Why is a major snowstorm still very possible in the MidAtlantic and New England?

Some media outlets have been waffling back and forth between the possibility of a significant or major storm and just the nuisance variety, based mostly on what the models at a particular hour are saying. I have not however. I still believe a major snowstorm across a large portion of the northern MidAtlantic and New England is likely Thursday into Friday, and below is a major reason why.

The first map below is the upper air pattern at hour 60 from the afternoon run of the NAM (12z) for this Thursday evening. It shows a positively tilted shallower trough, with it’s base/axis in the central part of north Georgia. The ridge out west is less amplified as well.

 photo 12znam500mb-hgt_rvort_us-60_zps91a6426a.png

Now, compare that to the next image below from this evening NAM (18z). The trough is less positive and more neutral, with the base is slightly west and south in the western portion of central Georgia, meaning it is slightly deeper, and also a little sharper (lines in the upper midwest are steeper along the front of the trough).

 photo 18znam500mb-hgt_rvort_us-54_zps703aedda.png

So most would think – who cares? Not that big of a difference, right? And you’d be correct, it isn’t a big difference. But that’s the point. These little differences higher in the atmosphere at 500mb can have profound effects on what happens at the surface.

The third map below corresponds to the first 500 mb map we looked at, yet at the surface, for the 12z NAM. The low center is directly south of Prince Edward island, Canada, and the precip shield is sparse to non existent over the Mid-Atlantic. 

 photo 12znam-mslp-qpf6-us_hr60_zps3330476f.png

Now look at the last map in the series below. This is the surface depiction off the 18z NAM. Those subtle changes at 500mb produce quite a difference at the surface, with the low about 150 miles west of its position versus the 12z. Additionally, the precip shield is much more robust, producing moderate to heavy snow over most of the area. 

 photo 18znam-mslp-qpf6-us_hr54_zps128ac938.png

Now, these differences were only 6 hours apart between two model runs. And all of the runs did this this afternoon at 500mb. So you can see how the next 12-24 hours may make quite a difference as more accurate data comes in and the models adjust. I expect that the models will continue to adjust a quite a bit of wind blown, high ratio snow is on the way for most of us. 


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Quick lunchtime update, but first a statement from the National Weather Service:


Translated, the storm appears tracking a little farther west than originally thought. Now, I know that many of you in the western portions of the area have seen nothing at all, and are thinking, ‘how can that be?’ The reason for this has to do with the low to our west – the sinking air between the coastal low and the western low effectively prevents any precipitation from occurring.

HOWEVER, the energy from the western low is currently being absorbed or phasing with the coastal low. This coastal storm is now undergoing rapid intensification, and there is intense convection over SE NJ moving NW at this hour. As the storm deepens and moves up the coast, the precip shield will expand northwestward. This will take some time, but snow should develop from southeast to northwest. Because the low may track slightly further west, this can shift some of the heaviest snows slightly west.

But, unfortunately, its not that simple. There will also be mesoscale banding developing, which in layman’s terms is where one town can get 2 or 3 inches of snow in an hour or two, and another a few miles away gets nearly nothing (again due to sinking air). There is no way to predict this ahead of time. Regardless, everyone in eastern PA will see snow this evening, with the most the farther east and north you go.

This is a complicated storm to say the least, but it is far from over, and in fact, is only just getting started. The heaviest snows will be occurring from late afternoon into the overnight hours. We’ll have more updates periodically as warranted. [doc]


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By now, most have heard of what’s about to happen in southern and central New England – 2-3 feet of wind blown snow with blizzard conditions this Friday and Saturday. This storm will be the biggest in decades for them and will likely shut down that part of the country into next week.

Given the relative lack of snow the past two years in the northern mid-Atlantic, the obvious question for snow lovers – and haters for that matter –  is what about us? Can we expect some of the same? Well, the answer is no – not 2-3 feet, and not blizzard conditions for our area. However, there are some things am I following which suggest we still can get a significant to major snowstorm in most of eastern PA and central and northern New Jersey.

Let’s start with the models.  The European model has by far been the most consistent, bringing this storm up the coast for the past 10 days now:


Prior to this point in time, it is prudent to look at the upper levels of the atmosphere, at 500 mb, to see whether this models forecast coincides with whats happening in real time. This is the 500 mb map for 4 am eastern time from the overnight run of the European:


I drew over the height lines over the southeast US. I know it’s somewhat difficult to see, but you get the idea of the rather gentle SW to NE orientation of the upper level winds at 500 mb as forecasted by the european at 4 am.

Now let’s look at the real time 500 mb chart (also at 4am):


You can see that the orientation of the upper level winds is sharper, and pointing more up the coast, than out. Yes, it’s not a large difference, but it’s differences like this at the infancy of a storm that can change forecasts down the road.

Now let’s look at the 0z GFS upper air map at 4 am:


Also flatter, in fact a little more flat than the European, and definitely flatter than the current upper level pattern above. It’s no surprise then that the GFS pushes the storm even farther east and offshore when compared to the European (see above) at the same time:


So, we have a current 500 mb (or upper level wind field) which is pointing more up the coast than certainly the GFS forecast at this point in time, but more importantly, the European model, which brings the storm the closest to us. An interesting observation, and one that would favor a storm closer to the coast, and more snow as well, for eastern PA and NJ.

How about the pressure, strength of the low over the northern gulf? You can see that the low is about 1009 mb just SW of New Orleans, LA at 4 am:


Compare this to the GFS forecast at 4am, at about 1012 mb:


and the European at 7am, 3 hours later, with still a low of 1011-1012 mb:


Both have pressures slightly higher than what is actually happening at this point in time. Not a big difference, but something to watch.

And finally, lets look at the current radar at 4:45 AM:


You can see that the southern precip shield is much larger than modeled (compare to the maps above), extending into the western half of Georgia, and even up into Kentucky. Additionally, precipitation to the north and west looks healthier than predicted by both models as well. Moisture is beginning to be drawn into the system from the Atlantic just NE of Jacksonville, FL in the form of thunderstorms.

In summary, we have upper level winds pointing more up the coast than out, favoring a storm track closer to the coast, more like the European model. We have a developing low slightly stronger than modeled. And we we have a more expansile precipitation field than forecasted.  I’ll be watching all of these factors throughout the day. Current observations and comparisons with the modeled forecast will be critical. How much snow will we get? I think at least 6 inches is a good start in eastern PA, which includes the Lehigh Valley, with as much as foot in the eastern counties along the Delaware river, especially in northeast PA, and in northern New Jersey, IF things come together just right. Southeast Pennsylvania will see the least, with perhaps 3-4 inches in the city of Philadelphia an it’s immediate suburbs.  Southern NJ and Delaware will see very little. These numbers can and will probably change, but regardless, this will be the biggest storm of the year for many here.

More updates on my facebook page(http://www.facebook.com/DocToochWeather) and at Eastern PA Weather Authority (http://www.facebook.com/easternpaweatherauthority and http://www.epawablogs.com)

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Every once in a while a storm comes along on the computer models we watch that causes us to pause. For many of these instances, such a storm will be seen once on the models, and the simply disappear. This can happen for a variety of reasons, but usually has to to with a problem with the models themselves – whether it be incorrect sampling of atmospheric data, or a defect in model physics. After all, the atmosphere dictates the weather, not computer models. They are at best imperfect tools sampling a continually changing atmosphere, and the farther down the road we go in time, the more uncertainty there is when it comes to using them to forecasting the weather.

So, when we first started talking about the idea of this storm which will have far reaching effects for many along the eastern seaboard later this weekend and early next week, we were somewhat skeptical as to whether such an event could happen. It truly is an unusual occurrence – most hurricanes taking the path Sandy is currently on are whisked out to sea, with only rough surf and maybe some clouds along the majority of the east coast; but not this one – not Sandy. 

Why So Confident?

As stated above, most storms seen 7 or 8 days down the road can’t be taken that seriously. The difference here was because of two factors. First, we started seeing it repeatedly come up on the best medium range computer model we have – the European. Second, and more importantly, what the European was showing made sense synoptically. This was the reason why we were able to at least mention the possibility this could happen back on Sunday. You could mention every big storm you see on the models a week out, but you wouldn’t be seen as very credible. So, we liked what we saw – enough to warrant a “heads up” a few days ago. But what exactly was the European showing?


The above image is a snapshot of the upper atmosphere this coming Sunday from the European model run this afternoon. The first feature I want you to look at is the trough (blue U-shaped structure) advancing through the upper plains in the image above. This trough is a deep one – extending from Canada into the southern United States. The depth of this trough is a result of the upstream ridge (upside down U) over the west coast of the U.S.  Those of you who have been following Jim Rinaldi’s videos over the last few days already know that. The degree of ridging out west allows for for a deeper trough downstream in the east. Normally, a trough like this would push a hurricane such as Sandy out to sea.  However, an extreme blocking pattern to the east of the United States – manifested by a large high pressure sitting over Greenland (H) and a huge Oceanic storm (L) in the central Atlantic – is preventing that from happening. Both prevent an escape path for Sandy. As a result, she’s trapped along the east coast. All of this makes sense synoptically, and hence the higher than normal confidence that an event such as this might occur. Now, as this trough advances toward the coast this coming Sunday, Sandy will interact with, instead of being pushed out by, this trough. The trough will effectively “capture” Sandy – and pull it into the coast, making a rather abrupt left turn.

This scenario, while once felt somewhat unlikely,  is supported by nearly every major model we have. The American model, or GFS, has finally come around to the European’s thinking, which is why just today you have been likely hearing about this storm on your local news.  Most of the tropical model we have are similar as well, illustrating a left hook pathway to the Mid-Atlantic coast resulting from the capture scenario discussed above (image below courtesy of Weather Bell Analytics, LLC):

Why all the Hype?

This “capturing” of Sandy by the trough will transition the storm from a tropical hurricane (which is a WARM core system with it’s highest winds and greatest rains near the center of the system) to a hybrid type or extratropical storm (or COLD core system, with winds and rain much more spread out). Additionally, this trough interaction will cause rapid deepening, meaning the pressure will fall quickly. This is important because as the pressure falls, the winds with the storm pick up, because of the pressure contrast with an area of higher pressure to its north.

In the image above from the European, also courtesy of Weatherbell, we see Sandy just north of her current position. The colors represent wind speeds in knots about a mile up in the atmosphere. Typically, winds at the ground surface are slightly lower. Her pressure at this point in time is around 953 mb, and the highest winds (just over 100 knots, or around 105 mph), are concentrated around the center of the storm – a warm core or tropical system.

Now notice how Sandy has changed as she’s moved north in the image above – she’s losing her tropical characteristics as she interacts with that trough, and is making the transition to an extratropical, or cold core system. She’s deepening, as her pressure has dropped to 932 mb, and the wind field has greatly expanded – sustained winds of greater than 45 miles per hour extend for over 1000 miles, while winds of greater than 65 mph, with gusts to 90 mph, extend for over 500 miles. Granted this is about a mile up in the atmosphere, so winds will be slightly lower at the surface, but you get the picture – this will truly be a widespread wind event as it roars ashore.

In addition, because of the blocking to the east, Sandy will be a very slow mover, affecting the area from around the Sunday evening through Wednesday morning time frame (give or take 6-12 hours), with the height of the storm likely Monday afternoon through Tuesday evening, based on the current model runs. This will result in a tremendous amount of rainfall, as shown below, courtesy of the Hydrometeorological Prediction Center (HPC):

As you can see, a general 5-10 inches, with local amounts slightly higher, are likely. Couple that rain, which will soften the ground, and sustained winds of 40-70 mph, with gusts to 90, and you have a recipe for extensive tree and property damage, flooding, and widespread power outages.

What to expect:

This setup is truly a historical, once in a hundred year event, if it comes to fruition in the way that nearly all model guidance suggest.  As a result, after a cloudy and rather rainy and breezy day on Sunday, conditions will deteriorate rather rapidly from southeast to northwest across the area starting sometime on Monday, and we can expect, in general across the entire area:

  • high winds sustained 40-60mph with higher gusts possible at the height inland, 60-80mph+ along the coastal areas… contributing to property, tree, and power line damage.
  • flooding rainfall on the order of 6-12″ in a 24-36 hour period, which will cause flash flooding, as well as possibly historic river flooding.
  • widespread power outages that can last for days if not weeks.
  • similar effects as Hurricane Irene last year for inland residents, if not worse, including all of eastern PA andNew Jersey.
  • massive beach erosion and damage, along with destruction to poorly constructed homes and businesses along shore points
  • coastal flooding and storm surge for a landfalling tropical system combined with a full moon, which enhances the wave action even further.

Because this storm will be so large, with such a massive windfield, the exact point at which the center crosses the coast won’t be as important if this was a purely tropical system especially away from the coast.  However, coastal areas that lie to the north of the storms center will have a much greater coastal flooding threat, due to the wind direction coming off the ocean for a prolnged period of time, over two or three high tidal cycles and during a full moon:


The center may make landfall anywhere from the Delmarva Peninsula (Ocean City, MD area) to eastern Long Island (Islip), with somewhere along the New Jersey coastline being most likely at the moment.

Finally, for those of you who would’ve like to have seen snow with this event, you’ll have to head pretty far west. The mountains of West Virginia and southwest PA seem to be your best bet for a foot or perhaps much more of heavy wet snow.

So, that’s it for now. Sorry for the long read… we’ll have more as we get closer to the storm.


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Tonight’s enhanced water vapor loop shows a weakening tropical depression Isaac moving up the Mississippi river between Arkansas and Mississippi, and will continue to bring welcome rains to the middle of the country over the next few days. The storm will eventually make it to the east coast, but in a substantially weaker form of enhanced rain along an approaching frontal boundary.


Given Isaac’s impending demise, we will turn our attention to the two storms now located in the central Atlantic.

Hurricane Kirk, which has strengthen quite rapidly over the past 24 hours, and now packs 100 mph winds, is located in the center of the Atlantic ocean, about 900 miles east southeast of Bermuda. Although it’s a very pretty yet compact storm, with a well defined eye, it will pose no threat to the US mainland and it moves north then northeast over the next few days.

Tropical Storm Leslie, a much larger storm, and much farther to the south, is currently located about 1000 miles east of the Lesser Antilles. Leslie has significantly increased in strength in just the last 24 hours, with winds of 50 mph. She is currently moving west at 18 mph as it travels along the southern periphery of a subtropical ridge to the north. Most models forecast a break in that ridge in about 2-3 days, which would allow for a more northwesterly motion and eventually the storm re-curving north and then northeast to the east of the US east coast. This potential break in the ridge will be the first factor which will ultimately dictate Leslie’s final path. If this weakness is less pronounced, the storm could get much closer to the US mainland. The European model, which did well (in comparison to most of the other models) with Isaac and his eventual path into the Gulf of Mexico, is taking Leslie close enough to cause rough surf up and down the east coast, but no landfall, through late next week on it’s most recent run this afternoon (courtesy of Weatherbell Analytics, LLC, at http://www.weatherbell.com):

The first potential fork in the road doesn’t come for at least another 48 hours, so there’s time to watch it. Although the chance is rather small, Leslie can’t be written off for the US east coast – at least not yet.

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Tropical storm Isaac continues to gradually weaken as it crawls off to the northwest, currently about 50 miles west of New Orleans with 60 mph winds:

Rainfall amounts are approaching 20 inches around parts of New Orleans, and the earthen levees to the south in Plaquemines Parish have given way with severe flooding in that part of the state. Winds gusted over 100 mph as it roared ashore last night, and the storm surge in some areas topped 12 feet. Despite all of that, fortunately, New Orleans itself has been able to dodge the bullet with the new levee system holding thus far.

Isaac will continue to weaken and slowly move off to the northwest and then north over the next few days, and bring welcome rains to the Mississippi valley and Midwest through the upcoming weekend:

As Isaac winds down, it’s time to once again look to the rest of the Atlantic for any future threats:

Tropical storm Kirk, with 50 mph winds, is located in the central Atlantic and currently moving west-northwest. This storm will gradually turn north and then northeast and move into the north central Atlantic. While it may reach hurricane status, it will pose no threat to North America and disrupt only the shipping lanes within this region.

A broad area of low pressure (Invest 98L), or the big red blob on the map above, is about 925 miles west of the Cape Verde islands farther south in the central Atlantic. This is beginning to show signs of organization and has a high chance (70%) of tropical depression formation in the next 1-2 days. Most models also recurve this storm into the north Atlantic as well, but it will need to be watched over the next 3-5 days given its southern latitude. Regardless, it is at least 7 days away from the US, so there’s plenty of time to watch it.

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Hurricane Isaac is ramping up in intensity rather rapidly, with winds of 80 mph and a central pressure of 975 mb as of 5pm. Unfortunately for residents along the south central Gulf, the dry air we’ve been talking about for the past few days is currently being evacuated from the storm center and the peripheral outflow has significantly improved this hour, resulting in a much more concentric and more classical hurricane look on satellite:

The eye appears to actually be contracting and the storm is tightening up and slowing down on the latest radar from the NWS New Orleans, as it approaches the mouth of the Mississippi at the southeastern most tip of the state:

This contraction and decrease in forward speed is not an uncommon occurrence, and has been seen with previous Gulf of Mexico storms such as Hurricane Ike which hit the Galveston, TX area in 2008 (credit goes to Garret Bastardi for the radar link from Colorado State University):

as well as Hurricane Charley, which devasted Punta Gorda, FL in 2004 (courtesy of Weather Underground hurricane archives):

The good news is the center of the storm only has another 6-8 hours over open water, assuming it maintains it’s forward speed of 8 pm to the northwest. The bad news is that there still can be some significant strengthening during that time, and I would not be surprised if Isaac makes it to category 2 status, and tops out at near or over 100 mph at landfall. Compounding the problem is the slow movement of the storm, which some models have raking the Louisiana area for the next 24-36 hours, leading to torrential rains, as evidenced by the rainfall forecast from the NHC:

and the storm surge, also discussed in previous posts:

The predicted surge from the National Hurricane Center illustrates the danger well, showing a 50% chance of the following levels being exceeded:

with a general forecast from the NHC of a surge of 6 to 12 feet for southeast Louisiana.

The bottom line is that Isaac will come ashore later this evening or in the early morning hours overnight, and crawl to a position very close to the New Orleans area, with winds approaching 100 mph sustained and gusts to 110-115 mph. The real story with this storm, however, will be the flooding, with 10-20 inches of rain over the next 24-36 hours and the storm surges discussed above. With all that, the new levee and pump system will get quite the test. Here’s to hoping it passes.

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One of the things  discussed during last evening’s update was the dry air that was impeding  Isaac’s development, and that appears to be continuing this morning. Although an eye can now be seen forming on satellite, the storm overall continues to have a ragged appearance, especially along it’s northern edge:


The convection to the east of the center is trying to close the eyewall off, and it’s difficult to say whether this will happen or whether it will be another failed attempt given the dry air that continues to plague Isaac’s development. This is not to say, however, that there has not been intensification; because, the pressure has in fact dropped to 978 mb, which is typically a category 2 hurricane based on pressure alone. At the very least, I consider this to be a cat 1 currently. Regardless though, the dry air entrainment and the rather large size of the storm, as pointed out by our own Jim Rinaldi at epawa.net, has largely prevented the winds from increasing to correlate with the pressure drop. This scenario is similar to what was seen with Hurricane Irene last year – very low central pressures, which normally would mean a strong hurricane, but a disconnect between the pressure and the surface winds.

Additionally, Isaac has appeared to jogged a bit to the west overnight, and appears, for the time being, to be moving more west-northwest. Hopefully, this trend will continue and prevent the strongest winds (and therefore the largest storm surge) out of the city of New Orleans. A few of the most recent model runs have the center passing fairly far south of the city, including the overnight European, which has the eye a good 50 miles to the south, shown below, courtesy of Weatherbell Analytics, LLC, at weatherbell.com):

The next 12 hours will be critical, as we will see whether (1) the storm continues in a more WNW rather than NW direction, and (2) how much intensification we get as it approaches the coast. Hurricane Charlie remains in the back of my mind, because that storm went from a category 1 to category 4 storm in a matter of hours, so New Orlean’s is not clear of this threat by a long shot. But one thing that is clear – there will be a tremendous amount of rain associated with this storm, which will cause a great deal of flooding on it’s own, especially if the storm slows down as it approaches the coast as modeled.

As usual, we will have updates on our Facebook page as warranted, and at weatherbell.com, which gives you in depth analysis by some of the best in the business, as well as models that are second to none, as a discounted price that is available only on our website, http://www.epawa.net.

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Currently, Tropical Storm Isaac remains just under hurricane strength with winds of 70 mph. The pressure has been steadily dropping throughout the day indicating an intensifying storm, currently at 981 mb, down from just below 990 mb earlier this morning. Often, the pressure will drop first, and the winds will respond a little later. However, Isaac has been fighting off dry air to its south which is becoming entrained into the circulation:

This is retarding development for the time being, but as Isaac moves to more to the northwest, this should diminish and intensification should continue as the storm moves into a more moist environment with very warm sea surface temperatures and little in the way of wind shear to disrupt the circulation.

Isaac is forecasted to remain heading in a northwesterly direction, and the model spread is in rather good agreement, bringing Isaac near or over the city of New Orleans, probably as a category 2 storm, perhaps a cat 3. Here’s the most recent GFS from 18z showing a very healthy Isaac just southeast of the city of New Orleans (model images courtesy of Weatherbell Analytics, LLC, at http://www.weatherbell.com):

This model, much like the European shown earlier this morning, slows the storm down as it approaches the coast, which allows for a prolonged storm surge of 6-12 ft+, over a period of 18-24 hours. Because the storm comes in from the southeast with respect to the city, the surge will come from the east from the Gulf and up the Mississippi river and into Lake Pontchartrain:

This slow movement also allows for tremendous amounts of rain over this period as well, to the tune of 10-20 inches:

These two factors, the prolonged storm surge, and heavy rains, will cause tremendous flooding in and around the city of New Orleans. If the dry air continues to win out, and development is minimal, this of course would lessen the blow. Alternatively, if the storm were to track more to the east, and make landfall over Mobile Bay, for instance, it would place the city in the western semicircle, with primarily northerly, or offshore winds. But both are felt less likely at this point. As I said this morning, although this storm will likely be weaker than Katrina when it comes to overall storm category and wind speed, the direction the storm is approaching from (southeast, not south like Katrina) and the increasing intensity as it approaches (Katrina was weakening, or decreasing in intensity), coupled with the problems of a slow moving storm described above, it could potentially be as bad as Katrina, even though it’s a weaker category 2 storm.

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Tropical Storm Isaac continues moving west-northwest into the open waters of the eastern Gulf of Mexico this morning. The pressure has decreased and winds are at 65 mph, and it’s signature on satellite has been improving:

The storm should become a hurricane by later today. Overnight hurricane track model guidance (all model images shown below are powered by Weatherbell Analytics, http://www.weatherbell.com/) has nearly overwhelmingly clustered on the southeast coast of Louisiana:

What’s even more concerning, is that most models have the storm slowing down and even stalling as it approaches the coast. Here’s the European from last night showing the storm moving very little over a 24 hour period from early morning Wednesday through early morning Thursday:

Wednesday, 3 am local time:

Thursday, 3 am local time:

Some models even take this to a category 4 (145 mph) as it approaches the coast, as shown by the latest HRWF:

While this is surely overdone, a category 2 or even category 3 storm is a possibility. Take this and combine it with the slow down in forward speed as it approaches the coast, and it’s a recipe for disaster in New Orleans, which is below sea level. This is different from Katrina, because although it may be weaker overall, it will likely be strengthening as it approaches the coast (Katrina was weakening), and it’s coming in from the southeast, increasing the threat of storm surge up the mouth of the Mississippi (Katrina was from the south).

The bottom line – although a major hurricane is a possibility, even if this storm comes ashore as a weaker category 2, a prolonged storm surge and copious amounts of rain could be just as bad as Katrina for New Orleans.

There will be more updates throughout the day on our Facebook page as warranted and as always on weatherbell.com, a premium service which is discounted exclusively through our website, epawa.net.

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