When it comes to the late weekend storm which we’ve been monitoring and reporting on all week, you have to say—it doesn’t get more interesting or challenging than this! NO forecast involving a storm is easy or completely clear cut. Storms are complex entities and the product of atmospheric processes which are happening on a huge scale which can make them tricky to measure with precision.
The challenge involved in the prediction of this weekend’s system hinges on several variables—the track the system ends up following and the degree to which lake enhanced snowfall gets involved in its snow production. Either or both could radically affect the amount of snow which falls, especially since Chicago is to sit on the northern flank of the storm where the north to south variation in accumulations could be stunning—potentially ranging from a dusting or an inch in spots to the north to almost a foot of snow to the south.
Storm taking shape over Texas
The storm in question is to come together over Texas Saturday then lift northeast into downstate Illinois and Indiana where it is to undergo intensification.
Storms intensify when air ascends with increasing speed. This increase often occurs as colder air floods into the upper reaches of the storm while warm, moist air sweeps in from below. This is a set-up which enhances air’s tendency to rise. When that takes place, air pressures drop, wind speeds increase and the flow of moisture into the system surges which, at the same time, precipitation development takes off.
Predicting snow amounts always challenging
Modern snowfall forecasting involves careful analysis of a whole range of computer models—scores of them—and their ensembles (versions of the models which average a series of forecasts produced by interpreting the initial state of the atmosphere in more than one way). But computer models aren’t the only game in town when it comes to predicting snowfall. Forecast techniques employing names such as the “B.J. Cook” or the “Garcia” methods come into play.
Believe it or not, the B.J. Cook technique focuses on temperature changes near the 39,000 ft. level. The rate at which warming takes place there offers an indication on the amount of snow a storm may produce. By looking at temperature changes aloft, light is shed on how fast warm, moist air from below is upwelling through the storm to produce its clouds and precipitation.
The Garcia method focuses on the moisture available to the storm. It uses what’s called the “mixing ratio” to reflect the amount of moisture in the air—which is expressed as the number of grams of water available in a kilogram of air.
The “B.J. Cook” and “Garcia” methods each suggest the potential with this storm of snowfall in the 4 to 6″ range over the city.
An average of nearly 100 different snow predictions out of a whole range of computer forecast models produces a snowfall estimate in roughly the same range. But, because winds are to blow into Chicago off the comparatively “warm” waters of Lake Michigan, lake-enhanced snowfall appears a good bet with this storm—and this could become a real wildcard.
Adding lake-enhanced snowfall to the storm’s ultimate snow tally could, if dry air riding the northeast flow into the area doesn’t dampen snowfall here, increase amounts downwind of the lake significantly.
Watches for significant snowfall issued downstate
Winter storm watches have been hoisted downstate and may later have to be expanded as the developing storm begins its trek to the northeast. Were the storm to track more east than north, the snow threat to Chicago would diminish and there would be no need to expand the advisories. But, a storm track farther north would boost snow prospects here. So weekend forecasters are going to have to monitor the movement of this system carefully.
Probability figures generated by the Weather Service’s WPC (Weather Prediction Center in Maryland) put odds of snowfall in the Chicago area reaching winter storm criteria at between 40 and 60%—a fairly high assessment of risk this far ahead of the event.
March sun sends temps to 40 Friday; likely to do the same again Saturday in all but shoreline areas of northeastern Illinois
The high temperature at O’Hare Friday hit 40-degrees while, Midway topped out at 41 and the lakefront held to 33-degrees. A repeat of 40-degree highs is likely Saturday away from the lake ahead of Sunday’s potentially wintry storm.
The normal high on March 23 is 50-degrees.