Snow chances coming into clear(er) focus? Science behind a “red sky”

19F in the Twin Cities metro this morning

16F average low temp for today, December 5th

36F high at MSP this afternoon (at 3:55pm)

Light rain & freezing drizzle possible anytime after 10pm tonight

Cloudy weather “crystal ball” clearing? Snow chances coming onto better focus

“Red sky at night, sailor’s delight.

Red sky at morning, sailor take warning.” – unknown

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Image: Paul Huttner-MPR News

Red sky in the morning:

It was a “red sky” this morning at the weather lab.

The science behind old sailor’s weather lore goes something like this.

“Red sky at night.” Whe the sky is red in the evening it’s often caused by the setting sun in the west illuminating the “back side” of a departing storm to the east, indicating fair weather ahead.

“Red sky at morning” When the sky is red in the morning, that usually means high cirrus clouds are moving in from the west at mid-latitudes…a sign of an approaching storm. A red morning sky is caused by the sunrise in the east hitting the “front side” of an approaching storm.

I like meteorologist Jeff Haby’s description of this piece of weather lore.

The approach of upper level cirrus from the west often indicates an approaching storm system. The sky will not be as red at night if a storm system is approaching because the sun is setting behind the clouds approaching from the west. A red sky at night implies “the storm system moving through has ended!”; The clouds have broken and the sun is shining on and reddening the exiting clouds. The sun will continue to shine on clouds for a period of time after the sun has dipped below the horizon (especially cirrus). Keep in mind this saying was developed before satellite, radar and modern meteorological knowledge. Much of the knowledge of an approaching storm system back then was cloud and wind patterns. Of course, this saying (weather folklore) has some profound problems such as:

(1) The sky can be “reddish” near the sun at dawn and dusk (with or without clouds)

(2) storm systems do not always move straight west to east

(3) cirrus can occur without a storm system approaching or leaving. Clouds can cover one side of the sky or the other without being directly associated with a storm system

(4) the meteorological sailor may delight at an approaching storm system even while taking precautions at the same time

(5) rarely do the clouds from an approaching or exiting storm system only cover 1/2 of the eastern or western sky. The saying represents an ideal case.

SUMMARY: If you can see the sunrise but the west part of the sky is dark: look out for approaching bad weather. If you can see the sunset: the weather conditions will be nice.

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Image: Caltech

Rain & snow chances coming into better focus today:

The weather lab crystal ball has been admittedly “murky” this week. Thankfully today the outlook for rain & snow chances seems to be coming into (somewhat) clearer focus.

Right now I see 4 distinct chances for precip through next Tuesday.

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#1) Light rain and possible freezing drizzle tonight.

A weak frontal system sliding through Minnesota will trigger a few more December rain showers tonight. Rain chances should increase after about 10pm or so, and linger into early Thursday AM. Temps may hover near freezing overnight, especially north of the metro. Watch out for some possible icy spots overnight.

Warmer air pushing in Thursday should boost highs into the lower 40s, easing any possible early AM icy threat.

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Image: NOAA

It will briefly feel like spring again by noon Thursday.

#2) Snow chances Friday:

Friday looks cold enough for “all snow.”

Weak low pressure zipping by to the south should be enough to trigger an area of light snow for southern Minnesota. This system has the potential to produce some 1″ to 3″ snowfall Friday onto Friday night, generally south of the metro to along the I-90 corridor. Redwood Falls, Mankato, Waseca, Rochester and Owatonna and the I-90 corridor cities may all see fresh snow and slick roads Friday.

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Image: NOAA NAM model via College of DuPage

The Twin Cities should ride the northern edge of this system. Right now the NAM and GFS have the northern edge of the snow cutting of right over the metro. If that holds the central and southern metro could see a coating to about 1″ of snow by Friday night…with just flurries or nothing in the north metro. The far south metro could see 1″ to 2″

Stay tuned…the northern edge of the snow area could shift with Thursday’s model runs.

#3) Saturday night & Sunday:

All the major models are still painting a stronger system passing (just) south of Chicago Sunday. This storm track typically favors heavy snows of 6″+ in parts of Iowa and Wisconsin. If you are planning travel south or east along I-35 or I-94 Sunday expect snow. Saturday looks like the better travel day.

Most of the models bring a northern extension…or “baroclinic leaf” into Minnesota Saturday night and Sunday. If that happens, a fairly long duration, “light snowfall” event of 18-24 hours could set up over much of Minnesota.

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Image: GFS at 6am Sunday via College of DuPage

If we can manage 18-24 hours of light snow as the upper low associated with the main storm to the south glides overhead, some decent snowfall totals could add up in the metro and much of central and south Minnesota.

It’s just too early to make a specific forecast of potential snowfall Sunday…but the Euro is cranking out .20″ liquid, and the GFS is closer to .37″ at this early point in the forecast cycle. At a simple 10:1…that might translate into 2″ to 4″ for some parts of southern Minnesota, and maybe the metro.

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Image: Iowa State University

Again with caveats and caution in place…stay tuned….but be aware of snow chances from about midnight Saturday night through Sunday.

#4) Tuesday AM clipper?

The GFS likes the notion of brining a follow on clipper through Minnesota late Monday night into Tuesday morning. This could lay down another swath of a couple more inches…and potentially mess with Tuesday AM rush hour.

Again…it’s too early to be more specific.

Big picture? We’re moving into a colder pattern with a more active jet stream overhead that can bring several weaker snow producing systems into Minnesota. We may just nickel & dime our way to a few inches of snow…and a white landscape over the next 5-6 days.

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Image: GFS 120 hour (5-day) snowfall totals via wxcaster.com

PH

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