Wednesday, December 17, 2014

Christmas Eve Potentially Significant Winter Storm

I'm watching the increasing potential for a significant winter storm in the Central US, created by what could be one of the strongest low pressure systems in the last few years, if not longer.

Instant Weather Maps
The above image shows 500mb geopotential height contours over the United States from the prestigious ECMWF model, valid for December 23rd. In this image, we see two pieces of energy in the Central US. One is a closed low, placed in the Northern Plains, while the other is a deepening trough in the Southern Plains, shown by the depression in contour lines. In a situation similar to that I described in my December 10th post, we see this trough begin to lift northward into the Central US, but not before the "bomb" goes off...
Instant Weather Maps
Just 24 hours later, on the morning of Christmas Eve, we find that the two pieces of energy have combined, and the trough as a whole has now attained a negative tilt, indicating it has reached its mature phase. As a result, the storm undergoes rapid strengthening, very near the criteria of bombogenesis. Bombogenesis is a meteorological term used to describe the phenomenon when extratropical cyclones rapidly strengthen, and their minimum central pressure values decrease by 24 millibars in a 24 hour period. Switching between December 23rd and 24th, we find that pressure values from the ECMWF lower by about 19.3 millibars, only a little ways off from being a true 'bomb'. Even though this storm doesn't fit the criteria, the strengthening is nothing to shake a stick at.

This solution is a tricky one. I don't have access to pay-to-view weather model graphics (yet), so I cannot see the precipitation pattern from this system. However, from what others are discussing, it appears that not only will the nearly-due-northward movement of this system foul up the precipitation shield, but temperature profiles are above freezing in many spots that would otherwise see snow. This could be placed on model error, or it may be a legitimate forecast. For now, it's just too far out to tell one way or another.

Weather Online
What is interesting, however, is the difference between the ECMWF model and the ECMWF ensembles. The above image shows mean sea level pressure values, valid on Christmas Eve (the same time as the second image we discussed). By simple comparison, note how the ECMWF model places the center of this storm somewhere in southern Ohio, while the ECMWF ensembles put northeastern Indiana in the center of this cyclone.

A further east track of this storm system could result in more of a snow impact to the Great Lakes, instead of primarily wind-driven snow in the Midwest and Ohio Valley from current runs of the ECMWF. But as I said earlier, we are still quite a while away from nailing down these details.

To summarize:

- Model guidance is beginning to sniff out a very strong storm system impacting most of the Central and East US on Christmas Eve into Christmas Day.
- This would severely impact travel.
- Snow would be confined primarily to the north-central Great Lakes into Canada.
- Very high uncertainty still exists.


Tuesday, December 16, 2014

Atmospheric Trifecta Preparing to Deliver Cold, Snowy January

A trio of atmospheric signals are gearing up for what could be a rather cold, snowy January.

Research I completed last night showed significant (10"+) snowstorms in the Midwest are most favored under the negative phase of the Arctic Oscillation, the negative phase of the East Pacific Oscillation, the negative phase of the North Atlantic Oscillation, and the positive phase of the Pacific-North American index. We look to have at least three of these factors locking down the atmosphere to round out December and kick off 2015.

Tropical Tidbits
The image above shows the GFS ensembles' forecasted 500mb geopotential height anomalies over the Northern Hemisphere, valid on the evening of December 26th. On this graphic, we see a prevailing negative EPO, as highlighted in this graphic with ridging over the Gulf of Alaska. A negative NAO has also emerged just west of Greenland, combining with ridging in Eurasia to provoke a negative AO regime. I'm watching for a positive PNA signal, but as a ridge is passing over the region, I'm content on acknowledging a trifecta for now.

It doesn't end there, however...

Tropical Tidbits
By December 30th, we find more than a few big developments have taken place. We see our negative EPO ridge has blossomed into a massive swath of high pressure, now extending along the West Coast into Alaska... and then some. Ridging over Greenland still exists, paving the way for a continued negative NAO and negative AO tandem. As a result, we continue to observe below-normal anomalies in the East US, signaling a very cold period to end December and start January.

My thinking is we'll see those below-normal height anomalies slowly progress west, as a west-based negative NAO (where the ridge is displaced to the west of Greenland) typically favors cold weather more into the Central US. This would be amplified/supported by the strong -EPO ridge, slowly evolving into a +PNA signal, it appears.

Now, onto the snow...

Tropical Tidbits
The image above shows the GFS ensembles projection on December 28th, showing jet stream wind speed values. There are a few things to note here.
First off, check out the extended Pacific jet stream. We're seeing that occur in the very near future, and it's no coincidence that this extended jet stream into North America is happening at the same time some storm threats are arising. The extended jet allows for cyclogenesis in the Northern Pacific, where energy may be shunted east into the United States, leading to those winter storm chances.
Second, we see a notable subtropical jet stream (STJ), as depicted by the green colors extending from the northeast Pacific into Mexico and the South US. With a negative NAO regime in place, as well as an enhanced subtropical jet stream, it's possible the East Coast could get rocking and rolling with these storm chances.

As for the Central US, I alluded to the below normal height anomalies pushing east. As far as snowfall impacts, clipper systems look to make their return, as the strong ridge pushing into Alaska produces a sustained northwest flow pattern (where the winds are out of the northwest). This could lead to not only episodes of snow in the North US, but also lake effect snow episodes, as all Great Lakes are currently well below last year's ice levels at this same time.

To summarize:

- The atmosphere is preparing to shift from a generally mild December pattern to a rather harsh January pattern.
- Sustained cold weather is likely for most of the nation east of the Rockies.
- Enhanced chances of snowstorms will be seen both along the East Coast, into the Central US (depending on individual storm tracks, of course).
- Buckle up, things are about to get fun.


Saturday, December 13, 2014

December 19-21 Potential Winter Storm

Model guidance is supporting the risk of a winter storm in the December 19-21 timeframe.
I'm trying to decide how to start off this post, since simply going through model guidance won't work. I'll begin by explaining each graphic, doing a compare/contrast as we do so.

Tropical Tidbits
The latest GFS model has a trough pushing into the Midwest on December 20th, as the blue colors and depression of contour lines shows. This trough is beginning to close off after becoming negatively tilted, and is pushing northward as a result. The surface low takes a track through the Ohio Valley on this run, dropping appreciable snows from east Kansas into southern Michigan. North Missouri sees snow over 6" from this system.

In this overview of the pattern, we can diagnose a few items arguing for this more inland track, as future guidance I'll show you will depict an East US track. First of all, we have another strong trough dropping into the West US, trying to work its way southward. This will try and force a ridge to develop in the central and eastern Rocky Mountains, as we can already see above. However, as we see a deep upper level low over Greenland, this ridge won't be able to exert too much influence (that ULL over Greenland defines the positive phase of the North Atlantic Oscillation/NAO, notorious for keeping the jet stream very 'non-wavy')
What does make a ridge form, however, is the troughing in the Bering Sea into the Gulf of Alaska, a textbook positive East Pacific Oscillation (EPO) pattern, which will act to limit cold air reserves in Canada, but also try to direct storm systems northward. The latter influence is seen well in that ridge centered over New York, just east of the storm system in question. That same northward influence could happen along the East Coast, but for now, the GFS favors this solution.

Tropical Tidbits
Here's the ECMWF 500mb geopotential height anomaly forecast, valid at the same time as the GFS graphic. We see more than a couple significant differences here.

First and foremost, the storm system in question seems to be developing a second piece of energy, signified by that second dip in the contour lines along the Gulf Coast. Looking at the forecast following this timeframe. it looks as if that second piece of energy will act to pull the main trough east, and develop it into an East Coast system. This is far different from the GFS, which maintains a single trough.
Additionally, we see the entrance time of the second trough into Western North America has been slowed by about a day. This allows the ridge in the central and eastern Rockies to flourish well into Canada, where the ridge up there is also quite a bit stronger than its GFS counterpart. Consequentially, the storm system is suppressed to the south in the ECMWF forecast. Also, note the lack of a ridge to the east of the storm system in the ECMWF image, compared to the GFS. This, combined with the extra piece of energy along the Gulf Coast, appears to favor an East Coast solution. The solution results in this pattern, valid 24 hours after the image above:

Tropical Tidbits
Purely for comparison, here's the GFS snowfall forecast I mentioned earlier, expressing the solution in opposition to the ECMWF:

Instant Weather Maps
I've been looking back and forth between the ECMWF and GFS images I've shown above, trying to figure out which one I think is the most valid, and I can't decide.

On one hand, the ECMWF projection appears to be obeying the positive EPO signal, as exhibited by troughing along the west coast and a strong ridge in central Canada.

On the other hand, the GFS is doing well with the emergence of a ridge just east of the trough, possibly as a result of the negative PNA orientation out west, as the aforementioned second storm system drops into the West.

It will ultimately depend on the timing of when the trough drops into the West, just how strongly the atmosphere responds to the +EPO signal, and (of course) if that secondary piece of energy forms along the Gulf Coast. It's worth noting none of the model guidance is having any consistency with the ridge in Canada, purely by looking at run-by-run comparisons. Additionally, today's 12z ECMWF run (examined above) is the first one to have that secondary piece of energy develop to the south of the storm, something that does not bode well for any consistency that either the GFS or ECMWF may have built up. I'll pass on giving my opinion right now, because this is a truly grotesque set-up.

To summarize:

- A winter storm is possible for the Central or East US in the December 19-21 timeframe.
- Model guidance is expressing little to no consistency on a defined track for this storm.
- Cold air availability will eventually become a concern.
- Anomalously low confidence exists.


Thursday, December 11, 2014

Christmas Potentially Significant Winter Storm

I'm now watching the risk of a strong winter storm rise as we move towards Christmas.

Tropical Tidbits
The image above shows 500mb geopotential height values (colored shadings), with mean sea level pressure (MSLP) values superimposed. In this graphic, valid on December 16th, we see a pair of very strong storm systems, one on each side of Japan. The prognosis is that a strong system will push into the Sea of Japan (located west of the country), while a second storm system will develop in the southern part of the country and move northeast-ward, skirting the eastern fringe of Japan as it does so. It is this second storm that we need to keep a close eye on, and is the one we will be discussing here today.

Using the Typhoon Rule, which states that weather phenomena occurring in East Asia is recipricated in the US about 6-10 days later, we can extrapolate this December 16th date out and predict a storm in the United States in a December 22-26th period... right in the Christmas rush.

But we can expand on this quite a bit more. The storm will be shooting north along the eastern coast of Japan. This does have an impact on the expected storm in the US. As you might expect, it raises the chance of this consequential storm also moving northeast-ward rapidly, and from there, we come out with two prevalent/possible storm tracks:

- A Panhandle Hook storm, where the system shoots north from the Southern Plains. These storms are climatologically favored to bring heavy snow to cities in the east-central Plains and Lower Great Lakes. This scenario is a possibility, as that strong storm in the Sea of Japan would likely correlate to a strong North Plains cyclone. This would keep that body of low pressure east of Japan in an area close-by, as the storms would eventually phase (not to mention low pressure areas are attracted to other low pressure areas).

- An East Coast storm. Because this body of low pressure is forecasted to merely skirt the eastern side of Japan, this could be a plausible scenario. We won't know if either of these are correct until we have more model runs to access.

The graphic above only shows the GFS model view... let's head on over to the European model projection.

Tropical Tidbits
Wow! Can you see the change?

This graphic, showing the same parameters as the GFS image, and for the same timeframe, portrays that strong storm in the Sea of Japan, but now the second storm skirting eastern Japan is more inland. It hasn't shifted much, but it has shifted nonetheless.

What does this mean? It means it's time for East Coasters to throw in the towel.

Not really, but a more inland storm does favor an inland track when the storm comes around in the US. The interesting thing is, this more inland track is an idea. Here's why.

Recall that, whether you learned it or just know it through logic, low pressure areas will try to move towards areas with the least resistance, in this case the least high pressure. My theory here is that the storm in the Sea of Japan, the stronger of the two (shown on the GFS as 993mb, 980mb on the ECMWF), is trying to pull the storm skirting east Japan towards itself. Down the road, model guidance shows the second storm absorbing the stronger Sea of Japan storm, rather than vice versa, and that's also a possibility.
My point here is, there is the possibility of a phased storm.

For those who aren't as knowledgeable with weather lingo, a 'phased storm' is a storm system which is made up of previously-two or more pieces of energy. Typically, phased storms end up stronger than either of the first two pieces of energy were. I'm not holding my breath on this Christmas storm phasing, but it probably isn't a bad idea to keep it in the back of your mind.

Regardless of if this storm phases, remember that the storm on the east coast of Japan is projected to be below 1000 millibars, so it's likely to be a nice little storm in itself.

Tropical Tidbits
We've now confirmed that not only are looking at a storm in the Christmas time period, but model guidance has amped up that threat since yesterday. Now, we have to diagnose the weather pattern here at home in that December 22nd - 26th timeframe, to see if we can pull any hints out.

I've posted the image above from the GFS ensembles, showing 500mb geopotential heights on Christmas morning. Warm colors depict ridging/high pressure, usually indicative of warm and quiet weather. Similarly, blues indicate troughing/low pressure, accompanied by colder and stormier weather. We have more than a few things to talk about with the above graphic.

First and foremost, we're looking at the Pacific driving our pattern to round out December. Tropical activity in the Equatorial Pacific will be dying off in the next few days (more knowledgeable weather folks know this as the MJO weakening), which will shift the weather pattern 'responsibilities' to the North Pacific.

We look to have a positive Pacific-North American (PNA) index pattern in place for this event. We can observe this positive PNA as a ridge forming in the West US, which allows the jet stream to buckle in the Central US. Such a pattern is climatologically favorable for a Central US storm track. In addition, a positive East Pacific Oscillation (EPO) signal will begin showing up, as ridging overtakes Canada. This will be in part due to that positive PNA, but the further east you go, the more the EPO influence takes over. A positive EPO doesn't affect the storm track so much as it does temperatures (above normal in the North US). When we factor into account slight ridging along the Eastern Seaboard, we start to see that signal for a storm system in the Central US, favoring development in the Central US.
I'm a bit skeptical, however, Many Northeast weather buffs may know that winter storms are favored in the East when the North Atlantic Oscillation (NAO) changes phases from positive to negative, or vice versa. Guess what the forecast for the NAO is around this storm's timeframe:

We've got a dilemma on our hands, with some variables favoring a Central US storm, and others favoring an East US storm. So is the world of forecasting...

To summarize:

- A winter storm appears to be in the cards for December 22-26th, likely impacting Christmas travel plans.
- A second storm system may need to be watched for the Northern Plains.
- The primary threat here may become a storm favorable for heavy snow, either in the Central/East US (ideally the Ohio Valley/Midwest) or along the Eastern Seaboard.
- Rather high confidence in the threat of a storm in this timeframe, but low confidence in who will be most affected.


Wednesday, December 10, 2014

December 22-26/Christmas Potential Winter Storm

I'm watching the potential for a winter storm in the December 22-26 timeframe.

Tropical Tidbits
The image above shows the forecasted 500mb vorticity values on the morning of December 16th, valid over the West Pacific. In this graphic, we see a negatively-tilted upper level low pushing east into Japan, as the elevated vorticity values indicate. We can observe the negative tilt by the height contours seeming to "dig" in a southeast-ward direction. When a trough/storm system is at its mature stage, usually its strongest stage in a storm's life, the storm is said to be negatively tilted.

Using the Typhoon Rule, which states weather phenomena occurring over Japan is reciprocated in the United States 6-10 days later, we might expect this storm to impact the United States in a December 22-26 timeframe... possibly right around Christmas.

But there's more to this story.

Tropical Tidbits
Take a look at this image, forecasting 500mb geopotential height values in colored shadings, as well as mean sea level pressure (MSLP) values in the contoured lines. In this image, valid just six hours prior to the time period in the first image we analyzed, we see a strong storm system in the Sea of Japan, correlated to the strong upper level low.

But wait... there's something else there.

Check out that sagging of contour lines just south of Japan, right where the arrow is pointing. It almost looks like another low pressure system! Other model guidance confirms this idea of a second body of low pressure forming south of Japan and skirting the nation to the east before phasing with the very strong storm in the Sea of Japan. The implications here could be huge.

If that really is another low pressure system, then we have a very interesting scenario on our hands for Christmas. We can't tell for sure just yet, but the storm shooting northward to the east of Japan could mean a few possibilities.
Let's first hypothesize that the item outlined above is indeed a storm system. The track to the east of Japan could either mean we would be looking at:

- A Panhandle Hook storm, where the system shoots north from the Southern Plains. These storms are climatologically favored to bring heavy snow to cities in the east-central Plains and Lower Great Lakes. This scenario is a possibility, as that strong storm in the Sea of Japan would likely correlate to a strong North Plains cyclone. This would keep that body of low pressure east of Japan in an area close-by, as the storms would eventually phase (not to mention low pressure areas are attracted to other low pressure areas).

- An East Coast storm. Because this body of low pressure is forecasted to skirt the eastern side of Japan, this could be a plausible scenario. We won't know if either of these are correct until we have more model runs to access.

Long range ensemble model guidance for the evening of December 23rd shows a relatively favorable pattern for a storm. We see strong ridging high pressure in the West Coast of North America, but another swath of high pressure also looks to exist in southern Canada. This could boost temperatures, or worse, suppress and kill the storm system altogether. Even though this is quite far out and should not be taken at face value, those in the Midwest looking for a winter storm should feel rather good from what model guidance is projecting:

Tropical Tidbits
This image from the GFS ensembles shows what I'm talking about. In this image, also valid for the evening of December 23rd, we see a few factors at work here. A strong positive PNA is currently agreed upon by these ensembles, which permits a strong ridge to form in the Western US. This ridge then allows a colder pattern to develop in the Central US, as the jet stream drops southward to accommodate the +PNA ridge. Sweetening the pot is evidence of a very slight ridge in the Southeast, something that could allow any potential storms to move more northward. That final piece to the puzzle will be a bit more difficult to come by, as that strong upper level low over Greenland will try to keep the flow over North America very zonal (west-to-east, jet stream not wavy). There's still, of course, plenty of time for this to change.

To summarize:

- A winter storm may be in the cards for December 22-26th, likely impacting Christmas travel plans.
- A second storm system may need to be watched for the Northern Plains.
- The primary threat here may become a storm favorable for heavy snow, either in the Central/East US (ideally the Ohio Valley/Midwest) or along the Eastern Seaboard.
- Relatively low confidence still exists due to the long-range nature of this threat.