Sudden Stratospheric Warming Could Mean Robust Spring

The chances of a late-winter sudden stratospheric warming could mean a robust severe weather season this spring.

Shown above is an animation of temperature anomalies at the 10 millibar region in the atmosphere. To give you an idea of how high up that is, we live at the 1000 millibar mark, commonly known as surface level. 500 millibars is where storm systems are most commonly seen, and 300 millibars is where airplanes fly. So 10 millibars is quite a high position in the atmosphere. If you look at the animation, you will see it start with sudden cooling after a large sudden stratospheric warming in January. From early February to now, we have seen a very chilly Arctic, meaning the chances of anomalously cold air reaching the lower latitudes and the polar vortex becoming more unstable are lower than normal. If we look towards the Himalayan mountain range, however, we can see some oranges (and more recently, reds) bubbling up in the mountain range. This does signify that the stratosphere is warming in that area, and sudden stratospheric warmings have been known to occur off of mountain ranges, particularly the Himalayan range.

From here, I can reasonably assume that the warmth in the Himalayan mountain range will continue to increase in the upper stratospheric 10 millibar level. In the next 1-3 weeks, we should then see movement of this body of warmer than normal temperatures propagate north and likely into the Arctic. This is almost an identical set up to the sudden stratospheric warming that was experienced in January- warming bubbles up in the Himalayan mountain range for a few weeks, then propagates north into the Arctic where it blossoms into a sudden stratospheric warming event.

So what does this have to do with spring? Well, the sudden stratospheric warming phenomenon involves warm air being forced up into the stratosphere. As a result of warm air going into the stratosphere, anomalously cold air must then be pushed out of the stratosphere due to air displacement. This cold air then moves down the stratosphere and troposphere until it hits the surface. This moving down takes a good 2-4 weeks, so if the sudden stratospheric warming event did come to fruition, we would still have to wait 3-6 weeks before we see its effects. This leads us into spring, the season of severe weather. If the atmospheric pattern is able to allow this cold air to flow into the US and not other parts of the world, we would most likely see additional chances for wintry weather, including snow and cold. The increased temperature gradient in a cold front would most certainly excite severe weather potentials, and this in turn could lead to more damaging wind and general severe weather events.

This all depends on if the Himalayan warmth even propagates into the Arctic, which there's not a 100% confidence in right now. However, based on past experiences with Himalayan stratospheric warming, I think what I typed above has at least a decent chance of coming true.

Andrew

Himalayas Provoking Upper Stratospheric Warming

The upper stratosphere is showing signs of additional warming over the Himalayas and Southeast Asia. Posted above is the animation over the past 30 days of the 10 millibar layer of the stratosphere, considered the upper stratosphere.

Looking down towards the Himalaya Mountain range, we see there has been a trend of increasing warm temperature anomalies. The mountain range is one of the ideal spots for stratospheric warmings, as air is easily provoked to be forced up and into upper levels of the atmosphere. It was in the past several weeks that we saw a sudden stratospheric warming originate from the very same mountain range. While we are at the end of February and it would take a month to see this warming propagate down to the surface, the stratosphere does not effectively 'shut down' for the winter until well into the spring and into summer. Until then, stratospheric warmings are still on the table.

The EP Flux forecast at the bottom of this multi-panel image will also be on the rise in the next several days. This increase means the stratosphere is becoming more supportive of warming in the stratosphere. Personally, I don't believe we will see another sudden stratospheric warming in the next 10-14 days. However, that increasing warmth in the Himalayas will be something to watch- if it is able to strengthen and detach from that mountain range into the Arctic, we could then see a solid sudden stratospheric warming event. Climatologically, that chance is decreasing by the day, but in weather, anything is possible.

Andrew

Serious Threat to Florida Citrus Crop in Coming Days

There could very well be a serious threat of damage inflicted on the Florida citrus business and other warm-season economies in the Deep South in coming days.

The forecast above depicts 850 millibar temperatures in light color contours, where below freezing temperatures are north of the thick blue freezing line, and above freezing temperatures to the south of that thick blue line. As you can see, the thick blue line is edging very close to the southern tip of Florida, leaving Jacksonville, Tampa and Orlando in below-freezing temperatures. Light precipitation over the latter two cities could indicate snow - yes, SNOW - would be falling in Florida. This solution has been shown a few times, so the idea of such cold temperatures hitting the state is not entirely out of the realm of possibility. The orange/citrus economy that is based in Florida could be severely damaged if such a solution came true- if snow fell, only time could tell what the impact on the citrus economy would be.

Andrew

February 24-27 High-Impact Blizzard

500 millibar analysis reveals that our storm system is currently in the Rockies, and is at the pre-mature stage of a positive tilt. The positive tilt is shown by how the vorticity values (in red outlines) seem to be trying to push to the southwest. In the next 24-48 hours, the system will attain a negative tilt, where the vorticity values will slide east and appear to be pushing to the southeast. In similar fashion to the most recent snow event, this storm will dump intense snowfall on the Plains and occlude (weaken) as it progresses east.

The time for analyzing forecast models has come and gone, as we begin to see swaths of the country stamped with various winter weather headlines. In purple, we see winter weather advisories, which are issued for generally lighter snow events. Pinks show winter storm warnings, and these are the areas that are most likely to see these heavy snowfall amounts in the next couple of days. In an orange-red color we see blizzard warnings, which extend from North Texas and western Oklahoma into southwest Kansas and portions of Colorado to the east of the Front Range. This is where the high-impact blizzard portion of the storm title comes from. These areas will see the worst of this storm in terms of travel impacts and ability to travel.

This is the short range NAM model, an American-based weather model that does fare well in these types of late-winter storm situations. Its forecast for this event holds as much as 31 inches of snow in western Oklahoma in that small red dot. That's nearly three feet of solid snow! The apricot color signifies 24-30 inches, and the light blues reflect 20-24 inches of snow. This is essentially a repeat of what we saw with the most recent snow event: It dumps all of its snow early, occludes and leaves the leftovers for the Midwest and Great Lakes. However, this time, the 'leftovers' may not be so meager. Multiple forecasting sources have informed me that cities like Chicago, IL , Gary, IN and even the region of southern Michigan could be in the running for more accumulating snow than what fell earlier last week.

I did put in city forecasts for this event in addition to the coloring scale. Light Blue is a general 1-3 inches, dark blue is 4-8 inches, and light pink is upwards of 8 inches. I used a blend of the NAM model and forecasts from the National Weather Service, as well as my own opinion on the matter. As far as commutes go, I am anticipating an incredibly messy one for Wichita and Amarillo, where blizzard conditions are expected, especially in the latter city. Chicago and Des Moines will really be 'game-time decisions', as upper air thermal profiles are still a little murky and will ultimately determine how much liquid and snow falls over these cities.

Andrew

February 24-27 Potential Significant Winter Storm

Current satellite water vapor imagery indicates that the storm system in question is currently over the waters just offshore the Pacific Northwest. Enhanced water vapor values across the Pacific Northwest would indicate the presence of precipitation, and radar analysis confirms that precipitation is ongoing in the oceanside states. Despite its appearance of close proximity to the United States, 500 millibar analysis reveals the storm is still too far away for weather balloons (that are launched in the US Mainland) to get into the storm and put data into the models that would help significantly improve their forecasts for this storm.

For this post, we will be using the European model and ensemble forecast out of consistency for this storm. American model remains out of touch with current trends, which will be reflected in this post. Shown above is the Hour 72 (times will be expressed in hours for this post. If you want to know how many days out that is, divide the number of hours by 24) from the European ensembles, forecasting 500 millibar heights. The system is clearly indicated by a deep depression in the Rockies and Plains. Such a strong depression does indicate a strong storm. Looking closer at the storm, we see it is almost pointing straight down with those light blues. This indicates a neutral tilt storm. This means that the storm's 500mb appearance is not pointed towards the southwest (positive tilt) or the southeast (negative tilt), it is simply pointing south. For future reference in this post, a negative tilt means the storm is at its strongest point.

Let's go back to the winter storm we just had across the Plains and Midwest. If you recall, the concern in the days leading up to the forecast was that it would reach a negative tilt too quickly, and would then occlude (or weaken) as it progressed northeast towards the Midwest. This did end up happening, and the Plains got the brunt of the storm. This was thanks in part to how early the storm attained a negative tilt. By the time it was exiting the Rockies, it already had developed a negative tilt. However, looking at the forecast above, we see it is holding on a neutral tilt- a good sign for those east of the Plains as far as snow potential goes. Another thing relating to this recent storm is where it exited the Rockies. I did mention in a recent post how it would be exiting the mountains in southeast Colorado. This is further north than what is being shown above. The European ensemble's forecasted south placement of this storm system relative to the previous storm is more good news for folk in the Midwest and Great Lakes that may be affected by this system.

We now progress ahead to Hour 96 of the European ensembles forecast, again looking at the 500 millibar level. We find that the storm has now exited the Rockies and has attained a negative tilt, shown by how the storm seems to be pushing into the southeast. This pushing motion/negative tilt does a few things. For one, it strengthens the storm. The presence of a negative tilt does mean the storm has reached maturity, as I mentioned earlier in this post. Second, the system begins to dig down towards the southeast. In a similar situation to the most recent snowstorm, there will be high pressure to the east of this storm system, and that feature is exhibited well by large arcs of greens and yellows in the East Coast and Northeast. This high pressure, combined with the strongest areas of vorticity pushing southeast (the technical definition of a negative tilt) results in the cold front catching up to the warm front, and this results in the final stage of occlusion. Occlusion of a storm means that the negative tilt swings north so the storm seems like it's pointing to the east or even northeast. When this happens, significant weakening of the system occurs and precipitation weakens in response. This is exactly what happened in the Midwest and Great Lakes with the most recent storm system. Luckily for those folk, the European ensembles believe that this will happen when the system is beginning to cross into Canada in the central Great Lakes just east of Michigan (the mainland Michigan, not the U.P.).

Now it's time to look at what the atmosphere will be supporting as far as storm tracks. Shown above are two different atmospheric indices, forecasted by two different government weather agencies. The ESRL/PSD agency has their forecasts on the left hand side of this multi-panel image, while the NCEP agency is on the right hand side. The two indices being forecasted are the Pacific North American index (PNA) and the North Atlantic Oscillation (NAO). For ease of viewing, I superimposed arrows pointing towards the date of this storm's timeframe. The two agencies generally agree on a slightly negative to neutral PNA. A negative PNA means there are negative height anomalies in the West US (low pressure), thus positive height anomalies must form in the East US (high pressure), as dictated by Newton's Third Law of Motion. The negative PNA discourages the big coastal storms known as Nor'easters, and are typically detrimental to the formation of any big winter storms in the central and East US, save the North Plains. However, as I stated with the European ensembles, there should be very little influence from the PNA at this time, which leaves more influence up to the NAO. Both agencies have the NAO negative during this timeframe. In the negative NAO, high pressure builds over Greenland. In response, the jet stream drops south into either the Central or Northeast US, bringing cold to the region. The negative NAO then enhances the subtropical jet stream, which is basically another jet stream along the Gulf Coast and South US. This is the jet stream that drives the Nor'easters up the coast. As the subtropical jet stream (STJ) is enhanced, high pressure in the East is suppressed, meaning that if there were a solid negative PNA, it would have to fight the negative NAO. Luckily, there will not be a strong negative PNA this time around.

Here's my current thinking on the system. I'm going to buy into the European model and ensemble's consistent solutions and insert a large swath where significant snow may fall. It should be noted that the significant snow area may have to be pulled south a bit if some indications of mine become stronger. I also put in a potential icy area. Temperatures are shaky right now in the icy zone, and those areas could see a potentially major icing event. Don't begin preparing yet- besides the system not being in our upper air network yet, ice storms are a much shorter-range forecast than snow, as ice is much harder to forecast. Nonetheless, keep up to date on the latest forecasts if you are in the icy zone.

Confidence checks (out of 100%)

Confidence that a significant winter storm will hit the Plains and/or Midwest: 85% confidence
Confidence that over a foot of snow will hit somewhere in the Midwest: 40% confidence
Confidence that a major icing event will occur somewhere with this storm: 90% confidence
Confidence that a snow day will be called for someone in this storm: 90% confidence

Comments are welcome, please don't ask for your location's snow or snow day chance. You will not get an answer, because we are too far away from the event.

Andrew