New York City Metro Weather

Complex winter storm to impact the area

By: NYCvort, 5:33 AM GMT on January 29, 2011

TUE 12 AM UPDATE

Like I alluded before, there will be two distinct parts to this storm. The first will be affecting us tonight and early Tuesday; the second late Tuesday night and Wednesday:



The first feature is mainly overrunning precipitation that will be enhanced by a weak upper level disturbance. This will lead to some light snow developing tonight and early Tuesday morning with under an inch of accumulation. Then, with a strong 850mb low to our southwest, we will come under the influence of increasingly strong southerly mid-level air flow, which will usher in a wedge of warmer air aloft:



But with a surface high pressure ridge extending to our north, supported by an upper level confluence zone (discussed in my last update), a northerly surface wind will keep cold air locked in at ground level and keep precipitation frozen:



This will lead to precipitation quickly changing over to sleet Tuesday morning and eventually freezing rain in the afternoon. That’s feature #1.The key word with feature #1 is light. In between the two features will be at most some light freezing drizzle Tuesday evening. As you can see in the image below, feature #2 has much better dynamic support associated with the main energy:



The increased dynamics will enhance lifting and allow for much heavier precipitation to fall with the second feature. However, as the low pressure system nears, the high finally loses in control and winds turn ESE, ushering in warmer air. So although we will begin as freezing rain Tuesday night, temperatures are expected to rise above freezing and quickly change the precipitation to rain during Wednesday morning. But still, before the changeover, there is the potential for some heavy freezing rain to fall. The key word with feature #2 is heavy. Just compare the dynamics of the two features (shading on the 500mb image above). More updates on part 2 of the storm tomorrow.
-NYCvort


Real-Time Weather Maps


Current Water Vapor (Courtesy of Rutgers)


Current Radar


Previous Discussions

MON 5 PM UPDATE

This storm will feature a two distinct parts and a mixed bag of precipitation. Right from the start I had a feeling that this storm would feature serious precipitation type issues. We have a strong ridge in the far eastern Pacific and a corresponding negative anomaly over the west coast of the US. This would ordinarily dampen any chances for wintry precipitation as a ridge would pump up in the east to counteract the western trough. However, we also have a strong polar vortex just northeast of Hudson Bay pressing down on any attempt for a ridge to build:



This interaction is leading to an upper level confluence zone over US/Canadian border and support for strong surface high pressure. As the main upper level energy associated with our storm system begins to eject, moisture over the Plains will be sent up and over the cold, dense air associated with the surface high pressure and lead to an overrunning event which will begin as snow tonight. However, while the surface is more than conducive to frozen precipitation, the mid and upper levels are not. With that ridging trying to build up over the eastern US to counteract the western trough, a strong southwest flow of 50 kts will set up early tomorrow at the 850mb level, and this will allow for a wedge of warmer air to advect in and lead to a changeover from snow to sleet:



This wedge will gradually expand downward and will eventually be large enough to result in freezing rain. The key to this frozen set up despite what is going on up above us is that surface winds will keep a northerly component. Once we lose that northerly component, all bets are off, and precipitation quickly changes to rain. This is what will occur later during the second half of the second round of precipitation on Wednesday. More updates to follow.
-NYCvort



ORIGINAL DISCUSSION



The energy that will be the main contributor to next week’s storm is already looking very nice spinning over the eastern Pacific Ocean. This energy will slowly move ashore and phase early next week with additional energy dropping down from western Canada. The energy will then eject into the northeast and has good potential to bring a mixed bag of precipitation into the area by the middle of next week. There will be a very broad area of moisture and also a sharp thermal gradient leading to an overrunning event. Good remaining dynamics from this offshore Pacific energy will allow a low pressure system to develop and take an inland track up the coast before redeveloping off the coast. The set up is very different with this storm than the last one, and this one does in my opinion pose more potential mixing issues for our area. With a strong high pressure area receding, precipitation will likely start off as snow but the question remains as to whether enough warm air filters in at the surface and aloft to allow for a changeover to rain.

Updated: 5:56 AM GMT on February 01, 2011

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Significant winter storm underway with heavy snow tonight

By: NYCvort, 1:51 AM GMT on January 23, 2011

Real-Time Weather Maps


Current Water Vapor (Courtesy of Rutgers)


Current Radar (Courtesy of NWS)



WED 12 PM UPDATE

Well, what a surprise this morning! Two days in a row now. Snow came in much earlier than expected because I was underestimating the extent of warm air advection precipitation. I think I was caught going against my own rule of not relying totally on the models for forecasting :) This is all icing on the cake snow—this is not the dynamic precipitation associated with the 500mb energy that I’ve been talking about on the backside of the surface low. Expect a lull this afternoon with just some scattered snow showers in between these two features, with snow picking up again early this evening. Additional accumulations of 5 to 8 inches can be expected in the city, with a sharp gradient still anticipated northwest of the city. The NAM has corrected itself, so last night’s snow map (which I made based more on consistency, disregarding the 0z NAM) would still be applicable; however, since we have already seen warm advection precipitation drop several inches, my map basically shows you how much additional snowfall to expect with the main part of the storm this evening and overnight. You can really see the upper level energy firing up over West Virginia on the water vapor loop above. That is what will be providing the dynamics for tonight’s heavy snow.



WED 1 AM UPDATE

Well, I might as well have just posted my map before, as I think I felt more confident about the numbers then than I do now. The models seem to be having a lot of trouble figuring out what to do with a double-barreled low structure. The latest NAM scaled back on precipitation and shows a weird distribution that looks to be the result of keeping the two pieces of a double-barreled low structure more separated than they were in earlier runs. This creates a swath of higher amounts over south Jersey and out across extreme eastern LI and SE New England, with a void of much lower amounts in between. The interesting thing is the GFS also has the double-barreled low but it consolidates the two into one single entity before approaching our area. Take a look at the comparison below. On the top we have the NAM (note the double low structure on the left panel and associated big void in precipitation on the right panel) and on the bottom is the GFS (note the consolidated low and less of a break in precipitation.



The NAM is basically suggesting that there will be a break in precipitation between these two convergence zones. So for clarification I also took a look at the GEM. The GEM does keep the double-barreled low structure around longer like the NAM, but it brings everything closer to the coast and doesn’t show a void in precipitation between the two like the NAM. It’s a very minute difference that quite honestly I can’t even comment on which solution would be more plausible. So I’ll have to follow consistency and hedge against the new NAM’s solution, but I have to admit, it does have me a little concerned. So I’m basically using the new GFS for placement of precipitation (I like how the new 0z run shows a nice sharp cut off to the north and west—it follows what I was saying earlier about the dry air intrusion just away from the mid-level low’s influence) and a blend of the 18z NAM and SREF mean for intensity. We need to keep watching the double-barreled low idea—remember, it’s not a question of whether there will be a double-barreled low (both models agree there will be one), but rather how quickly it consolidates into one entity and also its precise track.

So…here’s my snow map:





TUE 7 PM UPDATE: Everything on track for a substantial snowfall

Not much has changed; the 18z intermediate runs still follow my thinking. There still is some discrepancy between the models with the GFS showing a stronger and less sheared storm than the NAM. Right now I’m going to side toward the NAM because I still think as the storm starts getting picked up by the northern disturbance, the upper level energy will get a bit more sheared. It looks like this will be an all snow event for the city with at most a little sleet or rain mixed on at the onset if the warmer GFS solution pans out (not a big deal). Expect a wet snow with temperatures starting out above freezing prior to evaporative cooling kicking in. I started my snow map, but I’m going to wait until 0z runs come out before finalizing it. But right now I’m thinking about 4” to 8” in and around the city.



TUE 2 PM UPDATE: Looking good

Our burst of snow this morning was caused by the northern stream disturbance that was responsible for depositing the trough over Texas last night. That trough that was deposited and is now spinning over the deep south will be gathering up Gulf moisture and intensifying at the same time. Our “kicker” northern stream trough will then help pull it up the coast tomorrow. The strong southern stream trough coming up the coast will still be jam packed with plenty of upper level energy and associated lifting. This is a warm southern stream system with little northern stream interaction, but the low is expected to remain far enough off the coast preventing the warm air from moving into our area. In fact, forecast soundings from both the NAM and GFS support an all snow event for the city. The main weather story is actually what will be happening on the backside of the surface low:

You can see on the surface panel above that even with the low off to our east, there is still plenty of precipitation accumulating on the backside of the low.

Now, let’s take a look at the mid-levels. A weak 700mb low will be developing with moisture wrapping around the low:

But note the sharp cut off in moisture to the north and west. There will likely be a sharp cut off with dry air intruding outside of the low’s immediate influence.

On the backside of the low, intense upper level energy will act on the ample moisture to produce an area of moderate to heavy snow. This is an interesting set up and has the potential to produce a significant accumulating snowfall:


Now, you may wonder why it is possible for a storm to pass this far east near the benchmark if we have a ridge along the west coast. A more “favorable” teleconnective pattern for an I-95 snowstorm is to have a ridge focused near the Rockies, and I often look for this feature in long term storm prediction. However, right from the start I have felt that this storm is different and more of an exception to this rule. It’s important to realize that teleconnections should only be used to forecast the placement of anomalies relative to one another. Saying that the lack of a ridge over the Rockies means no I-95 snowstorm would only be true in an ideal situation. But the atmosphere is anything but ideal, and I made an image below to show you another factor in addition to lack of blocking that I discussed last night that allows us to see a snowstorm despite the less favorable position of the western ridge:



Take a look at what I circled. All of the positive tilt troughiness that I circled extending so far into the southwest is “stuff” going on behind our storm that will be basically pulling the negative anomaly counteracting the western ridge farther west.

Look closely at the image below:



Note how we have two distinct troughs (red lines)—one is our storm, and the other is the “stuff” going on behind. In between the two there is a weak upward bulge of ridging (blue line) extending north from the Mississippi valley. So we have two separate negative anomalies, with some degree of ridging between the two. There obviously needs to be plenty of space for there to be a trough, weak ridge, and then another trough all in the same mean trough east of the western ridge. Therefore, we have a longer wavelength to allow for all of this to occur inside the mean trough. The mean trough position is farther to our west like you would expect with a western ridge axis so far west, but since we have two distinct, separate negative anomalies in the mean trough, the negative anomaly associated with our storm system will still be able to stay off the coast, supporting a frozen scenario. A little hard for me to explain, but this was another reason (in addition to the lack of blocking and the “kicker” shortwave) why I felt the east solution would prevail despite the teleconnections. We can broaden the view and take a look at the Pacific-North American pattern as a whole:



The idea of having separate pieces of energy contributing to multiple smaller negative anomalies is similar to what’s going on off the west coast. There are a lot of separate pieces of energy in the flow pattern that is causing a longer wavelength pattern than one might expect. You can see how basically the entire north Pacific ocean is covered by troughiness/negative anomalies due to multiple separate pieces of energy in the flow. With the MJO in phase 8/El Nino-like phase, there are plenty of disturbances in the Pacific-North American flow for something like this to occur.

Check back tonight for numbers
-NYCvort



TUE 1 AM UPDATE: All models now supportive of east solution

Like I suspected, the models continue to show the east solution which would give us a good shot at an accumulating snowfall. There are still some discrepancies that need to be ironed out, but this is about as good as it can get for NYC with this set up. Let’s look in very simple terms at what is actually going on. What we basically have here is the northern stream deposits a trough tonight over Texas and then that trough moves very slowly over the deep South before getting picked up by that northern disturbance that I was calling the kicker shortwave. I made a visual representation of this. In the first image, we see the initial northern stream disturbance (circled) depositing the trough, then in the second image the northern disturbance moves away, and then in the third image we see the “kicker” shortwave (circled) picking up the southern trough.



There are several key reasons as to why I believed this east solution would play out despite what the foreign models were showing, and I’d like to go a bit more depth into one of them that I started discussing earlier—lack of blocking. I think one thing that might be throwing some people off is the fact that we’re now in a very different pattern than the one that we saw earlier in the season. I’ve been talking about how the NAO has become near neutral, and this index is an indicator that shows us the blocking is basically gone. Without going into it too much in depth, we may be finally starting to feel some of the effects of the positive QBO (opposite of last year) that we weren’t feeling because of the continued negative NAO. Last month a QBO index just over +13 was recorded, so I would suspect that the QBO is now somewhere in the positive low to mid teens, which for the QBO is a reasonably significant positive number, perhaps indicative of stratospheric wind patterns that would negate blocking. So everything has become very progressive; we saw that with the last system. So I had a very hard time believing that a system would be able to come inland without getting pushed along by the progressive northern jet stream. In fact, the new GEM says forget about our “kicker” shortwave, it’s just bringing another one right on down behind it and helping to force the storm out even further! This storm is just getting kicked along by the progressive northern stream, so I never liked the rain scenarios.

There’s a lot more that I’d like to say but it’s going on 2 am and I’d like to catch some Z’s and continue tomorrow :)
-NYCvort



MON 12 PM UPDATE

The models have been trending away from that west solution that I didn’t think looked right based on the pattern. That N. disturbance (what I’ve been referring to as the “kicker” shortwave) will play a key role in the ultimate track of this system. Even though last night’s EC and GEM are both farther west than the American models, upon closer observation they are actually each farther west for very different reasons. Let’s take a look at a comparison panel between last night’s EC and GEM and this morning’s GFS and NAM. I have circled the all-important N. disturbance in each image:



The EC is farther west because it keeps the N. energy more clustered up over Canada (which is oddly enough more similar to what the GEM was doing last week). On the other hand, the GEM sharpens that energy more than any other model (the complete opposite of what it was doing last week!) and keeps it west and tries to phase the S. disturbance with the northern stream, creating a bigger storm than the EC and even farther west. The S. disturbance needs room to grow, so if you keep the N. disturbance more packed up over Canada with less influence, then the S. disturbance would have more room, but on the other hand if you bring down the N. disturbance and phase it in, it can also draw the system in, which is what the GFS has been doing in some of the recent runs and thus its more western track relative to what it was saying earlier. The point that I’m trying to make here is that the EC and GEM cannot be trusted because even though they are similar, they are accomplishing a similar solution for opposite reasons.

If it does start to phase I think it will have to deal with shear and get ripped apart, so I don’t like the GEM depiction. The EC is an outlier now in how it keeps all the energy up in Canada, so I don’t want to follow that solution either, especially since we’ve seen a northern stream dominated winter so far. The GFS doesn’t look that good either because it’s trying to phase the S. disturbance with just a weak N. shortwave—doesn’t look right to me either. The NAM has a stronger N. stream disturbance, and it starts to phase the S. disturbance with it and shears the S. disturbance a little bit, so I think out of all of them some of the elements of the NAM look the best to me. Also of note is that we also don’t have good blocking, so it won’t be slowing down for wrapping up like it might have earlier in the season when we did have the excellent blocking. Keep your eye on that N. stream disturbance. If that N. disturbance does remain weak and up over Canada like the EC is showing, then the EC solution would have no trouble panning out. But I don’t think it will. I think the N. disturbance will be sharper and thus we’ll see a more sheared and weaker system that is farther to the east, much unlike last night’s GEM. Just as I’m finishing typing this, I’m very happy to say that the new 12z GEM has just come out and it follows my thinking as it has shifted much farther east and is more sheared.



ORIGINAL DISCUSSION

I’m in a hurry, so just a quick post on the Tue/Wed potential. Take a look at the panel below. On the left side we have the west solution. The top left is the current GGEM model depiction from this morning. On the bottom left is an old GFS run from Thursday night. This was back when the GFS had also depicted a coastal hugger with rain for the coast and snow inland. On the right side we have the east solution, which consists of this morning’s ECMWF (top right) and GFS (bottom right). The important thing to take a look at is what is going in the Minnesota/Wisconsin area.



Note how in the west solution (left) the black lines over the western Great Lakes area are more clustered, while in the east solution (right) the black lines are sharper, indicating lower pressures aloft. During many of the recent runs, the GFS, and even the NAM, have trended towards a faster, sharper northern stream shortwave, “kicking” the storm farther east, so I’m siding more with the east solution. 0z runs tonight will be important.
-NYCvort

Updated: 5:31 PM GMT on January 26, 2011

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Quick-hitting storm system will bring another round of snow

By: NYCvort, 10:14 PM GMT on January 19, 2011

THU 7 PM UPDATE





DISCUSSION

As it turns out, the ridge along the west coast will not quite be strong enough to send that PV I pointed out in my medium range outlook last weekend all the way down the chute, but it will allow a piece of energy from the PV to break off and phase in with our system, providing plenty of cold air and good snow ratios. But like I’ve been pointing out, the NAO has been higher the last week than it’s been the entire winter season thus far, and this illustrates how the lack of good downstream blocking will help to keep this storm a fast-mover. Over the past few days there has been a discrepancy between the ECMWF, which insisted on keeping the northern and southern streams more separated, while the NAM/GFS were better phasing the two. The trend over the past few model runs has been for a slightly less amplified, more progressive northern stream trough, which equates to less phasing. So we’re only dealing with a partial phase here—but enough of a phase would still ensure decent moisture and dynamics, and the trough goes negative, which will allow for more instability with colder air aloft starting to override the warmer surface low. But there is another big limiting factor aside from the poor blocking and only a partial phase. There will be a piece of “nonsense” energy, as I like to call it, out in front which will prevent our system from really getting going until the upper level energy associated with our system starts to wrap up into a closed low, which won’t happen until it’s past us. The nonsense energy prevents the 500mb isobars from separating out/good diffluence, which would allow for a stronger surface low.

Now here’s something else that’s of note. With our December blizzard we had a closed 700mb low come up the coast. In our snowstorm last week, the 700mb low redeveloped off the coast. With this storm, we won’t have any closed 700mb low to deal with at all. It’s not expected to develop until after the storm has gone past us. So there won’t be any wraparound moisture.

Combining all of these factors together, I would expect generally 2 to 4 inches of snow falling Friday morning, with maybe even some sunshine before the day’s out. This snow will be a quick-hitter, but it will be nice to get a fresh pack of snow on top of the mess of puddles and ice that was left behind by the melting the last couple days. Finally, with the arctic air mass coming down (that I discussed on Sat 1/15) for this weekend depositing very cold 850mb temperatures/low thicknesses, first it will feel very cold via windchills, and then our new snowpack will only further assist the process with some additional “homegrown” cold air as high pressure builds in, with little warming during the day and some very cold nights. Check back for more updates on the storm.
-NYCvort

Updated: 12:01 AM GMT on January 21, 2011

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Thoughts on the wintry mix situation tonight/tomorrow

By: NYCvort, 7:27 PM GMT on January 17, 2011

Precipitation comes in after midnight and it starts out in the form of snow but quickly changes over to sleet well before dawn. The sleet continues with maybe a period of freezing rain before changing over to all rain by late in the morning, with interior areas holding on a bit longer. In my opinion the models are overestimating the amount of snow potential with this set up. There will be a warm layer the mid-levels that is being underestimated by the models which will end up changing the snow over to sleet very quickly with little snow accumulation. I’ve seen this happen so many times before where the models show it staying as snow for too long. Let’s take a very close look at what is going on above us at 4 am tomorrow morning per the NAM:



The blue line represents the 0 C line. If the red line goes to the right of that 0 C line at any level above the surface, then there’s sleet because the pellet melts and then refreezes. Note that the red line is to the left of the 0 C line at all levels, and so the NAM is forecasting snow at this time stamp. But my feeling is that there will be a spot inside the circle where the temperature rises just above freezing to very roughly the area where I scribbled in red. That’s how we would changeover to sleet sooner than expected. My theory is that this may have to do with the idea that radiosonde analysis data can’t be plotted at an infinite number of points and in between the models need to do some rough extrapolating. Regardless I wouldn’t expect much in the way of snow—sleet/freezing rain may be another issue. I don’t really like the set up too much for CAD (cold air damming) with winds generally out of the east with only a small northerly component. Existing snowpack may not help too much either because like we saw last night, temperatures actually needed a good adjustment upwards from the MOS low temperature predictions because of wind. So once that wind gets cranking the snow will be of little help to “lock in” the cold air. But I still think the GFS has a better idea of keeping the low level cold air locked in longer than the NAM. So whereas in an ideal CAD set up I would generally take the coldest solution at the surface (GFS) and hold the sleet/freezing rain a little longer than it depicts, this time around I would say lean in that direction and maybe even chop it a bit. But the NAM does bring a changeover to plain rain too quickly for my liking, and its daytime high into the 40s under an easterly flow is suspect. If anything changes significantly I’ll update again later today.
-NYCvort

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Strengthening El Nino-like MJO signal will lead to an arctic outbreak

By: NYCvort, 11:31 PM GMT on January 15, 2011

I would like to take the time to show you some very interesting developments that will be occurring over the Pacific during the next week and will have a significant influence on our weather downstream. There is currently enhanced convection focused over the western Pacific Ocean. Below is the surface panel from the GFS with the Pacific Ocean as the domain. I circled a low pressure area and associated cold front with blue shading depicting convection. This may remind you of a cold front over the US during the late spring/summer months that would bring convective thunderstorms to the region:


This convection is associated with Madden-Julian Oscillation (or simply, MJO) wave propagation. This MJO wave is forecast by both the ESRL/PSD and the NCEP ensembles to orbit eastward across the Pacific and into phase 7. In case you’re not familiar with the MJO, below is a diagram from NOAA showing the eight phases of the oscillation. The green shading represents where the precipitation anomalies (associated with enhanced convection) are focused in each phase:



Based on the positioning of convection, right now the MJO is in phase 6 and it is headed into phase 7 and eventually phase 8. Here is the MJO phase diagram forecast by the GFS ensembles. I circled where we are as of yesterday, in phase 6. The green line is our forecast. Note that phases shaded in orange are generally more El Nino-like, while the phases shaded in light blue generate more of a La Nina-like pattern in the Pacific. We are headed into El Nino-like territory:



In addition to hosting an excellent model page, Allan Huffman also did a valuable research study on MJO phases and their subsequent impacts on the hemispheric wave pattern. Below you will find the hemispheric anomalies that are commonly associated with an MJO coming out of phase 7 during the month of January (based on his study):

The deep trough that you see depicted by the dark purple shading over the Pacific is the negative anomaly associated with the phase 7 MJO wave. That strong negative anomaly teleconnects to a strong positive anomaly downstream over the western US, and a subsequent strong negative anomaly over the east.

Using the MJO study which consists of a collection of analogs is great, but since no two years are exactly the same, it’s just as important to take a look at the particular situation that will be presenting itself to us over the next week (as forecasted by the models). So let’s take a look.

Below we have the GFS model depiction of the 500mb pattern over the Pacific Ocean for this evening:


I have circled the upper level support (upper low/trough) associated with the MJO wave that is being propagated by the enhanced convection noted in the surface map (first image way back at the beginning) for this evening.

Now let’s step ahead to Thursday evening:


You can see how the MJO wave of enhanced convection has moved eastward into the central Pacific. Note how the a corresponding ridge has built up along the west coast of North America, and you can also see on the far right side of the image there is a deep trough starting to dig down into the eastern US. I put an arrow to show how that cold polar vortex gets sent straight down the chute via the strong flow on the eastern flank of the huge ridge over western North America. This is what will deliver arctic air into the region for next weekend. This pattern would now be uncharacteristic of La Nina, even in the Pacific, with strong convection focused around the dateline.

For additional evidence, let’s also take a look at the ECMWF model, which is considered by some to be a more accurate model because of its higher resolution:

Note how once again we see the trough in the central Pacific associated with the enhanced convection/MJO wave in phase 7, and a corresponding ridge near the west coast and a trough digging into the east. You can probably recognize how similar this set up is becoming to the one that was presented in Mr. Huffman’s study of phase 7 MJO analogs.


All of these signs point to arctic air invading the northeastern US next weekend as that deep trough digs in further and the floodgates of the arctic are opened. Windchill values below zero are likely. Temperatures at 850mb will drop to near –20 C. A cold high builds in as well, which will lead to the very cold air remaining early next week, even after the support aloft recedes. Now the next question that comes up is will this lead to a cold and dry pattern or a cold and snowy pattern? Well, a lot of that depends on the interaction of disturbances that is impossible to determine this far out. However, there is one thing that I would like to show you relating to this. Below we have the NAO index forecasted by the GFS ensembles:


For the best support for a northeast snowstorm, we want to have the NAO in a weak negative phase. Anything lower than this and chances are better that the storm gets suppressed; higher and there often isn’t enough blocking to support a big storm. Note how the NAO ensemble spread is pretty large. This could be a big wild card in the extended range forecast. If we could get a weak negative NAO and combine it with what will be going on in the Pacific, support for a northeast snowstorm may exist. Then all we would need is for the right disturbances to interact at the right time. We’ll have to keep an eye on this as the new pattern starts to settle in. We will be entering a pattern that looks very similar to what was experienced much of last winter, now minus the strong negative NAO block. This would argue for a storm track farther to the north, should one form.

But if there’s one thing I’m certain about, it’s get ready to get cold next weekend!
-NYCvort

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Digging out after a significant snowstorm and looking ahead

By: NYCvort, 5:30 AM GMT on January 13, 2011

Areas to the northeast of the city did end up seeing the bullseye from this storm as the sharp thermal gradient led to strong banding of precipitation setting up shop over parts of interior southwestern Connecticut. There was a very sharp gradient in snowfall amounts to the northeast, with Central Park “only” ending up with 9 inches while localized parts of interior southwest Connecticut reported close to 30 inches. The high QPF that was forecasted by the models over western Suffolk County, NY and suspect to me didn’t verify and instead they actually got dry slotted for a period as the redeveloping 700mb low passed nearly overhead. Northeastern Suffolk did get in on the action a bit more than I expected.

You may recall that I spent a lot of time a few days before the storm talking about the importance of what would be going on near the coast of British Columbia. Remember my post “the GFS blinked” when it finally gave in to the ECMWF’s depiction of the offshore energy being more separated from the PV over Canada. That same day I also showed the NAM solution, which actually depicted the low as being separate enough to close off, but I thought that would end up being too exaggerated. I thought it would be interesting for me to show you what actually happened. This is the GFS initialization image from this morning:



Note how there is in fact a closed 522 contour near the coast of BC. The NAM did end up being right in the end. My initial concern way back last week that I commented on Blizz’s blog was that the strong positive anomaly near Alaska would force the PV to come down and beat down any western US ridge from trying to pop up. But note on our initialization from this morning that with the piece of energy partially split off from the PV, there is enough separation in between for that all-important ridge for east coast snowstorms to pop up near the Rockies.

Now as we all finish digging out we can expect some very cold air, unlike the days following the last big snowstorm. It was a bit unusual after the Post-Christmas Blizzard that it actually got milder after the storm as that storm was sort of a temporary “pattern changer” with rising heights following the storm’s departure. Recall how that storm featured a cut off low and a ridge that was already pressed to come in right behind the storm (it was so far east that such a storm would not have been possible without the low being cut off from the flow—thus the uncertainty right up until Christmas morning). This time around a mean trough will remain focused in the east, and that will only further assist the effect that a new snowpack covering the ground has on reflecting insolation and emitting longwave radiation (snow is a good reflector of solar radiation and also a good emitter of longwave radiation). Temperatures tonight will drop to near 20 but with gusty northwest winds it will feel much colder with wind chills in the single digits. Mid-level shortwave ridging will allow for plenty of sunshine on Thursday but with the snowpack reflecting much of the insolation highs won’t get out of the 20s. The winds will lighten Thursday night, so with much better radiational cooling (clear skies/light winds/snowpack) we will see lows fall into the mid teens in the city and far outlying areas may drop into the single digits. But at least wind chill factors won’t be an issue with a lighter wind.

Additional energy rounding the top of the flat western US ridge will dive into the trough on Friday with another piece following behind on Saturday:



These will be strong pieces of Pacific jet energy with broad shortwaves and a lack of dynamics and they will be moisture-starved by the time they reach us. Because of this, they will only provide us with more clouds and reinforcing shots of cold air while keeping us precipitation-free. So with more cold air and remaining snowpack, highs will be in the 20s once again on Friday with more clouds associated with the Pacific shortwave. With more cloud cover and slightly less ideal radiational cooling, it will not be as cold Friday night. Weak high pressure finally moves off the coast on Saturday, and southwest flow at the surface will boost highs into the 30s. There will be a sharper piece of energy, thanks to a polar component providing more amplification to the flow, passing through Sunday night. This one will be accompanied by a weak cold front and a chance of flurries. Behind the shortwave trough, it will get milder on Tuesday as ridging builds. This will set the stage for our next potential storm system. Just how much of a warm up we experience ahead of the storm will set the stage for precipitation type. Could there be any snow, sleet, or freezing rain at the onset, especially over the interior? Well, the ECMWF is certainly better than the GFS solution in terms of cold air. The EC has a sharper northern stream ridge over the Canadian Maritimes, leading to a surface high staying farther north over the Maritimes and easterly flow out ahead of the system. On the GFS solution the high moves off the coast, leading to more of a warm up as southeasterly flow prevails off the open Atlantic:



There’s still plenty of time to iron out these details before the storm. I’ll be keeping a close eye on it.

Lastly, I would be interested to hear how much snow you got in your area. Feel free to post your observations and reports.

Updated: 6:19 AM GMT on January 13, 2011

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Snowstorm will dump 6” to 12” over NYC, higher northeast

By: NYCvort, 5:23 AM GMT on January 11, 2011

Real-Time Weather Maps
Track the progress of our snowstorm in real-time!


Current Water Vapor Satellite (Courtesy of Rutgers)



Current 500mb Pattern/Surface Map (Courtesy of NOAA)



Current Radar (Courtesy of NWS)



Current 700mb Frontogenesis (left) & UL Divergence/LL Convergence (right) (Courtesy of SPC)


Snow Map




Discussions

WED 1 AM UPDATE

Moderate snow continues to fall across the entire area. Note how this is in conjunction with the best upper level divergence and low level convergence (on the SPC map above), providing plenty of upward motion. Also note how we no longer see any closed contours on the surface map over the interior--most of the energy has transferred to a developing low offshore. This is why we are seeing much of the precip fading away across far western areas. An upper low remains to our west, providing plenty of uplift through rest of the night.


TUE 1 PM UPDATE

All of my thinking remains the same for the coming storm. My feeling is that the lower GFS numbers will be more likely to verify west of the city (which will be “in between” the weakening primary 700mb low and the redeveloping low off the coast), while the NAM solution tweaked downward a little and shifted slightly northwest (where I believe strong banding will develop after the new 700mb low rapidly intensifies) will be more likely northeast of the city. I still don’t foresee mixing with sleet being an issue except along the south Jersey shore. Sleet could also mix in briefly on extreme eastern LI and possibly along the immediate south shore. This all suggests a general average of 3”-6” in NW Jersey and south Jersey on westward, 6”-12” in NE Jersey, NYC, Long Island, and up the Hudson Valley (with locally higher amounts possible across the eastern Hudson Valley), and 10”-16” with locally higher amounts across most of Connecticut. This is all shown in a visual representation on last night’s snow map.

Today I added an automatically updating water vapor satellite loop, radar loop, and current frontogenesis and upper level divergence/low level convergence mesoscale maps (great for showing where banding and heavy precipitation will develop).

Happy storm tracking
NYCvort


MON NIGHT DISCUSSION

My main algorithm for creating this snow map was to use the idea of the 700mb low redevelopment and thus a sharp gradient downward to the west of NYC and upward to the northeast (discussed in more detail in my last entry here). Some of the models have been showing the highest QPF out across eastern Long Island, but I think that we will see the better moisture and forcing combine for better mesoscale banding to the northwest of the developing mid-level low, and thus the higher amounts over much of Connecticut. I’ve found a map that does a reasonable job of depicting my reasoning behind the highest amounts being focused in this location:


0z/11 WRF model forecasted 850mb temp/3 hr precip ending 7 am Wed

Note how the WRF shows the highest QPF at this timestamp across eastern areas, but the better thermal gradient is a bit father northwest. That outer 850mb thermal gradient moving away from the storm to the northwest is usually where the best mesoscale forcing develops. Shift everything a little bit east (b/c I still think the NAM/WRF is a bit too far west), and that is where I believe the best mesoscale banding will in fact develop.

I also constructed this map with the idea that the models showing higher QPF over northeastern sections (i.e. the NAM) would be correct due to the great amount of available moisture for this storm once it really gets going. On the opposite end of the spectrum, the best shot at seeing a mix with sleet would be over the south Jersey shore.

Updated: 5:59 AM GMT on January 12, 2011

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Winter storm expected to impact the area Tue night/Wed

By: NYCvort, 5:13 AM GMT on January 10, 2011

MON 6 PM UPDATE

I just wanted to share a good analogy that NWS Albany used in their discussion. It helps better explain what I was saying last night.

GULF COAST SYSTEM QUITE DYNAMIC NOW...BUT GUIDANCE CONSENSUS
SUGGESTS DEAMPLIFICATION AND WEAKENING AS IT TRACKS TOWARD THE
COASTAL CAROLINAS. HOWEVER...THE ATMOSPHERIC DEEP MOISTURE AND
POTENTIAL VORTICITY FOOTPRINT WILL STILL EXIST...LIKE A FUSE THAT
JUST NEEDS TO BE RE-LIT
.

Now take a look at the image below and note the pinks (vorticity) and light grays (moisture) over the southeast. The red (stronger vorticity) out west as well as the baroclinity offshore (not shown) will help to "re-light the fuse."


Automatically updating current infrared satellite/500mb heights/relative vorticity (atmospheric spin)


MON 3 PM UPDATE

Not much has changed from my thinking last night. The new 12z model run package has remained consistent, and the NAM/SREF mean still remain farther west with the surface low than the global models, which have shown the best continuity thus far.

I want to show a comparison of the 700mb set up between our last snowstorm (the “Boxing Day Blizzard”) and this one. Below on the left we have the observed mid-level pattern from that storm and on the right is the mid-level pattern forecasted by the NAM for the middle of this storm:



Note how there are major differences. I highlighted the area north and just west of NYC which experienced the highest snowfall totals. Note how in that location we had a sharp thermal gradient. That area also coincided with the best moisture wrapping around the low (I’m using an old print out for that—can’t seem to overlay it on the image). For the Boxing Day Blizzard we had the 700mb low come up the coast and deepen at it did so. Now take a look at the right side. For this storm we have a 700mb low redeveloping off the coast, shown by the arrow. This is a very different situation, not to mention the fact that the low will be much shallower with lower amounts all around. The main reason why I’m using this comparison is to show why I feel there will be a sharp accumulation gradient to the west. That secondary low will not have developed soon enough for those areas west of NYC, and they will be left in between an already weak primary mid-level low and the undeveloped secondary low. This time I expect the higher amounts to be focused northeast of the city as that new developing secondary 700mb low really gets things cranking. This is sort of a precursor to my snow map which I’m still working on.


SUN NIGHT DISCUSSION

After seeing yesterday’s change in the models’ portrayal of the higher latitudes features influencing the overall set up for this storm and continued good consistency, I feel very confident that a winter storm will impact the NYC metro area from Tuesday night into Wednesday. The main question that remains is the exact track of this storm, which will significantly influence accumulations, dry slotting, and any mixing that could occur along the immediate coast. As I discussed last night, the 0z/9 NAM was alarming in how slowly it transferred the energy from the primary low west of the Appalachians to the secondary low offshore. A solution like that one would have incurred serious mixing issues and brought the dreaded dry slot into question. As I suspected, the NAM has been correcting itself. Tonight’s 0z NAM is a real coastal hugger with the 700mb low overhead and possible dry slotting issues east of the city, but it is a cold solution with no real mixing issues. Regardless I still don’t trust the NAM because the new GFS and GGEM both continue to show the low farther offshore with much better run to run consistency than the NAM.

So far HPC has proven correct in their statement that this storm would be easier to predict based on the fact that we are not dealing with energy coming down from Canada and influencing the storm. For the first time this season we have seen the more usual (a) ECMWF predicts a hit, (b) GFS is too far south and east, and then (c) GFS corrects itself. The main players in this storm are already on the field (the field being areas with good data initialization/radiosonde launches), so I suspect the consistency that we’ve seen between the global models for the past day or so will continue.

Yesterday I went over the higher latitude set up for the storm; now let’s take a look at the actual features that will be influencing its track.

Take a look at the image below. On the left side of the image we have the 500mb pattern/vorticity field as depicted by the NAM for tonight. On the right side of the image we have tonight’s 700mb pattern/moisture field:



Let’s first examine the left side. In the orange circle we have a sub tropical disturbance. This disturbance is pulling moisture out of the Gulf of Mexico. Now take a look at the right side. Note the large green area around the deep south. That is the ample moisture field associated with the sub tropical disturbance. Now look back at the left side and note the blue circle. The dark red shading inside that circle represents our Pacific disturbance. Take a look back at the right side image and note how there isn’t that much moisture associated with it. But there are plenty of dynamics, shown by the vorticity (red) on the left image that was generated by a strong jet. Now all we need for a nice storm is to get those dynamics to work on the moisture field. That’s what will be occurring after the two streams phase. The upper low currently over Montana will then dive into the backside of the phased jet and add additional energy into the equation. There will be so much energy after the two streams phase that an upper low will close off.

Like I said in my last entry there aren’t any other features around to rob moisture from this storm, so I think the GFS is underestimating the amount of moisture that will be involved, with possible corrections in later runs.

So right now, based on everything discussed above, I expect a significant winter storm will likely impact the area. I’m going to wait for one more full model run package before talking about specific accumulations, but we’re looking at the potential for upwards of 6 inches.

Much more tomorrow
NYCvort

Updated: 11:03 PM GMT on January 10, 2011

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Pondering the potential for a northeast snow event

By: NYCvort, 6:51 PM GMT on January 08, 2011

SUNDAY 12 AM UPDATE

Well, the GFS blinked. Since this morning the model has shown a consistent trend (18z, 0z) of increasing the separation between the PV and energy splitting off near the coast of British Columbia:



The area of increased “split” over western Canada has allowed for more ridging along the western US coast and a more amplified midwest trough. The model has also been pushing that eastern Canada ridge farther to the west and increasing its amplification. The combination of the changes in the two higher latitude features has allowed the upper low to close off near the Ohio Valley and pull the surface low much closer to the coast. Seeing the GFS trend so significantly toward the opposite model camp with those higher latitude features has significantly increased by confidence that a low pressure system will affect us this coming week. In fact, the solution has shifted so dramatically that there is support for a primary surface low to develop west of the Appalachians before energy transfers to a secondary development offshore (where there is a much better baroclinic zone). This transfer must happen fast enough or else we would be dealing with serious mixing issues along the coast. The NAM is now “too much of a good thing” with a low actually splitting off completely near the coast of British Columbia. This allows for a more amplified ridge right along the west coast and closed contours extending as far back as the Rockies. The resulting surface low west of the Appalachians would be too strong and too far west and would lead to a slower transfer of energy to the secondary low offshore. This would not be good, but I don’t buy into the NAM solution because not only is it an outlier, but the NAM is notorious for its inaccuracy that far out. I often tell people not to worry about the NAM because it’s the easiest to change. Eventually it will give up its solution to the global models. The new GFS and GGEM look great and the EC was great this morning (not staying awake for tonight’s run!). So let’s keep our fingers crossed but it’s looking very good right now!



ORIGINAL DISCUSSION

Good Saturday everyone. You may have woken up to weather reports of a possible coastal storm next week. I’d like to go over some of my thoughts about this potential storm. Let’s first take a look at this morning’s observed 500mb pattern:



Note that we have an upper low centered over southern British Columbia. It is the remnant energy of this upper low that will be fueling our potential storm. But I don’t want to focus on that so much today. I would like to take a look at the pattern going on at the higher latitudes that will be helping to set up this potential. So let’s take another look at the image above. Also note that we have a strong ridge over the eastern Pacific up through Alaska, and an upper high sitting over north central Canada. We also have an upper low (PV) over far northwestern Canada. These features will be significant in regards to next week’s storm.

The blocking upper high over north central Canada will weaken, and at the same time, a new high will cut off over Alaska:



The orientation of the new upper high over Alaska and its resulting changes in the flow over western Canada will be important in the set up for this storm. Also important will be the weakening high over north central Canada which opens into a ridge that begins slowly moving along in the main flow. This ridge will be positioned somewhere to our north during the time in question. It is more likely for a low to close off over the eastern US if there is a strong ridge directly north of it as that would keep the polar jet flow farther away.

As we’ve all seen lately, the models have not been very reliable in helping us to forecast potential snowfall. The HPC hopes that this time will be different because we’re not dealing with Canadian-hailing energy directly affecting our storm. It makes sense as this set up does look a whole lot cleaner with actually very little flow to our north at all. However, there has been an important difference in how the models handle a key feature in the blocked up northern latitudes. I assembled a graphic yesterday to compare the two different solutions:



On the left we have the GFS model and on the right we have the GGEM (Canadian) model. Find the upper low (PV) over northern Canada in each image and compare them. Note how on the GGEM, the PV is more squished by the ridges on each side of it. We can see how it is trying to stretch out the influence of the upper high near Alaska and create more separation between the PV and a piece of energy that splits off from it and closes off near the coast of British Columbia. The area of separation allows for more ridging along the west coast and thus a more amplified trough downstream over the midwest.

A second difference to note is how the ridge over eastern Canada is more amplified and farther to the west. The fact that the ridge is stronger and farther west on the GGEM allows the energy to have an easier time closing off over the eastern US and sooner.

So if you combine the additional energy resulting from the more amplified midwest trough with the good support for closing off of an eastern US upper low, that new formed upper low then pulls the surface low closer to the coast.

Like I said that was yesterday’s map, so now let’s go take a look and see if similar discrepancies remain between the models today. In fact the models are still struggling over these features. To give further evidence to this idea, let’s take a look at a comparison between this morning’s runs of the NAM and GFS, both American run models:



The NAM only goes out to 84 hrs, so we can’t see past here, but it’s clearly getting ready for a coastal hugger, while the GFS looks flatter and much farther offshore. But this is no surprise if you look up across Canada. The NAM (left side) is trying to split off a low near the coast of British Columbia (in the black circle), while the GFS (right side) only shows a shortwave in the same location with no evidence of attempting to split it off. Also note how the ridge in eastern Canada is stronger and father west on the NAM than on the GFS, further promoting an easier/earlier closing of the eastern US low.

The reason why I think all of this is important is because these are large scale features that will set up this potential. It’s not the disturbance itself crossing the country so much as the set up that we need to focus on. The energy is present with the disturbance, we’re not questioning that, but the bigger pattern needs to be supportive of this storm in order for it to occur. The set up for this storm is far from a slam dunk. We don’t have our ridge near the Rockies that I like to see, and we also have an NAO that will be trending neutral. The AO is still in the depths, but the arctic air is focused over western North America. So the teleconnections really aren’t very supportive of this storm, but that doesn’t mean it can’t happen. One thing in favor of this storm is that without any other features near it and more of a “clean slate,” there’s nothing to rob the available moisture from the storm. My suggestion is that rather than focusing on what is going on in the US with regards to the formation of this storm, keep an eye on these features that are changing across the higher latitudes. This will have an important influence on what actually plays out.

Just as I’m finishing typing this, the new Euro came out, and it’s a big hit. But once again the ridge over eastern Canada is more amplified and farther west, and the shortwave off British Columbia is showing more “split.”

Check back for more updates and have a great weekend
NYCvort

Updated: 5:18 AM GMT on January 09, 2011

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NAM and GFS continue to show very different solutions for snow event

By: NYCvort, 5:02 AM GMT on January 06, 2011

FRI 12 AM UPDATE

Last night I discussed the differences between the GFS and the NAM and how the orientation of the PVA/energy aloft would ultimately determine which solution played out. Well, starting in this morning’s 6z run, the NAM immediately began trending westward and bundling its PVA more to the west, and by 12z the NAM looked very similar to the GFS at 500mb. The NAM remains consistent in its latest run, and I just did a quick loop of the 500mb panels of the SREF ensembles, and for the most part they all keep the energy bundled to our west. Since the GFS solution is clearly panning out, if I had it to do over again, I would lower the amounts a bit on my snow map, with maybe 1” to 3” in most areas, locally up to 6” to the north where we have the better forcing with the enhanced upper level support and also better snow ratios. This is a daytime event, and it is a workday, so keep in mind that with a lot of traffic on the roads and warm boundary layer temperatures the snow will have trouble sticking on the roadways. The GFS MOS guidance high temperature forecast is in the upper 30s, but this will be too high. 2m temps on the GFS are near or just above freezing, and this is similar to the NAM MOS guidance, so I would go with low to mid 30s for highs tomorrow. This would normally promote at most about a 10:1 snow to liquid ratio. Remember, any snow on the roads is hazardous, regardless of how much, so drive carefully.


WED NIGHT DISCUSSION

The NAM and GFS continue to show vastly different solutions for the Fri/Sat snow event, with the GFS giving us a brushing of 1 to 3 inches from southwest to northeast across the area, while the NAM continues to depict 5 to 10 inches or more across a good portion of the area. The main difference between these two models, and the main feature that I keep checking before I even bother to look at the QPF output is the vort max that is depicted in our vicinity by the models, which is a key component to the upper level support for this set up. Compare the two images below, both for late Friday:

On the top we have the GFS. The PVA is much better focused to our west around a small closed upper level low. On the bottom we have the NAM, which is remarkably different in its upper level depiction. I circled the elongated sharp trough featuring excellent PVA heading straight into our area. If we take this PVA and combine it with the surface convergence zone, the NAM spits out heavy snow on its precipitation panel, as shown in the black circle on the right. A few of the SREF ensembles have this feature aloft but they keep the axis of the low level convergence zone (i.e. the inverted trough) farther to the north and east, so we still get spared.

With so much uncertainty existing, even in the upper levels, things still do have a little more time to change. If the NAM solution were to verify, we would need the good PVA AND the inverted trough over us. This doesn’t look very likely in my opinion, especially considering that these Norlun troughs are climatologically focused more north and east.

So in tonight’s snow map, I’m going to keep my general idea of seeing up to 6” in the heaviest swath associated with the inverted trough axis. Right now I still think this axis will set up in SW CT and the lower Hudson Valley. Elsewhere we have a general 2" to 5" accumulation, with up to 2" as you head well south and west of the area.



Keep an eye on the vort max on the 500mb charts in later model runs. If the PVA energy is all bundled up in a low to our west (GFS), then the snow amounts will be lighter. If, however, we can get that energy more spread out in a sharp upper level wave to our south and keep the inverted surface trough axis near our area (NAM), then amounts would need to be increased. I don’t foresee the latter scenario occurring at this point.

In case you didn’t get a chance to look at it yet, in my previous entry I discussed the overall set up for this storm in more detail. You can still find that discussion here.

Updated: 5:29 AM GMT on January 07, 2011

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Moderate snow event expected in our vicinity later this week

By: NYCvort, 6:13 AM GMT on January 05, 2011

We start out this evening with an upper low/polar vortex (PV) near western Hudson Bay and a strong blocking high over Greenland:


The Greenland high will be retrograding into northern Canada by Thursday. As it does so, this will force the polar vortex to break up into pieces. One piece will head northwest and away, but the other two pieces will reconsolidate into a new PV over southeastern Canada:


Now let’s take a much closer look at what will be going on over the US. On this evening’s map we have three key disturbances:

#1 is a southern stream disturbance and #2 is a Pacific jet disturbance. #3 is the polar vortex near Hudson Bay that was discussed above. Let’s first focus in on disturbance #1 and #2. It looks like disturbances #1 and #2 will phase and create a storm system well offshore that will not affect us. However, this storm is important because it is this storm that will basically be shielding us from a major snowstorm. There are two reasons why this is the case.

Take a look at the images below. This compares the 850mb temperature gradient panels prior to the first storm developing well off the coast (left hand side) and after the first storm has exited (right side):

Note how the dashed lines are relatively closely spaced off the northeast coast on the left, showing good baroclinity, while the right side image shows the lines much farther apart near the coast and closer together offshore. The first storm has pulled the baroclinic zone well off the coast. This is important because now as the next storm comes along, it wants to develop farther offshore near the good baroclinic instability that it can feed off of.

So now let’s take a look at the panel below. The strong upper low that is shown over the eastern Great Lakes is the same energy that was associated with disturbance #3 which pinwheeled down and across from its position on the first image:

The entire area where I drew a dashed blue line shows good PVA, or positive vorticity advection. Consequently, note how on the right hand surface panel, we see an elongated area of low pressure ahead of that line where the best divergence aloft is located. We could basically have our low pressure system develop anywhere in that area. The upper level support is there. So the location of the baroclinic zone will be the determining factor, and like I showed above, the baroclinic zone will be located well offshore because of that first storm.

Take a look at the right hand side of the image below:

The top and bottom are two consecutive progs with only a 6-hour separation. The arrow shows how the low seems to magically jump from over our area to well off the coast and outside of the benchmark very quickly. Angry New York snow lovers might say that it is the unfortunate deterrent bullseye around their homes that is preventing the storm from developing there :) But in fact, there is a physical reason behind this jump, and that reason has to do with the first storm moving the baroclinic zone well off the coast, so this second storm will intensify there instead of near the eastern seaboard.


Reason #2 for why the first storm is limiting the second storm’s potential is that in order to have a strong low pressure system at the surface, we need to have an area of diffluence (downstream outward separation of isobars) in the upper levels of the atmosphere. For this we need to go back and take a look at the left side of the image above. The area that I circled associated with the first storm is acting as a convergence zone ahead of the second storm which is limiting the second storm’s potential to intensify by keeping the isobars from fanning out and preventing good diffluence. There is some diffluence, and thus our low pressure system, but it isn’t that strong.


Now despite all of the limiting factors (like the storm being so far off the coast and not being allowed to deepen easily), there is a good potential for a moderate snow event for somebody this weekend. This potential has to with the elongated stretch of PVA that I talked about earlier. On the image below, I highlighted the troughs at the various levels of the atmosphere. Together, these are acting like a front:

The lines at the different levels of the atmosphere are acting similar to a front. You can imagine the inclination with the surface trough to the north, mid-level portion a bit farther south, and then the upper level portion the farthest south. The surface representation of this front is an inverted trough, extending outward from the central low pressure system on the northwestern side. An inverted trough is similar to a convergence zone that is generated by a surface front. Combine this convergence with the good upper level dynamics and someone sees a moderate snowfall. But the exact alignment of this trough is difficult to determine this far out, and that is the key component to determining who receives this snowfall. The GFS has been consistently aligning the trough such that areas to the north and east of NYC end up with the moderate snowfall, but the city still receives a light accumulation.

This is how the WRF high resolution model sees the precipitation set up right now. It is similar to the other models:

I more or less purposely chopped off the QPF color key for the output because I don’t think we should be looking at exact amounts at this point (and the WRF is usually overdone anyway), just the set up for the inverted trough. Right now it looks like the best shot at a moderate snow event would be north and east of NYC. I think areas that end up within that swath will generally see a 3 to 6 inch snowfall. This will be for Friday into early Saturday.

Be sure to check back for more updates. I’m also working on an update to my winter season forecast that I’ll be posting by the weekend.
NYCvort

Updated: 3:13 PM GMT on January 05, 2011

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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.

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