Clipper Snowfall

Still looking at a volatile and challenging system for the Northeast tomorrow and Saturday. My first and final accumulation map is below:

Snowfall Forecast: Thursday Night-Saturday Evening (Click to enlarge)

There could still be some significant variability in the snow totals, especially in the area bounded by Syracuse-Albany-Worcester-Hartford-Newburgh, NY. Snow will move in tomorrow morning. From about Trenton, NJ south this looks to be a very low impact event. There may be a brief period of heavier snow to cause some havoc on roadways, but accumulation will be limited. The main area of impact for this storm should be the Catskills, Mohawk Valley, Adirondacks, Berkshires, Litchfield Hills, and perhaps parts of SW Connecticut and Long Island. The main event will wind down Friday evening, but lake enhancement will begin to deliver heavier snow to parts of Upstate NY through Saturday night, leading to the chance of higher amounts there. The Norlun trough effect could take over in Maine and southern New Hampshire Saturday night, leading to higher totals than advertised above (remember this map ends Saturday 7 PM). Max amounts could top off around 12″, with a few higher amounts, primarily in the mountains/upslope of Upstate NY courtesy of lake enhancement or pure lake effect.

Models continue to show a favorable pattern for a large storm for midweek next week. More on that after this system passes.

Select snow amounts through Saturday evening:

DC/Baltimore, MD: Trace
Philadelphia, PA: Coating-1″
Trenton, NJ: Coating-2″
New York City: 1-2″ average
Morristown, NJ: 1-3″
Bridgeport, CT: 2-5″
Syracuse, NY: 3-6″
Utica, NY: 4-8″
Albany, NY: 3-7″
Hartford, CT: 2-5″
Boston, MA: 1-3″


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Norlun Troughs?

Going to get a little technical briefly. Going to put this pretty plainly: What’s coming Friday will more than likely not be forecasted very well. Why? We’re dealing with a phenomenon that’s pretty common in the Northeast in winter…it usually rears its ugly head at least once. We call this an inverted trough…Norlun trough…or instability trough. What is it? Well, typically, when you think of an atmospheric trough, you think of a “U” or “V” shaped pattern. In an inverted trough, this, well, inverts. It looks more like an upside down U. The lower pressure, or lower heights end up on the north side, rather than the south side. These sorts of setups can produce some nasty weather, heavy precipitation, and thunder.

Here’s more info on Norlun troughs (recognized in the 90s and named for the meteorologists who discovered it).
A technical paper on Norlun trough events.
And here’s more on inverted troughs.

So what’s it to you? Well, the models are beginning to hone in on how this storm on Friday and Saturday is going to behave. And they are indicating that an inverted/Norlun trough type setup may occur. The bottom line is that there will be snow. The question is twofold: Will the Norlun event occur, and if so, where will it setup? This morning’s models pegged it right over North Jersey and NYC. This evening it looks to be shifted a little further north. This is important, because outside of the Norlun trough there will be a generic sort of snowfall. But under the narrow Norlun trough, there should be significant snow, with the possibility of thundersnow and more.

Right now, the NAM model (a mesoscale, small scale, higher resolution model) is showing extremely heavy snow (and has on three straight runs today) for areas immediately north of NYC, Long Island, the Catskills, and the Western Mohawk Valley. The GFS model is suggesting this will be further north and east into New England and the Adirondacks.

Latest Snowfall Risk Assessment for Friday and Saturday

So my snow “assessment” has a broad area of potentially significant snow showing in red. I right now have this from about the Bronx, east-southeast into Long Island (yes, that places the division of snow vs. heavy snow within NYC). It stretches up into the western Hudson Valley, Catskills, western Mohawk Valley and southwestern Adirondacks. I also include much of southwest Connecticut as well, including Bridgeport and New Haven. The problem is that it’s next to impossible to pinpoint exactly where within this idealized zone the prolific snow will be. The yellow territory indicates the basic “cone of error” of sorts. I don’t foresee the risk of heavy snow shifting south of NYC much, if at all. The risk to me is clearly on the northern and eastern sides. This could shift further north, so the axis of the heavy snow stretches from Syracuse/Utica through Albany, the Berkshires, and Central Connecticut to the coast. And it could even shift further north too.

The heavy snow band will be awesome though, with extremely heavy rates of snow, likely some thunder, lightning, and whiteout conditions. I could see max amounts of 10-15″ in this band. You will only need to go a few miles from that core of heavy snow however to find amounts of 6″ or less…that’s the type of storm we’re looking at: Very narrow gradient, very sharp gradient, with extremely variable snowfall totals. Thus, this makes forecasting this system extremely difficult, if not impossible at this point. So if you live from NYC into Central New York and all of Southern New England, stay tuned. As I said, the forecasts for this will more than likely be challenging, vague, and possibly even wrong initially. But that’s the story.

Outside of the heavy snows, expect a brief period of maybe some heavy snow, with a coating or so, up to about 3″ in spots across much of NJ, PA and central New England. Closer to the heavy snow axis, there may be some 3-6″ amounts. But those will be isolated.

New model data indicating the possibility still of a major snowstorm Tuesday of next week. There are still a multitude of questions as to whether it will track in the sweet spot for a DC-Boston hit, as tonight’s GFS is showing, or if it will pass further south. More on that after this first event passes.


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Quick Weekend Snow Risk Assessment

Not much to say beyond what was said yesterday. Just looking at the models today and going off past experience, I feel this first system Friday-Saturday is not going to be a backbreaker on a large scale. I think we’re dealing with a more localized storm in areas more suited to handle heavy/surprise snow…an Alberta Clipper system that starts to explosively develop off the New England coast, which may for some point carry one of my favorite descriptions: A ballistic clipper. These types of storms can really hammer areas north of I-80 if the situation is right.

Snowfall Risk Assessment for the Northeast Friday-Saturday (Click to enlarge)

So anywhere from about Rochester or Syracuse eastward is fair game for this possibly overachieving. Right now, I think the odds of the “clipper” overachieving are highest in the Catskills, but this could easily be extended north toward the Mohawk Valley or even the southern Adirondacks and Berkshires. The secondary area is where the intense coastal development will take place, which is off the Massachusetts coast, possibly putting Portsmouth, Boston, Providence, and Cape Cod under the gun for a period of heavy snow. I’m non-commital on that area though, and more enthused with the idea of parts of the interior being “surprised” by this. For now, you can see my highest risk areas. These may be expanded or contracted later. Additionally, the area of 3″ accumulating snow could end up pushing a little further south, but if I lived south of I-195 in Jersey, I wouldn’t be too concerned or enthused for snow at this point. I think this first event will be primarily a north of I-80/east of I-81 storm. I’ll keep you posted.

We still have this other system showing up on the models for midweek next week, and that one I’m still not writing off or committing to at this point…but I will say, given recent model trends and the overall pattern, it would make sense that someone will see a significant winter storm for the middle of next week. Stay tuned.


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Polar Pain

First a quick reminder that S of O is now on Facebook, so if you haven’t already, “Like” the page, and I’ll be posting intriguing links from time to time, as well as posting snowfall forecast maps as needed a little faster than you’ll get them here. Click here to “like” State of Occlusion, and feel free to tell anyone you pass snow forecasts on to!

In what’s turning out to be quite an interesting winter across the entire US this season. We’ve had a blizzard in the Northeast, record low pressures in the Upper Midwest, a blizzard there too, one of the coldest Decembers on record in spots, snow in Vegas, graupel in Phoenix, and snow in many of the foothill and desert communities north of Los Angeles, record rainfall and snow in California, flooding in the Northwest, record brutal cold and snow in Europe, and on, and on. And that may have just been the appetizer.

6-10 European Ensemble 850 mb T Anomalies - Credit: Allan Huffman's Weather Model Site:

What’s coming over the next 1-3 weeks is nothing short of awesome to watch unfolding on the weather maps. Over the next 10-15 days there will be some of the coldest air of the season draining into the Central part of the country and possibly into the Southern and Eastern parts as well, at least 2-3 *chances* at storms in the Northeast/Mid-Atlantic, the potential for the Southeast as well, and possibly even a winter storm deep into Texas. All *possibilities* in this pattern.

The image I posted (click it or any other image to enlarge) is a map from Allan Huffman’s model site (, which illustrates the European ensemble mean 850 mb temperature anomaly over the period 6-10 days from now. Basically, just an average of how much warmer (yellow) or colder (blue/purple) than normal this model is predicting over the period 6-10 days from now. And notice how almost the entire US (except Maine) is in the below normal range. Keep in mind, 850 mb temperatures are just temperature about 5,000′ above sea level. Translate this down to the ground and under clear skies and/or calm winds…that is cold, even for January, even for these areas. Also keep in mind that these temperature anomalies are in degrees C, not F, so 23F colder than normal aloft could be 30-40+ degrees colder than normal at times on the surface. And I’m not posting it, but the 11-15 day forecast looks equally as cold, if not even colder.

Does it get any better? One of the main drivers (probably the easiest to explain, but certainly not the only driver) is the Arctic Oscillation, or a measure of atmospheric blocking in the Arctic region. High pressure can set up there aloft, and it basically displaces the cold air that’s supposed to be there and dumps it southbound into the US, Europe, etc. The AO tanked in December and really has not recovered, and it seems to be reinforced this month. This happened last winter and helped make it one of the colder winters in recent years for the Southeast and parts of the Central US. This looks as impressive as anything that was seen last winter or this past December. Coupled with things we’re seeing elsewhere, this adds up to what looks to be a long-duration and impressive period of cold in the US. The biggest question is whether or not the extreme cold can spread south and east far enough to impact larger population areas. But for the Northern Rockies and Northern Plains, this looks awful.

GFS Forecast for Saturday Morning, credit: NCEP

So what about snow? This will be the more challenging forecast to make. The map to the left is the GFS forecast for this coming Saturday. Last night’s European model absolutely bombed areas from Baltimore northeast into Boston with extremely heavy snow. Today’s GFS tries to do something similar, displaced further north a bit and behaving differently. This is the first of several threats. I’m not on board with this being a major storm at this point, but a weakish “clipper” type system with a widespread light snow, isolated pockets of heavy snow (potentially a squall line type of scenario with embedded thunder as it moves through NY/PA and New England). Right now there’s a low risk of this being a big system for most of PA/NJ and south, and a medium risk for New England. Stay tuned.

GFS Forecast for Next Wednesday Evening, credit: NCEP

The more important storm comes early to middle next week. The image to the right shows the GFS forecast for next Wednesday evening. Depending on how things play out over the next week, this one could be the one that presents a major threat to the I-95 corridor again. There are threats beyond that, but primarily for Texas and the Deep South….in other words, it may be “too cold to snow,” as they say.

I’m going to be non-committal on any of these threats at this time, but if you live in the Northeast or Mid-Atlantic, keep an eye on this Friday-Saturday and next Tuesday-Thursday, and load up on cold weather essentials…when us as meteorologists start saying that the winter weather is interesting, that’s usually not a good sign. I will keep you posted however.

Happy New Year Preview!

Happy New Year from State of Occlusion. I hope I can provide you with good forecasts and insights this year as time permits.

We have a number of things to discuss, but that will wait til Sunday night or early this week.

First off, if you haven’t “liked” the blog on Facebook, go ahead and do so, as I’ll be posting cool links, snowfall maps (before they get posted in the blog) and other fun things from time to time. Click here to like it, tell your friends, and please pass on any feedback or ask questions on the wall there!

Looks like the cold we saw in the East through much of December is on pace to “re-rack” and bring back the pain for many of you later this week. With that will come a couple more opportunities for snow and cold, with the first notable one coming Thursday-Saturday. Did I mention it looks cold? So enjoy the relative warmth if you have it. Enjoy today’s bowl games and of course the Winter Classic tonight in Pittsburgh (appears the rain will be more sporadic for the rest of the day, and while an occasional shower or two can’t be ruled out, despite the later start time, the weather won’t be nearly as ugly as it would have been this afternoon).

I hope 2011 is good to all of you!