New York City Metro Weather

Changes in the Pacific will lead to a mild New Year’s

By: NYCvort, 11:24 PM GMT on December 30, 2010

So you probably noticed it started getting milder soon after our big snowstorm. It continues to get milder with each passing day. I’m sure many people are wondering why this has been occurring, even with the deep snowpack on the ground. While we were all busy tracking last weekend’s blizzard, there were some important changes going on out in the Pacific which will be impacting our weather over the next few days. Let’s recap the important changes that took place in the Pacific over the past week that have affected the hemispheric wave pattern. Here is the 500mb height/anomaly pattern (from the Penn State E-Wall) that was observed across the northern hemisphere last Saturday (Christmas Day):

It doesn’t take much observation to notice how amplified the pattern was on Christmas on this side of the hemisphere. Note the strong positive anomaly ridge near the Aleutian Islands and over the Rockies, and a strong negative anomaly in the Gulf of Alaska. This all teleconnected to a deepening trough in the east, which resulted in our blizzard.

Now take a look at the observed pattern from Tuesday afternoon:

Notice how much less amplified everything is on this side of the hemisphere. A large piece of the very strong ridge that was once near the Aleutians broke off and is now the strong upper high over eastern Asia. Once this occurred, there were significant implications downstream. There was no longer any support for a deep, amplified trough extending southward from the Gulf of AK, and this trough tilted negative, came ashore, and broke down the western US ridge. Consequently, the trough in the east moved out into the Atlantic. Also note that where there was once troughing over the eastern Pacific has now been replaced with a southern stream ridge off the west coast. The pressure gradient between the less amplified Gulf of AK trough and the eastern Pacific ridge to the south has created a fast zonal Pacific flow coming in and cutting across the country. This is why very soon following the storm, even despite all of the snowpack covering the ground, you may have noticed that it has become a lot milder than its been in quite a while. In fact, LaGuardia Airport had hit 40 degrees already back on Tuesday.

Now, even if the pattern in the Pacific isn’t supportive of cold air in the east, this doesn’t mean that we can’t have cold weather anyway. It actually could easily happen, depending on what is going on in eastern Canada and the north Atlantic to help suppress the Pacific jet stream. But note how the Greenland block is gone as well, with only a weaker NAO compliments of our storm coupled with a ridge just south of Greenland. Note how there is no vortex over eastern Canada, but rather a well-amplified ridge. We even have a little southeast ridge popping up.

Another important thing to note in terms of our weather going forward is that as of Tuesday we had the eastern Pacific ridge in the southern stream. But also note that there was strong ridging (the leftover piece of the Aleutian ridge) coming along in the northern stream. Once this met up with the southern stream ridge, we now have very good support for a strong ridge extending all the way from the Gulf of Alaska down through the eastern Pacific, which has teleconnected to a deep western US trough. This will in turn continue pumping up the ridge in the east, leading to a significant warm up by New Years:

Now, you may also notice that on this morning’s 500mb hemispheric pattern, we see that some blocking has returned to Greenland (depicted by the dark red shading), but also note that this positive anomaly extends toward the United Kingdom. This is more of an east-based negative NAO block, which is far enough away to allow for a ridge to build into the eastern US, which, as you can see, has already occurred to some extent.

So the combination of the changes in the Pacific as well as the breakdown of the north Atlantic blocking has led to a significant warm up to begin 2011. We saw the pattern completely flip from an amplified eastern Pacific trough, western US ridge, eastern US trough to a fast zonal pattern earlier this week, and now to an amplified eastern Pacific ridge, western US trough, and eastern US ridge. This was all caused by a minute change—that ridge pumping up so strong in the central Pacific that a high broke off at the top and now remains over eastern Asia.

Enjoy the mild weather while we’ve still got it and have a great New Years
NYCvort

Updated: 12:20 AM GMT on December 31, 2010

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Winter storm will dump a foot of snow from NYC east

By: NYCvort, 5:45 AM GMT on December 26, 2010

MON 1 AM UPDATE

Well, the best areas for frontogenetic forcing are now to the north of the area (as depicted on the mid-level frontogenesis map below), but we still have moderate to heavy snow compliments of very strong dynamically produced precipitation. The vort max shown on the 500mb image below (in my 6 pm update) is greatly helping to dynamically lift the air and produce this snow. This dynamic lifting continues to affect much of eastern New Jersey, the Hudson Valley, southwest Connecticut, New York City, and Long Island. The dry slot that was over eastern LI earlier has pushed northward into parts of southern New England. You can see it on the water vapor image below (this is also self-updating):


This dry air has worked its way around the 700mb low and is notorious because no matter what kind of lifting mechanisms are present, without the necessary amount of moisture you just can’t produce the precipitation. Luckily for New York City area snow-lovers, this time around the dry slotting was just far enough to the east so that we weren’t affected by it.

Due to the very cold air temperatures in the upper teens and low 20s, we could have seen high snow to liquid ratios. However, the National Weather Service pointed out that very strong winds have limited these ratios. I’ve actually never heard of this before today, but a met on Tri-State Weather Boards pointed out that high winds can break up the flakes of snow into smaller pieces, and this would in turn cause them to better compact on the ground. It makes sense. So we could have had higher accumulations with this amount of liquid and these very cold temperatures if it weren’t for the wind.

Another thing that has also helped to limit the impacts from this storm was the NAO, which is now trending more weakly negative. This has limited blocking and is allowing the storm to exit rather quickly even though there is a cut off low in the upper levels. So less blocking = shorter storm duration.

Pressures are dropping rapidly and the winds are howling. We’ve seen sustained winds of over 40 mph at Kennedy and consistently in the 30-40 mph range for the last few hours. And of course, the snow is really piling up outside. I would be very interested to hear your local storm reports from around the area.

Enjoy the night
NYCvort


SUN 6PM UPDATE: Post-Christmas Blizzard for NYC

First and foremost, please do not drive unless it is absolutely necessary.

Boy is it windy outside! Right now the storm is rapidly intensifying as the pieces of jet energy that were once strung out begin to converge into one entity as the upper low deepens and pulls the jet towards the center. This creates a strong pressure gradient and an intense storm. Take a look at the strong clustered vort max just offshore as forecasted by the NAM for later this evening. Also note that the storm will be in the left front quadrant of a strong 130+ knot upper level jet streak:


Right now we have some nice frontogenesis and dynamic lifting, bringing moderate to heavy bands of snow through much of the area. The areas that are favorable for the heaviest bands are where the strong frontogenesis and dynamic lifting overlap.

Take a look at the map below, which shows frontogenesis (purple lines):



Then, note the following image, which depicts areas of low level convergence and upper level divergence. It is in the areas where these two features overlap with the frontogenesis shown above where you will roughly find the heaviest snow bands. These maps will continue to update themselves as the storm all night long so you can follow along:



As of 5 pm, there was some dry slotting evident across eastern Long Island. So even despite whatever mesoscale lifting factors were available (i.e. LL covergence, UL divergence, frontogenesis), the moisture source was cut off, so there was little available moisture to lift to produce precipitation. Moderate snow bands were observed as one continued west on Long Island and Connecticut, with heavy bands from New York City on southward through the Jersey Shore. Lighter precipitation was observed west of there.

Keep in mind that all of the heavy precipitation that is now across coastal central and southern New Jersey still has to come NNEward, with additional intensification likely as the storm strengthens further.

The Blizzard Warning has been expanded by the NWS to include the entire metro area and beyond. This is a very rare occurrence as frictional factors usually limit winds enough for inland areas to prevent the mention of the word "Blizzard" in those areas. I have to say, it is difficult to actually get blizzard conditions to verify, so the far interior areas would have less of a chance at seeing actual blizzard conditions.

So, I guess that's about it for now. Be careful everyone, stay inside, and enjoy the storm

Until later
NYCvort


SUN 1AM UPDATE

Blizzard Warnings have been posted by the NWS for the coast. Winter storm warnings are in effect elsewhere. We are looking at a major winter storm that will affect the area from Sunday afternoon through early Monday.

I have posted my snowfall accumulation predictions for this storm. I tend to be a more conservative snow forecaster, but with this kind of a rapidly intensifying coastal storm, I think that some of the high QPF forecasted by the models may very well actually verify.

I would expect to see averages of 10-14 inches in Manhattan, metro NJ, the lower Hudson Valley, and SW Connecticut with 14-18 inches from eastern Queens out through Nassau and most Suffolk County, as well as the Jersey Shore. I do think there will be a brief period of mixing on the south fork of eastern Long Island tomorrow evening, limiting accumulations a bit there. Amounts of under a foot will be found across western NJ:



I think there will be a “bullseye” of highest amounts, perhaps locally higher than 18 inches, and the best chance for this to occur in my opinion would be across the north shore of eastern LI. This is where I do not expect any mixing, and the highest QPF as a result of being closer to the storm as well as mesoscale influences (i.e. Sound influence).

The water vapor satellite image (which I like since it gives us an actual look at the atmosphere right now, not just what models are portraying, which is subject to errors), is looking excellent this evening:

We now have a well-phased trough, with additional energy rushing straight down the Plains to help cut off an upper level low. Everything is looking great in real-time.

The intense energy that is over the Gulf Coast right now will pivot around the upper low that forms, and this will allow a low pressure system to not only rapidly deepen but also back into the coast.

Happy storm tracking
NYCvort

Updated: 6:03 AM GMT on December 27, 2010

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Significant winter storm imminent for the NYC area

By: NYCvort, 4:58 AM GMT on December 25, 2010





Sat 4 pm update
I'm running out to dinner now, but I think we're looking at at least 8-12 inches, possibly more.


Sat 2 pm update
OK, this is it now. The Euro remains on board, and all of the models are in reasonable agreement. The WV satellite is showing a nice phase taking place. We're a go.



Sat 11 am update

Considering my growing skepticism with the set up for the storm as well as the data initialization problems that were suggested even last night by the HPC, I decided that it would be best if I at least waited to see some consistency in westward trend of the models that began yesterday evening. After looking at this morning’s new model data and the water vapor satellite images, I feel confident that a significant, potentially major, winter storm will affect the New York City metro area. In my last entry I discussed the factors against the storm, and I said that although I thought it was unlikely, it would be possible for the storm to hook back towards the coast should the upper level feature cut off from the main airflow. Well, the unlikely is now likely. The important upper level phasing process to produce this storm has begun. More details to follow.
Until later…
Merry Christmas

NYCvort



Sat 1 am update

After following all that’s happened today, I’m bewildered. I think Santa’s at work here :)

The models have all trended significantly westward, and at this late in the game, this is big. I admit, I’m still a little skeptical after just one run, and I don’t want to jump into anything here, but if the models continue trending the way they are now at 12z tomorrow, then this is it, a snowstorm is coming. I wasn’t planning on updating on Christmas Day, but seeing the situation at hand here, I’ll try my best.

Updated: 8:44 PM GMT on December 25, 2010

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Factors against the post-Christmas snowstorm

By: NYCvort, 6:42 AM GMT on December 24, 2010

I’ve always been the type of weather forecaster who doesn’t like to just take something at face value. I always want to know the reasons why something is happening. I never had any interest at all in climatology (i.e. studying El Nino/La Nina, etc.), and I always considered myself to be interested only in meteorology (the study of shorter term atmospheric patterns) and not in climatology (the study of longer range atmospheric patterns). However, as time went on, I began to realize that the only way to actually know why something is going on in the short range is to find the answer in the longer range climate patterns, so I suddenly became more interested in climatology. For this reason, I obviously want to know the reasons why this storm is not working out as originally planned.

Even though I didn’t get to post on here last weekend, I have to say, I was pretty excited about this storm right from the start. I was actually beginning to look at the pattern when the last storm was still in question, and then last weekend I got a chance to print out the panels from the models and take a closer look. I really liked the pattern that I was seeing, and despite the fact that the models were not in full agreement, they were in reasonably good agreement on the pattern for that far out. As the week went on, though, my skepticism grew with each passing day.

So I decided to go back and pull out those maps from last Saturday that first got me interested in this storm and take a look at them one more time. There was something very interesting that I noticed. The main features looked oddly similar to the ones we are looking at on the models right now, but on last Saturday’s 12z run of the GFS, the model predicted a nice large storm with a good track and east coast hit. The trough over the eastern Pacific is in the same relative location, the ridge over the Rockies is in a similar spot and it looks ideal, but wait…the trough over the east was already moving into the Atlantic, whereas now the forecast calls for ridging still over the Atlantic at the same time. On the graphic below, to the left is the 500mb height forecast for Christmas morning made by the GFS last Saturday. On the right we have the 500mb height forecast for Christmas morning made by the GFS today:

You could basically divide the two images in half, and the left (west) sides of both the images are very similar, while the right (east) sides are starkly different. Note how northern stream energy that should have already phased and been off the coast is still all the way back over the northern Plains, and a ridge that should have been out over the Atlantic is still over our area. But everything over the west is the same, just as predicted. This is what I was trying to point out in my last entry when I talked about the delayed timing over the east and how the Pacific energy would not be waiting around. The Pacific flow was pressed to come in toward the coast on Christmas morning. The ridge axis was in a perfect location on Christmas morning. If this storm was going to happen, in my opinion, it was going to happen on Christmas. This was a Christmas storm, right from the start. Once the stuff happening in the east was going to be delayed, the storm was doomed because everything going on out west was still coming in fast and furious, and the stuff coming in out west is just going to push everything out to sea. Now, like I commented the other day on Blizz’s northern Mid-Atlantic blog, if the low was completely cut off from the flow, like the models were suggesting for a time, the ridge being too far east could have still been okay. But now with the southern stream energy racing by in the fast La Nina driven Pacific flow, the phasing is not going to happen early enough for the low to cut off in the right spot, and it just doesn’t work. The new 0z/24 GFS run shows more separation from the flow and more closing of the upper low, and this will be something to keep an eye on, but right now I think it is more likely this storm heads out to sea. Note how the trough axis over the east and western Atlantic ridge are finally in a similar location to the one forecasted for Christmas morning by the models last weekend. But now it is Sunday night and the ridge is well east of the Rockies as the eastern Pacific trough energy has pushed inland. This displacement is causing the flow to buckle over southeastern Canada and is attempting to cut off the low (if the low were to close off enough, then we could get snow out of it; unlikely in my opinion at this point):


So I guess in his sleigh Santa is delivering a nice positive PNA ridge over the Rockies in the right spot on Christmas morning, but unfortunately everything else is not going to line up to take advantage of his present for all the New York snow-lovers :)

Now, my theory is that maybe the reason why this didn’t work out is because while the models accurately forecasted the fast La Nina flow via the sub-tropical jet, the northern stream flow is stronger and more amplified and slower due to the extreme high latitude blocking we’ve had. This is just a theory.

Merry Christmas (sorry it won’t be very white)

NYCvort

Updated: 7:13 AM GMT on December 24, 2010

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Dreamin’ of a white Christmas

By: NYCvort, 6:22 AM GMT on December 21, 2010

I don’t know about you, but some of the models sure are, and they have been for a while. Let me start off by saying that next weekend’s potential storm is a very different game than the one last weekend. For one, the pattern is simpler. The pattern with last weekend’s storm was crazy with a mess of features all over the place, and the models just didn’t know what to do with them. This time around, the stronger ridge out west is going to be a hindrance factor to any energy trying to get over it, and we also don’t have the messy polar vortex over southern Canada to deal with. I really don't think we’ll see drastic flip-flops with this as we get nearer to the event—I have a feeling the models will gradually converge on a solution over time.

The models have been consistent in depicting a well-developed piece of Pacific energy coming into California by mid-week. The energy from last weekend’s storm will become a strong 50/50 low later in the week. This will help to suppress the storm. So I guess even if this storm was a miss, at least it will be a helping factor in our next storm! At the same time, the NAO will be rising into a very weak negative phase, which would argue for a track near the coast. The strong negative AO will provide plenty of cold air. A rising PNA index supports a stronger ridge over the Rockies, and at least up until now, the ridge depicted on the models has looked good to me in terms of both location and amplitude. There is relatively good pattern agreement for this point in the game between the models. In fact, the new 00z/21 GFS and GGEM models that just came out show a major hit.

Now for the bad stuff. Even though the operational GFS has been suggesting the possibility of the storm tracking inside of the benchmark, the majority of its ensemble members have consistently shown a track much farther out. In fact, what has bothered me is that as the trough approaches the Mississippi valley, it is still positively tilted. This means that it will still be digging at that point as the strong jet energy remains on the west side of the trough. In my opinion, this would support the ensemble idea of a track farther out to sea. Also, every time I look at this storm on the models, it seems to be pushed back a little. Not only does this make the idea of children waking up and seeing snow outside like a wonderful classic Christmas morning less likely, but it also makes me question that while everything is getting backed up in the east—what is happening back west? If there was nothing to be learned from last weekend’s miss, it was at least a good reminder that we are in a full-fledged La Nina pattern, with a progressive jet. So while we’re looking at the addition of Polar energy here in the east blocking everything up, we need to continue to realize that the Pacific jet is waiting for no one out west. So while the ridge axis may be in a good spot initially, additional La Nina driven fast Pacific energy feeding into the west coast will be attempting to push it father inland. This is how the storm could end up missing us to the south and east. The 12z/20 Euro shows a raging Pacific jet pushing the ridge well east of the Rockies by Sunday, and I would tend to support this idea over the new GFS, which holds back the energy a bit more.

So we have the good and we have the bad. Despite the bad points, I like the overall set up for this storm, and I have from the beginning. I think this storm does have a decent shot right now, and I for one will certainly be monitoring it over the next couple of days as the features come into better agreement. I don’t think this is going to be up in the air right up until the end like the last one, though, for the reasons discussed above.


Have a great night,
NYCvort

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Potential snow event for Sunday/Monday?

By: NYCvort, 9:14 PM GMT on December 16, 2010

Over the past couple of days, I’ve just been inserting some subtle comments about a potential storm for late this weekend/early next week because the models have been very inconsistent. The thing that I think is so eerie about this is that not only is the storm in question forecasted for the same exact day as last year’s crippling snowstorm on Dec. 19, but also if you recall that storm last year was considered to be an “against all odds” type of thing. The pattern wasn’t favorable for something that significant, but all of the pieces came together just right. Similarly, this year we saw this potential snowstorm on the Euro last weekend, but it was rather quickly evident that solution would not pan out. However, the GFS kept us in question as it depicted a completely different upper level pattern set up that would yield a similar result. The Euro just wouldn’t buy it, until last night’s model run, when coincidentally the GFS had lost all hope and almost completely dismantled the idea, all of a sudden the European model came out and gave anyone still awake a little late night special. Of course, it wasn’t too surprising to see the GFS follow suit in this morning’s model run, as it often does. The question at that point was, are they on to something, or was it a little blip in the analysis that set everything off? Then the Euro showed the storm once again in today’s run, even stronger and farther west. Nothing is by any means set in stone yet, but I have to say this is where we want to see the models trending at this point. To be honest, I would like to see the European come out tonight before making a first call on any storm potential. This storm is now something that is definitely worthy of keeping an eye on.

I assembled an image using this morning’s 500mb analysis chart for North America. This shows where all of the disturbances that will be involved in forming this potential storm are right now.





Sub tropical disturbance cuts across the southern US, Pacific disturbance ejects from quasi-stationary Gulf of Alaska low and dives into backside of sub tropical disturbance, amplifying a trough. Then, the polar disturbance spinning around the upper low over southeastern Canada helps to potentially form a cut off.

Updated: 9:21 PM GMT on December 16, 2010

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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.