Oct 16

First Look At Thoughts On Winter 2017-18

When we look how our atmosphere responds to perturbations, this is what we call weather. The weather we experience is cyclical, both climatologically and globally. What may happen in Siberia will have some type of impact on the United States’ weather in the future, a general rule of thumb is around 21 days. Why do I bring all of this up? To bring an appreciation to the science of long-range and seasonal outlooks. Season outlooks are not meant to perfect, but more of a guide as to how our atmosphere will handle these long-range perturbations in our atmosphere. It is by no means an easy task, but it is certainly a craft that many have dedicated their lives to and continue to dedicate their lives to.

One of biggest drivers of our weather and climate patterns is oceanic behavior. How warm or how cold the temperatures of the water play an absolutely vital role in what weather we experience day in and day out as well as long-term weather patterns that occur throughout a meteorological season. The atmosphere’s response to changes in the ocean has an amplified effect on the variations of our winter weather (or lack thereof) every year.

I’m sure many, if not most of you, have heard of ENSO (El Nino-Southern Oscillation) and its phases. The El Nino phase of ENSO is where the waters of the central and eastern tropical Pacific are warmer than average. This phase occurs every 2 to 7 years. On the contrary, the La Nina phase of ENSO is where the waters of the central and eastern tropical Pacific are cooler than average. The latest outlook discussion from NOAA suggests that the northern Hemisphere will be in a developing La Nina phase of ENSO this winter, indicating a better than 50% of La Nina conditions during the Fall and upcoming Winter months. How the atmosphere responds to ENSO phases is noted by the “Southern Oscillation” in the term, ENSO. The atmosphere’s peak response to ENSO phases generally occur throughout the Winter months across the Northern Hemisphere.

With a La Nina phase, the primary jet stream tends to drive the storm track through the Pacific Northwest. This generally produces wet conditions during the meteorological Winter months. This same pattern also produces a track that would drive low pressure systems into parts of the Midwest, and the Ohio and Tennessee Valleys resulting in above average precipitation for these regions for the same period. On the counter side, this pattern would favor periodic ridge building across the southern half of the United States on the foundation of high pressure systems. This would produce drier conditions across much of the southern half of the country while bringing milder temperatures across Texas and eastward through the Gulf States and Southeast US.

The greatest opportunity for colder conditions during a La Nina phase stretches across Montana and across the High Plains as the dip in the jetstream across that region which serves to usher in colder air from Canada. The Midwest, Northeast and New England are accustomed to seeing colder than average temperatures during a La Nina phase. Keep in mind though, for as much as ENSO plays a vital role in our winter, it is only one factor among several.


In the figure above, we note the cooler than normal (0) SSTs stretching across the central and eastern equatorial Pacific. This is characteristic of the La Nina phase of ENSO.

The figure above illustrates the common pattern setup resulting from the La Nina phase of the ENSO cycle. The setup of the jetstream (thick, blue arrow stretching across CONUS) will be critical for winter storm tracks throughout meteorological Winter.

Season models have been fluctuating all over the place on their projections for air temperature departures, meaning how much warmer or colder, compared to normal, various regions of the US will be this winter. I personally do not see great use in utilizing air temperature projections. As I discussed earlier, oceanic behavior and sea surface temperatures convey a more telling message for the weather patterns we might experience over the Winter months.

According to the JAMSTEC seasonal model, as characteristic of a La Nina, the equatorial waters of the central and eastern Pacific are modeled to be cooler than normal, while across the northeast Pacific, average to above average warm waters are modeled to evolve through the Winter months. Looking at the JAMSTEC SSTAs through meteorological Winter, one particular area of interest when I do my research is how warm or cool the waters are across the Northeast Pacific, especially from the Alaskan Gulf, southward to just off the west coast of the United States. If there is a warm signal that is modeled to develop during the Winter months from the Alaskan Gulf down through waters off of the western US coast, this tells me that there are favorable, seasonal conditions that would conducive for blocking high pressure features to develop somewhere in this region, usually near the Alaskan Gulf which would promote ridging in the western US with a troughing in the eastern US. The magnitude of the La Nina phase will determine how amplified such a pattern will become. This pattern is known as the positive phase of the PNA (Pacific-North American Oscillation).

Analyzing teleconnections and their trends give us critical insight into what we might be able to expect in the upcoming winter months. Teleconnections are large-scale, long-duration shifts in atmospheric circulation that can have impacts across the globe. Critical teleconnections for the United States during the winter months include the following: the PNA (Pacific-North American), PDO (Pacific Decadal Oscillation) and NAO (North Atlantic Oscillation) patterns.

If the warmer waters do prevail as modeled from the Alaskan Gulf to the California coastal waters, this would suggest the positive phase of the PNA pattern would evolve. With a +PNA, above normal geopotential heights would prevail across the western US with below normal geopotential heights the trend across the eastern US. This would lead to ridging in the west (milder air) and troughing in the east (colder air). With ENSO in forecast to be in the La Nina phase, though, contrary to an amplified ridge/trough setup, we could see a pattern more characteristic of a zonal ridge/trough setup, meaning the ridging and troughing throughout the Winter months may need become as deep or pronounced. Something we will have to monitor very closely in the coming months and throughout the Winter season.

If the waters immediately along the western coastline of the US become even slightly below normal, as models are hinting at in the early stages, this would promote above average temperatures and below average winter precipitation across the Southeast US characteristic of the cold phase of the PDO pattern. One thing to watch closely with the PDO and the La Nina phase is depending on how weak or strong La Nina will actually become will give us insight as to how deep troughs that form in the east will develop throughout the Winter. If the ridge/trough setup does remain more zonal (flattened) in nature, we could be looking at the jetstream pushing more into the Tennessee and Ohio Valleys, bringing a greater opportunity for a winter storm track for these regions. On the contrary, if the ridge/trough setup becomes even slightly more amplified, the jet stream could become more suppressed bringing more opportunities for a few winter events across the Southeast, though I don’t suspect terribly large amount.

Finally, the NAO phase is forecast to remain mostly positive through at least the of October. This would result in a wetter pattern with many storm (winter storm) threats due to the upswing in upper level winds. Also, with the NAO in the positive phase, above average temperatures are the trend associated with this pattern. In this case, “above average temperatures” does not infer spring-like temperatures, but temperatures above normal for that time of the year in that particular region, climatologically. It is important to note that a wetter pattern requires temperatures supportive of moisture in the atmosphere. Bitterly cold temperatures essentially shut down the moisture supply in the atmosphere.

All these considered, I suspect this will be a winter that brings a variety of weather to the entire country. I’ve broken down the map below into numbered sections. Each associated number will provide a preliminary concise, generalized winter outlook for that particular area.

Region #1: Mostly episodes of rain to through December with chances for colder temperatures and snow during the second half of Winter, especially through the interior regions.

Region #2: Frequent episodes of cold, Arctic air, especially during the second half of Winter. A number of chances for clipper systems to pass through.

Region #3: Frequent chance for cold and snow. Watch for the annual “winter battle zone” (ice, freezing rain) to be the trend along and south of the Ohio Valley and into the Tennessee Valleys.

Region #4: Mostly mild and drier with occasional rain chances throughout the Winter months.

Region #5: A fairly average winter with seasonable snow and cold chances. Colder across the southern Plains over the second half of Winter.

Region #6: Cooler temperatures and wet, more rain chances than snow chances, although, I suspect a few snow/cold chances might be the case over the second half of Winter

Region #7: Mild temperatures with occasional chances for rain. Might be hard to buy much snow for this region with best snow chances centered across the eastern Carolinas.

While seasonal and long-range outlooks give us some insights as to what we can expect over an extended period of time, they are by no means concrete. The atmosphere is constantly changing and is an extraordinarily fickle beast. As I say every year, the smallest change in a variable that is unseen can have a great impact on the overall outlook. Please keep in mind, the information presented in this outlook is not gospel truth and therefore should not be treated or shared as such. This outlook was developed as a preliminary outlook and will be updated or adjusted as need in the coming months.

 

 

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