Forecast: Winter storm warning Friday

Welcome to meteorological winter and our next shot of winter weather in Minnesota.

A "hybrid" Alberta Clipper/Pacific storm (A Wyoming Winder?) is tracking for Minnesota. The system will spread a swath of dry powdery snow into Minnesota Friday and Friday night.

Winter storm warnings and advisories have been hoisted for much of Minnesota.

The system has several factors going for it to produce significant plowable snow over much of central and southern Minnesota.

1) Temperatures are cold enough for ALL SNOW. No rain/snow line to deal with here...so every flake that falls will pile up on roads and your driveway.

2) The colder air will make for an efficient snow producer. The forecast models are cranking out snow:water ratios between 15:1 and 20:1 with this system. This will be a "Champagne Power" snowfall like they get at Steamboat. (Saturday should be a GREAT day for skiing around the metro, maybe one of the best this winter.) Because this will be a dry snow and have a little more moisture than usual, the system will crank out relatively high snowfall amounts for a "clipper type" system.

3) Temperatures around 10k feet will be in the perfect range (-12C to -16C) for maximim "dendritic growth" capable of producing big, high qulaity snowflakes that can accumulate rapidly. Here's the text from the Twin Cities NWS Thursday PM forecast discussion.

TEMPS IN THE DENDRITIC CRYSTAL GROWTH AREA ARE FORECAST BETWEEN -12 CELSIUS AND -16 CELSIUS...VERY CONDUCIVE FOR LARGE SNOWFLAKES AND HEAVY SNOW.

There are still two more critical model runs (tonight & Friday AM) to come in before the snow flies. With that in mind, here's my best shot at a timeline, forecast snow totals, and effects for the storm in Minnesota Friday & Friday night.

The System: "Hybrid" Alberta Clipper-Pacific storm. The track is more southerly than a traditional Alberta Clipper. (Surface low coming through Wyoming and dropping south into Kansas) Because of this track the system may have a bit more moisture to work with than a traditional Clipper. There is also still the potential for the storm's snow shield to jog south, and reduce snow amounts for the metro accordingly.

The Timeline: Snow should break out in the eastern Dakotas and western Minnesota (Ortonville, Morris) Friday morning, and expand slowly east Friday. It looks like snow may hold off in the Twin Cities until late afternoon. The first part (and maybe all?) of PM rush hour could be okay in the metro...with snow moving this way. Still, it might be a good idea to get going home early Friday PM, especially if you are heading west.

Snow should continue from Friday evening through about 6am Saturday, then pull away rapidly to the east. The peak of the snow intensity should occur between 9pm Friday evening and 3am Saturday morning.

The Track: The surface low is tracking unusually far south with this system. That's a scary scenario for forecasters...as there is a chance it may displace the snow shield southward along with the surface low track...and drop forecast snow totals on the north edge of the storm.

The upper low (which also supports snow accumulations) is slated to track just southwest of the Twin Cities...and this should help keep snow going in the metro Friday night.

The snow axis will run NW-SE, generally parallel to I-94 and the Minnesota River Valley.

It appears the heaviest snow band will set up just southwest of the Twin Cities metro, on a Willmar-Mankato-Rochester line, and could spread into the SW metro.

Snow Totals: Ah yes..the moment we've all been waiting for.

I'm using the "24 hour rule" here. I usually like to give snow totals about 24 hours in advance of the onset of snowfall. This is often the best balance between giving people ample time to prepare and adjust plans, and giving the forecast models time to latch onto the storm.

The modles insist on cranking out anywhere form about .32" (GFS) to .45" (NAM) of water with this system. That would translate into a pretty good pile of snow at at 15:1 or 20:1 snow:water ratio.

Heaviest snow band: 4" to 9" totals by Saturday morning 30 miles either side of a line from Ortonville-Willmar-Mankato-Waseca-Rochester. Includes Morris, Montevideo, Redwood Falls, New Ulm, Hutchinson, Litchfield, Fairfax, Glencoe, and Owatonna.

I-94 Corridor: 4" to 7" by Saturday morning.( Fargo-Fergus Falls-Alexandria-St. Cloud-Twin Cities Metro-Eau Claire.

Best chance of 6" to 7" in southwest metro communities of Waconia, Norwood, Chaska. Best chance of 4" or less northeast metro Forest Lake, Cambridge.) 4" to 6" looks like a good range for the downtowns right now.

Models and NWS cranking out 5" to 8" totals for Twin Cities Airport.

(Click for bigger image)

North and south? Generally amounts should drop off to 1" to 3" north and south of these areas. Think Hinckley, Brainerd, Duluth, Worthington, Pipestone.

NAM model lays out heaviest snow band just SW of metro.

NAM model cranks out snow:water ratios in the 20:1 range.

(click for a bigger, more easily readable image)

What could go wrong?

There is still a chance that the storm could jog south. That could reduce snowfall totals in the Twin Cities, and increase thotals accordingly along the I-90 corridor if it happens.

Bototm Line: Expect snow to spread across Minnesota Friday and Friday night. Roads will be snow covered and slippery. Prepare accordingly!

By the way I LOVE forecasting snow. It's one of the most challenging and exciting forecasts a weather geek can make. (It's also often humbling...or downright humiliating!)

There are many approaches forecasters take to forecasting snowfall, and I find them fascinating, and even amusing. I won't name names, but if you look at various forecasts you can see that some forecasters shoot form the hip a long way out in front of the storm...and scatter varying snowfall amounts around like buckshot. In this scenario you may hear "several" snow forecasts with a big "range." This can be tough for the customer (you!) to sift through and use as valuable information.

The other extreme is some forecasters who use what I call the "gun to the target" analogy. The wait to issue snowfall totals until they walk the barrel of the "forecast gun" right up to the target's bull's eye...then pull the trigger. (Sometimes after the snow has already begun!) Pretty easy to get a good forecast then, but how much value to the viewer/listener?

For the record I think we in the Twin Cities are very fortunate to have such a capable, well trained crop of meteorologists at NWS and on TV & radio. We're right far more often then wrong...and just letting people know the timing and general magnitude of snowfall has great value...even if the "inches" forecast is off a bit.

Also, I can be wrong as often as the next guy/gal, so I say this with a sense of great respect, humility and humor. I just find it fascinating to watch how different forecasters approach winter storms.

It seems even NWS forecasters can (and I would argue should!) have a healthy debate on how much snow will fall. Again, from the NWS PM dicsussion Thursday.

AFTER MUCH DEBATE...WILL UPGRADE THE WINTER STORM WATCH TO

WARNINGS/ADVISORIES WITH EXCEPTION OF RUSK COUNTY IN FAR WEST

CENTRAL WISCONSIN.

On the plus side of the snow...this will be a high quality snow for winter enthusiasts in Minnesota...enjoy!

PH