Meteorological Autumn has begun, with the autumnal equinox following later this month. The National Weather Service and Climate Prediction Center have released their fall advisory data, which has provided Indiana some guidance on what to expect.
Nationally, the country is preparing for the probable onset of La Nina. The El Nino Southern Oscillation (ENSO) is showing signs that the average temperature in a portion of the equatorial Pacific Ocean will be more than half a degree Celsius below average. This relative chill near the equator affects weather patterns in the Pacific Northwest and Southeastern US in predictable ways. In particular, those two regions become wetter than normal with a corresponding dry pattern in the Southwestern US.
In Indiana, a La Nina event in the autumn does not suggest any particular precipitation regime.
“The challenge is Indiana’s location, which is well inland from the ocean coastlines that tend to see bigger impacts from La Nina events,” according to Beth Hall, Indiana State Climatologist.
Where La Nina events do creep into Indiana predictions will come in winter when the jet stream shifts and the entire state trends a little warmer and wetter. There are two climate regimes in Indiana, however, the snowy north and the temperate south. Warmer, wetter temperatures in the north may mean more snow for the Michiana area. Warm and wet for the south means less chance of snow and more rain. However, the likelihood of La Nina conditions persisting through the winter at this time is 55 percent, according to the Climate Prediction Center.
Hall adds that this uncertainty from rather weak ENSO phase outlooks makes seasonal predictions difficult this far in advance. Even the dates of the first hard freeze event show no significant preference during La Nina, Neutral, or El Nino years.
If La Nina does not form and the autumn and winter turn to ENSO-neutral conditions, the cycling of the other climatic oscillations will have more influence over local weather. The Arctic Oscillation, North Atlantic Oscillation, and Pacific-North American Pattern can actually interfere with the ENSO trends expected when those oscillations are strong enough. None of those oscillations tend to have the staying power of ENSO, however, as they tend to look at the time scale of weeks rather than seasons.