The NWS released their latest forecast for December and it is calling for very strong odds of a warmer-than-average December for Illinois and a large portion of the United States. They put the chances at 70 percent or higher in northern Illinois and the Great Lakes region. The odds are between 60 and 70 percent for the rest of Illinois. Those are about the strongest odds that I have ever seen in a monthly forecast.
This forecast for December is strongly supported by the various forecasts out to 14 days, which are also calling for very strong chances of above-average temperatures.
The precipitation forecast is not as striking – increased odds of wetter-than-average conditions in the south and along the East Coast. That’s a classic El Niño precipitation pattern in winter months for those regions.
The winter storm over the weekend left a narrow swath of significant snowfall across the central Midwest. The heaviest amounts of 5 or more inches (blue) extended from the High Plains through Iowa, northern Illinois, southern Wisconsin, northern Indiana, and Michigan.
Here is a close-up of Illinois. The largest amounts are along the Illinois-Wisconsin border. Amounts of 5 inches or more (blue) were common across the northern third of the state. The largest snowfall totals in Illinois was 17.1 inches at Roscoe (Winnebago Co.) followed closely by 16.9 inches in Mundelein (Lake Co.). It was the second largest November snowfall for both Chicago (11.2 inches) and Rockford (8.8 inches). See graphics below created by NWS Chicago.
Snowfall amounts tapered off southward, decreasing rapidly from 5 to 1 inches (green), and south of Interstate 70 was essentially snow-free.
Today the NWS Climate Prediction Center released their forecast for December, winter, and spring. Of course, the strong El Niño in the Pacific Ocean is the dominant player in the forecast. It is expected to remain strong this winter, before transitioning to neutral conditions in late spring/early summer.
In general, over the next six months Illinois will have an increased chance of being warmer than average, while parts of Illinois will have an increased chance of being drier than average. The strongest effects of El Niño will likely be felt in northern states (warmer and drier than average), and in the Southwest and Southeast US (cooler and wetter than average).
For December, Illinois has an increased chance of being warmer than average. There is no increased risk of being wetter or drier in December.
For winter, Illinois has an increased chance of being warmer than average. The eastern half of Illinois has an increased chance of being drier than average. The western half of Illinois has no increased chance of being wetter or drier than average.
For spring, the northern half of Illinois has an increased chance of being warmer than average. The southern half has no additional chance of being warmer or cooler than average. Parts of eastern and northern Illinois have an increased chance of being drier than average. The rest of the state has no increased chance of being wetter or drier than average.
A large, slow-moving low-pressure system passed through the central US this week, bringing widespread rains from Louisiana all the way up the Mississippi River Valley. Areas in yellow and beige received 2 to 4 inches of rain, including large parts of Illinois. Some counties along the Illinois-Indiana border received amounts in the 1 to 2 inch range. Meanwhile, areas in Missouri and Arkansas received 5 to 8 inches.
The largest amount reported in Illinois in the last three days was 3.95 inches in White Hall, IL. White Hall is in Greene County and where my great-grandparents lived.
Here are the rainfall departures for November so far. Areas in green are between half an inch and two inches above average. Areas in blue are two to four inches above average. This should pretty much erase any concerns about dry conditions earlier this fall.
By the way, these maps came from a NWS product at http://water.weather.gov/precip/
I have noticed that the average date of the first snow is a popular search term on the blog, so here is the reposting of the median dates of the first measurable snowfall of the season in Illinois. This map is based on 1971-2000 data. While it is not based on the current 1981-2010 averages, the map is still relevant for the purpose of getting an idea of the dates. Measurable snowfall means at least a tenth of an inch.
In the northern third of Illinois, the first snowfall occurred around Thanksgiving. The dates switch from November to December once you reach central Illinois (just north of a line between Quincy and Champaign). By the time you reach Carbondale, the date can be as late as December 20. From this you can see that we are by no means behind schedule this year.