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Potential Winter Threat - December 6-11

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Meteorologist Larry Cosgrove

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I do not often use the words "ominous" and "potentially dangerous" to describe a storm threat. But we could have a long-lived doozy of a storm that tracks from the International Dateline into Baja California, then through Texas and the Gulf Coast, turning up along the Atlantic shoreline between December 6 - 11. Since the numerical models will be shifting around trying to get a handle on this disturbance, it makes no sense to plot an exact track scenario. But there is some consistency in the numerical models in that in every run, at least one scheme shows a Miller A system with the variety of severe thunderstorms (S TX into FL), heavy rainfall (in some cases past 6" total) and a mantle of ice and snow in much of the cold sector of the storm from the lower Great Plains into Appalachia, and -possibly- sneaking into the Interstate 95 corridor above Richmond VA. All of this accompanied by a fairly vast Arctic regime which, with any luck, may moderate and lift out after the storm passes east of New England and the Maritime Provinces. This system is worthy of close scrutiny, even if the computer schemes lose the outline of the storm during the next few runs (which is often the case during the early medium range).

 

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But I need to bring to your attention some small changes which I think point to pattern shifts as we get closer to the Christmas/New Year's period. The highest rainfall rates are now over the eastern third of the nation, while the previously wet Great Plains and Texas have turned somewhat drier. My read on this alteration is that a pattern of storms moving along the Gulf Coast and off of the Eastern Seaboard will soon take hold. Just like the threat for a major winter-type, Miller A cyclone in the 6 - 10 pattern, but probably ticking a bit more below and to the right of the coastlines. This is what the analog mean suggests will develop, in the last 5 to 7 days of December.

 

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There will be a system coming from the California that will be the potential storm maker. But certain things must happen in certain ways in order for us to have a chance at any winter weather, and one of those is the track of this upper low.

For us to have the best chance for winter weather, a low needs to track along the Gulf coast and exit in the Atlantic off of the Georgia coast. Tracks like these are called a "Miller A" storm track, and these two images help to illustrate that.

2031518780_millera.jpg.5d3477e028a892d31b191e9c96b153f2.jpg

497374586_millera-1.png.1ad8fc852a7207541026e15b25f6f3d8.png

 

The next image is a look at the latest GFS and its forecast for the position of the trough. Notice the flat flow that is carrying this feature east. We're not seeing a lot of digging south with it right now (yellow arrow) but instead a more direct easterly trajectory (orange arrow), and in this case, that would mean mostly rain for north Georgia. Time for changes still, so we'll keep watching.

Snap330-02.thumb.jpg.b36bde38f5b55f3c7164f12aee8904dd.jpg

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31 minutes ago, Dawg said:

How's the Euro looking?

Best run you will EVER see! 

44BAD1C0-1076-48A0-91EC-784667C0986C.png

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That’s inches!!!😳

For GA peeps!!!

1BA06711-B1E9-4EEF-AF94-B96C0DA2CA9D.png

Edited by SNOW
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With so many models in agreement it is hard not to look at it.  I do not think it will be as extreme as the maps are showing but I feel this may be one storm that we actually get something.  Only time will tell.  I have seen storms like this for us be strong in this time frame only to vanish completely.  

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Just want to say this.... you are going to see a lot of wild snow maps over the next week, please take all of those with a grain of salt. The way I look at snow maps is that they are a good indicator of maximum potential, but almost never the actual snow that falls, as many other processes will work to reduce those totals.

ALWAYS keep in mind that here in Georgia, it's not just temps at the surface that we have to worry about. Here are a few others:

  • Temps at the surface as well as higher altitudes. To get snow, the air must be sufficiently cold fairly high up in the atmosphere.
    • 925 mb (~2500 feet)
    • 850 mb (~4800 feet)
    • 700 mb (~9900 feet)
  • Dew points - Once the precip starts to fall, temps start to fall toward the wet bulb temp. This can work to drop the temperatures, but it also means that until the air is saturated, the winter weather that is falling is evaporating as it falls. Too cold and too dry means that a lot of precip is used up moistening up the atmosphere. 

Just know, there are a lot of details that have to line up in order for us to get snow, and we're going to see LOTS of those changes over the course of this week. 

Love it!!! 🙂

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36 minutes ago, BishopJean said:

What does the timing on this look like? Sunday? Late Saturday night? Please don't say Saturday evening.

At the moment, the GFS brings it in late Saturday and continues the event into Monday as a mostly snow event. The Euro starts roughly the same time and has more sleet and ice. It also continues into Monday as the low appears to slow down off the coast. 

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NWS Atlanta

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 I have gone with a model blend for now with a chance for some ZR/IP early
Saturday as the low approaches and a RW/SW mix Saturday night/early
Sunday as the system sweeps up the east coast. Confidence remains
low at this point.

 

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2 hours ago, BishopJean said:

What does the timing on this look like? Sunday? Late Saturday night? Please don't say Saturday evening.

I'll be a great internet friend and take those ATLUTD tickets so you can be safe at home.  😁

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