Sunday, December 28, 2014

January 3-5 Potential Major Winter Storm

** This post is dedicated to the lives potentially lost on the QZ8501 flight, reported missing last night. **

The potential for a winter storm in the January 3-5 period appears to be growing.

The above image shows 500mb vorticity values across the United States from the GFS model, valid for the afternoon of January 2nd. Here, we see a rather potent piece of energy pushing eastward across the southern Plains at a neutral tilt. This energy is the piece we will be watching to impact a significant portion of the Central and East US. As of now, there are two tracks: one takes the system north and lays down plowable snowfall in the Great Lakes (Madison, WI or Chicago, IL), while the other track goes into the Ohio Valley, putting down snow in that area, even into the Northeast.

I'm going to go through the different mechanisms for this storm, and why ingredients are here for both a north and south track.
Shown here is the mid-level wind speed chart from the GFS model, valid at the same time as the first image we looked at. Notice the streak of enhanced wind speeds rounding the base of the trough/storm system in the South Plains. This streak of strong air is a textbook signal for a strengthening storm system, as the storm will then attempt to attain a 'negative tilt', where those vorticity values in that top image will try and "push" in a southeast direction. This negative tilt indicates the storm has reached peak intensity, and is now a mature storm. Additionally, it allows the storm to curve northward, and qualify the northward track.

Tropical Tidbits
Here's the snowfall projection off of the most recent GFS model forecast, giving you a good idea for where snow would fall if this northern track were to verify. Northeast Iowa into much of southern and central Wisconsin would see amounts near or in excess of 6", with similar amounts nearing 12"+ in Michigan. The GFS has continued to trend north as time has gone on, and is now reaching 'outlier' status, as other model guidance comes in south.

Tropical Tidbits
Here's the Canadian GEM model forecasted 500mb geopotential height values (color) and MSLP values (contoured lines) for the afternoon of January 3rd. We see our storm system displaced well south of the GFS track, with this forecast leading the low pressure system over western Tennessee, eventually into the Mid-Atlantic, producing accumulating snow as it does so. The GFS-Parallel model, the successor to the current GFS model, is also siding with this GEM projection of a further south track. This southern track would deliver snows to portions of Indiana, Ohio, and into the Northeast, finally adding a bit of winter for those regions.

Now, it's time for a bit of analysis.

Let's use the same image now as we did to kick off this post. As we see the energy push into the Southern Plains, notice how the longwave trough over Canada seems to almost converge with that ridge in the Southeast, right over the Midwest. This tightening up/coming-together of contour lines is a process called 'confluence'. Confluence works by piling up air, as those contour lines press together. At the surface, this results in high pressure. My concern is that model guidance (the GFS model in particular) is not accurately accounting for this high pressure. The formation of high pressure right where the storm is supposed to track would usually see the storm forced southward, similar to what the GEM and GFS-Parallel are showing. 
Adding to this concern is that all model guidance has a known bias to be too slow with the progression of Arctic air southward. Lo and behold, we have a strong Arctic high pressure system on the heels of this storm system, and if models are retaining this bias, it's quite possible the storm ends up shunted to the south, giving snow to the Ohio Valley and Northeast.

Tropical Tidbits
The image above shows the projection for 500mb geopotential height values (colored regions) and MSLP values (contour lines) for December 29th over the West Pacific. We see a low pressure system has formed on the southeastern coast of Japan, and is moving east-northeastward along the coast. As time progresses, this low will eventually shift out to sea and away from Japan.

Our Christmas Eve storm was expressed well by this Typhoon Rule application, and this storm looks rather similar to the track the storm took in Japan preceding the Christmas Eve event. The only difference here is this time around, the low pressure system drifts out to sea instead of cutting up the eastern coast of Japan. This is a huge red flag, and tells us that this storm may very well go further south and east than current model guidance is suggesting. Given the success this predictor has had in the past few years I've used it, there is reason to believe this system could go further south and east. 

For now, I'll set my sights on this storm tracking to the south, more in line with what the GEM/GFS-Parallel are showing. The GFS model, in addition to being too far north, may be too quick with the storm, so other guidance appears to be good to use at this time. The combination of potential model error, as well as expected high pressure issues, in addition to the Typhoon Rule, is too much for me to put faith in the northern solution. The GEM snowfall prediction is shown below (this event's snowfall would be through the upper Ohio Valley into the Northeast).

Tropical Tidbits

To summarize:

- Model guidance is now confirming the possibility of a storm in the January 3-5 timeframe.
- A southward track is expected at this time, with snow hitting the upper Ohio Valley and Northeast.
- Substantial uncertainty still exists.


1 comment:

Anonymous said...

Funny NOAA and accu weather are saying the opposite. This storm is a week away.It will change from day to day.