Chances of a White Christmas in Illinois

What are the historical chances of a White Christmas?

We define a “white Christmas” as having at least an inch of snow on the ground on December 25. The map below shows the odds across the state. It should come as no surprise that the highest odds are in northern Illinois.  In general, the odds are about 40-60 percent in the northern third of Illinois, 20-40 percent in central Illinois, and 0-20 percent in southern Illinois.

There can be large differences between nearby sites. Snowfall is notoriously difficult to measure with blowing, drifting, and melting. Two nearby sites may have different results due to exposure to the sun and the wind as well as the dedication of the observer to report on Christmas Day.

WC_Chance_8110

What happened last year?

Areas north of Interstate 80 had snow on the ground Christmas morning, December 25, 2016. Chicago O’Hare reported 2 inches of snow on the ground, while Rockford reported 6 inches. The snow disappeared quickly with the mild weather in late December.

 

ims2016360_usa
Map courtesy of NOAA at https://www.ncdc.noaa.gov/snow-and-ice/snow-cover/

 

What is the most snow on Christmas Day?

Here are the largest snow depths on Christmas Day (12/25) for selected locations around the state:

  • Chicago, 17 inches in 1951
  • Rockford, 14 inches in 1951 and 2000
  • Quad Cities, 12 inches in 1909
  • Peoria, 10 inches in 1909
  • Springfield, 10 inches in 1915
  • Champaign-Urbana, 9 inches in 1983
  • Carbondale, 9 inches in 2004

What are the odds this year?

Christmas is still 14 days away when this is being written. Single day forecasts that far out, especially for snow are too difficult to make. However, we can get a glimpse of how the models are trending for the week up to Christmas. Temperatures are expected to be near-normal; however, precipitation is expected to be below normal. Therefore, I think the odds of a white Christmas this year look very slim.

814prcp.new

Advertisements

November in Illinois – Cool and Dry with a Dash of Snow

November Summary

The preliminary numbers are in for Illinois and November was slightly cooler and drier than normal.

The statewide average temperature for November was 41.7 degrees, 0.8 degrees below normal. The warmest reading in the state was 79 degrees at Belleville on November 2. The coldest reading in the state was 11 degrees at Stockton on November 24.

The statewide average precipitation was 2.13 inches, 1.34 inches below normal. The largest precipitation amount was 5.60 inches at Paxton. Some snow was reported in northern and central Illinois. The largest snowfall amount was 1.5 inches at the Chicago Botanic Garden.

Fall (September-November)

For the traditional fall months of September through November, the statewide average temperature was 56.2 degrees, 1.8 degrees above normal and the 15th warmest fall on record.

The statewide average precipitation for fall was 7.72 inches, which is 2.22 inches below normal. Dry conditions near St. Louis have resulted in the US Drought Monitor calling for moderate drought (D1) to severe drought (D2) from Randolph to Calhoun Counties (see map at end of post).

MAPS

Precipitation for November

Here are the precipitation maps for November, accumulated amounts on the first map, departures from normal on the second map. In general, more precipitation fell in eastern and southern Illinois (2 to 4-inch totals were common), than in western and northern (totals less than 2 inches were common).  By the way, precipitation represents both the rainfall and the water content of any snowfall.

IL-prcp-mpe-m2d-totIL-prcp-mpe-m2d-dev

Snowfall for November

Parts of northern and central Illinois received snow in November; however, the amounts were generally less than an inch. As mentioned earlier, the Chicago Botanic Garden near the shores of Lake Michigan received 1.5 inches for the month.

map3

Temperature Departures for November

Illinois experienced wide swings in temperature in November. In the end, the colder-than-normal and warmer-than-normal days nearly canceled each other out, leaving us just 0.8 degrees below normal for the month. Northern Illinois ended up 1 to 2 degrees below normal (shades of green) while the area around St. Louis was 1 to 2 degrees above normal. These warmer-than-normal temperatures coincided with the area with little precipitation in November.

map4

US Drought Monitor

20171128_IL_trd

Snow – Date of First Snowfall and Normals for Illinois

Date of First Snowfall

Here are 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 usually occurs around Thanksgiving. The dates move 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.

date of first snow

In about 10 percent of the cases, the first measurable snow occurred as early as November 5 in northern Illinois to November 20 in far southern Illinois.

In about 10 percent of the cases, the first measurable snow occurred as late as December 20 in northern Illinois. South of a line from Quincy to Champaign (Interstate 72), it can occur after the New Year. From Carbondale southward, Illinois, it can be January 20th before the first snow arrives.

How are we doing so far?

Here are the snowfall accumulations so far in the 2017-18 season as of November 27, 2017. I would say that we are not too far off the schedule for snowfall in Illinois.

map

Normal Annual Snowfall Totals (1981-2010)

Here are the normal annual snowfall totals for sites across Illinois. You can find many more maps on normal snowfall by month here and tables of normal snowfall by site here. To give you a rough idea of how the normal snowfall changes across Illinois, it’s close to 3 feet in northern Illinois, 2 feet in central Illinois, and 1 foot in southern Illinois.

Illinois-snow-00ANN-normals

La Nina and New Winter/Spring Outlooks

Summary: According to NOAA, La Niña has arrived and there is a 65-75 percent chance that it will persist through this winter before fading sometime in the spring. We talked about La Niña in an earlier post here. Illinois has an increased chance of being wetter than normal for this winter and spring with an increased chance of being warmer than normal in spring in central and southern Illinois.

November So Far

November this year is starting out much colder than last year. The statewide average temperature for November 1-16 of this year was 41.7 degrees, 1.4 degrees below normal. Most stations have reported lows below freezing at some point in November and many sites in northern Illinois reached down into the low teens. By contrast, the statewide average temperature for November 1-16 of last year,  was 52.3 degrees, 8.1 degrees above normal. And stations in northern Illinois had only reached down into the mid-20s by this point.

December

The outlook for December is not very interesting for Illinois. We have equal chances (EC) of above-, below-, and near-normal temperatures and precipitation. In other words, there are no clear climate signals to indicate any unusual conditions during December. Wetter than normal conditions are favored in states to the north of us and drier than normal conditions are favored in states to the south of us.  Click on the maps to enlarge.

Winter

For the winter months of December, January, and February, wetter than normal conditions are favored for Illinois, as well as the northern and Great Lakes states. Neither warmer nor colder than normal conditions are favored in Illinois. However, many states to the south of Illinois are favored to be warmer than normal. Northern tier states from Washington to Minnesota are favored to be colder than normal. Hmm, colder and wetter than normal conditions expected in the Dakotas and MN this winter – sure sounds like a recipe for more snow to me.

Spring

For the spring months of March, April, and May, wetter than normal conditions are favored for all of Illinois. Warmer than normal conditions are favored in central and southern Illinois. Meanwhile, neither warmer nor colder than normal conditions are favored in northern Illinois (labeled EC on the map).

 

October in Illinois was 10th Wettest

Based on preliminary data, the statewide average precipitation in Illinois for October was 4.94 inches, 1.70 inches above normal and the 10th wettest October since 1895. The largest monthly total was 15.90 inches of precipitation in Glen Ellyn. Seven other sites in that area reported over a foot of precipitation.

The other noteworthy precipitation event of October was that several sites in northern and central Illinois reported seeing snow on October 28. In most areas, it was nothing measurable, just flurries. However, one station, New Lenox (Will County), did report 0.1 inches. Continue reading

NWS Outlook for Winter and La Nina

ññSummary: The NWS rolled out their winter outlook today. There is about a 55 to 65 percent chance of a La Niña developing this fall and winter. However, it is expected to be weak and possibly short-lived. Wetter than normal conditions are favored in Illinois this winter (December-February); however, that may not translate into more snow.

Temperature

For Illinois, only southern Illinois is more likely to have warmer-than-normal temperatures this winter. The rest of Illinois has equal chances of above, below, or near-normal temperatures because there is not a strong climate signal in these areas to suggest warmer or colder conditions.

IMAGE-Outlook-map-temp2017Flt2-101917-3300x2576-original_0

Precipitation

Meanwhile, wetter-than-normal conditions are favored this winter across all of Illinois. This is part of a wetter pattern associated with La Nina that covers the Great Lakes and Ohio River Valley, as well as the northern Rockies. However, this may not translate into more snow. We can have significant amounts of winter precipitation fall as rain rather than snow, especially in milder winters.

By the way, predicting how much snow Illinois will get in the coming winter is always difficult. One or two big storms can make or break the forecast. Two years ago, our janitor Doug won our office pool on the total amount of snow for the winter in Champaign.

IMAGE-Outlook-map-Precip-2017Flt3-101917-3300x2576-original

La Niña Pattern

The “typical” winter La Niña pattern is shown below with the jet stream positioned over the Great Lakes more frequently, leading to more storm activity. But there are other factors that can influence our winter weather and many of these occur in a much shorter time frame. For example, the Arctic Oscillation and the dreaded Polar Vortex cannot be reliably predicted more than two weeks in advance.

LaNina-Jet-Wintertime-Pattern

Last Winter

The weak La Niña episode last winter did not produce the expected results in Illinois. Instead of wetter conditions, we had mild temperatures and little snow or rain (maps below, click to enlarge). However, it was wetter in the other Great Lakes states and the Ohio River Valley with above-normal snowfall in the upper Midwest, but the colder temperatures never materialized.

Winter Temperature Trend in Illinois

Finally, here is the plot of winter temperatures averaged across Illinois from 1895 to present. The black line is the 1981-2010 average. We have warmed by 1.7 degrees over the last century, according to the linear trend. Perhaps more meaningful is that only one of the ten coldest winters has occurred since 1980 (2013-14). On the flip side, seven out of the ten warmest winters in Illinois have occurred since 1980 and four out of ten have been since 2000. I’ll show the snowfall plot for Illinois in a future post when I get a chance to update it for the winter of 2016-17.

graph1

 

 

New Fall Outlook for Illinois, Reduction in Drought

Lots of new information released today. First of all, the latest US Drought Monitor reflects the impacts of the weekend rains in northern Illinois. The rains tapered off in central and southern Illinois so the changes were less dramatic. The areas considered in “moderate drought” have been reduced to isolated pockets. Click on the figures to enlarge. The NWS expects more rain this weekend with amounts in the neighborhood of 1 inch falling in central and southern Illinois.

NWS Seasonal Outlooks

The NWS Climate Prediction Center released their latest outlooks for November and beyond. Nothing to report for November in Illinois or the Midwest (first row of maps). They do indicate that Illinois has an increased chance of above-normal temperatures in November- January (second row). Continue reading