Christmas Snowstorm Discussion (Northeast Affected) (Updated 12/17)

Confidence Level: Very Low

The system we are watching for a potential Christmas snow event is the area of dark blue water vapor to the left of the circle-shaped disturbance. It is the one that looks somewhat like a line. This disturbance will move southeastward and eventually make landfall into the Southwest. From there, the models are indicating the potential for this disturbance to slide eastward in the Southern half of the country and eventually 'bomb out' (strengthen very rapidly) as it comes into contact with the East Coast.
The area affected would be the Northeast if this all panned out. The Relative Predictability measures you may have seen in previous posts are not in a good enough agreement for me to feel confident about posting the image and having to explain possibly false information.
Just for looks, we will bring up the GFS Individual Ensembles (click to enlarge image).

GFS Individual Ensembles at Hour 174 of the 6z run.
Thick blue line is rain/snow line- anything above it is theoretically snow.

The GFS ensembles are also mixed up right now, but most are coming to grips with it, and that prospect is encouraging. Our confidence level is very low right now because the models are not stable this far out. We will, however, continue to monitor this storm system and keep you informed for a potential white Christmas-bringing storm.

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.

ECMWF Ensembles Picking Up on Pattern Change with Positive PNA

The ECMWF Ensembles are picking up on the long-awaited pattern change by displaying the typical PNA index in its positive phase, as shown below as well in 500mb heights.

Typical Positive PNA 500mb heights in January.

A positive PNA features a trough in the East and a ridge in the West. Putting that in simple terms, a positive PNA means many low pressure system occurrences in the East and many high pressure system occurrences in the West. As the stratosphere warms, it is only fitting that the PNA, AO and NAO (all 'essential' for cold air and snow for the East) will be turning into much more favorable phases. January could be, and likely will be if everything goes right, THE month for this winter. February could be snowy as well, but January will really start cranking the snow machine.

Side note: For those seeing my exaggerated feelings on this winter or this storm, I am a snow lover, and, no matter how hard I try to avoid sending those feelings on here, they do get on here. Winter is typically a turbulent time for my emotions and patience with storms and winter overall, so y apologies in advance if I start to go overboard.

December 20 Snow Event Discussion (Updated 12/16)

Confidence Level: Medium

System that will eventually become the Dec. 20 storm system.
Defined as the dark blue circle of increased Water Vapor.

The system that will eventually turn in to the December 20th storm for the Midwest and Ohio Valley is currently just offshore Baja California. The system is defined above as the dark blue circle of increased water vapor. The circular shape is similar to that of a cutoff low, which this storm system actually is. A cutoff low basically is disconnected from the jet stream, or cut-off, and can spin around for days in a single area before eventually being nudged somewhere else by a different atmospheric feature (another disturbance, for example).
This cutoff low will be progressing eastward into the US at some point in the next couple days. From that point on, the system should sit by itself, possibly losing some of its strength to the storm-ravaging Rocky Mountains. The cutoff low should be moved eastward by another disturbance sliding southeast-ward from the Pacific Ocean. It can be compared to a 'one leaves, another takes its place' sort of pattern.
This first cutoff low should be nudged far enough eastward to be influenced by the jet stream and be pulled back into the flow. One question is just how strong the second system that will be moving southeast will be. It is possible that a stronger system would move the first low farther east, thus moving the track farther east. We want to find out what the models think will happen, but models are known to not be good with systems in the Pacific due to lack of upper air observation data. So we turn to the ensembles.

Measure of Predictability using NCEP Ensembles for the December 20 period.
Darker colors indicate more confidence in a certain atmospheric event happening.

As the system ejects eastward out of the Southwest, the NCEP ensembles are believing that a strong ridge (high pressure system) will be in place in the Southeast. This would do two things. One, it would stop eastward progression of the disturbance due to the ridge being in the way. Two, the ridge would take the disturbance and pull it northward, moving the disturbance around the ridge. This would ideally take the system through the Midwest and Ohio Valley before entering the Northeast.
Something we are monitoring, however, is how strong the ensembles take this ridge. The ensembles appear to be really pushing this ridge northwestward, farther northwest than previous ensemble predictability images have seen. This increased northwest ridge presence would alter the disturbance's track, presumably taking the storm more northward.
The big concern with this is how the ensembles may not be picking up the system. We can see an area of light orange colors in the Texas region, around where the disturbance is projected to be by models. However, the shade of colors would indicate that 50-70% of the ensembles are predicting this disturbance to be present, with more than 80% predicting that the ridge will be present. We do believe that the disturbance will be in place, but with the ensembles not agreeing, the final solution may be altered due to the disagreement.

Models are Junk Today

We don't have much to share with you today, other than the fact that the models are junk with the Dec. 20 storm. Winter forecast probably won't play out as expected, and we are debating a revision.