a good place to find an answer. The Drought Monitor gives drought ratings on a scale from D0 (abnormally dry) to D4 (exceptional drought). Before Isaac, all of Missouri was in drought, ranging from D2 to D4. Over 35% of Missouri was in the worst stage, D4, and 97% of the state was in D3 or greater.  Less than 3% of our state was in the D2 stage.

After Isaac, we have seen some real improvements.  But to answer the question, yes, we are still in drought.  The D4 area has been reduced to 3% of the state. Some areas remain at D3 levels (parts of northwest and central Missouri), and most of the rest of our state is rated at D2 levels.  There is even an area in northeast Missouri rated at D1.  But officially, the entire state is still under drought.

There are also a few other ways to look at drought. The U.S. Seasonal Drought Outlook from NOAA’s Climate Prediction Center for the period September 6 to November 30 shows “Improvement” for most of Missouri.  I sure hope they are right.

Another interesting guide is the Crop Moisture Index.  This map looks at short-term need vs. available water in a shallow soil profile. While this is not intended to assess long-term droughts, it does give interesting information for the short term. The Index released on September 1, actually shows northwest Missouri in a neutral range, while most of the rest of Missouri (except the Bootheel) is in the -1.0 to -1.9 inch range.

Perhaps a better assessment of the long-term severity of the drought is the Palmer Drought Severity Index. An interesting graphic, which shows the amount of additional precipitation needed to bring the Palmer Index to -0.5 inches, is available on the web.  The map released on September 1, shows northwest Missouri as needing 6 to 9 inches of precipitation to bring us closer to normal. Southern Missouri ranges from 9 to 15 )inches of needed precipitation.

So yes, we are still under drought.  Isaac helped, but we still need rain. Precipitation deficits are a good way to look at our needs, and I will take a look at how far we are behind for precipitation in northwest Missouri in a future column.
(By Tim Baker, Northwest Region Horticulture Specialist, University of Missouri Extension)