Monday, November 3, 2014

Thanksgiving Potentially Significant Winter Storm

This is an update to yesterday's post, discussing a potentially significant winter storm in the November 24-30 timeframe.

Tropical Tidbits
The image above shows the ECMWF 500mb geopotential height values in color, with superimposed mean sea level pressure (MSLP) values and appropriated high/low pressure marks. In this image, valid for November 8th over the West and North Pacific basins, we see an incredibly strong extratropical storm circulating over the far western Aleutian Islands, with a minimum SLP value of 920 millibars.

To give you an idea of how strong this reading is, if this forecast from the most prestigious weather model in the world were to verify, we could very well break the record for the lowest extratropical minimum SLP value ever recorded (the lowest SLP value ever belongs to Typhoon Tip, with 870 millibars, but that was a tropical system). But the most intriguing thing?

It could actually happen.

This whole process looks to begin with the remnants of Typhoon Nuri, currently offshore Japan, which will race northeast and quickly strengthen as it approaches the Bering Sea. While storm systems tend to strengthen in the Bering Sea, this one may strengthen much faster and even stronger than most due to that tropical component from Typhoon Nuri.

But wait! Why should we care about this? What about the Thanksgiving storm? 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. Therefore, if we take the storm in the image above (valid on 11/8) and extrapolate it out using the BSR guidelines, we come up with a potentially significant storm in the November 25-29 period, give or take a day or two.

Tropical Tidbits
This next graphic above shows the same 500mb height values and superimposed MSLP values over the West Pacific, like the ECMWF model forecast, but now from a different model. This is the GFS-Parallel model, rumored to be the next-generation forecasting model that will soon replace the GFS model. For the time being, this is an experimental model, and won't become operational for another month or two, but it's still worth using for examination purposes.

Here, we see in this forecast for November 8th, the minimum SLP is down to 914 millibars. That's about 6 millibars stronger than the ECMWF forecast, and indicative of a likely-record-breaking storm, but the premise remains the same. Both model guidance systems are showing a very strong storm system in the Bering Sea in this November 8th timeframe. If you're a true analyst, you might notice that the GFS-parallel forecast is just a bit east of the ECMWF forecast. That does make a difference in this forecast, and I'll explain it a bit later in this post.

Tropical Tidbits
In the spirit of including the old with the new, we'll go over the currently-operational GFS model and its forecast. Here, we see the projection for November 8th calls for a 923 millibar cyclone to hit the Bering Sea. This is the weakest of the three forecasts we've analyzed thus far, but that's a pretty odd thing to say when the weakest member of the three is "only" at 923 millibars. Despite the slightly weaker outlook, the consensus is still for a very strong, possibly record-breaking storm to hit the western Bering Sea.

Tropical Tidbits
Lastly, to show I'm willing to include all angles and not, as some may accuse, only the strongest projections, we'll go over the ECMWF ensemble set. The ECMWF ensembles, commonly called the most accurate ensemble set in the world at this time, are forecasting a minimum SLP of 947 millibars for this storm system on November 8th.

If you aren't sure how to interpret this, I'll sum it up: WOW!

Ensemble guidance is composed of a number of 'members', or versions of the original model that have been slightly altered, intentionally, to produce a 'spread', or variation, in the forecast. This then adds accuracy to the forecast. It's very surprising to see a set of ensembles - from the ECMWF, no less - to be calling for a 945mb storm to strike the Bering Sea. This average of 52 - yes, fifty-two separate ensemble members - indicates that certainty for such a strong storm to hit the north Pacific is on the rise. It's quite plausible we see this minimum SLP from the ECMWF ensembles drop in future runs.

But we're not just watching for a powerful winter storm- we're watching for potentially significant cold!

The animation above shows temperature anomalies at the 10 millibar level, in the far upper reaches of the stratosphere, over the past month or so. According to this animation, we recently saw a very early stratospheric warming event overtake the Eurasia and North Pacific basin. If we remember that the warmth in the stratosphere typically results in very cold weather at the surface about 2-4 weeks later... put the pieces together, and we could very well see a strong body of cold air intercept this potentially significant winter storm system, making for one heck of a situation that could unfold.

You might have noticed a combination of rather ominous language about the storm, as well as more than a few caveats. Both are well warranted, as we'll have to wait and see if this storm ends up being as strong as projected in the Bering Sea.

To summarize:

- The potential exists for a significant winter storm around Thanksgiving.
- With stratospheric-induced cold air possibly intercepting this storm, winter weather may be a serious issue to contend with.
- High uncertainty still exists, but appears to slowly be waning.
- As of now, the Central and East US look to be affected by this storm (i.e. Midwest, Great Lakes, Ohio Valley, Northeast...).