Sun. Jan. 31, 2021 6 P.M.
A weakening storm system which created large snowfalls over northern IL and IN yesterday gave us the overnight rain. Today a very weak surface low (the remains of yesterday’s stronger storm) has been very slowly moving east over southern IN. The passage of the low has changed our winds to a westerly flow and brought in colder air. As we’ve seen today, not much happens in the center of a low pressure system – especially a weakening one.
As the upper air system moves eastward to merge with the developing “Noreaster” along the east coast, we should see the added influx of energy allow the noreaster to become a major winter storm from the mid-Atlantic states to New England. Very slow movement will keep this storm active over New England into Tuesday.
Meanwhile, as the upper air system moves east the so-called backlash part of the storm moves over us. Basically, some of the moisture (from southerly winds) ahead of a storm becomes trapped in the lower atmosphere. The counterclockwise flow around the storm takes this moisture northward, then pushes it westward, and finally southward again. This wrap around effect takes the small supply of moisture into the colder air following the storm. The cold air then squeezes the moisture out of the air. Whether it’s rain or snow depends on temperatures.
Around a developed low pressure area the northwest quadrant pretty much promises two things. 1). there will be precipitation, and 2). the amount of precipitation will be light. Precipitation totals usually run about .02″-.03″ inches per six hours.
All that background to lead to tonight’s forecast…
Periods of light drizzle/rain this evening will change to light snow before midnight and continue overnight. Temperatures will be slowly dropping to about 32 for the morning rush hour. By that time, grassy areas will have less than an inch of snow. Roadways will have little or no accumulation, so roads should be in good shape, mostly. Possible exceptions, as always, will be bridges and overpasses. If any icy conditions develop those areas are the most likely spots.
Additional light snow/flurries are possible tomorrow into tomorrow night.
The U.S. has a forecast model, the coupled forecast system, version 2 (CFSv2) that is designed to detect weather/climate trends out to about six months ahead. For the past few months it has continuously predicted the whole country to have above to much above normal temperatures for February. As recently as last Tuesday, the warm February forecast covered most of the U.S.
Then, something strange happened. Wednesday, the forecast changed to “near normal”. Thursday, the forecast changed again. Below normal temperatures covered about 2/3 of the country. Just the southwest part of the U.S. stayed at near-to-above normal.
Today’s forecast continues the cold projection. This model normally changes very little on even a weekly basis. Even when it does, changes are slow. But, a total forecast reversal in two days just doesn’t happen. Until it does!
There has been some sudden fundamental change in the atmosphere that the CFSv2 has picked up on. Could be a cold and stormy month lie ahead. That would be fun for a change!