Winter Weather Update 11 AM

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Latest NAM data just became available.  QPF values nearly the same, with the heaviest precip falling during the afternoon hours.   Morning hours: 0.20 inches  afternoon: 0.47, evening 0.06 water equivalent.

The trend is towards lower temperatures at the critical heights of 3000 and 6000 feet.   Temperatures at these levels remain right at or below freezing and do not rise later.  Temperatures in the upper atmosphere, as reflected by the 500 mb thickness level is marginal for snow, but still supports wet snow.

So maybe we do get that 4 or 5 inches on grassy surfaces just outside of the city.   Roadways significantly less.

Winter Weather Update 8:30 AM

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So  far, the models have been correct with the timing (starting about 8 AM) and the precipitation type (snow).   I was beginning to be concerned about the accuracy of the forecast when I woke up and the outdoor temperature was 34 degrees.

But once the precipitation started, temperatures dropped to 31.

The 2AM run of the NAM has the QPF almost 0.80 inches, but much of the heavier precipitation will fall in the afternoon, according to the NAM.   This most recent NAM run has the temperatures at 6000ft (800 mb) moving above freezing during the later half of the afternoon,  allowing the snow to mix or change to rain or a mix…..so another parameter has shifted towards decreasing snow totals.

The grassy surface vs road accumulation difference is already apparent.   Getting the final accumulations numbers correct with this storm is probably both meaningless and impossible.  I think you get the idea.

I’ll update when the next NAM FOUS data becomes available about 10:45 AM.

 

Midnight Update

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Latest GFS model data in. Here is the trend. QPF values remain about the same at 0.65 inches water, and it has the precip  heavy both  in the morning and afternoon.  Looks to be all snow.

Looking at the GFS and again at the at the NAM  FOUS data, the temperature profile is trending colder.

A tough call-

I’m going back to my original forecast:  4, possibly 5 inches of snow on grassy surfaces in the immediate suburbs, less in the city. Less on roadways. Up to 6 inches in far northwestern suburbs. Precip ends about 10  pm

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Update

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Latest NAM model data in.  Here is the trend.   QPF values remain about the same at 0.67 inches water, but the heaviest preciptation has shifted to the afternoon and lingers later into the evening.

With the heaviest precip occuring during the warmer afternoon hours, somewhat lower snow totals in the city and the the immediate suburbs.  Two, possibly four inches of snow on grassy surfaces in the immediate suburbs, less in the city.  Less on roadways.   Up to 6 inches in far northwestern suburbs.  Some mixing with rain possible late afternoon.  Precip ends about 9 pm

This snow will look worse from your window than it will really be.   A messy day, weatherwise.

Storm Forecasts & Other Things "up in the air".