New York City Metro Weather

Winter Forecast 2010-2011

By: NYCvort, 4:45 AM GMT on December 01, 2010

Welcome to Winter 2010-2011!

No doubt this winter will be like a visit to Six Flags Great Adventure. It may be in some ways even more exciting than last year because we will have a La Nina working against a negative NAO, whereas last year everything was working together making it more predictable (sour grapes). Below today’s weather discussion you will find my winter forecast and some new changes this winter to the National Weather Service.

December starts off mild, but big changes are on the way

Meteorological winter starts tomorrow, but it won’t feel like it at all. We will see highs getting up well into the 50s ahead of a cold front. A strong trough full-latitude trough will turn negative and usher the strong cold front through our area tomorrow with rain associated with the front/wave of low pressure. A strong blocking upper high will set up just south of Greenland. This will keep a negatively-tilted trough in the northeast into this weekend.

This set up is very conducive for sending arctic air in our direction. With the blocking high and a strong negative AO, the cold air has nowhere else to go but to pool in the northeastern US. A deep upper low may be spawned as a result of all of this pooling arctic air, and it could remain in the vicinity of New England through most of next week. It would remain unseasonably cold by virtue of anomalously low heights. This would also keep Pacific energy suppressed to our south, along with any major precipitation events. In that sense, this feels like déjà vu of last winter, when many of the major storms were suppressed due to the strong –NAO. Even a complete global switch from El Nino to La Nina can’t crack the curse we are under! If I didn’t see the trough off the west coast, I would say this pattern looks like winter 2009-10 all over again.

Before laying out my winter forecast, I would like to take a look back at where we’ve been and where we are now.

Where we’ve been…The El Nino that led to a memorable winter last year turned into a La Nina, and this changeover helped to make for a hot and humid summer 2010. It also resulted in an active Atlantic hurricane season, despite the fact that not one hurricane made landfall in the United States. The 2010 hurricane season ends today.

During late September into early October, an El Nino-like trough developed in the Pacific, and this built a strong ridge downstream over western North America with a trough in the east. Consequently, we saw a period of cool/wet and stormy weather. Follow along on the 500mb height/anomalies image below:

This trough in the eastern Pacific was uncharacteristic of La Nina. The trough then shifted a little farther east, which pushed the mean trough off the east coast and bouts of cool and drier weather prevailed.

Where we are now…The eastern Pacific trough then turned into a strong ridge in November, which resulted in a mild turnaround. A ridge in that position is more like what we should be seeing in an east-based La Nina. We still remain in a mild Pacific-North American pattern (negative PNA).

Now for where we’re going…

2010-2011 Winter Forecast

I believe that while this winter will start out cold and potentially more stormy in mid-late December, milder weather may be on the way for January and February. My reasoning behind this change is discussed below:

Right now we are in a full-fledged La Nina that is slightly east-based. This is important because an east-based La Nina generally leads to ridging farther east in the Pacific. The ensembles of the CFS long term climate model are indicating that as we head into January, this will reverse itself as the western Pacific becomes colder and the eastern Pacific warms up. This change would lead to a more west-based La Nina, with ridging father west and a trough near the west coast. If this occurs, then I think we will see a higher probability of ridging in the east. Take a look at NINO 4 and NINO 3.4 (western Pacific) in the image below, and compare them to NINO 3 and NINO 1+2 (eastern Pacific). It appears NINO 4 is beginning to take a nosedive.

Another factor is the +QBO (the opposite of last winter), which may lead to less high latitude blocking. When you combine a +QBO with a west-based La Nina, this would mean a progressive Pacific jet cutting across the country, leading to a stream of mild air. Just to give you a sense of the change from last winter, the QBO index was around negative 16 last year, and the QBO index is now above positive 10. A major turnaround.

There is considered to be a statistical correlation between the Antarctic oscillation (AAO) and Arctic oscillation (AO). A –AO (right now we have a strong, plummeting –AO) generally leads the way for colder weather in the east, while a +AO keeps much of the arctic air up near the North Pole. Earlier this year, the AAO was negative and turned strongly positive. Note how the AAO has been consistently positive:

I personally don’t think that the rough statistical correlation over the past 30 years is good enough to say for certain that the –AO will turn positive later in the season for us, like the AAO did in the southern hemisphere. However, for me this is just the icing on the cake when it comes to projecting a turn to milder weather for Jan/Feb. It is just more evidence that such a change will take place when you combine it with the La Nina turning west-based, and the all around +QBO. If the AO does in fact turn positive in January, then the thing I’m most concerned about is that there will be a lack of cold air in the potential storms that form. Note how the AO has been showing signs of a possible turnaround:

The one thing that I think will be interesting throughout the winter is the persistent –NAO that we’ve been seeing, perhaps in some correlation with the warm AMO Atlantic anomalies. Note how the NAO has been consistently negative for the majority of the time:

This negative NAO is more of a wild card and could lead to some surprise storms, like we saw at one point earlier in the fall. The surprise fall storm was in the form of rain, but a surprise snowstorm is a much bigger deal. This may help to dam some low level cold air with maybe some ice storms, especially for interior suburbs.

Overall, we have generally seen a strong northern stream jet and a weak southern stream. This is characteristic of La Nina, and I feel confident that this will continue through the winter. For this reason, I think that while last year we saw nor’easter after nor’easter, this winter will be characterized by clippers.

As it is right now (over the next two weeks), we have a strong upper low move into the Gulf of Alaska, and mild Pacific air gets flooded into the US, but the –AO/–NAO keeps it well suppressed to our south. If we lose the –AO later in the winter, that mild air would have little difficulty streaming right into our area, with an east-based –NAO. This is why I expect that the pattern will change later in the winter—right now we are only holding onto cold air because of that –AO; the overall Pacific-North American (PNA) pattern does not support it. We’re hanging on by a thread—that thread is the (strong) –AO. We’re all excited about a rising PNA, even though it is still negative. A –PNA is not supportive of a trough in the eastern United States.

The only reason why we’ll have a persistent trough over the next week and a half despite the –PNA is because of the negative AO combined with the negative NAO.

So to reiterate, my forecast calls for a –PNA/–AO/–NAO for December, and then a –PNA/+AO/–NAO for January/February. I think the slightly east-based La Nina may strengthen a bit and become a slightly west-based orientation La Nina. I expect to see a positive QBO throughout the season. This would all lead to a pattern supporting a cold December, and potentially stormy mid-late December after the NAO rises out of the depths, followed by a milder January/February.

Finally, keep in mind that even though my forecast calls for a turn to milder weather later in the season, this doesn’t mean that we will all be sitting in outdoor pools with bathing suits on in January. All it means is that the overall weather pattern is more likely to support temperatures that are slightly above normal, as well as a less favorable storm track for snowstorms. It doesn’t mean we won’t get any snow—it just means that it will be more difficult to produce a snowstorm in the overall large-scale weather pattern.

NWS Forecast Zone Changes

Starting tomorrow, our local National Weather Service will be breaking up some of the forecast zones in the area. This change was announced early in November, and from the first time I read about it, I was very pleased. I think they made a really good decision to do this. The forecast zones that will be affected include Bergen, Essex, and Union counties in New Jersey, and Nassau and Queens counties in New York. I’m not as familiar with the Jersey counties, but I can comment on the New York ones. The climates of the north and south shores of geographical Long Island are very different. It’s an indisputable fact. In late spring and early summer, we often see sea breezes cause Kennedy Airport to be 20 or more degrees cooler than LaGuardia. In the winter, I know people who live on the inlets of southern Queens/Nassau counties who say that while most other people farther north on the island are experiencing snow, they often get rain or a rain/snow mix. This wasn’t so much of a big deal last winter because it was more of a “snow or no” sort of thing with southern areas generally picking up more. However, this winter I expect more mixing at the coasts, so I think the NWS has picked (by chance or not) a very good time to make these changes. I think there will be times this winter when we see winter storm warnings, watches, or winter weather advisories issued for northern Queens and northern Nassau without having to include the entire counties unnecessarily. We will begin to see it used tomorrow in regularly updated public zone forecasts. I have no doubt that this will be a useful change. You can read more about it here.

Here’s to an exciting winter season in New York!



Mild overall pattern continues for now, despite a few kinks

By: NYCvort, 6:44 PM GMT on November 19, 2010

The holiday season is fast approaching. WLTW/106.7 Lite FM started playing Christmas music for the New York area at noon today. Now all we need is some snowflakes to go along with it and get us all in the spirit! That’s not going to happen anytime soon, though, so long as we are in this pattern. But there are some signs of change coming in the long range. This is going to be a long one, so get some hot cocoa, turn on Lite FM, and enjoy!

We did have an upper level trough over the eastern half of the country yesterday that delivered some chilly air to the region, but that trough is already progressing well offshore. We will still feel the effects of the cold air that was deposited by the trough (kink #1), but a moderating trend has already taken effect in the upper levels. The fact of the matter is we just don’t have support for troughing in the east.

The culprit is actually all the way upstream in the eastern Pacific. (Since weather patterns generally flow from west to east like a river, upstream would be to our west and downstream to the east.) We may have to get used to a ridge in this area of Pacific with the orientation of the La Nina that we are in. I will be discussing this further in the winter forecast that I am planning on posting next week (hopefully).

This time of year, an upper level ridge with an axis in the Gulf of Alaska supports a trough over the western US and a ridge in the eastern half of the country. If this were August, we would all be complaining about the heat again in this pattern. This time of year, most people just enjoy the mild dry weather that we get in association with the ridge. In fact, with the ridge in no hurry to leave the Gulf of Alaska, mild weather should continue through much of next week.

Now for the more detailed forecast…Flat ridging will build in aloft later today through tomorrow. High pressure will settle near the southern mid-Atlantic coast. The ridge will begin to pump up late this weekend into early next week. With the jet near the US/Canadian border, this will give support for a strong surface high to our north and a backdoor cold front that will come down Saturday night (kink #2).

So after our moderating trend tomorrow with highs breaking 50, we will be back in the 40s on Sunday with not much of a diurnal temperature rise with very little surface flow. Once we get the ridge axis to our east and that high out of here, southwest flow will take hold and temperatures will be above normal early next week.

Back to the overall pattern…A strong upper trough will finally manage to overcome the ridge and put a trough in the Gulf of Alaska late next week. You know what that means. All of the trough energy over the west will eject and shoot eastward. This will bring a chance for rain around Thanksgiving. It will also put an end to the mild weather after Thanksgiving. As we officially begin the Christmas season, it will certainly feel like it as the Gulf of Alaska trough lingers and allows a deep trough to settle into the eastern half of the country. This is more than a kink—it will likely be (at least) a temporary pattern change.

The GFS goes crazy with this trough, as if it is taking the opportunity while we have support for a trough in the east to pummel arctic air in our direction. It shows 500mb heights plummeting to below 520dm by next weekend, with sub-516 thicknesses!

The ECMWF takes a much more moderate approach, with heights staying well above 530dm. Take a look at the GFS/ECMWF day 8-10 comparison below. Note the darker blue over the east in the image on the right hand side (GFS), depicting lower heights, as opposed to the ECMWF on the left:

I’m not doubting the fact that it will get cold by next weekend. The AO index is forecast to drop to at least 3 standard deviations below normal, and this will send an impressive polar vortex into Canada. There is also stratospheric support for a polar vortex on this side of the pole. Combine all of this with at least a 2 standard deviation below normal NAO and a rising PNA, and we’ve got ourselves the ingredients for a cold pattern! The strong -NAO/AO gives support for the deep upper low depicted by both models over southeastern Canada. Just how cold will it get is the question, and will there be snow flurries to go along with it, or like me, has the GFS been listening to a little too much Christmas music too early? The GFS does make a good case for a stronger intensity/depth of the trough. I personally don’t like the amplitude of the trough in the eastern Pacific that is depicted by the GFS. It may be too elongated to the south, and considering we are in a La Nina, I would favor a solution with less southern stream amplification. Since the ECMWF trough in the Gulf of Alaska has less amplitude, the resulting trough downstream over the eastern US would not dig as much.

The ECMWF is also more progressive in moving the trough out late next weekend. I think in that aspect it might be incorrect because teleconnections relative to the quasi-stationary trough in the Gulf of Alaska would argue for troughing to persist in the east.

To summarize, my best guess at this time would be that the ECMWF may be right about the trough in the east being less amplified, but the GFS correct about keeping the trough for a more prolonged period of time. But I’m really going out a limb here.

Updated: 2:35 AM GMT on November 20, 2010


Quasi-stationary low continues pumping mild air our way

By: NYCvort, 5:01 AM GMT on November 12, 2010

Mild air aloft, that is, but the low levels will take a little longer warming up. A pesky cut off low continues to indirectly affect our weather pattern as a compensatory ridge remains in control to the west of the cut off low. However, despite the fact that heights are above 576dm at 500mb (which is high for this time of year), a strong area of high pressure remains over northern New England:

This is producing subsidence limiting the ability to mix down the warmer temperatures aloft. In addition, with the high in that position there is a northeasterly flow, which means that we have an onshore component to the surface air flow, and water temperatures are in the 50s. Even with both of these factors working against us, we have managed to become a bit milder. As the low sinks a little farther south and meanders in the vicinity of Bermuda and energy cuts across southeastern Canada, the support for a strong high over northern New England diminishes. This will allow for temperatures to warm up over the weekend.

While we deal with a delayed warm up, there is currently a strong upper high building over the eastern Pacific. As a result, heights will come down over the mid-part of the nation late in the weekend to balance out the higher heights over the Pacific:

The southwesterly flow ahead of this trough will keep temperatures mild through early next week with a little counteraction from some ocean influence with a surface high moving over the Maritimes. Then as the upper high over the eastern Pacific begins to flatten out by the middle of next week, the residual energy will eject itself through our area. This may result in a low pressure system and some rain after quite a nice stretch of days. However, we will be in a progressive pattern, which means the low will be in and out. Be on the look out for much colder air behind it.


A little unexpected taste of winter across parts of the area

By: NYCvort, 3:47 PM GMT on November 08, 2010

Hi everyone,

I have taken a little unanticipated “vacation” of sorts from posting on the blog due to my very busy schedule as of late. Occasional time for looking at weather maps, but unfortunately not enough time to write about it. Anyhow, I’ll just jump right in. What an unexpected morning it has been for some, waking up and seeing the first snowflakes of the season falling. Well, even though this may have been unexpected, it is not so outlandish when you consider the pattern that we are in right now. What I think is a little funny is that late yesterday I finally got a chance to upload some weather maps onto my screen, and I noticed a relatively narrow line of intense energy over the area in the upper levels, associated with a strong jet combined with cyclonic curvature:

When I went outside, I saw the stars, and I considered how interesting it was that we had so much anomalous energy overhead, and yet a clear sky. Despite the deceiving sky, all of this energy was actually helping to carve out a deep upper low offshore that would cyclogenerate a coastal low pressure system and force it to retrograde toward New York.

The anomalously deep upper low is currently centered just southeast of Cape Cod. To be very honest, the 500mb pattern across the east is one that reminds me very much of last winter. The upper low is pulling on a low pressure system and causing it to back toward the area. This situation is very interesting, and to illustrate why this is the case, I am referring to the 9 am observations from across the area. At 9 am, every airport recording station across Connecticut that showed precipitation reported rain as the precip type. On the contrary, all recording stations on eastern Long Island reported snow. The reason behind this has to do with the fact that the strong upper low has allowed the warm conveyor belt of mild air to wrap around the center, such that the warmest air is focused to the north of the low, while the coldest air has wrapped around to the southern side of the low. This does happen, but it’s the exception and certainly not the norm. I remember this occurred last year during one of our snowstorms. So at the same time as it was snowing over eastern Long Island, temperatures were close to 50 degrees all the way up at 850mb in northeastern Maine (note the red lines north of the low on the image below, as opposed to the blue line south of the low), which is obviously not even close to being conducive to frozen precipitation of any kind. In fact, temperatures there at the surface are near 50 as well.

So if you wanted to see snow flurries or a shot at a little sleet, you needed to be on the southwestern edge of the low, where the coldest air associated with the deep upper low has been pooling. Coincidentally, the areas closest to the low on the southwestern side included the places that saw snow/sleet.

I was able to find light snow and/or sleet reported across parts of the Hudson Valley, Long Island, and Connecticut. There were also flurries reported at Kennedy Airport. I would be interested to hear any of your own wintry precipitation observations from this morning.

As always, thanks for reading,

Updated: 4:37 PM GMT on November 08, 2010


About NYCvort

I am a meteorologist from New York who has been studying and forecasting the local weather for years. I especially enjoy tracking winter storms.