Posted: 5:00 am Friday, December 16th, 2016

Ice? Not so fast 

By Kirk Mellish

MAPS UPDATED 5am Friday 12/16…

I continue to be unconcerned about this weak mini threat.

One of the typical ways Georgia gets snow OR ice, is with “the wedge” pattern. That’s a layman’s term for what in meteorology is known as a “CAD event” or cold-air damning.

We get these wedge patterns many times in the fall and winter and to a lesser extent in spring.

You can look up the science details. Its a notoriously tricky to forecast weather pattern, and only occurs in areas of the world near mountains. Our proximity to the Atlantic helps add to the complexity.

In short, shallow cold dry air gets trapped or wedged or damned by the mountains while warmer air flows and rides up on top over the colder air. This often results in condensation to the saturation point leading to clouds and precipitation. The precipitation falling into the dry air can lead to evaporative cooling and dropping temperatures.

The fine details geographic and topographical are critical to what type of precipitation falls, where and how much. The horizontal AND vertical temperature and moisture profile are crucial, and tiny differences and changes and make for major differences or changes in the actual weather.

The problem is our computer models do not have the kind of resolution needed to resolve these fine details. Thus the HIGH LEVEL OF DIFFICULTY in forecasting wedge related weather compared to more normal large-scale weather patterns.

OFTEN the models give very poor guidance on the temps and precip in wedge patterns! I’ve seen them be off the mark too warm or too cold by 5-25 degrees, especially several days out but sometimes even the day of an event.

Google the term CAD weather event and explore for yourself.

We’ve been in and out of wedge patterns frequently of late and more are ahead.

ONE EXAMPLE OF WHAT A WEDGE FRONT LOOKS LIKE ON SURFACE WEATHER MAP:

screen-shot-2016-12-14-at-3-18-03-pm

The above map is the forecast surface chart for Saturday morning 12/17/2016.

The next wedge we get relates to the risk of some minor amounts of sleet or freezing rain in NE Georgia Friday night to very early Saturday morning.

The Atlanta National Weather Service office put out a nice graphic about it here:

image_full1

Here are some maps on the “event”, such as it may be. I will update these maps as needed so check back for updates, and of course my local forecast online and on the radio.

PROBABILITY OF AT LEAST 0.01 inch of ice:

screen-shot-2016-12-16-at-5-02-49-am

The NAM model output below only spotty amounts to make things briefly slick in pink zones if it’s right:

screen-shot-2016-12-16-at-5-26-36-am

CHANCE OF VERY LIGHT FLURRIES OR SLEET SUNDAY EVENING:

screen-shot-2016-12-16-at-5-06-40-am

 

 

Obviously things can and do change, so stay tuned. And Merry Christmas and Happy New Year to all no matter the weather. Follow me on Twitter @MellishMeterWSB

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