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                   Monday, December 3, 2012 - 12:17PM CST

Storm observing forecast update for December 3-11

By DAN ROBINSON
Editor/Photographer
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This is turning out to be quite the fun December as a Midwest storm observer. Not only are lightning expeditions in the offing for later tonight/Tuesday morning and later this week, but a major storm system next Sunday-Monday has been solidly shown by the models the past few runs. I'm not one to sound the alarm for severe weather events 7 days out, but it's December, and I'm anxious to start talking about what might be a big event for the region. With the typically more-reliable European model on board with this, I think it's not too far-fetched to mention it now.

By next Sunday night-Monday morning (the timeframe shown by the images above), a very large warm sector is shown as being established over a big chunk of the country, with 60°F+ dewpoints possibly making it as far north as Iowa and northern Illinois by afternoon. Aloft, a massive upper level trough is shown spreading 40 to 60 knot southwesterly midlevel flow overtop all of this very unseasonable warmth and moisture. A very cold surface air mass is shown to the northwest of the warm sector. This baroclinicity (high-contrast warm/cold boundary), combined with the strong upper support, is shown spinning up a strong surface low in the Plains. Low-level wind fields will strengthen and 'back' ahead of the low in the warm sector. Combine that with the midlevel and upper-level wind fields, and the result should be a directional/speed shear profile that would support supercells and tornadoes if the instability factor of the equation develops.

The wildcard (as with any cold season setup like this) will be early cloud cover and precipitation. Widespread storms are shown ongoing on Monday morning, particularly up in the Midwest, which would limit daytime destabilization in the warm sector. Nonetheless, with the ambient low-level moisture, the surface low and the upper support, this could be a significant severe weather generator even without the help of the sun. On the cold side of this system, a heavy snow/blizzard event is possible in parts of the central Plains. It's safe to say that if the models keep showing this system, you'll be hearing a lot more about it in the days ahead.

Having said all that, we're still 7 days away, so I'll limit my 'geeking out' over a long range model forecast to this, for now (I hope you'll forgive me for this December indulgence). At any rate, it will be a fun system to watch through the week, and if all goes well, an expedition will certainly be in the cards.

Looking closer - tonight's lightning expedition in Missouri and Illinois still looks like a lock, with my preferred starting point looking like Hannibal, MO at sunset. Stay tuned to the Twitter feed for semi-live posts of photos, if the expedition is a success, that is. A full blog post will eventually come later Tuesday.

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