Sunday, September 14, 2014

The Story Behind The Polar Vortex

Ever since last winter, the polar vortex has taken over as the go-to thought as soon as the prospect of cold weather is brought up. Let's take this time today to go over what the polar vortex actually is, and disprove some inaccuracies I've seen swirling around the world.

1. The Polar Vortex Exists Year-Round
The phrase 'polar vortex' is used to describe a strong low pressure system (vortex) that is semipermanently placed over the Arctic region (polar). In the summer, the vortex significantly weakens, and is replaced by high pressure. During the fall months, however, the vortex gains strength yet again and low pressure dominates the Arctic as the polar vortex gears up for winter.

2. The Polar Vortex Extends into the Stratosphere, Not Just the Troposphere

NCEP
The image above shows the 10-day forecast from the GFS model in the stratosphere, at the 30 millibar level. That's pretty high up the sky when you consider us humans are at the 1000 millibar level. In this forecast, we see relatively weak low pressure establishing itself over the Arctic Circle, with weak high pressure surrounding it. That low pressure body is the "birth" of the polar vortex for this cold season, as it begins to come back to life and strengthen for the winter.

Some people assume the polar vortex is no different than your average storm system that brings snow to your neighborhood. The vortex is actually quite different. In order for it to be the polar vortex, it must exist at both the tropospheric and stratospheric levels. That's how big and controlling it is in the atmosphere.

3. There is only ONE Polar Vortex

This is something I've been hearing ever since January 2014, when this polar vortex business really took off. The belief is that there is more than one polar vortex in the atmosphere. This might stem from the aforementioned idea that some believe the polar vortex is no different than your average storm system, but regardless of its origins, the belief is false.

ESRL
Take a look at the image above. In this image, we see a view of 500mb height anomalies across the Northern Hemisphere on January 7th, 2014. Warm colors correspond to high pressure, which brings warmth and generally quiet conditions. Blues and purples indicate the presence of low pressure, which permits stormy and chilly weather. Just taking a glance over this map, we see one strong upper level low that seems to be the strongest one in the entire hemisphere. That upper level low, seen scraping the northern US and inundating Canada, is the tropospheric version of the polar vortex. As you can see, there is only one low as strong as the one in North America, hence there is only one polar vortex. Those other low pressure systems are just disturbances compared to the polar vortex.

ESRL
To give you a better idea of how there's only one polar vortex, let's take a look at the stratosphere on January 7th, 2014. In this image, we see what appears to be two strong areas of negative height anomalies, being pinched by two bodies of ridging/high pressure on either side of the globe. If you thought that this image shows two polar vortexes, you are incorrect!
What's actually happening is the two bodies of high pressure are trying to tear apart the polar vortex, something that happens rather often each year. The difference between other years and this past year is that the ridging forced the main part of the polar vortex into North America; that other area of negative height anomalies in Asia is just a weaker part of the single polar vortex that was torn off of the main body. Some of you more experienced weather enthusiasts may know this type of set-up as Wave-2 stratospheric activity, as two bodies of high pressure push into the Arctic.

To close out this post, here's a stellar graphic made by the folks at the National Weather Service office in New York in early January, when the hype over the polar vortex was in full swing.

NWS
To summarize:

• The Polar Vortex exists year-round, varying in strength. It does not (and will not) "come back" for the winter, as it's already present and cannot "come back".

• The Polar Vortex exists across both the troposphere (where we live) and the stratosphere (where Felix Baumgartner jumped from his capsule in October 2012); it does not exist at the surface.

• There is only one polar vortex; our world would be much different if more than one polar vortex existed.

Additional points of summary are included in the NWS graphic above.

Andrew

SOI Continues To Plummet As El Nino Emerges

The Southern Oscillation Index, a key indicator for the emergence of an El Nino or La Nina, continues to free-fall as our El Nino looks to finally come to the surface.

BOM
The image above shows a 30-day average of the SOI values from January 2012 to the present day. The Southern Oscillation Index is calculated by examining the pressure differences between Tahiti and Darwin, Australia. When values drop below -8, conditions are considered optimal for El Nino formation. Values above +8 are considered optimal for La Nina conditions.

 As the chart shows, we had been seeing predominantly-positive anomalies this past spring and early summer, which is likely a reason why the the historic Kelvin Wave never brought about the Super El Nino so many (myself included) had discussed. However, beginning right around the start of July, we saw SOI values tank, and they've remained below zero for the entire period since. Values have gone both above and below the El Nino 'benchmark' of -8 since that time, but have remained below zero the entire time.

Due to potential copyright infringement concerns, I will refrain from posting a forecast of the SOI I came across yesterday. What it shows, however, is continued negative SOI values for the next few weeks, and possibly beyond.

CPC
The animation above shows a depth-by-longitude animation of equatorial temperature anomalies, in Celsius, since mid-July to present day. In this animation, we see the Kelvin Wave-induced warm waters spreading east and pushing to the surface as they do so, now just beginning to hit the surface.

The SOI being negative indicates that trade winds along the Equator and areas in the neighborhood of the Central Pacific have shifted to a state that will allow this warmth to stay and ferment on the surface, likely into an El Nino. Back in April, with the historic Kelvin Wave warmth, trade winds wee unfavorable, and prohibited the warmth from sustaining itself on the surface.

In coming days and weeks, we'll likely see this winter's El Nino bring itself to the surface, likely at a weak, potentially moderate strength. I'll have more on the projected El Nino strength in the near future.

Andrew

Saturday, September 13, 2014

September 27-October 1 Potential Storm System

The September 27th to October 1st period is being monitored for a potential storm system.

Tropical Tidbits
The image above shows 500mb height anomalies over the West Pacific area on the afternoon of September 21. In this image, blues indicate the presence of below-normal height anomalies, which can then lead to cooler than normal temperatures, as well as stormy conditions. Warm colors are associated with positive height anomalies, which generally bring warm and quiet conditions.

As we see in the image above, a trough is moving through Japan on the 21st, according to the latest GFS Ensembles forecast. We see negative height anomalies across the entire country of Japan, but the real point of interest is the depression in the contour lines. This depression indicates a trough moving through the area, and that's what may provide us with the chance for a storm system in the September 27th - October 1st period.

ESRL
Forecasted teleconnections can give us an idea of where this potential storm may end up.

On the top-left, we see the forecast for the Pacific-North American (PNA) index. We see that at the end of the forecast period, on September 27th, the PNA is projected to be descending from a strong positive state. This tells us that ridging in place across the Western US may be dissipating by the time this potential storm rolls around. Typically a positive PNA means the Midwest and Ohio Valley are most favored for storm systems, but it could be more for the Plains if the PNA is descending to neutral or negative territory.

Additional updates will be provided in the near future.

Andrew

Long Range Regional Outlook (Ohio Valley): September 20-October 12

This is the latest Long Range Regional Outlook forecast for the Ohio Valley area, valid for September 20th through October 12th.

CMC Ensembles
I have elected to use a combination of the CMC and ECMWF ensembles for this outlook, after finding the GFS ensembles to be in disagreement with the two aforementioned systems, which do hold a consensus.

The image above shows the CMC ensemble mean 500mb height contours on the date of September 22nd. In this forecast, depressions in the contours indicate troughing (cold and unsettled weather) while arcing motions in the contours depict ridging, which results in warm and relatively quiet weather. In the graphic shown above, we see a broad depression in the contours across the Plains and into the Midwest, though we then see slight ridging emerging over the Ohio Valley. This would tell me that the forecast period of September 20th to October 12th likely opens with some seasonable to above-normal temperatures, before colder weather moves in.

ECMWF Ensembles
Now shown above is a two-panel forecast from the ECMWF ensemble system. We see forecasted 500mb height contours and anomalies (with legend on the right) in the left-hand panel, with the ensemble 'spread' on the right image. An ensemble 'spread' indicates the degree of uncertainty among individual ensemble members on a particular area and its forecast. For example, we see deep purples on the right panel over the Great Lakes, which the left panel says will experience troughing (due to the depressed contours). This tells us that the ensembles are uncertain as to how likely this particular factor is to actually occur, and results in a wider spread of ensembles.

In the image above, valid for September 21st, we see a similar layout as that of the CMC ensembles. We see suppressed ridging over the Western US, created by some stormy weather in the Gulf of Alaska (see green shadings of below-normal height anomalies). This ridge in the West results in cooler weather for the Plains and Midwest, as the CMC ensembles depicted, but that then leads to some slight ridging over the Ohio Valley. This jives well altogether with the CMC ensembles.

Tropical Tidbits
In the image above, valid September 17th, we see a swath of negative height anomalies overtaking Japan as a rather strong upper level low scrapes the nation to the north. Seems pretty mundane, sure, but the consequences here at home are far more than mundane. As has been discussed consistently for the past couple of years, the weather in East Asia can have a significant impact on weather here in the United States. Utilizing the East Asian correlation of ridging over Japan equals ridging in the US 6-10 days later, and the same situation with negative height anomalies, we can foresee long range weather patterns weeks out at a time. This mechanism is referred to as the Typhoon Rule, and states that weather patterns found at the 500mb level can replicate themselves over North America 6-10 days later after they appear over Japan.

If we use this rule for the Ohio Valley, we might expect to see some cold weather, though I wouldn't call it a 'cold blast'. In this forecast, the upper level low looks displaced north enough that the core of the cold would likely stay in Canada. This fits in with the CMC and ECMWF ensemble projections of a weak trough moving east from the Plains and Midwest (as we discussed earlier), and is a reason why I disagreed with the GFS ensembles.

* The image used above is a forecast from the GFS ensembles, but the part I disagree with is for its forecast in the Northeast Pacific, not over Japan. Still, this part of the forecast must be monitored closely for the discrepancies described above.

Tropical Tidbits
Once again using this Typhoon Rule, we see the forecast on September 21st calling for continued deep troughing over Japan, now pushed deep into the country. Using the guidelines set forth for this rule, we might expect some chilly weather in the Ohio Valley around the September 27th - October 1st period.

After going over the factors listed above, the temperature and precipitation outlooks for the Ohio Valley over the September 20th - October 12th period are as follows:

Temperature Outlook:

The Weather Centre
Temperature Outlook
Temperatures for the Ohio Valley are expected to remain around average to slightly below-average, due to a warm start to the forecast period and a few weak cool shots.

Precipitation Outlook:

The Weather Centre
Precipitation Outlook
The precipitation outlook calls for predominantly below-normal precipitation over the forecast period, due to somewhat dry signals over the Japan region for the next week or two, among other factors.

The next Long Range Regional Outlook will be published Saturday, September 20th.

Andrew

Friday, September 12, 2014

Long Range Regional Outlook (Northeast & Mid-Atlantic): September 20-October 12

This is the premiere post of the Long Range Regional Outlooks, this one valid for the Northeast over the September 20th to October 12th period.

CMC
After examining long range ensemble guidance, a split became evident between the ECMWF/CMC ensembles, as well as the GFS Ensembles. Have elected to side with the ECMWF/CMC projections, as GFS Ensembles are generally going against the grain of the other two ensemble sets, as well as my own thoughts.

The CMC ensemble mean 500mb geopotential height forecast is shown above. As a general rule of thumb, depressions in the contour lines tend to indicate troughing and cold/stormy weather, while arcing of the contour lines indicates ridging of high pressure, as well as warm/quiet weather. In the image above, we see 500mb height contours valid for September 20th. We see a rather zonal flow (non-wavy pattern) set-up over the northeast Pacific, with the jet stream pushing into the west coast of North America. This is in contrast to the GFS ensembles, which predicted deep troughing in the Gulf of Alaska. There is some slight ridging in the Southwest, leading to some weak troughing across Central and Eastern US areas.

ECMWF
The ECMWF ensembles are shown above, with the 500mb geopotential height anomalies and contours shown on the left panel, as well as the 'spread' (degree of disagreement among the ensemble members) for this forecast of September 21st on the right. This forecast is similar to the CMC ensemble projection above, as we see the Pacific jet stream pushing into the Western part of North America.

According to the height anomalies and contours, we do see some substantial troughing in the Gulf of Alaska, though it is not to the degree of the GFS ensembles. We then see some weak ridging in the Southwest, as was also seen with the CMC ensembles, though it is suppressed. Some weak troughing evolves in the Central US before weak ridging in the East.

Tropical Tidbits
In the image above, valid September 17th, we see a swath of negative height anomalies overtaking Japan as a rather strong upper level low scrapes the nation to the north. Seems pretty mundane, sure, but the consequences here at home are far more than mundane. As has been discussed consistently for the past couple of years, the weather in East Asia can have a significant impact on weather here in the United States. Utilizing the East Asian correlation of ridging over Japan equals ridging in the US 6-10 days later, and the same situation with negative height anomalies, we can foresee long range weather patterns weeks out at a time. This mechanism is referred to as the Typhoon Rule, and states that weather patterns found at the 500mb level can replicate themselves over North America 6-10 days later after they appear over Japan.

If we use this rule for the Northeast, we might expect to see the northern parts of the region get some substantial cold around a September 23rd - 27th period. This fits in with the CMC and ECMWF ensemble projections, and is a reason why I disagreed with the GFS ensembles.

* The image used above is a forecast from the GFS ensembles, but the part I disagree with is for its forecast in the Northeast Pacific, not over Japan. Still, this part of the forecast must be monitored closely for the discrepancies described above.

Tropical Tidbits
Once again using this Typhoon Rule, we see the forecast on September 21st calling for continued deep troughing over Japan, now pushed deep into the country. Using the guidelines set forth for this rule, we might expect continued cold in the Northeast around the September 27th - October 1st period.

After going over all of the factors above, as well as extra analyzations not mentioned in this post, the outlook for the September 20th to October 12th period over the Northeast is as follows.

Temperature Outlook:

The Weather Centre
Temperature Outlook
Precipitation Outlook

The Weather Centre
Precipitation Outlook
I expect the Northeast to remain predominantly below normal for temperatures in the aforementioned time period, while precipitation should end up a bit below normal, close to neutral.

Andrew