December 20 Snow Event Discussion (Morning of December 17)

Confidence: High

Water Vapor imagery of East Pacific. The Dec. 20 system is the dark blue circle of increased
water vapor.

We are watching the system that will soon become the December 20 snowstorm spin offshore Baja California right now. It is a cutoff low, characterized by the distinct circular shape- some of its vapor is being transferred east through the jet stream. You can see this from a blue line stretching eastward from the cutoff low. The cutoff low itself will move onto US soil soon by the line of dark blue to the west of the cutoff low. This is another separate disturbance that will push the original cutoff low eastward, through the Southwest, and eject into the Southern Plains.
The cutoff low you see in the water vapor image above does indicate that this system has quite a bit of energy to work with. After moving into the Southern Plains and giving them at least a fair 4 inches of snow, the system will begin to work northward along a ridge in place.

Measure of Predictability map from NCEP Ensembles.
Darker colors indicate more confidence by NCEP Ensembles that a certain atmospheric
feature will be present.

The NCEP Ensembles are showing two things. The disturbance cut-off low, and the ridge of high pressure. The ridge of high pressure is the orange colors with the arcing isobars in the Southeast. Compared to yesterday, it looks like the ensembles are thinking twice about the extent of the ridge's influence in the Midwest. More influence would push the storm track of this disturbance farther north. We will have to closely monitor how the ensembles continue to handle this.
The second feature we see is the disturbance itself. Unlike previous runs, this run of the ensembles is in ally showing confidence that the storm will be present in a strong manner. Confidence is indeed high, with widespread 80%+ colors in the region of the proposed trough. The disturbance is characterized by the isobars taking a dip southward as seen around Texas in the image above.

Yes, we did show this image at the beginning of the post.
But now it's recap time.

To recap, we expect the disturbance to come out of the Southwest guns blazing, and likely put down a sizable accumulation in the panhandles of Texas and Oklahoma. After that, the storm system will be weakening as it crosses through the Midwest, with accumulations there topping out at 4 inches. The low will then move into the Northeast and likely strengthen as it is near the oceans, just like coastal storms. That said, some sizable accumulations may also fall in the Northeast. The entire time the storm will have to navigate around the ridge of high pressure in the Southeast, and may have some Gulf Air accompanying it to help strengthen and/or add more precipitation to the storm.