Thursday, January 29, 2015

January 31 - February 2 Potentially Significant Winter Storm

A potentially significant winter storm is taking aim at a large swath of the country this weekend into the start of next workweek.

Tropical Tidbits
Click images to enlarge
The image above shows the latest GFS forecast for precipitation, mean sea level pressure (MSLP) values, and 1000-500mb thickness values (dashed blue and red lines) on the evening of February 1st. The prognosis calls for energy coming from the Southwest, moving eastward where interaction with additional energy out of Canada may occur. The more interaction these two systems have, the more snow & overall precipitation may fall. The caveats associated with this storm include storm track, strength, snow amounts, interaction between the two pieces of energy, among other items. We'll address some of these here today.

I'll start off right away and say that I think we see this storm either maintain its current position, or even shift south a bit more. Why? I'm glad you asked.

Tropical Tidbits
Here's a look at 500mb vorticity values from January 22nd, over the West Pacific. Notice how we see a positively-tilted piece of energy making its way eastward over Japan, with another piece of energy sliding southward from higher latitudes. Does this set-up look familiar? It should, because the Typhoon Rule says this will be how our storm will evolve. After this point in time, the two pieces of energy approached each other, but did not phase/interact until they were well east of Japan. To me, this means our storm coming up this weekend may not phase as easily as model guidance is saying. It is known that weather models tend to phase storms too readily, and this could be a prime example of that bias showing through. If that is true, this storm would likely stick further south.

CPC
Additionally, and perhaps most importantly, this storm will occur under predominantly northwest flow, with a strong lobe of the tropospheric polar vortex pressing southward from Canada. In this 500mb height contour map, valid for February 1st, we see that lobe of the vortex creating for a very tight gradient in the North US. This could very well act to suppress the storm further south, possibly more than what is currently being advertised. Second, notice how the height contours in Canada seem to be diving southeast into the US. This is what is called 'northwest flow', where the air in the United States is coming from the northwest. Because this is happening, I find it rather plausible we see the storm pushed further south, as strong surface high pressure dropping southward from Canada will also be playing a role here. In case you haven't guessed yet, I'm not finding many factors favoring a north shift.

Instant Weather Maps
One positive thing I am seeing is that model guidance is likely under-forecasting precipitation for this storm. Above, we see the GFS model forecast for low-level relative humidity on February 1st. You can see just how moist this storm will be, with high RH values stretching from the eastern Rockies all the way to the waters offshore the East Coast. This moisture will be driven from the Gulf, hence why I find it plausible, if not likely, more precipitation occurs in this storm than what is being modeled right now.

The question then arises, could this favor prove to stab the wintry portion in the back? Thunderstorms in the warm sector of winter storms have been known to rob the wintry sector of moisture, leading to more rain than forecasted and less snow than forecast. This will be something we'll need to watch when the storm commences.

Tropical Tidbits
Storm track forecasts should really begin to solidify by Sunday(!!), when both pieces of energy relevant to this event move into the United States. Right now, the energy from the Southwest is slowly moving east onshore California, which should aid in model accuracy, gradually, in coming forecasts. An interesting question being posed is, what if latest model guidance inching north in the last day or so has something to do with that energy near California coming onshore? Could it spell a further north track down the road? Anything's possible, sure, but I'll stick with a southern track for now.

Here's my forecast graphic for this event.
The Weather Centre

To summarize:

- Model guidance is in agreement on a potentially significant snowstorm impacting the North US this weekend.
- Amounts in the hardest-hit regions may exceed 12", but this may need revision in later updates.
- High uncertainty still exists.

Andrew

Wednesday, January 28, 2015

Long Range Outlook (Made January 28, 2015)

This is the long range outlook post, made January 28th, 2015, valid for the next 7-31 days.

We'll begin with a look at what's happened in the last week, as well as a verification check on the calls made in our last long range outlook post.

ESRL
Over the last week, we saw rather persistent ridging building over the Western United States into southwest Canada. This forced the development of some colder weather in eastern Canada and the northeast United States. The tropospheric polar vortex continued to be in a disrupted state, with multiple bodies of ridging being forced into the Arctic Circle.

In our last outlook, we predicted a period of cooler than normal weather for much of the Central and East US. That call didn't verify as well as we hoped, with warmth taking hold in the Northern Plains, as well as some rather mild conditions across the Southeast. However, the general idea for cold being maximized in the Northeast did work out well.

JMA
We'll now go over tropical forcing across the globe, and show how our pattern has been affected by it.

This chart shows a lot of things at once, but for now, we'll take it piece by piece. The first thing to recognize is the blue color shadings on this map. The color shadings are indicative of Outgoing Longwave Radiation (OLR) anomalies, where negative/blue depictions show enhanced convection (thunderstorms), and positive/orange depictions show suppressed convection. Arrows on this image will point away from the blue shadings, as thunderstorms force air up and away, while arrows will compress towards orange shadings, since sinking air (due to lack of convection) drags air down towards the surface. Lastly, the green contours show the intensity of divergence, the action of air being pushed up and away by thunderstorms, while reddish/purple contours show convergence, the action of air being pulled down and compressed towards the surface as the air sinks.

We've begun to see enhanced convection make its way across the waters due south of Eurasia, as our new Madden-Julian Oscillation (MJO) wave forms. This convection is currently located west of India when viewing it from a longitudinal aspect. Such positioning of convection usually favors a colder and stormier pattern in the East US, as was evidenced by the recent blizzard that hit parts of the coastal Northeast. We're still seeing enhanced tropical convection west of the 180 degree longitude line, and this may have led to the unexpected warmth that messed with our call for a cool period this past week. As this convection moves east and dies off, the newly-developing body of convection should take control.

Now that we've discussed what has happened, let's start to go over what will happen.

ESRL
The image above shows 500mb geopotential height anomalies over the Northern Hemisphere, where blues and purples depict negative height anomalies, usually indicative of cold/stormy weather. Similarly, greens and reds indicate the presence of positive height anomalies, a precursor to warmth and generally quiet weather. In this panel of the ESRL ensemble forecast, valid for 9 days from today, we see that the persistent lobe of the tropospheric polar vortex has retracted north into Greenland, a classic positive North Atlantic Oscillation (NAO) signal. However, due to the blossoming ridge in the West, stormy weather still continues in the nation's midsection. This does not last long, however. This forecast matches our projection in the last Long Range Outlook for a brief cold spell around February 6-7.

ESRL
By the 336-hour forecast mark, ensembles are in agreement that the positive NAO will permit that ridge in the West US to bleed east, resulting in a warmer than normal pattern for much of the country. Negative height anomalies are shown along the Gulf Coast, but an unfavorable set-up upstream (to the west) of the United States means that warmth should prevail. This matches our earlier call for warmth after about the first week of February. A consolidated tropospheric polar vortex certainly does not help matters for winter weather fans, either.

Now that we've looked over the next 7-14 days, let's start looking out even further.

Tropical Tidbits
The image above shows 500mb geopotential height anomalies, forecasted over the West Pacific on the evening of February 2nd. Note the presence of a trough just to the east Japan, with a very powerful ridge just to the west of the country, arguably influencing the nation more than the trough. This trough appears to be a storm system that may impact us here in the United States down the road, just before this ridge may take hold, but that potential will be investigated in a later post. Using the Typhoon Rule, which states weather phenomenon occurring in the West Pacific is reciprocated in the US about 6-10 days later, we may expect a general warm period, possibly cooler in the East, around February 8th - 12th.

Tropical Tidbits
Long range models have been hinting at the idea that a strong upper level low or deepening trough will slide into Japan around February 6th or 7th. For now, ensembles are taking a more progressive and generally weaker approach with this potential, as is expected. For now, we'll watch a February 12th - 16th period for cooler weather, throwing a wrench into our outlook from the last post.

ESRL
I want to now go over the teleconnections over the next two weeks, which can help us diagnose the pattern heading into the 14-31 day period.

Top left: PNA Forecast
Top right: NAO Forecast
Bottom left: WPO Forecast
Bottom right: EPO Forecast

A quick refresher on the PNA, NAO, WPO and EPO...

The Pacific North American index involves what the atmosphere does in the northeast Pacific and the western coast of North America. When we see a stormy pattern in place over these regions, we call such a pattern a negative PNA, due to the below normal height anomalies in this region. In a similar sense, when high pressure dominates that same region, we call that a positive PNA. A negative PNA will bend the jet stream to give the storms to the Plains and the Deep South regions, frequently initiating high pressure system formations over the Central US. A Positive PNA will bring about an opposite response to high pressure (HP) over the West, and will have the stormy pattern evolve over the East US.

The North Atlantic Oscillation involves the presence of a high pressure system over Greenland (negative NAO) or the presence of a low pressure system over Greenland (positive NAO). In the negative NAO, the jet stream will buckle into the Northeast to allow storms and cold to thrive in that region. The positive NAO denies this region any of these benefits.

The WPO (West Pacific Oscillation) and EPO (East Pacific Oscillation) are very closely related. In the negative phase of the WPO, a strong ridge exists over the Bering Sea, which can allow for sustained cold weather in the Central and Eastern United States. The negative phase of the EPO gives similar results, though the ridge is positioned in the Gulf of Alaska instead. The positive phase of both the EPO and WPO see warm weather prevail in much of the US, as stormy weather replaces the ridges in each respective region.

The forecast for the PNA includes a decrease in positive values as of right now, before a spike back well into positive territory in the long range. This works well with the ESRL ensembles we were analyzing earlier. The NAO forecast generally stays positive in the long range, a red flag for that positive PNA ridge to shift east into the Central and East US. A dip to negative territory does occur in the January-February transition, which may create a storm threat (upcoming posts will address this), but nothing too serious. Both the WPO and EPO are negative in the long range, which is good for winter weather fans. However, the consensus has been for this -EPO/WPO regime to weaken heading beyond the 16 day period, which may only enhance the threat for warmer weather in the middle of February.

Finally, let's use the skills learned in our OLR analysis earlier, to forecast the long range.

CPC
As in the JMA graphic, cooler colors define negative outgoing longwave radiation anomalies, which mean the presence of thunderstorm activity. Warm colors depict positive OLR anomalies, otherwise known as suppressed storm activity. In the 6-10 day forecast panel, we begin to see our new MJO wave really strengthen and define itself as it moves east, just south of the Indian subcontinent. Notice in this same panel that our current MJO wave near the 180 degree longitude line is nearly dissipated by this forecast time period. At the 11-15 day forecast mark, the MJO wave has strengthened considerably, and has now switched to pro-warmth phases. These pro-warmth phases can generally be identified when -OLR anomalies move east of the Indian subcontinent, and this should happen even beyond the 11-15 day forecast period. Beyond then, confidence is too low to accurately produce a forecast.

To summarize:

- A period of rather chilly weather should kick off the month of February.
- A brief period of seasonal to cool weather is expected to impact the Central and East US around February 6-7.
- In mid-February, the atmosphere is in favor of a warm pattern. However, disagreement from the Typhoon Rule means this part of the month could really go either way. We will re-examine this in the next long range post.
- An early look at late February continues to show a predominantly warmth-favoring pattern.

Andrew

Sunday, January 25, 2015

January 28 - February 1 Potential Winter Storm

We're watching for a potential winter storm in a January 28th through February 1st timeframe.

Tropical Tidbits
Click Images to enlarge
The above graphic shows mean sea level pressure (MSLP) contours superimposed on 500mb geopotential height values (colored shadings), valid from last Thursday morning. Last Thursday, we saw a positively-tilted trough forcing a rather strong low pressure system to form just east of Japan, dipping below the 1000 millibar mark on this image. When we apply the Typhoon Rule, which states weather phenomenon occurring in Japan is reciprocated in the US 6-10 days later, we come out with a potential winter storm in the January 28 - February 1 timeframe.

Instant Weather Maps
Initially, the ECMWF takes this system down through the Midwest and Ohio Valley, as the above image shows on the evening of January 29th. We see a minimum sea level pressure value of about 1008 millibars, if not a bit below that. This won't be a significant storm, per current forecasts, but could drop some wintry precipitation across the aforementioned regions.

Instant Weather Maps
By the evening of January 30th, the ECMWF model sees this storm transferring offshore the Mid-Atlantic, strengthening at an appreciable pace to a minimum sea level pressure value of ~997 millibars. This would likely produce accumulating snowfall for parts of the Northeast, as the GFS model is also alluding to, but again, nothing incredibly significant.

To summarize:

- A winter storm may affect the US between January 28th and February 1st.
- This storm may affect the Midwest, Ohio Valley, and Northeast the most, if at all, given the relatively-weak strength of this storm.

Andrew

Friday, January 23, 2015

Short Range Outlook (Made January 23, 2015)

This is the latest Short Range Outlook post, where the pattern for the next 0-7 days is addressed. This forecast is valid for January 23rd to January 30th.

ESRL

500mb geopotential height anomalies (right panel) and means (left panel) over the last 7 days show a general ridging pattern across the western portion of North America, extending a bit into the Central US and southern Canada. This pattern resulted in generally mild conditions for much of the nation, particularly in the Plains region. This warmth was not as enthusiastic in the New England region.

NOAA
Satellite analysis of the Northeast Pacific shows abundant moisture making its way onshore in the Pacific Northwest from as far west as Hawaii. This will lead to rainy weather in that region for the foreseeable future, while drier conditions should persist in the Southwest. Some of this moisture is associated with a system that will be moving into the Midwest and Ohio Valley over the next few days, dropping accumulating snow as it does so.
Instant Weather Maps

Model guidance sees the aforementioned piece of energy sliding southeast-ward as a weak snow-producing system. From the most recent NAM model, the heaviest snow would appear to fall in northern Indiana, central Ohio, and into the mid-Atlantic, with scattered snow also showing up in Wisconsin, Minnesota, Iowa, Illinois, and points east, all falling this Sunday through Tuesday. A different storm system looks to drop plowable snow in the Northeast in the same timeframe.

PSU
After this weak system moves eastward, model guidance sees another small storm system dropping into the Midwest, likely laying down some low-accumulation snows. Latest model guidance indicates this would be another 1-3" snow event.

To summarize:

- Heavy snow looks to impact the Northeast in the next four days.
- A weak storm system will drop from Canada and produce accumulating snow in the Midwest and Ohio Valley in the next four days.
- Following that storm, a weak clipper may lay down another 1-3" of snow in similar regions.

Andrew

Wednesday, January 21, 2015

Long Range Outlook (Made January 21, 2015)

This is the long range outlook post, made January 21st, 2015. This post will address the upcoming pattern over the next 7-31 days.

We'll begin with an analysis of the pattern over the last several days.

ESRL
Click images to enlarge
Over the last week, we saw ridging present across the Western US, as exemplified by the green and yellow colors, with that ridge making its way east to provide warmth for many in the Central US. Ridging was also dominant over Greenland and south, into the North Atlantic and Canadian Maritimes. These two ridges combined to force an upper level low into northern Canada, rather than further south towards the United States, a likely scenario if the former ridge out west had been more dominant.

On a more synoptic scale, we recognize high pressure has forced the tropospheric upper-latitude vortex to weaken and splinter, with the prevailing lobe located in western Europe, and other splinters scattered across lower latitudes. This tells us that the cold air up in Canada is more free to move around, rather than maintain its position, locked in the Arctic Circle.

JMA
Let's now discuss tropical forcing across the globe to recognize how it has affected our pattern the last several days.

This chart shows a lot of things at once, but for now, we'll take it piece by piece. The first thing to recognize is the blue color shadings on this map. The color shadings are indicative of Outgoing Longwave Radiation (OLR) anomalies, where negative/blue depictions show enhanced convection (thunderstorms), and positive/orange depictions show suppressed convection. Arrows on this image will point away from the blue shadings, as thunderstorms force air up and away, while arrows will compress towards orange shadings, since sinking air (due to lack of convection) drags air down towards the surface. Lastly, the green contours show the intensity of divergence, the action of air being pushed up and away by thunderstorms, while reddish/purple contours show convergence, the action of air being pulled down and compressed towards the surface as the air sinks.

Over the past several days, we've seen an area of enhanced thunderstorms make its way eastward from Oceania, and we now see it beginning to weaken off the western coast of the South and Central Americas. You weather enthusiasts may recognize this as a weakening Madden-Julian Oscillation wave. However, look towards southern Africa. We see a new plume of thunderstorms beginning to develop. It is expected that these storms will propagate eastward and form the new MJO wave that will also push east with time, but that's for discussion later on in this post.

Now that we've gone over the pattern developing in the last several days, let's start looking into the future, beyond the 7 day window.

ESRL
This is the ESRL ensemble projection of 500mb geopotential height anomalies for 7 days out. Here, we see a strong ridge pumping north in the Western US, resulting in a deep trough (and associated cold weather) in the East US. This fits in well with the expected progression of our new MJO wave east, as the placement of tropical convection just southwest of India supports this type of cold regime. Looking across the northern hemisphere, we can also identify an upper level low over Greenland, a signal for the positive phase of the North Atlantic Oscillation (NAO). The +NAO supports a more low-amplitude jet stream, which is technical-speak for weaker ridges and weaker storms, as well as a faster progression of those ridges and storms to the east. However, at least for now, that +NAO is being overruled by the strong ridge in the West, ensuring a cold period to end January (something we've been discussing for over 2 weeks now).

ESRL
Going ahead to 11 days from today, the 500mb geopotential height anomaly shows a different picture than the one we analyzed above. The ridge in the West US has been swallowed up by a more dominant ridge stretching across the Bering Sea and into the North-Central Pacific. With support from an inferior ridge along the western North America coastline, it appears a cross-polar flow situation would be in the works, where cold air would be transported from Eurasia, across the Arctic, directly down into the Northeast and Canadian Maritimes, based on the placement of that trough. Things start to improve in the Central US, for those wishing for spring. That +NAO signal we discussed earlier is now in play, as we see the aforementioned West US ridge 'bleeding east' into the Plains. This fits in with the continued progression of the MJO wave east (weather enthusiasts know this part as Phase 3, the image above this one resembled a Phase 2 event). However, I do think we see the ridge retract itself a bit west as the time between this forecast and present day decreases.

We've examined the outlook for the next 7-14 days, so let's start digging into the outlook in the next 14-31 days.

Tropical Tidbits
The image above shows 500mb geopotential height anomalies, forecasted over the West Pacific on the evening of January 22nd. Note the presence of a trough in Japan, in the midst of two ridges on either side of the country. This trough appears to be a storm system that may impact us here in the United States down the road. Using the Typhoon Rule, which states weather phenomenon occurring in the West Pacific is reciprocated in the US about 6-10 days later, we may expect a storm threat around the January 28-February 1 timeframe. As of now, this storm wouldn't be particularly strong, and should be followed by a warm-up. However, it could perk the interest of some severe weather enthusiasts.

Tropical Tidbits
If we fast-forward a bit, we find ourselves looking at 500mb height anomalies on the evening of January 25th, with a much different prognosis than what we saw in the image above. A strong ridge now encompasses all of Japan, bringing a 'heat wave' to the region. This ridge persists for more than a couple of days, which could very well validate my outlook for a mild middle of February. Using the Typhoon Rule here, we look to find ourselves in a warm period around a January 31st - February 4th period, possibly for longer, based on the looks of ensemble guidance further out. This warmth is then interrupted by a brief cool shot before that warmth may return.

ESRL
I want to now go over the teleconnections over the next two weeks, which can help us diagnose the pattern heading into the 14-31 day period.

Top left: PNA Forecast
Top right: NAO Forecast
Bottom left: WPO Forecast
Bottom right: EPO Forecast

A quick refresher on the PNA, NAO, WPO and EPO...

The Pacific North American index involves what the atmosphere does in the northeast Pacific and the western coast of North America. When we see a stormy pattern in place over these regions, we call such a pattern a negative PNA, due to the below normal height anomalies in this region. In a similar sense, when high pressure dominates that same region, we call that a positive PNA. A negative PNA will bend the jet stream to give the storms to the Plains and the Deep South regions, frequently initiating high pressure system formations over the Central US. A Positive PNA will bring about an opposite response to high pressure (HP) over the West, and will have the stormy pattern evolve over the East US.

The North Atlantic Oscillation involves the presence of a high pressure system over Greenland (negative NAO) or the presence of a low pressure system over Greenland (positive NAO). In the negative NAO, the jet stream will buckle into the Northeast to allow storms and cold to thrive in that region. The positive NAO denies this region any of these benefits.

The WPO (West Pacific Oscillation) and EPO (East Pacific Oscillation) are very closely related. In the negative phase of the WPO, a strong ridge exists over the Bering Sea, which can allow for sustained cold weather in the Central and Eastern United States. The negative phase of the EPO gives similar results, though the ridge is positioned in the Gulf of Alaska instead. The positive phase of both the EPO and WPO see warm weather prevail in much of the US, as stormy weather replaces the ridges in each respective region.

The forecast for the PNA is positive for the next two weeks, and a sustained strong positive signal at that. This tells me we're looking at that ridge sticking around the West US for a prolonged period of time into February, though it may very well bleed east into the Central US as we already discussed. The NAO forecast is sustained at a moderate level for the entire period, meaning the possibility of that +PNA ridge bleeding east is rather high.
The WPO forecast starts negative, goes positive, and then goes into strong negative territory as that intense ridge on the ESRL ensembles takes over. This should continue for a bit as the MJO wave moves through Phase 3. The EPO follows a nearly-identical path, and both should permit the persistence of cold in the East US. I will refrain from including the Central US in that cold forecast due to the risk of that ridge in the West US bleeding east.

Let's now use tropical forcing to see what we may expect later on in February.

CPC
This four-panel image shows OLR anomalies, using the same color definitions as the JMA chart we discussed earlier in this post. In the top panel, we see current OLR anomalies, and that dying MJO wave is observed moving eastward in the next 1-5 days. By the 6-10 day period, our new MJO wave evolves in Phase 2, favoring the cold weather we have discussed earlier in this post. By by the Days 11-15 panel, our MJO wave has shifted east, to Phase 3, favoring a warmer nation as we move into mid-February. As this wave moves eastward over time, it is expected that the wave will go into phases even more favorable for warm weather, which is why I'm maintaining my call for a warm period in mid-late February. Beyond that 31 day benchmark, confidence is too low to forecast further.

To summarize:

- A period of colder than normal weather across the Central and East US is favored next workweek, likely from Tuesday to Friday to round out February. This cold will be maximized in the Northeast.
- A period of warmth may overtake the Central US in the opening days of February, though the Northeast will remain cold.
- Cooler weather should return to the Central and East US for a brief period around February 6th or 7th, before warm weather takes over.
- A warm pattern may persist into the middle portion of February, possibly into the later part of the month, for much of the nation.

Andrew