The NWS Climate Prediction Center released their latest outlook for October and beyond. It looks like the warm-than-average weather is expected to continue for the next several months. The primary driver in the forecast is the ongoing El Niño in the Pacific Ocean basin.
Illinois has an increased chance of above-average temperatures for October. There is not much to report on precipitation in Illinois. We are between drier-than-average conditions to our northeast and wetter-than-average conditions to our southwest.
The first half of September in Illinois was warmer than average, about 3.5 degrees above average.
Precipitation for the first half of September is mixed. A few areas east of St. Louis and along the eastern border with Indiana have seen 3 to 4 inches, which is well above average. Much of the rest of the state has seen a respectable 1 to 3 inches. Western Illinois has been drier.
A number of Midwestern regional climate service partners, including state climatologists, regional climate centers, and NOAA offices, put together a two-page fact sheet on El Niño status and impacts in the Midwest.
Cooler than average daytime highs (first map, blue shaded areas) and warmer than average night-time lows (second map) this summer were common across much of the Midwest, according to data from the National Centers for Environmental Information.
It is typical in Illinois for this to happen in summer after a wet spring. We marched through summer with above-average soil moisture, streamflows, and lake levels. As a result, more of the sun’s energy went into evaporating this water instead of heating up the land surface and the atmosphere. The end results were higher humidity levels and lower high temperatures.
At night, the higher humidity levels kept temperatures from dropping as much. The old forecasting rule of thumb was to consider the dew-point temperature as the floor to night-time temperatures. Therefore, the higher dew-point temperatures led to higher night-time temperatures.*
Finally, all the extra humidity in the atmosphere turned into more rainfall for thunderstorms, maintaining the wetter conditions through at least July.
There is a 95% chance that El Niño will continue through winter and gradually weaken through spring 2016, according to the NOAA Climate Prediction Center today. That remains essentially unchanged from last month’s outlook. As noted in a earlier post, this increases the chances of a mild winter in Illinois.
Here are the recent sea surface temperatures, as a departure from the average, in the Pacific Ocean basin. The units are in degrees Celsius, so just double the number to get the approximate departure in degrees Fahrenheit. This represents an impressive amount of heat, considering that these warmer water temperatures cover thousands of square miles.
The traditional area for El Niño formation is circled and labeled on the map. The one wild card in the current situation is that we have exceptionally warm waters farther to the north than what I have seen in past El Niño episodes (green circle). This could potentially change the impacts of El Niño over North America.
Illinois and the rest of the Midwest are off to a hot start in September. As the map below shows, areas in shades of brown were up to 9 degrees above average. This includes southern Illinois and much of the southern states in the Midwest. The areas in pinks and red were even farther above average, some 9 to 12 degrees above average. This includes most of the northern half of Illinois, as well as northern Missouri, Iowa, Minnesota, Wisconsin, and most of Michigan.
Cooler and more seasonable weather is expected in Illinois after today.