Sunday, November 2, 2014

November 24-30 Potentially Significant Winter Storm

It's becoming evident that we may be dealing with a potentially very strong Thanksgiving winter storm.

Tropical Tidbits
The image above shows the GFS model's combined forecast of 500mb geopotential height values (colored shadings) and mean sea level pressure (MSLP) contours over the West Pacific and north-central Pacific basins. High and low pressure demarcations are also seen. This forecast is valid on the evening of November 7th, and may have a significant effect on our weather here.

The potential for a significant winter storm evolves out of the Bering Sea for this forecast. The method, referred to as the Bering Sea Rule, takes the occurrence of high and low pressure events in the Bering Sea, and expects a similar weather phenomenon to occur in the US about 17-21 days later. This is almost exactly like the Typhoon Rule I commonly discuss when analyzing winter storm potentials, except now the area to watch is the Bering Sea, and the timeframe from occurrence in the Bering Sea to reciprocation in the US is now 17-21 days.

Getting back to this model forecast, we can see a very strong extratropical cyclone impacting the far western Aleutian islands, with a minimum central pressure of 930 millibars. If we utilize the Bering Sea Rule here, and extrapolate the forecasted time frame above of November 7th out seventeen to twenty-one days, we find ourselves with a potentially significant winter storm in a rough November 24-30th timeframe.

The evidence supporting this storm doesn't end here; we have a long way to go...

Tropical Tidbits
The model now shown above is the Canadian GEM forecast model, with the same 500mb geopotential height and MSLP contour parameters as the GFS model (for future reference, all model guidance we will analyze here today will use those 500mb and MSLP parameters). This forecast is valid on the afternoon of November 8th, nearly a day later than the GFS forecast above. Despite this time difference, we still see that very strong storm system from the GFS model now appearing in the GEM's forecast, with a minimum central pressure of 944mb spread across a heavy majority of the Bering Sea. Once again, this supports the prospect of a strong winter storm, if not solely a strong blast of cold air, to the US in late November.

Tropical Tidbits
Let's now examine the ECMWF model, commonly referred to as the most accurate weather model in the world at this time. The forecast panel above is valid for the evening of November 7th, and even though the storm hasn't reached the Bering Sea yet, its minimum central pressure is already down to whopping 930 millibars!

Tropical Tidbits
If we fast forward that same ECMWF model forecast up twenty-four hours to the evening of November 8th, we find that the storm system has gotten a bit stronger, and quite larger. The minimum central pressure is now down to 928 millibars, with the storm's influence spread from Russia to Alaska! The ECMWF model joins the GFS and GEM in supporting a strong winter storm for late November, possibly in time for Thanksgiving.

I could keep adding more and more model guidance projections to prove my point, but the general premise is already pretty clear by now. The three aforementioned models, in conjunction with the GFS-Parallel, ECMWF ensembles, and GFS Ensembles are favoring a very strong storm system in the Bering Sea for around November 7th or 8th.

The next question becomes, who could it target?

Well, if these model forecasts do verify as they appear now (which is nowhere from certain), each model projection would hit a difference part of the nation. The forecast members with the storm further west in the Bering Sea would find the Thanksgiving storm system in the Central US, while the model guidance members further east into the middle Aleutian Islands (or even further east) would likely find the consequential Thanksgiving timeframe storm in the East US, possibly along the coast.

Because this potential storm is still over 7 days away from appearing in the Bering Sea, and thus guidance can be expected to change with uncertainty, it's not worth putting down an accurate estimate as to where the storm may hit. However, just glancing over trends from these three shown models, as well as others available, would only slightly support more of a Central US/inland East US storm system.

To summarize:

- Model guidance is favoring a very strong storm to hit the Bering Sea in the next few days.
- If this storm ends up as strong as projected, it may result in a very strong winter storm around the Thanksgiving timeframe.
- A lot of uncertainty exists with this storm at this time.


November 8-12 Potential Storm System & Cold Weather

The November 8-12 period is being monitored for a potential storm system, as well as a shot of cold air.

Tropical Tidbits
The graphic above shows vorticity values at the 500mb level of the atmosphere, below the level where airplanes fly but still pretty high up in the sky. In this image, we see vorticity values forecasted for the afternoon of November 2nd. Notice the strong, negatively-tilted cyclone to the northwest of Japan, highlighted by the enhanced vorticity values. This trough quickly circles up and occludes as it pushes north and east of Japan.

This particular storm potential is a tough one to discuss, as the storm will be very strong and will affect Japan in some way, but the trough itself won't actually hit Japan; it'll occlude too fast and will be shoved north. As a result, for now, the outlook calls for a cold front with associated chilly air to affect the US around this November 8-12 period. I'm not quite confident yet in calling for a storm system, but will leave the possibility open in the event some sort of disturbance can be whipped up, or become associated with the cold front pushing through around this timeframe.