Winter storm warnings: Snow likely Thursday night and Friday

Yes, I hear the groans of latent wintry anguish on the @MPRweather Twitter feed.

I feel you. Trust me, I am kicking the Doppler and Weather Lab Supercomputer as hard as I can to spit out a different solution, an escape hatch of sorts from the forecast of heavy snowfall with this system. So far no response.

Here are the latest headlines on our evolving late season winter storm taking aim at Minnesota.

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  • Winter storm warnings for most of central Minnesota including the Twin Cities metro area late Thursday and Friday.

  • Light wintry mix possible overnight with some icing on streets/walkways possible.

  • Main body of storm moves in Thursday night into Friday morning with heavy snow potential

  • Snowfall rates could reach 1 to 2 inches per hour Thursday night and Friday morning.

  • Thundersnow is possible Thursday night and Friday morning.

  • Overall snowfall totals of 6 to 12 inches or more are possible and may include the Twin Cities metro

  • Snowfall totals of 7.4 inches or more with this storm would be a "Top 10" all-time April snowfall for the metro.

  • Temperatures in the 40s this weekend. Thankfully, April snow doesn't last too long!

422 warn
Twin Cities NWS

Still on track

The forecast models show some pretty good consistency (that's more than a little scary) in bringing a major low pressure system from Kansas City, Mo., to Madison, Wis.

The latest loop from the National Oceanic and Atmospheric Administration's National Digital Forecast Database.

422 2allfcsts_loop_ndfd (1)
NOAA

Slight northward shift?

If there's any discernible change in the latest model trends, it's the advance of warmer air edging slightly further north. That could keep the rain/snow line (850 millibar 0-degree Celsius isotherm) right over the Twin Cities a little longer into Thursday evening.

If that happens, mixed precipitation may last longer and that could reduce snowfall totals accordingly in as you go south.

The 850-millibar freezing line is often the best indicator of the rain-snow line for winter storms, and NOAA's Global Forecast System keeps it hanging around the metro at 6 p.m. Thursday -- and into part of the evening.

422 850
NOAA

Critical changeover from rain to snow later Thursday

Some of the tried and true "old school" methods the old timers used to predict rain to snow changeover include time of day in springtime. The extra incoming solar energy in the afternoon on spring days can delay the transition from rain to snow a few more hours than it might in say, December.

Other factors include "dynamical cooling" when precipitation rates and downdrafts  increase so quickly that colder air is literally sucked down from aloft -- and rain quickly changes to wet heavy snow. There may be a little of both going on Thursday afternoon and evening. With surface temperatures above freezing we may also melt the first couple of inches that falls Thursday evening.

Thundersnow Thursday night/Friday?

Eventually the rainy mix changes to all snow Thursday night. Strong upper lift may provide enough lift for mini-thunderstorms embedded within the overall areas of stratiform snow. The result?

Thundersnow.

thundersnow
Wiki Commons

Snowmageddon

Even a late changeover to snow Thursday evening still leaves a good 12 hours of potentially heavy snow, with models cranking out snowfall rates of 1 to 2 inches per hour.

If the rain snow line is south of the metro by late Thursday evening, that should mean no problem in getting at least 6 inches and possibly as much as 12 inches of heavy wet snowfall in much of the metro, especially the northern/western half.

The GFS is sticking to its guns on laying out the heaviest snow band near and north of the metro.

422 gfs latest
wxcaster.com

In areas that stay all snow north and west of the metro and into northwest Wisconsin, I will not be the least bit surprised if some 12- to 16-inch totals come in by late Friday.

Here's a nice system overview from the Twin Cities office of the National Weather Service.

Historic April snowfall?

As I said this morning, if the storm stays on track we could see a "top 10" April snowfall in the Twin Cities with this system. Thanks to Pete Boulay for sharing more with me this morning from the Minnesota Climatology Working Group.

Minnesota Climatology Working Group

The largest April snowstorm in the Twin Cities happened on April 14, 1983. A strong area of low pressure had ample moisture to work with and deepened over northern Iowa as it moved to the east and northeast. The event started as rain on the 13th and changed over to snow during the wee hours of April 14th.

Persons in east central Minnesota were surprised to wake up to a foot of snow on the ground with brilliant morning sunshine. The grand total at the Twin Cities airport from that storm was 13.6 inches. The Metrodome deflated for the third time since its construction in this storm. (The first two times were Nov. 21, 1981 and Dec. 30, 1982.)

Over 400 schools canceled class on April 14 and the University of Minnesota shut down at 2:30pm. Residential mail deliveries and pickups were canceled for only the third time in 20 years. The Twin Cities International Airport was shut down for 5 hours from 9am to 2pm on the 14th.

Stay tuned as I may still have to tweak storm snowfall with any potential changes tonight and Thursday before the big snow hits Thursday night.

Winter or spring storm?

Yes I hear many of you comment that it is now spring. Why does the National Weather Service still issue winter storm warnings?

My view is we're talking about semantics vs. effective communication here. I'm with NWS on calling these winter storm warnings, even in April -- or November for that matter. The reality of what best descries the weather conditions in these systems is winter. Minnesotans know how to react to a winter storm warning. But how do I prepare for a spring storm warning?

My two cents. Discuss among yourselves!

You be the forecaster!

Let's try something new with this storm. I know there are many of you who are expert and amateur meteorologists out there who read Updraft. Thank you for that!

Above is my best crack at the incoming storm. I'm told by colleagues my track record is good on winter storms, but none of us are prefect forecasters. I can be as wrong as the next guy on this storm.

What do you think as you look at the incoming system? Are there trends you see that might cause big changes in the forecast in the next 48 hours?

Let's discuss. Please feel free to share your view on the incoming storm in the comments. Sort of a forecasters and Minnesota weather lovers support group?

I'll chime in as I am able.

Stay tuned, and be nice!