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.

Andrew

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.

Andrew

November 5-8 Potentially Significant Winter Storm

I'm watching the potential for what could be a significant winter storm around the November 5-8 period.

PSU

The image above shows a four-panel forecast from the GFS model, valid on November 7th. The top-left panel of this image shows 500mb vorticity values, where reds show positive vorticity (trough/low pressure) and purples depict negative vorticity (ridging/high pressure). On the top-right panel, we see the forecasted mean sea level pressure contours, with appropriated high and low pressure demarcations. The red dashed line in the US shows, in essence, the rain/snow line. Since we are in the winter season, temperature profiles might not be as supportive of snow as one may think, but we'll address that later. The bottom-left panel shows relative humidity values in the lower levels of the atmosphere, while precipitation is seen in the bottom-right panel.

What do we notice about this forecast? There's a massive low pressure system barreling into the Northeast, with enough cold air in its wake to indicate the threat for heavy snow, according to this GFS forecast. This strong storm system looks to originate from Canada before gathering its strength along the coast, as many coastal storm systems do.

According to this forecast, the storm would certainly drop big snows, but let's see if other model guidance is catching on.

PSU

As a matter of fact, other guidance does support this idea. The above image shows the ECMWF's depiction of the atmosphere, valid for the same timeframe as the GFS image, and the first thing we see is one heck of a storm. In the 500mb height anomaly panel on the top-left, where blues depict ridging/high pressure and reds show troughs/low pressure, it's quite apparent that a major storm is overtaking the Northeast. The MSLP chart on the top-left once again shows a very strong storm system, similar to the GFS projection. Strong lower-level winds in the bottom-left panel, and sub-freezing temperatures only a few thousand feet above the ground confirm that this would likely be a very strong winter storm, IF this forecast were to verify, in line with the GFS.

Are there any other model guidance systems showing this sort of winter storm?

Yes.

PSU

The chart above shows the 180-hour projection from the NAVGEM model, with each panel the same parameters as those on the GFS chart earlier in this post. This model only goes out to 180 hours on this particular web site, but when we compare the 180-hour forecasts from the GFS and ECMWF models, the atmospheric set-up prior to this storm at 240-hours is incredibly similar among the three model guidance.

Those of you who are weather enthusiasts know that the NAVGEM (formerly the NOGAPS) model isn't worth putting much stock in; I'm mentioning it here just for pure discussion.

Those models aren't the only things in the arsenal favoring a winter storm, possibly of significant proportions.

PSU

This final graphic here shows you 500mb height anomalies across the Northern Hemisphere on October 27th. If we look closely over Japan, and if you animate it here between frames T-10 and T-6, you can see a small but powerful trough rotating around into central Japan. Using the Typhoon Rule, which states weather phenomena occurring in Japan is reciprocated in the US 6-10 days later, we might expect a blast of cold air with an associated possibly-strong storm system in a November 2-7 time period, very close to (if not completely encompassing) the projected timeframe for this coastal storm in the Northeast.

Finally, just for a scope of how tricky these temperature profiles might be since it's fall, check out the 12z GFS snowfall forecast with this storm.

Instant Weather Maps

We do see some slight accumulation in New York, but it's not until the storm hits southern Canada, west of Nova Scotia, that heavy snowfall projections start to come up. This will, of course, change in coming days, and I'll update as needed. High uncertainty remains an issue in this forecast.

Andrew 

2014-2015 Winter Forecast Update: Potentially Frigid Winter Ahead

This is a brief update to the temperature outlook for the 2014-2015 winter, where I'll close out discussions on the October-related patterns, such as the Snow Advance Index, Lezak Recurring Cycle, and October Pattern Index.

Rutgers

The image above shows snow cover anomalies over the Northern Hemisphere on October 19th. This is the latest we can get these charts, as data outages have lost snow cover records from October 20th to present day. Regardless, we can see the massive swath of above-normal snow cover extending across Eurasia, as the blues show. This snow cover has been expanding at a steady rate, and has maintained a well-above-normal stature.

We can apply this to seasonal weather trends in the coming winter. According to Judah Cohen, creator of the SAI concept, the rate of snow coverage across Siberia in the month of October can predict the following winter Arctic Oscillation (AO) phase, which can then play a big role in determining if we will see a cold winter. When snow coverage for October is well above normal, like this year, the AO will usually be negative in the following winter. Similarly, below normal snow coverage spells a positive AO for the following winter. Needless to say, the Snow Advance Index is favoring a potentially-strong negative AO this winter, which is likely to increase the risk of cold weather outbreaks here in North America.

OPI

Second, we have the October Pattern Index, or OPI. The above image shows anomalies of the October Pattern Index over the last several days, since the start of October. The OPI, the concept of which was brought about by a group of Italian scientists, says that monitoring of the atmosphere during the month of October can yield great hints at what the coming winter will bring. October is a month well-known for big winter-predictors showing their cards for the coming cold season (i.e. the LRC, and Judah Cohen's Snow Advance Index (SAI)), but it may interest many to know what the OPI may be one the best, if not the best predictor of the upcoming winter season out of the three mentioned above.

The explanation page of the OPI tells of the index's incredible accuracy, around 90%, of being able to predict the December-January-February Arctic Oscillation. In the winter, a negative phase of the Arctic Oscillation (AO) means the polar vortex is weak and is more prone to sending cold outbreaks to the mid-latitudes, while a positive AO indicates a strong polar vortex, hence a warmer winter increases in probability.

The OPI has been in sustained negative territory throughout the month, not once touching positive marks. This tells us with overwhelming certainty (or at least as much certainty as we can have at this point) that the coming winter's Arctic Oscillation will be negative, and this is backed up by the aforementioned SAI discussion.

Lastly, let's go over the Lezak Recurring Cycle.

HPC/WPC

Something I've discussed on here more than a few times is the concept of the Lezak Recurring Cycle, or LRC. The LRC was developed by meteorologist Gary Lezak, and discusses the idea that weather patterns which develop in October leave a 'footprint' of sorts that is repeated in a regular interval, between 40-60 days through the winter and following spring. In other words, the weather patterns that develop in October repeat themselves for the better chunk of the next year.

Since mid-September, we've seen predominantly below-normal temperatures for large swaths of the Central and East US. One of the more impressive atmospheric set-ups came with this very strong upper level low positioned just north of the US/Canada border. Notice how the influence of this low extends all the way to the Gulf Coast, per the contour lines. My worry is that this upper level low will end up being another piece of the polar vortex, like we saw last winter, that might push south and bring intense cold back once or twice through the coming winter. I'm not going to speculate any further, but some sharp Arctic blasts may be in the works for the next few months.
As we dip into a warm period right now, this will likely reflect in the LRC with some warm periods in the coming winter, but I'll go into that further down the road when we know how long this warmth might last.

To summarize:

- Indications are that a potentially frigid winter is once again in the works.

Andrew

Gulf of Alaska Waters Continue Cooling; Winter Implications Changing

As I had noted in my Long Range Discussion post last week, the Gulf of Alaska had begun a cooling trend as stormy conditions overtook the basin. Since then, waters have continued cooling to the point where much of the basin is now below normal.

ESRL

The image above shows sea surface temperature anomalies over the North Pacific basin on October 1st. On that day, we saw well above normal SSTAs across the Bering Sea, with more variable conditions to the southwest. The waters east of Japan were predominantly cool, in contrast to the very warm Gulf of Alaska and waters off the coast of California, Oregon, Washington state, British Columbia, etc.

I've seen many forecasters across the Internet push for a cold winter due to the waters being very warm in the Gulf of Alaska, myself included. It's not a bad path to choose- after all, it's that same swath of warm waters last winter that allowed for such brutally cold weather to enter North America. With those waters present again leading into fall, if they stay put, it's quite plausible that we might see another cold winter for some of the same reasons as last winter.

Those warm waters are gone.

ESRL

The graphic above shows sea surface temperature anomalies for the same area as the image on October 1st, but this graphic is now valid on October 22nd, just a few days ago. The changes in only a three-week period are astounding. The Bering Sea has cooled off dramatically, now predominantly below-normal instead of the nearly-3-degrees-above-normal anomalies in place on the 1st. The waters to the east of Japan are still below normal, but that swath of below-normal anomalies now extends far to the east, and has decimated the warmth in the Gulf of Alaska.

I had said that this would be a possibility with all of the storm activity in the Pacific, but many believed I was losing my mind...

So, we've lost the factor that provided such a cold winter for us last year. Now what?
The alignment of sea surface temperatures leads us to believe we are now entrenched in a positive-PDO pattern. If you look closely, you might be able to make out a semi-circle of warmth along the west coast of North America encircling the cool waters further west. This horseshoe-like alignment is a textbook example of a positive PDO, as the graphic below shows.

NCSU

The phase-definition chart, my personal label to the graph that tells us which phase the PDO is in, also confirms the positive PDO orientation.

NOAA

So, we've lost the big pool of warm water anomalies in the Gulf of Alaska, and as of right now, we're neck-deep in a positive PDO event.
Sounds like a real downer for cold weather fans, right?
Not exactly.

ESRL

The image above shows correlations with the Pacific Decadal Oscillation (PDO) and surface temperatures during a December-January-February period. What this chart is saying, is whenever the PDO is positive, temperatures in the West US will be warm (due to the positive correlation), and temperatures in the East US will be cold (due to the negative correlation). Similarly, when the PDO is negative, temperatures in the West US (East US) will be cold (warm), due to the positive (negative) correlation. So long as the PDO remains positive this winter, the risk of a cold winter would still be maintained for most of the Central and all of the East US. It remains to be seen if the cold waters will keep pushing east and erase the positive PDO completely, but as of right now, this would be a beneficial development for both cold weather fans in the East US, and warm weather fans in the West US.

Interestingly enough, positive PDO winters tend to bring wetter than normal winters to the Northern Plains...

Andrew