Winter Forecast

Transcription

1 Winter Forecast

2 Inside This Forecast Winter Outlook Overview La Niña/ENSO and a Word on the PDO What Winter Influencers are left? Winter Outlook: October - November Winter Outlook: December - January Winter Outlook: Feb. - Mar. - Apr. Long Range Team Meteorologists: Ken Elliott Mike Mihalik Brian Marmo Cody Hewitt Andrew Barney WeatherWorks was founded in 1986 in an effort to bring quality meteorological expertise to both the public and private sectors. WeatherWorks objective has remained the same ever since: to provide the highest quality of weather consultation based on years of training, experience, and the scientific principles of Meteorology. Now in its thirty-first year of publication, the WeatherWorks Winter Forecast is a small part of that effort. Please direct any comments or suggestions about this winter forecast to mikemihalik@weatherworksinc.com WeatherWorks Winter Forecast Winter Forecast Overview In somewhat of a format change this year...we re going to get right into our seasonal forecast! As the map to the right shows, we re expecting a colder than normal season for the northeastern quarter of the nation. While the cold is likely to focus over the Upper Plains and Great Lakes, several colder periods are likely all the way to the coast. The expected cold stands in stark contrast to the mild winter, which was highlighted by extreme warmth in December. These differences lead us to believe this winter will feel even colder than it would otherwise. Unlike in recent years, the cold air may be spread out over the winter, rather than in one long-duration outbreak. Particularly, we re focusing on mid to late December, late February and March for the season s coldest temperatures (compared to normal). In between, fluctuations to milder weather are likely, which is not all that surprising given that the US (and the world) in general has been pretty warm over the last several years. As always, the snowfall forecast is significantly more challenging since even when it is mild it still can snow, particularly in January. It seems, however, that ample cold air and an active pattern support slightly above normal In order to more effectively communicate the seasonal and monthly temperature and snowfall forecasts for , the graphical layout will continue to be presented in a probabilistic format, similar to that of our long-range web page. As a result, instead of just using vague terms such as above or below average, etc., a probability of occurrence will also snowfall for the eastern Great Lakes into the Northeast, on the strength of favorable conditions, particularly in February. Right now, it appears that the overall risk for significant snow producing nor easters is a little below normal, which is why we re not forecasting higher probabilities of above normal snowfall. Expect greater numbers of smaller and some mixed events which will serve to somewhat limit seasonal snowfall. Before heading into the monthly forecasts, let s examine the main players in our forecast rationale. Probabilistic Forecasts Explained be given to augment the forecast. For example, in the snowfall outlook map above, much of the Northeast has at least a 33% chance of above normal snowfall, with the greatest odds in northern New England, where there is a 40% probability to see above average snowfall during the season as a whole. Page 2

3 WeatherWorks Winter Forecast Final Look at La Niña/ENSO Several months ago, it seemed that a La Niña event for the fall and winter was nearly a foregone conclusion. However, a late summer trend towards more neutral conditions seems to be holding. Looking at the current sea surface temperatures (SSTs) below, there is some semblance of a La Niña across the Pacific, but the cool anomalies are weak and even mixed with warm anomalies near the South American coast. In addition, if a La Niña state were to persist or strengthen during the winter, we would see more of an atmospheric response in the equatorial Pacific (namely, low level easterly wind anomalies should be strong along with upper level westerly wind anomalies, but this is just not taking shape). Fig. 2: ENSO model predictions for the upcoming winter. Fig. 1: Sea Surface Temperature anomalies from October 12th, shows only a weak La Niña (circled) With that in mind, we currently favor an ENSO state that can be termed as cool-neutral, which was discussed in Winter Preview #2. Essentially, a cool-neutral ENSO event is like a partial La Niña, where ocean temperatures are on the cool side, but not significantly. It should be pointed out that the European Model is a bit less emphatic on the idea of cool-neutral, and even suggests that neutral conditions (often referred to a La Nada ) are still in play this winter. Given the lack of an atmospheric response, this notion cannot be dismissed. Going forward, we ll continue to closely monitor ENSO in our Long Range forecasts over the next one to three months and adjust as necessary, particularly in Winter Risk. Even though ENSO may be rather neutral, it is still an important part of the winter forecast. A couple past winters that match up favorably to the recent ENSO trends suggest a chilly and active pattern, with hints of February and perhaps even December being key wintry months. The Pacific Decadal Oscillation (PDO) hinges on anomalously warm and cool water in the northern Pacific (think of it as the northern cousin of El Niño/ La Niña). Over the past several years, a warm pool has existed in the Gulf of Alaska, lending to a positive state, which tends to pump a ridge into Alaska, British Columbia, and the western US. This in turn drops a trough and cold air into the eastern half of the country. Interestingly, this warm pool has shifted A Word on the PDO a bit west over the last several months, with even some analog years insinuating anomalously cooler water building along the western coast of North America. This would suggest a -PDO, a troughy west coast and warm east coast...basically a total pattern reversal. However, the PDO is typically slow to change state (as per the decadal in its name) and we have been in a positive, warm phase for the past 3 years. That said, we ll side with a semi-persistent west-based warm pool, which will Page 3 likely shift the ridge slightly west and coldest winter air into the north central portions of the US. Fig. 3: Expected Winter Pattern

4 Fig. 4: Siberian Snow Cover October 12, 2016 WeatherWorks Winter Forecast What Winter Influencers are Left? With La Niña not expected to be significant this year and the PDO being slightly finicky, other forecast predictors become more important than usual. Unfortunately, many of these items have low predictability beyond a 2 to 4 week time scale and are subject to change during the season, such as the Atlantic Ocean oscillations (AO & NAO) and the ever popular Madden-Julian Oscillation (MJO which tracks thunderstorms mainly in the tropical Indian and Pacific oceans). Luckily, there are a few things that are already starting to take shape that can aid in the forecast process. Chief among them is the extent of sea ice in the Arctic. While it does not directly impact the weather in the eastern US, it certainly acts as a cold air source for Arctic outbreaks through the season. In short, if there is not substantial ice buildup in the fall, it casts doubt on cold outbreaks during the winter. It is no secret there s been a lot of melting recently in the Arctic, but, with impressive cooling over the last month, sea ice has increased by 25% from its late summer minimum. This ice buildup brings current sea ice coverage close to that of this time last year, despite more melting in In addition, there has also been a marked increase in Siberian snow cover over the past 3 weeks. Its influences are similar to Arctic sea ice in that it contributes to cold air outbreaks in the US. However, just because there is cold and snow in the polar regions doesn t mean it will easily flow into the country. Sometimes the cold can remain bottled up in the Arctic if nothing displaces it. So do we have any indications the cold will be dislodged? The answers lie in the Atlantic Ocean oscillations. Although they are highly variable, there are growing indications that the state of the AO (Arctic Oscillation) in October foreshadows its overall state for the winter. So far this month, it has been strongly negative, which means the infamous polar vortex is not favored to remain bottled up during the winter, lending to cold patterns capable of producing snow storms. Fig. 5: AO index for October Monthly Outlooks: Getting into the Nitty-Gritty October / November 2016 Forecast After a shot of cold air and potential wet snow showers/ flurries for the Great Lakes (10/22-10/25), October temperatures return back to normal or above for the Eastern Seaboard and Midwest. November will likely remain near normal temperature-wise for the I-95 corridor and above normal south. However, shots of cold air will be more frequent & progressively colder in the Upper Plains and Great Lakes, but lack staying power. The storm track is likely active, but the East Coast will see mostly rain events The Midwest will be favored for some early season snow as the area will be on the cold side of the storm track. That being said, the region can expect slightly above normal snowfall. The eastern lakes may see a significant Lake Effect event late in the month. Page 4

5 WeatherWorks Winter Forecast December 2016 Forecast In a departure from recent seasons, we re anticipating cold air to arrive in December this year. Exactly when during the month is still questionable east of the Appalachians, but odds favor a seasonable start, before below normal temperatures take over. Enhanced areas of snow are likely in the Upper Plains and the downwind areas of the Great Lakes, where storm type varies between clippers and Lake Effect. Despite a near normal snow forecast, there is some concern for a nor easter, given the contrast between cold air near the Great Lakes and warm Atlantic waters. Even if such a storm develops, it may be too mild for the immediate coastal plain to see significant snow. The chance of a white Christmas is above normal as the risk of snow (and cold) increases through the month, especially in the I-95 corridor. January 2017 Forecast Partially in response to an anticipated west-to-east zonal pattern across the US ushering in maritime Pacific air, temperatures are likely to return to at least normal levels in a fairly widespread January thaw. The overall pattern is favored to remain active...with several fast moving systems crossing the nation. Mixed precipitation events seem favored region-wide, but particularly areas just inland from Massachusetts through Maryland in response to the lesser amounts of cold air. Plain rain events may even occur from the Corn Belt to the East Coast. Refreeze threats are likely to be fairly numerous from brief cold shots behind systems, but the cold air will be progressive and won t last. There is a risk for a clipper gone coastal type of storm to wind up into northern New England, which accounts for some of the above normal snowfall. Page 5

6 WeatherWorks Winter Forecast February 2017 Forecast The mild, zonal pattern from January is expected to break down toward the middle of the month with progressively colder outbreaks. Any above normal temperatures will be relegated to southern VA into the Carolinas. The most probable storm track will be through the Ohio Valley into southern New England. In association with cold air pressing in from the north, the Corn Belt states to areas north of the Mason-Dixon will likely deal with above normal snowfall. Overrunning events (surges of warm, moist air from the south into cold, dry air to the north) will be the predominate storm type, with modest snowfalls (typically ranging from 2-6 inches) north and mixed events along the Ohio River east into the Mid-Atlantic due to cold air damming. The greatest risk for a major snow event will be in New England as storms may wrap-up along the coast, especially during that last week of the month. March / April 2017 Forecast The building cold in February continues into March with the entire northeast quarter of the country below normal. The most prolific cold will be over the western Great Lakes. Although the cold will be in place, it may be rather dry (especially in the Great Lakes) as high pressure centers will frequent the area and persist. As a result, snowfall will be below normal in the Midwest (even with the cold) with the storm track pushed further east along or off shore of the Eastern Seaboard. This will favor wet snow events versus mixed storms. The above storm track increases the potential of a coastal storm for the entire East Coast through mid-month, but the heaviest snow may just scrape the region. A pattern shift is expected towards the end of the month, with April quickly transitioning to seasonable to even above normal temperatures. April may also be a bit wetter than normal from the Midwest through the Northeast. Page 6