Second Polar Vortex Intrusion Possible

We may have to deal with another intrusion of the polar vortex in just a handful of days.

Shown above is the GFS ensemble mean forecast of 500mb height anomalies over North America. We see a situation eerily similar to the one that brought about the severe cold in early January 2014. We see a Rossby Wave pushing north into eastern Alaska and Canada, as the strong positive 500mb height anomalies shows. As a result of this strong high pressure, we see a lobe of the polar vortex dropping to the south from Canada. This lobe of the vortex is illustrated well by the deep greens and low contour numbers superimposed on those deep negative height anomalies.

The GFS Ensemble 850mb level (5,000 feet level) temperature forecast for that same timeframe shows how cold conditions may get if this lobe of the polar vortex decides to scrape the United States just to our north again.

The GFS Ensemble Control 850mb temperature anomaly forecast is even stronger, with anomalies as low as -28.4 degrees Celsius in the core of the lobe of the polar vortex.

There's good reason to be concerned about a second intrusion of a lobe of the polar vortex: The Lezak Recurring Cycle supports it.

500mb chart from January 6th

The Lezak Recurring Cycle, or LRC, is a tool developed by meteorologist Gary Lezak that, in essence, can enable forecasters to predict the overall weather pattern months in advance. The gist of the LRC involves a cycling weather pattern that develops in October and November of each year; no pattern is the same from year to year. Around mid November, the LRC begins to repeat, meaning we start to see a similar weather pattern in mid November that we saw in early October. This means that the cycling pattern has begun, and it will continue to cycle on a regular, somewhat-unchanging 40-60 day interval for the next ~10 months before it dissipates over the following summer. This season, the length of the LRC has been pegged at 57 days. If we take the January 6th date and move ahead ~57 days, we arrive in the timeframe of late February, a couple of days within the very cold weather the GFS Ensembles showed above. Considering the LRC has been an efficient tool of use in not only this winter season, but in prior winter seasons, this is likely a real threat of a lobe of the vortex coming south once again.

To sum up:

•The atmosphere looks primed for a strong cold blast in late February that will likely produce anomalously cold weather.

Andrew

Kelvin Wave Provoking El Nino; Atmosphere in Disagreement

A Kelvin Wave currently moving across the eastern Pacific appears to be trying to provoke an El Nino, but the rest of the atmosphere is in disagreement.

The graphic above shows water temperature anomalies across the Equatorial Pacific, with longitude values on the bottom legend and depth values in meters on the left legend. The Kelvin wave is clearly shown by the strong positive water temperature anomalies from the 100 to 200 meter depth in the central/western ENSO monitoring area in the Equatorial Pacific. With a Kelvin Wave, you have a mass of warm water pushing east under the surface, and pushing east across most (if not all) of the Equatorial Pacific. This wave warms the waters below the surface, and if those subsurface waters are mixed to the surface, we can see an El Nino develop. Due to that process, Kelvin Waves are typically associated with El Nino formations, and that is why we are monitoring this event for possible El Nino provocation down the line in spring.

However, the rest of the atmosphere does not agree that we will see an El Nino anytime soon.

One feature we see over the Eastern Pacific is a swath of anomalously strong upper level winds, as the purples and yellows on the attached image denote. Thanks to the Earth System Research Laboratory (ESRL), we can compare these uppr level winds to see what ENSO state the atmosphere is currently in- a La Nina, El Nino, or neutral-ENSO state.

If we look at the ESRL's 200mb wind anomaly composite for February and March over the Equatorial Pacific, we see that El Nino's are typically recognized in those time periods by strong upper level winds due west of Mexico, with anomalously weak upper level winds over the ENSO monitoring area. Taking a look back at the latest observed upper level winds, it is clear that the atmosphere is in more of a La Nina state, as we usually see strong upper level winds over the eastern Equatorial Pacific during an El Nino instead of weak upper level winds we are currently seeing.

Another item we can use to judge what state the El Nino-Southern Oscillation phenomenon is in is the sea level anomaly. In other words, we can see if the waters over the eastern Equatorial Pacific are higher or lower than normal, and use that to help tell us what phase the ENSO is in. If we check out the composite graphic above, it is clear that the El Nino phenomenon is identified by higher than normal sea level anomalies, with the La Nina producing lower than normal sea level anomalies. The November 1997 sea level composite on the top of this graphic shows the El Nino sea level anomaly, while the February 1999 composite below that depicts sea level anomalies for a La Nina.

Taking a glance at the latest observed sea level anomalies, it is more apparent that while we aren't in an El Nino, which would show up as a swath of yellows and oranges to indicate higher than normal sea level anomalies, we also aren't in a defined La Nina, which would appear as a swath of deep blues on the right hand side of this chart. We do see some blues in the Equatorial Pacific, which could argue for a weak La Nina if you really want to be picky about it, but I'm not really seeing any defined ENSO state going one way or the other.

This chart, showing the anomalous depth of the 20 degree (C) isotherm over time, gives you a hint at what the progression of a Kelvin Wave looks like, with two clear examples of Kelvin Waves observed in November 2013 into January 2014, and from September 2013 into October 2013. The latest observation of this 20 degree isotherm confirms that we are looking at a Kelvin Wave. with this wave looking to be the strongest out of the last three events. It's possible that this sends us into an El Nino pattern when spring and summer come around, but for now, an El Nino does not look to be on the way, at least for the near future.

Andrew

February 20-22 Potentially Significant Severe Weather Event

Model guidance is beginning to converge on the idea of a potentially significant severe weather event over the February 20-22 period.

The Storm Prediction Center has already outlined a large area of severe weather potential for February 20th, centered from northern Louisiana into eastern Arkansas, eastern Missouri, a large portion of Illinois, and eastward into Ohio. States like Indiana, Kentucky, Tennessee, Mississippi and Alabama may be affected the most by this severe weather event.

For February 21st, the Storm Prediction Center then outlines an area of Georgia to southern New Jersey for an enhanced severe weather risk as the frontal boundary associated with this dynamic storm system moves to the east. In this post, because the threat area is a little more specific, we will focus on the February 20th severe weather risk.

Note: All model guidance images in this post will be valid for the evening of February 20th.

Model guidance indicates we will see a strong storm system lift into the upper Plains, tilting negatively as it heads towards the upper Midwest on February 20th. The jet stream projection above for the evening of February 20th shows the highly dynamic situation at hand here. We have a jet streak in the range of roughly 110 knots to 130 knots, centered over the southern Midwest and Central/Southern Plains. Analysis of upper level divergence suggests strong values located over those same areas, and for that reason we are looking for the strongest storms to hit the area of southern Illinois, southern Indiana into western KY/TN, and Missouri into Arkansas.

A forecast image of the 850mb level, located about 5,000 feet above the ground, gives us the full picture for the evening of February 20th. We see a strong moisture fetch from the Gulf of Mexico, shown by not only the northbound wind barbs from the Gulf Coast, but also the swath of high relative humidity values along the frontal boundary. We see a sharp gradient in theta-e values along that frontal boundary, denoted by the dashed pink values, and this also spells out the potential for at least a decent severe weather event. Enhancing this event's potential even further is the 50 to 60 knot lower level wind speeds right over the severe weather threat areas we discussed above, and these lower level winds ought to really help out with storm development. However, the lack of directional wind shear (changing of wind direction with height) means any storms that form are unlikely to contain a vast number of tornadoes. I expect tornado reports from this event, but not that of an outbreak level.

A forecast graphic for the 700mb vertical velocity on the evening of February 20th shows intense lifting located over the same regions we discussed above, and this lifting will lead to the development of strong thunderstorms. Precipitable water values exceeding 1" will ensure that this will be a heavy rain system, and model guidance (as well as the SPC) seems to agree on the idea that this will be a squall line event, rather than multiple tornadic supercells. The squall line lowers the tornado threat, but greatly raises the damaging wind threat. It appears that the damaging wind threat will really maximize this severe weather potential.

Adding fuel to the fire is how the subtropical jet stream will really help this severe weather event become the beast I expect it to be. See that long string of water vapor extending from the Central US all the way down towards Hawaii? That is a classic mark of the subtropical jet stream, close relative of the polar jet stream. During the springtime, the subtropical jet stream acts on storm systems crossing the US, and pumps warm, humid air into the warm sector of storm systems to enhance severe weather prospects. Additionally, we typically see stronger wind speeds as a result of the STJ, and this can enhance both directional wind shear and vertical speed shear (the change of wind speed with height).

All in all, I've got a feeling the Storm Prediction Center will nail its current outlook for the February 20th period. Model guidance seems to agree on a squall line event which will likely produce damaging winds, as well as an isolated tornado report.

Andrew

Long Range Discussion: Severe Weather, Snowstorms To Kick Off March

This long range discussion will focus on the expected weather to end February and kick off March.

The image above shows the latest 500mb height anomalies across the Northern Hemisphere on the left, with cloud cover, pressure contours and high/low pressure denotations on the right. If you've been with The Weather Centre for a while, you know how we use a rule created by Joe Renken that states a storm system in East Asia then results in a storm in the United States 6-10 days later. This situation is no different- today, we saw ridging/high pressure emerge in East Asia. If we extrapolate that out 6-10 days, we arrive at a warm-up in the February 20-24 timeframe. I'll discuss this more a little later down this post.

Lately, as the image above shows, the weather pattern has been dominated by west-northwest to northwest flow, thanks to deep troughing in the Gulf of Alaska and suppressed ridging in the Southwest producing a pattern favorable for East Coast snow events. This whole pattern is expected to flip in the medium range.

The changes begin as ensemble members foresee strong ridging shifting east into far eastern Russia, which will then shift the Gulf of Alaska storminess to the east, resulting in the loss of high pressure in the Southwest US. That Southwest ridging then propagates east into the contiguous United States, producing mainly zonal flow, which keeps anomalous warm or cold outbreaks out of the picture. However, with the ridging now gone, the storminess in the Gulf of Alaska can drop south into the Western US. The result is a stormy West US, and a very warm East US.

Model guidance (with the exception of the CMC model on the far right) is in agreement on a pattern change evolving, where we see the troughing drop south into the West US, as high pressure prevails yet again in the Gulf of Alaska. The storminess in the West US isn't as well defined on the ECMWF forecast (left) or GFS forecast (center), primarily because this image shows projected 500mb anomalies over the 8-10 day forecast period. This means the forecasts may seem a little less 'intense' than they end up being, because the anomalies are averaged out over a 2 day period, rather than a 6 hour period like most models print out.

Regardless, the pattern will involve high pressure setting up along the East Coast in response to troughing out West, and this sort of pattern just screams for not only a snow event, but a severe weather event. We will see anomalously warm and humid conditions present across the Central and Eastern US as the jet stream bends north to accommodate for the ridging out East. This ridge also tells me we should keep an eye out for a storm system cutting north through the western Great Lakes, something model guidance is already picking up on. I have a feeling such a storm cutting north would result in not only a northern Plains snowstorm, but also a severe weather event for the Central US, something I discussed in yesterday's post.

As we head into the last days of February, ensembles give the Central and East US another shot of cold weather, as shown by the GFS Ensembles' portrayal of the trough at the 500mb level on the left panel. Again, the pattern we discussed above would strongly support this solution. It wouldn't surprise me to see additional chances for storms, both snowy and severe, before the month of February ends, apart from that other storm system we discussed yesterday in the link above.

For the opening days of March, I wouldn't be surprised to see us start on a chilly note before rebounding to rather seasonal temperatures. Depending on how the sudden stratospheric warming event continues to evolve, we could be looking at a cool March, though the chance of that would be maximized in the north-central US (primarily the Plains), and minimized along the East Coast and West Coast, where ridging and warm weather would be expected.

Andrew

February 15-16 Potential Blizzard

It looks like there is now the potential for a blizzard this Saturday and Sunday.

The GFS model shows extreme wind speeds along the coast of the Northeast as another strong storm system rides up the coast in similar fashion as the storm that just ended for the East Coast. We see 10 meter wind speeds over 44 knots just offshore land, with widespread 25-35 knot wind speed forecasts at the height of this storm.

The GFS model indicates the storm will drop accumulating snow along the coast, as the 984 millibar low comes close to land. Despite the proximity of the low pressure system to the coast, the precipitation here will not be incredibly intense. Accumulation is expected, and the wind will be that of borderline-blizzard conditions, but the accumulation will not be significant.

The real fun begins overnight of Saturday into Sunday, as the GFS projects this storm will rapidly strengthen to drop significant snow accumulation on Maine, and this should continue into the day on Sunday. The system will be moving rather quickly, so while significant amounts are expected, it will not be a prolonged event. This also means good news for those of you who like to watch snow come down at high rates (i.e. 2 inches an hour or higher), as we'll likely see some pretty high snow rates in Maine from this storm.

This 6 hour snowfall forecast shows the intense snowfall overnight Saturday into Sunday, with the GFS depicting amounts as high as 12 to 14 inches of snow falling in that 6 hour period. If you were to average that out, it would be equivalent to 2" of snow falling every hour, but as we all know, snow does not fall in uniform and consistent rates like that. It's more probable that we would see very high snow rates (possibly 4" an hour at best) for a period of time, with lighter rates mixed in (i.e. 1" an hour or lower).

When all is set and done, here is the total snowfall put out by the GFS model. We can see those high amounts in Maine, but also some good totals in coastal portions of Massachusetts and Connecticut. Again, totals aren't looking to be too bad (I'm not so sure we see the GFS' predicted 10" of snow hit those areas in MA/CT), except for in Maine. The wind will certainly make it feel like a big snowstorm for those further south, though.

The NAM model has a different take on this event, bringing heavy snow to only the extreme eastern portion of Massachusetts, with that jackpot in Maine also moved to the east. Considering we saw the snowstorm along the East Coast a few days ago also further east than what model guidance suggested, it's possible we see a solution like this work out.

I'm wrestling with which solution I like better in this case. On one hand, we have the GFS solution, which seems somewhat-reasonable (compared to the NAM) with the slightly-lower inland snow amounts, but on the other hand, the NAM model being to the east may verify, considering the most recent East Coast storm appears to have been further east than modeled. Needless to say, this forecast will see some fine-tuning in the hours ahead.

Andrew