Last winter was a tough winter for many seasonal forecasters. In reality, short range forecasting became easier as the season progressed last year, almost to the point where one could look at a few pieces of guidance 144 hours in advance, and have a good idea of what the forecast would look like. However, many folks who annually produce a Seasonal Winter Outlook generally performed subpar except for a few exceptions. Last winter was much milder than normal with well below normal snowfall in the Greater Philadelphia Region. For one, the Northern Branch of the split flow jet was extremely progressive, and only on a handful of instances was it able to dig south and amplify. Even when we did see amplification, there generally was not enough cold air in place to give the region snowfall. Sure, we had situations last winter where snowfall fell, but that which accumulated did not fall close to our seasonal average of just over 20 inches of the white stuff (4 inches snowfall total with a few instances where traces were recorded).
With all of the factors that are thrown into seasonal forecasting, it is tough to sit down, analyze all of the factors, and just arrive at a conclusion for a winter forecast. Take for instance some of the patterns that go into "professional" outlooks: Julian-Madden Oscillation, PNA, NAO, QBO, El Nino, La Nina, PDO, Circumpolar Vortecular Flow, etc. It's tough to fathom how one can generate a forecast for an entire season based on many of these factors which have not even been thoroughly studied and analyzed. In our Winter Outlook 2002-2003, we will draw from a few of the factors that have been researched (ENSO and Atlantic SST Anamolies) and gather a few seasonal analogues from winters past that may fit in well with our Winter Outlook.
It is important to remember that last winter, ENSO was forecast to remain relatively weak during the winter months and slowly become moderate as the spring season went along. This indeed was exactly what ocurred, but was NOT what we at SNUWW based our outlook on last year. Last year, we drew our outlook from many of the current circulation patterns that were ocurring, failing to take into consideration that even a weak El Nino could influence the pattern enough to cause anamolies. As stated prior though, there are also other global features which caused the mild winter last year, such as very little mid-high latitude blocking and a stubborn Southeast Ridge and H5. Things wont be any easier this season in terms of deriving an outlook for the winter, suffice to say we agree that we have a much better grasp of how to arrive at a solid seasonal forecast.
The El Nino is predicted by the fine folks over at NCEP, to remain moderate during much of the fall months, but positive SST anamolies in the Equatorial Pacific are forceast to spread into all Nino regions throughout the winter months.
In regards to the Atlantic SST anamolies, we found a very strong analogue to the 1993-1994 winter season. With temperatures in the Northwestern Atlantic Ocean well above normal, this correlates well with above normal precipitation in the Northeast. Many will recall the 1993-1994 winter by the abmormal amounts of ice storms that the Philadelphia Area suffered through. All in all, the winter was right on average in terms of snowfall amounts for the season.
One of the key features that we will be seeing this winter is an active subtropical jet or southern jet branch. With regards to the STJ, it's positioning will be farther North in general throughout much of the upcoming winter. This could lead to a few above normal periods here and there throughout the next several months. On the same hand, this should lead to more systems phasing, and thus greater temperature fluctuations.
On the flip side, the Northern jet will be a steady source of energetic shortwaves and pieces of energy. However, the flow in the North will not be as progressive as last year, with High Latitude Blocking becomeing a prominent feature this season as well. Base on the recent overall pattern we have seen, this mid fall pattern is quite similair in regards to fall of 1995: a mild outbreak for a couple of days, followed by a drainage of much cooler air, and reinforced 2-3 times before the next mean ridge progresses Eastward to warm us up for a couple of days once again, followed by cooler air reinforcing itself as the pattern repeats. We will see how long this pattern sticks around, but we feel that positioning of the jets will play an active role in real weather for the Philadelphia Area this winter.
The flow of the Northern jet is being influenced primarily by a Polar Vortex pinwheeling around Hudson Bay. This is common in mid-winter, but usually not during late October. One large factor in allowing the Hudson Bay Vortex to expand and remain almost stationary over the same area lies partly in the fact that we already have a decent snowpack in place across parts of Central and Southern Canada, and this snowpack is steadily pressing farther South and Eastward. Not only does this allow colder air to develop just to our North, as the snowfall line conitnues to move generally Southward, this demarcation become a breeding ground for winter storms.
Believe it or not, even with the increase in precipitation during recent weeks, our area is still well below yearly average in precipitable water. Things during the longrun look no better based on monthly precipitation forecasts, as almost the entire Northeast is forecasted to be well below normal in precipitation through at least late winter. However, based on our research, we are going against reports of a very dry winter at this point. Since most of these precipitation forecasts are based on current patterns (a dry pattern when the forecast was generaged 4 weeks ago), we have a hunch that when the monthly QPFs are generated in the next few days, the "drought" will not be nearly as bad as currently predicted.
We also believe that temperatues throughout the winter will be a direct function of the Hudson Bay Vortex. Keep in mind that a Polar Vortex near Hudson Bay will likely not remain during the entire winter, for surely there will be times when the feature relocates completely. It will however be a feature that should remain at least noteworthy this winter. With this in mind, a fairly regular source of Polar and Arctic Air will seep South and Eastward from Central, Southern, and Eastern Canada.
There is also reason for me to believe that the emergence of a mean Western Ridge will continue to remain a stubborn feature during the winter. We have already seen strong hints of Blocking taking place across Western Canada and British Columbia. Especially reminiscent of El Nino, the split flow pattern should be a predominant feature with blockiness in and across Western Canada causing energy to undercut the ridge, or split as it heads East.
On average, El Nino years tend to be milder than normal across the country. On the contrary, as we sit in a cold phase of the PDO, one could argue that we should average out as colder than normal during this winter. El Nino years also typically tend to generate more Noreasters than we would see in a "neutral" year. With this said, we feel that we will average out near normal in temperature average during the winter months. We also agree with the tendency of El Nino years to produce increased Noreaster activity (as we have already seen so far this fall), we will average near to above normal in precipitation for most of the winter.
So now the question most people came to hear answered . . . what does this all mean in terms of "Wintry Weather" for the Philadelphia Region during the upcomng winter months? I think it is safe to say that we will surpass our snowfall totals from last year without a question. Last year our area saw little more than 4 inches of snowfall for the entire winter season. This was due in part by a very progressive Norhern Jet. Not only was it dry, but it was mild last winter as well averaging several degrees above normal throughout most of the country. We should be close to normal this year in temps, perhaps slightly above for the overall average. In regards to snowfall, the Philadelphia Region should average near or below average.
So how do we think the season will unfold? November will start out unseasonably cold, but will rebound to above normal a few times during the month. There is the chance that the Philadelphia Region could in fact see it's first measureable snowfall towards the middle or end of the month as the Hudson Bay Vortex strengthens and sends spokes of energy rotating around and into the lower 48, thus spinning up a coastal low or two.
December will start off relatively cool, but by the second week of the month, temps rebound just before a shot of pre-Christmas cold and a strong shot at a White Christmas this year (75%). The storm responsible could be a large one and may become notorious for stranding last minute Christmas shoppers just before the big holiday. Colder air will remain in place later in the month with the chance of one minor 'event' during the last week of the year.
January will start off cold and dry for the most part, with increased storm activity after day 10. Our "January Thaw" will be short-lived this year, if in fact we even see the "thaw". There is a good shot at an overunning event around the 20th with potential for an ice situation in parts of the area.
February starts off with yet another possibility of an ice event in part of the area and temperatures close to average. This year will feature a large coastal storm (snow or rain for us) on or very close to St. Valentine's Day. One more serious arctic outbreak invades just after the storm with one or two clipper systems affecting the area by the third week of the month. February ends on a stormy note with another Nor'easter affecting the region in the final days of the month.
March will start off above normal in temperatures and soggy. One more cold outbreak early in the 2nd week of the month with potential for another coastal storm. Remainder of the month will be near or just below average with one or two rain systems ending as short periods of snow in the area.
So in short, near to just slightly above normal in temperatures on average during the winter months with precipitation being above normal. However, with that said, total snowfall amounts are forecasted to be near or just below normal for the winter. Yes, compared to last winter, this winter will be better if you are a fan of wintry weather. Still feel that we miss out on one or two of the larger storms with rain, but at least one Nor'easter and several clipper systems bring us close to average in snowfall. Confidence in the outlook is average. Enjoy the winter months for what they are and be safe!!
Email: Steve
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