Ag Weather Forum

Drier Hint in Late-September Forecast

Bryce Anderson
By  Bryce Anderson , DTN Senior Ag Meteorologist
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Rainfall totals, on average, are forecast to be notably lighter during the late-September to mid-October time frame. (NOAA graphic by Scott R Kemper)

During July, the U.S. six-week forecast, a creation of NOAA's Combined Forecast System (CFS) model, called for heavy rain in the first part of September for the northern Midwest: Minnesota, northern Iowa and Wisconsin. That model prediction verified well, as many folks in the upper Midwest will attest.

Now, that same forecast model looks ahead to the four-week stretch from Sept. 23 through Oct.21. While it's not entirely dry, there is less rain on average indicated for the central U.S. than we have recently experienced. On average, rainfall totals are pegged in the 2.5- to 4-inch bracket for the western and northern Midwest through the northeastern U.S. along with eastern Canada. (Central Ontario through Quebec, along with northern New England, has 4- to 6-inch totals projected.)

The question, of course, is whether this forecast will verify. A couple factors come into play regarding that question: 1) the CFS model has a decent track record for its performance this season; and 2) tropical storm systems in the Gulf of Mexico and the Atlantic Basin seem to not be very active. We've seen this summer and early fall how tropical systems can ramp up the rain in a big hurry. The heavy rain and flooding in Louisiana during August was due to a stalled, nameless tropical low just off the coast. The heavy rain during the post-Labor Day week also had a tropical component in the form of Hurricane Newton, which formed off the Mexico Pacific coast and tracked northward into Arizona and New Mexico, adding a ripe moisture component from the Pacific to central U.S. conditions.

On the side of questioning the model's depiction, there are several reasons to bring out. First is that the precipitation pattern simply has not been very dry in the Plains and Midwest. One of the principal axioms in forecasting is to respect persistence -- and, following that tenet, one would go with a wetter rather than a drier scenario. A second feature is that the northeastern Pacific continues to offer bursts of upper-air energy in a general westerly jet stream flow across the central U.S., and that can induce storm formation. And finally, strong, hot high pressure in the southeastern U.S. continues to form a strong air mass boundary layer, to provide the lifting mechanism for storm development, and circulation around the southeast high lets Gulf of Mexico moisture drift northward into the central U.S. to become part of the rainfall profile.

How the next six weeks evolve will indeed be interesting -- and, obviously, very important -- in terms of harvest impact, both for grain quality and field conditions. Mud is no one's ally at this time of year.

Bryce Anderson can be reached at Bryce.anderson@dtn.com

Follow him on Twitter @BAndersonDTN

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