The NWS Climate Prediction Center released their latest seasonal forecasts today. Here are the results for Illinois. The biggest news is that Illinois has an increased chance of above-average temperatures and below-average precipitation for the winter months of December, January, and February. This forecast is based largely on the developing El Niño in the Pacific Ocean.
While the forecast of a milder winter may sound appealing, I would not leave the winter coat in the closet and throw away the snow shovel just yet. Two things to consider are: 1) this is not a 100% guarantee, other factors come into play in determining our winter weather, and 2) even a mild winter can contain short periods of intense cold and abundant snowfall.
With snow this weekend and another round of cold temperatures expected in coming weeks, it feels like spring will never get to Illinois. But it will arrive someday – I promise.
In coming weeks, many will point to the spring equinox as the start of spring, which is March 20 this year. The equinox is an astronomical event when the Earth’s axis is at a 90 degree angle to the sun (see figure). In theory, the length of day and night are the same around the world. However, due to how we calculate sunrise and sunset, and how the earth is not a perfect sphere means that the days and nights are not exactly equal in length on the equinox. However, that’s a story for another day.
The bigger problem with the equinox definition is that the spring and fall equinox*, as well as the winter and summer solstice, do not line up well with the annual march of temperature (see figure below for Chicago). With spring, temperatures on average are already warming significantly by March 21, compared to temperatures in January and February. By the arrival of the summer solstice, temperatures are already getting very close to their annual maximum.
As a result, many climatologists and meteorologists use calendar month definitions that line up better with the average annual changes in temperature. It works best for summer (June-August) and winter (December-February) in capturing the warmest and coldest periods of the year. In spring (March-May) and fall (September-November), it works well at capturing the strong transition in average temperatures. Several professional have called the period March-May “meteorological spring”, but it is based on climatological data so the better term is “climatological spring”.
Of course, all of this is a bit academic. We all carry our own definition of spring that is probably more in line with nature – the first signs of green grass, the buds swelling on trees and shrubs, increased activity in wildlife (especially birds). I think the one thing winter does best is to make us appreciate spring even more.
*the use of words like winter, spring, summer, and fall, are in reference to the Northern Hemisphere.
There were a lot of new outlooks released today from NOAA. First are the new temperature and precipitation outlooks for April and this spring. It looks like the below-average temperatures are likely to continue in April. We have had below-average temperatures in Illinois for every month since November.
The April-June outlook has northern Illinois with an increased chance of below-average temperatures. Areas labeled E.C. mean that there are equal chances of above-, below-, or near-average temperatures or precipitation (depending on the map type).
An unusually cold and wet winter across the Upper Mississippi Basin, Great Lakes region, Ohio River Valley, northern Middle Atlantic, New York and New England has produced an above normal amount of water in the current snowpack and a deep layer of frozen ground much further south than typical. With significant frozen ground in these areas, the flood risk is highly dependent on the amount of future rainfall and the rate of snowmelt this spring. Recent snowmelt has increased the near surface soil moisture and elevated the potential for rapid runoff from rain events. In addition, significant river ice increases the risk of flooding related to ice jams and ice jam breakups.
Moderate flooding is expected in southern Wisconsin, southern Michigan and portions of Iowa, Illinois and Indiana, as a result of water in the current snowpack and the deep layer of frozen ground coupled with expected seasonal temperatures and rainfall. Specific rivers at risk include the Mississippi River between Davenport, Iowa and Burlington, Iowa, the Illinois River between Beardstown, Illinois and Henry, Illinois and many smaller rivers in the area. In addition, a potential for exceeding minor river flood levels exists across the upper Midwest and east into New England.
More rain fell over Illinois over the Memorial Day weekend. The heaviest amounts were in the central part of the state and ranged from 2 to 6 inches (yellow to dark red in the map below).
Right now the statewide average rainfall for May stands at 5.03 inches, based on preliminary data. More rain is forecasted for today and much of this week. So this total is likely to increase as we go through the week. By contrast, Illinois received only 2.5 inches in May 2012.