Storm trends south: Metro forecast 1″ to 3″ Heavier S & E

The weather crystal ball is still “partly cloudy”…but we’re beginning to see some discernable trends with Tuesday’s weather system.

Let’s run down the variables.

Storm track:

Today’s model runs continue the trend of pushing the surface low track further south.

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That means the axis for the heavy snow band shifts south too, and puts the Twin Cities into the more precarious lighter snow accumulation zone on the system’s northwest fringes.

Timing & precip type:

It still looks as if precip will expand and move north Tuesday from along the I-90 corridor early toward the metro by midday.

The precip may be mixed during the daytime hours Tuesday, but should change to all snow by evening. Expect snow to end from west to east Wednesday morning, with a windy & colder day behind the system.

Snowfall rates:

Snowfall rates could be heavy at times Tuesday night, especially in southeast MN & western WI. I expect the potential for 1/2 to 1/4 mile visibilities (1/2S to 1/4S+) in moderate to heavy snow at times Tuesday night in these areas. There is the potential for “thundersnow.”

The snowfall intensity should overcome warm pavement in these areas Tuesday night for slick road conditions.

Snow totals:

I still expect this to be a “good” strong snow event for southeast Minnesota. Right now I am leaning toward general accumulations of 3″ to 6″+ (with a few 8″+ totals) along the I-90 corridor including Albert Lea, Austin, Rochester & La Crosse. The heavy snow band should also include Winona and Eau Claire and may stretch as far north as Owatonna & Red Wing.

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In the metro I think the best snowfall range at this point is 1″ to 3″ by Wednesday morning. The best chance for higher end accumulations (3″+?) appear to be in the far south & east metro.

Springtime snow accumulations difficult

There are several factors in April that make accumulating (and forecasting!) snow more difficult than during the winter months.

Many springtime factors work against heavy snowfall accumulations.

-Time of day:

In the spring, solar intensity is about 5 times greater than during the winter months. Even with a thick overcast, the lowest mile of the atmosphere can see several degrees of warming in April.

This means that precip can stay rain or mixed during the daytime (and not accumulate), when it would be all snow compared with the same system in winter. That can reduce snowfall totals accordingly.

-Warm ground:

In winter with snowpack and pavement temps below freezing, every snowflake that falls accumulates. Not so in spring. Ground (surface) temps are in the 40s in most areas.

As a result, snowfall rates must be heavy for a prolonged period of hours to overcome surface melting from below.

-Air temperatures:

Temperatures with these systems often remain above freezing, even when it’s snowing. This system is forecast to have metro temps above 36 degrees for most of the event, with only a few hours down to near 33.

That means melting is occurring from above as well as below for most of the event, reducing accumulations accordingly. It takes some really heavy snowfall rates to overcome above freezing temps below and above ground.

One scenario is that 3″ to 4″ of snow falls with this system in the metro…but only 1″ to 2″ can accumulate.

Bottom line:

Expect a rain/snow mix to spread gradually north Tuesday. Snow could become heavy Tuesday night. Highest accumulations will be in southern Minnesota & western Wisconsin, where 3″ to 6″+ may fall by Wednesday morning.

In the metro I expect most areas will see 1″ to 3″ on the ground by Wednesday morning, with some isolated 3″+ totals possible, especially south & east.

Let’s see if tonight’s model run continues the southward drift of the system…

PH

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