Tuesday, November 4, 2014

Thanksgiving Potentially Significant Early-Season Snowstorm

This is the latest discussion about the possibility of a Thanksgiving early-season snowstorm. This discussion will provide a comprehensive overview of model guidance, in addition to the expected cold air influences and potential location of the storm.

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 923 millibars.

This has been the theme over the past few days, as the remnants of Typhoon Nuri are expected to race northeast into the Bering Sea, and undergo 'bombogenesis', or incredibly rapid strengthening (weather folk call it "bombing out" to mean a storm quickly becomes stronger), to develop into this projected 923 millibar beast. The ECMWF model has this storm located the furthest west out of all the guidance we will go over tonight. Make sure to keep an eye on placement of the low in all of these forecasts, as it will make a significant difference on where this resultant storm may end up.

Tropical Tidbits
We'll move now to the GFS model projection, again of 500mb geopotential height values, and superimposed MSLP forecasts. These two parameters will show up on the remainder of model guidance, to keep things as simplistic as possible. In the GFS forecast, valid for the same November 8th timeframe, we find the storm placed substantially east of the ECMWF outlook, and with a minimum SLP value of 924 millibars.

A brief background on why we're focusing on the Bering Sea so much 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. 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. 

We can see the theme of a low 920s millibar storm hitting the Bering Sea, so let's keep pushing ahead.

Tropical Tidbits
Next up, we'll examine the GFS-Parallel forecast. This is the model expected to succeed the current GFS model, with higher resolution and corrected model biases. This model shows the Bering Sea storm bottoming out at 919 millibars, the strongest forecast of the three observed thus far. This storm appears to be a combination of the ECMWF and GFS models; you can click on one and move back and forth between images to get a sense for such a shift.

We've evaluated three major models thus far, all of which combine for a mere five millibar spread (minimum 919mb to maximum 924mb). We can now see confidence greatly rising in this potential for a very strong storm to hit the Bering Sea; let's see if ensemble guidance agrees.

Tropical Tidbits
Moving ahead to the ensembles, first and foremost the ECMWF Ensembles, we find the average of all 52 members - yes, that's fifty-two individual ensemble members - to place this storm at a strength of 938 millibars. With the storm only 96 hours away from entering the Bering Sea, and ensemble guidance continuing to strengthen with time, confidence only rises in this possibility of a near-record-breaking extratropical cyclone to strike the Bering Sea. 

As a side note, notice how the ECMWF ensembles are slightly east of the ECMWF model itself. We'll discuss this more later on.

Tropical Tidbits
Finally, we'll take a look at the Canadian GEM ensembles. The GEM model itself was unavailable for use in this post, so we'll use the ensembles as the next-best-thing, if not the best thing. The GEM ensembles show the average of all ensembles (around 22 individual members, if I recall correctly) to be slightly east of the ECMWF ensembles, with a mean strength of 948 millibars to be the weakest with this storm out of all global guidance. 

So, now that we've evaluated all model guidance available at the time of posting, let's summarize a few things to take away.
- Major operational model guidance is down to a consensus that the storm will likely land somewhere around 920 millibars in strength. That's a VERY strong storm.
- Ensemble guidance is still in disagreement, but does find a conclusion that there will be a strong storm in the Bering Sea for this timeframe.

I said I would discuss why location is key in the models, so let's quickly discuss. From the team at KOPN, it appears that weather in the Shemya Island region of the Aleutian Islands correlates roughly to weather in north-Central Missouri. Thus, one could believe that a storm northwest of Shemya would see a storm in the Plains, a storm south of Shemya might see a Texas storm system, etc. 

The pinpoint above shows the location of Shemya, Alaska. Doing a quick visual comparison to the five model guidance graphics we went over earlier in this post, but also remembering this location correlation is NOT proven quite yet (as of now, it's interpreted to act as a guideline), this Bering Sea storm would likely correlate to a powerful storm in the Upper Midwest, Plains, Midwest, Great Lakes, Canada... this particular correlation method is not an exact science, and should not be interpreted as such. Hence why it's only a guideline for now

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.

Let's summarize all of this.

- Confidence is growing in the threat of a near-record-breaking extratropical storm system impacting the Bering Sea.
- If this forecast goes as planned, a powerful storm system would be expected to traverse North America around or just after Thanksgiving.
- Abundant cold air provided by the stratospheric warming may enable a larger threat of snow, if such a threat does evolve.
- A high amount of uncertainty still exists with this storm potential, hence the continued caveat-laced language in these posts.


November 8-12 Potential Winter Storm

Model guidance appears to be toying with the idea of a winter storm in the November 8th-12th timeframe.

Tropical Tidbits
The image above shows the ECMWF model's forecast of mean sea level pressure (MSLP) contours and 850mb temperature values, valid for November 11th of next week. In this image, we see a strong storm system pushing north into western Illinois, dragging down sub-freezing air behind it only a few thousand feet off the ground. This 995 millibar storm would bring predominantly snow to areas north of the thin red freezing line, which would include Wisconsin and Iowa, among other regions.

I cannot post graphics due to copyright concerns, but this ECMWF run spits out over 12" of snow in parts of Wisconsin due to this storm. The ECMWF has shown this storm inconsistently for a few runs now, keeping uncertainty high.

Tropical Tidbits
The image above shows the GFS-Parallel model's forecast of MSLP values and 500 millibar geopotential height values, valid on November 10th. In this image, we see a messy set-up for this storm, with one low pressure system located in northwest Missouri, apparently attached to a very weak low pressure system in Michigan. This set-up would likely keep most snow out of the picture, save for the Upper Midwest, and is a plausible solution, given that this model will succeed the GFS model in due time.

This whole potential for a winter storm arises from what we saw happen in Japan a few days back.

The above image shows observed 500 millibar geopotential height anomalies over the Northern Hemisphere (left panel), as well as cloud cover and high/low pressure demarcations (right panel). Note how we saw a very strong storm stirring up near Japan on this date, which was November 2nd. We saw a strong trough wrap up and occlude just north of Japan, still bringing through a respectable shot of cold air. I had discussed earlier about how this could provide the basis for a potential winter storm, and it appears that this potential is being realized now.

Because this storm appears to occlude north of Japan, but still scrape the western side of the nation as it does so, I'm still content believing the risk for a winter storm remains. Just how great that risk is must be examined further, but the chance remains.

To summarize:

- Model guidance is indicating the potential for a winter storm to hit the Central US in the next week or so.
- Very high uncertainty exists with this storm potential.
- Depending on cold air availability, as well as if the storm actually happens, accumulating snowfall may be a concern.