Anatomy of a forecast model bust for Twin Cities snow

Twin Cities meteorologists are scraping more egg off our faces than snow off our driveways on this Friday. A significant forecast model bust has kept most of the snow south of the greater Twin Cities area.

What’s interesting is that virtually every forecast model run Thursday cranked out significant to heavy snowfall for the Twin Cities Friday, and every forecast model — including the usually trusty European Centre for Medium-Range Weather Forecasts — missed the surge of dry air that’s keeping most of the snow south and east of the Twin Cities Friday.

There is still heavy snow and wind as expected south of the Twin Cities across southern Minnesota, Iowa, and Wisconsin, but the Twin Cities is largely getting missed by significant snowfall. I can’t recall a system that was handled this badly for the Twin Cities by all forecast models.

So what happened?

The system

The big picture with this intense Midwest storm is unchanged. A powerful low-pressure system is dumping heavy snowfall that will reach a foot in many locations from Iowa through southern Wisconsin to the Chicago area. Blizzard warnings cover most of Iowa and parts of Wisconsin and Michigan as well as parts of southwestern Minnesota.

Six to13 inches of snow has fallen in Iowa as of this post, and it’s still snowing heavily in most areas. So the main body of the storm system to the south of the Twin Cities is delivering heavy snowfall as forecast.

Forecast models missed dry air surge

It’s always dicey to forecast the northern edge of major winter storms in Minnesota. Dry arctic air can eat away at moisture on the system’s northern edge. If the forecast models miss the extent of the dry air intrusion, model snowfall forecast can be overestimated. And boy, did the forecast models miss this dry air surge Friday.

Check out Thursday’s dew point forecast for Minnesota Friday. See how the models forecast a dew point of 16 degrees in the Twin Cities on Friday at 10 p.m.?

Now let’s look at the actual dew point numbers Friday. Below is the automated observation from the Federal Aviation Administration sensor at Minneapolis-St. Paul International Airport. The dew point hovered around the zero mark Friday morning and midday. Dew points are likely to peak in the single digits and start falling.

That’s a lot less available moisture at the surface to saturate the atmosphere for possible snowfall.

Big change in back edge of snowfall

Below was a typical forecast model output Thursday for Friday’s system across Minnesota. See how the snow zone was forecast to cover much more of Minnesota?

Now check out Friday’s NOAA High-Resolution Rapid Refresh model loop from noon Friday to 6 a.m. Saturday. See how the back edge of the snow barely grazes the Twin Cities area?

If this had been the forecast model solution Thursday, it would have led to a very different forecast with much less snowfall around the Twin Cities Friday.

So there you go. This is how one of the biggest forecast model busts for snow in the Twin Cities that I can remember went down.

It‘s interesting to note that all forecast models performed badly with this system over the Twin Cities. This wasn’t a case of just one or two badly performing models. It was the whole suite.

So that’s why pretty much every forecast from media and the National Weather Servivce overestimated snowfall for the Twin Cities Friday.

Bitter cold and wind continue

Perhaps the more important part of this storm is still on track. Expect gusty winds through Saturday and subzero wind chills into early next week. Temperatures will bottom out around minus 10 in the Twin Cities and southern Minnesota Sunday and Monday morning.

Wind chills will be in the minus 20s south to minus 30s north.

Winter will pack a bite this weekend.