The wedge threat for Monday 

Posted: 8:09 am Thursday, January 4th, 2018

By Kirk Mellish

Forecasters in Atlanta have been talking about “the wedge” weather pattern long before I arrived here 30 years ago. I’ve explained it’s role in our weather many times on-air and in my blogs over the years.

I won’t do a deep dive on it here, there are plenty of articles online about it that you can google if you’re interested. The science term is CAD or CAD EVENT. Cold air Damming.

A few key points to know are that no two are ever alike, models struggle with them because of limits to their resolution and terrain characteristics, being off in temperature prediction surface or aloft by just a degree or two can make all the difference in the world. Being off a few degrees in a temperature forecast is normal!

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IF the cold air remains entrenched when moisture moves in you get (1) snow when the cold air is from clouds to ground, or (2) sleet if snow partially melts in an elevated “warm nose layer” or (3) freezing rain if snow melts completely on the way down but surface temps are still <32. It’s just rain where and when surface temperatures are well above 32F.

Compare the PREDICTED SURFACE WEATHER MAP below to the generalized patterns from above, its not a classic CAD but ballpark…


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DO NOT USE the following maps verbatim, a lot can and will change.

GFS model late Sunday night early Monday:

Screen Shot 2018-01-04 at 7.02.33 AM

GFS model mid-day Monday:

Screen Shot 2018-01-04 at 7.03.26 AM

ECMWF model Monday AM:

Screen Shot 2018-01-04 at 7.59.26 AM

ECMWF model Monday afternoon:

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Canadian model Monday AM:

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Canadian model Monday afternoon:

Screen Shot 2018-01-04 at 8.16.50 AM

So there’s general agreement from the Global Equations that precipitation will start as a wintry mix sleet/freezing rain in at least some areas, then transition to just all cold rain later in the day.

But as I stated earlier models often get the details wrong and since it’s 5 days away a lot could change.

Hopefully the warm air coming in is stronger and the moisture comes in later, then it would just be a rain day.

With the big football game on Monday, Presidential visit, big trade conventions, back to school etc. the last thing we need is bad weather to add to traffic woes.

Too early to have confidence in specifics of precip type or amounts or timing. So stay tuned.

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