Thursday, October 30, 2014

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 

Monday, October 27, 2014

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

Saturday, October 25, 2014

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 

Friday, October 24, 2014

Extended Forecast Discussion (Part 2)

This is the second part of my Extended Forecast discussion, and will focus on the 2-3 week forecast range. A third part will be made for the final 4th week forecast this weekend.

For Part 1, click here.

AAM Transport
ESRL
The image here shows atmospheric angular momentum transport anomalies over the past few months. We want to focus on the anomalies in the top-right corner, the last few days of recording. On this graphic, we see a swath of yellows and oranges seeming to push diagonally upward along the image. Those yellows and oranges define positive AAM transport anomalies, and they are beginning their long-awaited movement to the north, which will throw a big wrench in our weather pattern. 

According to the oranges and yellows on the screen, the positive AAM anomaly transport values have made it as far north as about the 40N parallel, and they are strengthening. Because positive AAM anomalies generally can mean an enhanced jet stream, we might expect high pressure ridging to build northward with time as well, as the jet stream pushes north with the +AAM transport anomalies. This is already being observed with warm spells beginning to intrude upon the Central US.

This spells a predominantly warm weather period in the next few weeks, as this strong jet stream will eventually make it to the Arctic and keep the Arctic Oscillation (AO), and by extension the North Atlantic Oscillation (NAO) positive. Such a positive AO, which helps induce warm weather, is already on the forecast from the Climate Prediction Center, as shown below. 

CPC AO
The spaghetti-like alignment of the red ensemble forecast members for the Arctic Oscillation shows uncertainty associated with this pattern, as the very strong AAM transport anomalies push poleward, but eventually the ensembles should even out to a positive AO forecast over the next two or so weeks.

We can expand on the AAM for a more global look to this piece of the forecast.

ESRL
Relative AAM
Take a look at the chart above. Here, we see anomalies of the relative Atmospheric Angular Momentum (AAM) over the last few months, with a globally-averaged line on the bottom panel. If we look closely at conditions over the past few weeks, we can just barely see those yellow splotches of positive AAM anomalies that seem to be drifting in an upward-diagonal direction, meaning they're moving poleward. You wouldn't suppose this has to do with the AAM transport anomalies moving poleward too, would you?...

After the AAM transport anomalies make their move into the Arctic, guidance has been consistent on the idea of the AAM dropping well into La Nina-like negative territory. If this should happen, we would likely move into Stage 1 of the GWO stages. The stages, and their descriptions from the ESRL, are listed below.

The four primary phases of the GWO are described below, along with generally cold season (November-March) probable weather impacts for the USA. The GWO recurrence interval, or "time it takes to make a circuit", ranges from a broad 15-80 days. Two of the stages project strongly on El Nino and La Nina circulation states, which are also characterized by positive (Stage 3) and negative (Stage 1) global AAM anomalies, respectively.  Stages 2 and 4 are transitional.

Stage 1 (La-Nina like) – the global relative AAM anomaly is negative. The negative anomaly is primarily due to easterly upper level wind anomalies that extend from the Eastern Hemisphere tropics to the Western Hemisphere mid-latitudes. A retracted Pacific Ocean jet stream is a key feature in the total field.  Troughs are probable across the western USA with a ridge over the southeast.  High impact weather is favored across the Plains.

Stage 2 – the global relative AAM tendency is positive. This means that negative AAM is being removed from the atmosphere by surface friction and mountains. At the same time, westerly wind anomalies are intensifying in equatorial regions of the Western Hemisphere. Fast Rossby wave dispersion events in both hemispheres are a coherent feature of this stage and Stage 4.  A cold regime is probable across the central USA.

Stage 3 (El-Nino like) – the global relative AAM anomaly is positive. Westerly wind anomalies move into the Eastern Hemisphere, broaden in latitudinal extent and link up with deep westerly flow anomalies over the mid-latitude Western Hemisphere. An extended Pacific Ocean jet stream and southward shifted storm track is observed  favoring high impact weather events along the USA west coast.

Stage 4 – the global relative AAM tendency is negative. Positive (westerly) AAM anomalies are being removed by surface friction in the Western Hemisphere mid-latitudes and through mountain torques across the Northern Hemisphere topography. The next phase of the oscillation (if there is one) is represented by easterly wind anomalies intensifying over equatorial regions of the Western Hemisphere. This stage has enhanced subtropical jets and closed lows in the subtropics favoring rainfall events over the southwestern USA.
I italicized the Stage 1 description, as we would likely be heading for Stage 1 so long as projections remain consistent in coming days. Let's analyze that Stage 1 description as best we can. Reading through it, we notice that in Stage 1, the AAM anomaly is negative. With the forecast for the AAM dropping deep into negative territory, we can cross that off our checklist. Note how the description includes troughs in the Western USA, with a ridge over the Southeast. Model guidance has been latching onto that idea for a while now, as this shot of the GFS Ensembles 500mb height anomaly forecast valid for November 1st shows.

PSU
Lastly, check out the final sentence in that Stage 1 description. "High impact weather is favored across the Plains". 

Huh?

Well, let's decipher this. Stage 1 features the aforementioned troughs in the West US. If it also features a ridge in the Southeast, we can make a relatively safe assumption that they're talking about a predominantly-negative PNA pattern.

NCSU
The image above shows a typical negative PNA pattern across North America. Note the downsloping jet stream entering the West Coast; that tells us of troughing/storminess in the Western USA, just like we saw in the Stage 1 description. Conveniently, we also see a strong ridge positioned in the Southeast, oddly enough just like what we read in the Stage 1 description. Coincidence? No. 
Lastly, look where those two primary jet stream-like features lead- right into the Plains. That confirms our suspicions of a negative PNA pattern in coming weeks as we transition to that GWO Stage 1, placing the primary storm track over the Plains, and maybe a bit into the Rockies and Midwest at times.

SOHO
I want to briefly touch on what the Sun is doing right now. The chart above shows a sunspot identification image, where oranges and blacks indicate the presence of sunspots. Notice that large complex of sunspots in the center of the sun. This complex has been rotating to face Earth for the last week or so, and has finally gotten the Earth in its direct line of sight. The sunspot complex will be rotating away with time, but if it happens to expel a Coronal Mass Ejection (CME) or other expulsion of energy, our weather could turn warm rather quickly within the next month. For now, it's something to monitor.

JMA

The graphic above shows Outgoing Longwave Radiation (OLR) anomalies across the tropics regions, in a global view. Here, we can identify areas of tropical forcing, where enhanced or suppressed convection may be driving or enhancing our weather pattern here at home. Glancing over this image, we do find a swath of negative OLR anomalies, indicating enhanced thunderstorm activity, placed south of India and a bit southwest of the subcontinent as well. This swath of convection has an entirely different meaning for our weather pattern, which will be discussed later on in this discussion. But for now, we'll analyze it as the Madden-Julian Oscillation (MJO).

The MJO states that enhanced or suppressed tropical convection over certain parts of the Equatorial Pacific basin or Indian Ocean can have different effects on the global weather pattern. In this case, when comparing observed negative OLR anomalies in the image above with the Bureau of Meteorology's average negative OLR placement for each MJO phase, we can determine what MJO phase we appear to be in. See if you can figure it out on your own.

BOM
If you guessed Phase 2 into Phase 3, you're right! Phase 2 and 3 MJO events typically see enhanced tropical convection just south of India, very similar to the enhanced tropical convection we're seeing now in that JMA graphic above.

Now that we know what phase we're in, let's see if we can identify how this phase MJO is driving our weather pattern.

Americanwx
The image above shows typical 500mb height anomalies for Phase 3 MJO events in the month of October. Blues indicate stormy and cool weather, while greens, yellows and reds highlight warm and quiet weather. In "normal" October Phase 3 MJO events (the word 'normal' placed in quotes, since no MJO phase is really ever 'normal'), we tend to see high pressure extending from the West US into the Central US, with a swath of negative height anomalies from Greenland to the East Coast. Interestingly enough, we're currently seeing that same scenario play out, though not as defined to the naked eye as you might want it to be. Regardless, we are seeing that progressive ridge formation from the West extending into the Central US, with some stormy East US weather as a result. Though we might not make it to Phase 3 of the MJO, and the MJO isn't the only part of these negative OLR anomalies, we're certainly observing Phase 3-like conditions. Consequentially, if this continues, we might expect a continuation of such warm spells in the next few weeks or so in the US, as this may be enhanced with the strong +AAM transport anomalies driving north, and the resultant positive AO

That little section above was primarily from the first part of this Extended Forecast Discussion. Now that we're into Part 2, however, we can finally discuss the other use behind those negative OLR anomalies: The Hadley Cell.

Michael Ventrice
The chart above shows 14 day-averaged omega anomalies on a pressure-by-longitude cross-section. Though it may seem daunting at first, it's not all that complicated. The line marked with the '0' is the Equator, while the 30S and 30N demarcations are lines of latitude. On this chart, we see a swath of blues and purples extending from the surface (~1000 hPa) to about the jet stream level (200hPa). Checking the legend at the bottom of the graph, we recognize this to be a swath of negative omega values. If you remember what the concept of omega usage is, negative omega values tend to indicate rising air, while positive values indicate sinking air.

Interestingly enough, we're looking right at a cross-section of the Hadley Cell.


This example image above, depicting the Hadley Cell, shows how this rising and sinking air works. We will see warm Equatorial air rising, as the negative omega values depict, usually rising into the upper atmosphere. From there, the air cannot rise further, so it is carried northward (or southward, depending on hemisphere) to around the upper regions of the lower latitudes, i.e. 30N or 30S. From there, the warm Equatorial air has cooled, and now sinks back to the lower atmosphere. This is how we identify the positive omega values on the chart above. From there, the cool air is carried southward (or northward, again varying with hemisphere) back to the Equator to start the process over again.

Applying that example graph to the omega chart, we can roughly make out where the Hadley Cell is currently placed. We see a body of sinking air around 40N, where the circulation has been weakening in recent days, with another swath of sinking air possibly forming such a cell in about the 20N region. The Hadley Cell circulation at the 30N area was much more defined in this graph a few days ago, shown below.

Old chart showing the defined Hadley Circulation Cell from the Equator to about 40N.
As the very-knowledgeable blizzardof96 pointed out with respect to this Hadley Cell, its placement around 40N is making it difficult for troughs to form in the area (you Typhoon Rule enthusiasts may recognize this as a warm signal for the US). However, with the cell apparently weakening, and model guidance already re-introducing troughs to East Asia in the near-future, it looks like our week 2-3 forecast is turning cooler.

Summarization

- Very strong positive AAM transport anomalies are moving poleward, meaning warm weather can be expected for the next week or two, before the anomalies hit the Arctic and sustain a positive Arctic Oscillation.
- The positive Arctic Oscillation will then provide the chance for warmer weather for the next few weeks, depending on how long that positive AO can be sustained.
- Dropping AAM anomalies after the passage of the positive AAM transport anomalies suggests a GWO Stage 1 alignment, supportive of a cold West US, stormy Plains, and warm East US in the 2-3 week forecast.
- The Hadley cell dissipation is likely to allow troughs/storms to return to East Asia, setting up the Typhoon Rule for a gradual return to stormy/cooler weather in that 2-3 week timeframe.

Look for the final installment, Part 3, to debut this weekend.

Andrew

November 2-6 Potential Cold, Stormy Weather

I'm monitoring the threat for the season's first wintry blast of Arctic air in the next few weeks.

Tropical Tidbits
The image above shows the ECMWF model forecast for 500mb geopotential height contours and normalized anomalies, valid on the morning of October 27th. Blues depict areas of troughing, which lead to cold and stormy weather, while reds indicate the presence of ridging high pressure, which permits the formation of warmth and quiet weather. The graphic above shows a pretty strong upper level low dipping south into northern Japan on October 27th. We can observe this storm system pushing due east or a bit east-southeast as it moves in the aforementioned direction, and as it does so, the upper level low is expected to scrape northern Japan, if not actually centering itself over the northern part of the country.

When we apply the Typhoon Rule, which states weather phenomena in Japan is reciprocated in the United States about 6-10 days later, we might expect a pretty sharp cold blast of air in the November 2-6 timeframe.

Tropical Tidbits
For the sake of diversity, we'll analyze the GFS model's 500mb forecast as well, this one valid for the morning of October 28th. The GFS has this strong upper level low displaced a bit south of the ECMWF's location, and obviously slower than how the European model has the low progressing. But the kicker here is how the upper level low is stronger in the GFS forecast as opposed to the ECMWF. If the GFS ends up with the more accurate forecast, it's certainly a possibility that our cold weather in this November 2-6 timeframe may be more severe than currently thought.

To summarize:

- A sharp blast of cold air is expected to hit the United States around a November 2-6 timeframe, potentially with wintry weather.

Andrew