Saturday, December 8, 2012

December 14-16 Potential Major Winter Storm

This is an article concerning the potential for a major winter storm from December 14-16.

We are going to go through all of the last 4 American GFS model forecast runs. They include the 0z (oldest forecast), the 6z (second-oldest forecast), the 12z (second newest forecast) and the 18z (newest forecast) model runs. The above image is the sea level pressure (SLP) anomaly for December 15th, from the 0z model forecast, meaning this is the oldest forecast. As you can see, there is a dark blue area across the Midwest, and that little 'L' shows the center of this low pressure system as 1004 millibars- not that strong. I don't have much to say about this forecast except to say watch the change from the 0z forecast to the next 3 runs of the GFS.

Now, on this 6z model forecast (second-oldest forecast) of the GFS, valid for the afternoon of December 15th, we now see a sudden color change. Instead of a small area of dark blue, there is now a deep protrusion of light greens. These cooler colors show a deeper than normal SLP anomaly. Now, that 'L' in the above image shows our low pressure system with a 991 millibar reading- a substantial increase in strength from our 0z run.

This forecast image from the second-newest forecast from the 12z GFS, valid in the morning of December 16th, shows our low pressure system is still around, but is now further north than previous runs have shown. We are still finding dark green anomalies in the Upper Midwest, and the storm system is only 1 millibar weaker than the 6z run, with a strength of 992mb. At this point, also notice the expanded dark blue and green colors to the south. This means that the GFS is toying with the idea of possibly another low pressure system developing in the South, possibly eventually moving up the East Coast, but I caution you- this possible event is still over 5 days away.

Image from WeatherBell
Now, with our newest 18z model forecast, we see the low pressure system in nearly the same place as the 12z run, but it is stronger, as even lighter green colors have emerged to display this low pressure system as having a minimum central pressure of 988 millibars, making this run the strongest of the last 4. Again, we see green colors in the South US, possibly indicating another spin-off low pressure system for the East Coast, but once again, we are a long ways out.

We've seen the last 4 American GFS models, which have fair consistency in having a disturbance in the area in this timeframe. The track is the big unknown factor. Now, we turn to the slightly more reliable European ECMWF model.

Image from WeatherBell
This forecast of the ECMWF model shows SLP values, not anomalies, hence the color change. We see our developing low pressure system out in the eastern Plains on the morning of December 15th in this forecast. The system is already at an alarming 990 millibars, leading me to question how strong this system could get if it strengthens further in the Midwest- a very plausible idea. Note the high pressure system in the Northeast and off the Mid Atlantic coast. This tells me that the storm will take a track more to the northeast than to the direct east.

Image from WeatherBell
As expected, the system takes a more northeast turn for the morning of December 16th, but the system has not strengthened, nor has it weakened. Its central pressure remains at 990 millibars, something I don't believe will happen in the end. Then again, this forecast is unlikely to happen precisely. Some parts may happen, others won't, we just don't know.

In summary, the two most reliable models in the world are seeing this potential for a possibly major winter storm hitting the Lower 48. At the moment, I am leaning towards a Plains track, due to the likelihood of the negative PNA persisting (which favors storms going into the Plains). However, I would watch that spin-off low pressure possibility I mentioned with the GFS, as a negative NAO could easily take such a system and hammer the Northeast.

But, as I always say, WE'RE STILL A LONG WAYS OUT.



Anonymous said...

Any ideas of potential snowfall amounts in Iowa?

Andrew said...

Too far out to tell at the moment.

Travis Holbrook said...

How about western kentucky ,Tennessee line

Aran Jacobs said...

Since it is now 5 days out (still far out but better) I am a little confused.On your first forecast for this it was a huge storm for the Northeast and now it it is over the Plains and Midwest.Does this mean it has changed it's track completely?

Anonymous said...

Yeah, this one is pretty uncertain, but I agree, there WILL be a big storm somewhere with this. I live in the Chicago area and Tom Skilling was talking about this and suggesting a possible blizzard unfolding, but now high temps could reach 50 by Saturday and the track looks way off to the northwest.

Andrew said...

Travis: The model images seem to affect the Plains and Midwest, so probably not your region at the moment.

Aran: That ECMWF made the error of not abiding by the negative PNA. This one is more accurate.

Anonymous: This whole thing is looking VERY interesting.

Anonymous said...

Latest GFS model has shifted the track of this system further east again. We're really not going to know until midweek at best.

Anonymous said...

Yes, the latest GFS for hour 144 on Saturday has 2 low pressure systems, one centered over the UP of Michigan, another over northeast Oklahoma, and takes that way farther south than the ECMWF, south of the Ohio River now.

Anonymous said...

ECWMF has trended a bit further southeast as well, coming into better line with the GFS.