Remember all that snow Chicago was supposed to get today? It's 41°F and raining right now. Forecasts were always honest that it might not snow, but seeing the potential for up to six inches of snow on Christmas Eve vanish in a cold, dreary rain is still a shock to the system. Here's why that happened.

For the past week or so, we've been talking about a strong low pressure system that would develop over the Gulf Coast and race towards the Great Lakes as it strengthens into a strong cyclone. The tight pressure gradient would bring strong winds while fronts brought heavy rain to the southeast and megalopolis. On the west and northwestern sides of the low, we were supposed to see heavy snow develop, possibly dropping six or more inches in some spots that saw the heaviest precipitation.

It didn't snow, and that's not a surprise! If you listened to any forecasts, up until yesterday, many forecasters had lower-than-average confidence in the chance for snow on the western side of this storm. The entire snow event was (and still is) dependent on the strength and the track of the low pressure system. If the low tracks stronger and a little farther to the west, Chicago would get slammed with heavy snow. If it tracks weaker and a little farther to the east, they would get nothing but a cold, light rain. It's similar to the situation we see play out time and time again with coastal storms that threaten cities on the East Coast.

Here's what I said yesterday:

The exact track of the low will determine who sees the most snow. Above is a map of forecast snowfall totals from the National Weather Service. Chicago looks like it's positioned to see the most white doom from this storm—if the band of heavy snow manages to set up right over the Windy City, some spots could see more than six inches of heavy, wet snow by Christmas morning.

And on Monday:

The exact location of the heaviest snow is a little uncertain right now, as the exact track of the cyclone will determine where the heavier bands of snow set up. Runs of both the European and GFS models this morning show that a coating of snow will fall from the northern half of Illinois and across the lake into western Michigan and the U.P. The parallel GFS is painting the heaviest snow across eastern Wisconsin right now, as it takes the low farther to the west than the other two models.

For the past couple of days, the models showed the storm strengthening early enough and far enough west that Chicago would see snow. Here's an animation of the GFS run valid for 6:00 PM today, beginning on Monday morning and running every twelve hours through this morning.

That didn't happen. With each successive run of the model, the low trends weaker and farther to the east. Here's the surface analysis as of 12:00 PM CST, showing a 994 millibar low analyzed over western Ohio:

The track of the low is only one half of the story. The other piece of the puzzle that failed to fall into place was atmospheric temperatures. The low setting up stronger and closer to Chicago was supposed to help the atmosphere cool down enough that temperatures would be sufficient to sustain snow. As I write this post, it's 41°F at O'Hare. A surface temperature of 35-36 is usually considered the upper-limit for snow before it completely melts into a cold rain. The current profile of the atmosphere over Chicago shows that temperatures don't fall below freezing until you're about 2,000 feet above ground level. It's just too warm for snow, and again, that's thanks to where the low wound up forming.

Don't take it from me, here's what the NWS office in Chicago had to say about the distinct lack sky ice in this morning's forecast discussion:

HAVE CANCELED THE WINTER WEATHER ADVISORY FOR THE ENTIRE AREA. IN A NUTSHELL...THE POTENTIAL PITFALLS WITH THIS SYSTEM IN A VERY MARGINAL THERMAL SETUP ALL CAME TO FRUITION. ITS LIKELY THAT THE PLENTIFUL CONVECTION NEAR THE GULF COAST YESTERDAY INTO LAST NIGHT PLAYED A ROLE IN A FARTHER EAST LOW PRESSURE TRACK THAN EXPECTED...AS WELL AS DISRUPTING THE MORE RAPID INSENSIFICATION OF THE UPPER LEVEL AND SURFACE CIRCULATION. THIS IS KEY BECAUSE THE DRIVER OF WHAT WAS ANTICIPATED TO BE A FLIP TO WET SNOW WAS DYNAMIC COOLING IN AN INTENSE FGEN DRIVEN DEFORMATION BAND TO OVERCOME THE MILD LOW LEVEL TEMPERATURES. GIVEN SURFACE TEMPERATURES WHERE IT IS ACTUALLY RAINING NOW IN THE UPPER 30S TO LOWER 40S...EVEN A GRADUALLY COOLING COLUMN WILL AT BEST EVENTUALLY SUPPORT A MIX OR MAYBE A BRIEF CHANGE TO SNOW ESPECIALLY TOWARD THE FAR EAST CWA LATE IN THE DAY. THERE WILL BE LITTLE IF ANY ACCUMULATIONS AND THUS NO TRAVEL IMPACTS OTHER THAN WET ROADS SO FELT COMFORTABLE CANCELING THE WINTER WEATHER ADVISORY.

If you're a snow lover in Chicago, don't completely blame the meteorologists for your lack of a white Christmas—have some holiday mercy on your friendly neighborhood weatherpeople. Meteorology is an imperfect science, and instead of saying "yes it will snow" or "no it will not," almost every forecaster made sure the public knew that there was at least a chance that this wouldn't happen. They issued winter storm watches and advisories ahead of the storm because the models consistently showed the worst case scenario playing out and dumping snow over Chicago. Could you imagine the hell that would have broken loose if they predicted rain and seven inches of snow fell, instead?

Forecasters now predict that the heaviest snow will be confined to parts of western and northern Michigan, where up to seven inches could fall in the heaviest bands. Again, this could bust if the low fails to act as currently predicted.

If you're a snow lover and you're upset that it didn't snow, remember that it's only December. We've got a long winter to go.

[Images: WPC, WeatherBELL, CoD, NWS]

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