About the Coastal Occlusion on December 27

By: Lee Grenci , 8:16 PM GMT on December 29, 2012

The secondary low-pressure system that developed east of the Appalachians and dumped heavy snow on northern New England on December 27 (snowfall map) occluded and deepened early on Thursday. For confirmation, compare the central pressure of the occluding low at 06Z surface analysis on December 27 with the barometric reading at 09Z. Note that the pressure fell from 989 mb to 987 mb in three hours.


The 09Z surface analysis on December 27, 2012. Note the central pressure of the low along the New Jersey Coast was 987 mb, 2 mb lower than it was 06Z (despite the fact that the low had entered the early stages of occlusion). Courtesy of HPC.

So the low-pressure system intensified during the early stages of occlusion (not unusual). Aloft, the closed 500-mb low associated with the negatively tilted short-wave trough was moving toward a vertical alignment with the surface low (compare the 12Z NAM model analysis at 500 mb with the 12Z surface analysis).

The movement toward a vertical "stacking" is characteristic of the occlusion stage (note the vertical alignment of the surface low and the 500-mb low at 12Z). Even after the vertical stacking takes place, the central pressure of the occluded low is still pretty close to its minimum central pressure. Indeed, check out the 18Z surface analysis and note that the minimum barometric reading was 988 mb.

When the 500-mb low and surface low are vertically stacked, the upper-level divergence that had supported the surface low during its mature state and the stages of early occlusion has all but vanished. Keep in mind here that the upper-level divergence that sustains the surface low typically occurs to the east of the 500-mb short-wave trough. So, when the system is vertically stacked, the surface low is flat out of luck when it comes to upper-level divergence.

How does the occluding surface low respond as it starts to run out of upper-level divergence? Simple answer: It starts to move back into the cold air. At first glance, such a move seems self-defeating. Why's that? Well, above the center of a low-pressure system, the air column that extends from the earth's surface to the top of the atmosphere weighs the least compared to all the neighboring air columns. Keep in mind that, when the atmosphere is hydrostatic, surface pressure corresponds to the weight of the overlying air column (for convenience, we assume the column has unit cross-sectional area).

At any rate, the column of air with the lowest weight (above the center of a low) has the highest average air temperature (more precisely, the highest virtual temperature) compared to neighboring air columns. In turn, the column with the highest average temperature also has the lowest average air density (which is consistent with the lowest weight). The bottom line here is that an occluding low moving back into the cold air, which has a higher density than warm air (at the same pressure), appears, at first glance, to be counterproductive for a low's well being.


An idealized cross section showing the movement of an occluding low back into the cold air and the tropopause undulation (TUL) associated with a lower tropopause in the cold air mass behind the low and a higher tropopause in the warm air mass ahead of the low. Larger image. Courtesy of the American Meteorological Society.

So how can an occluding low move back into the cold air and still have its central pressure decrease (like the low along the Northeast Coast on December 27)? To answer this question, check out the idealized cross section of an occluding low above (larger image) (from Tropopause Undulations and the Development of Extratropical Cyclones by Paul A. Hirschberg and J. Michael Fritsch). The rightmost "L" along the bottom represents a surface low about to enter the occlusion stage at some time, t sub 0. The two "L"s to the left indicate the position of the occluding low at two later times. Back deep in the cold air mass, the tropopause, indicated by the thin dark curves (solid and dashed) roughly between 200 mb and 500 mb, lies at a relatively low pressure altitude. I say "relatively" because the tropopause is higher to the east of the low (in the warm air mass). Meteorologists call these variations in the heights of the tropopause a tropopause undulation.

To understand why the tropopause lies at a lower altitude in the cold air mass, recall that pressure decreases relatively rapidly with height.

In the cold air mass, sinking motion associated with collapsing heights prompts air to sink from the lower stratosphere, dramatically warming on descent. This dramatic warming of sinking stratospheric air creates a warm pocket at roughly 200 mb (and higher up). In turn, winds at 200 mb blow warm air at 200 mb eastward. This warm advection at 200 mb transports relatively warm air into the column of air over the center of the occluding low. The arrival of warm air at 200 mb over the center of the low increases the average mean temperature in the air column over the low. Thus, the average air density in the column decreases. In other words, the air column over the center of the low loses some weight, and, as a result, the surface pressures decreases (even as upper-level divergence vanishes).

So the low cuts back into the cold air, making it seem like an ill-advised move. But the low is actually moving back toward the warm pocket of air at 200 mb, banking on warm advection at this altitude to increase the mean column temperature and, thus, lower the mean column air density and weight. Eventually, the low moves far enough back into the cold air where warm advection at 200 mb ceases. Now the stage is set for the low to rapidly weaken and eventually dissipate.


The 12Z GFS model analysis of 200-mb isotherms (dashed contours in degrees Celsius) and 200-mb streamlines (thin blue contours with arrows). Larger image. Courtesy of Penn State.

Let's get back to the coastal low on December 27. Revisit the 12Z surface analysis. Okay, let's look (above) at the 12Z GFS analysis of 200-mb isotherms (dashed, green contours in degrees Celsius) and 200-mb streamlines (thin, blue contours with arrows). Larger image. Note the pocket of relatively warm air off the DelMarva Peninsula (inside the closed contour of minus 48 degrees Celsius). Note how 200-mb winds blew warm air toward the surface low (warm advection), helping the surface low to deepen (or to maintain its strength) as it moved back into the cold air during the early stages of occlusion.

Lee Grenci

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20. liuzheng
12:30 PM GMT on January 09, 2013
19. WunderAlertBot (Admin)
4:04 PM GMT on January 03, 2013
24hourprof has created a new entry.
18. bappit
5:36 PM GMT on January 02, 2013
Quoting georgevandenberghe:


I'll take this as a valid constructive criticism since at synoptic scales 0-3 days out I'm close to the blind model embracer.

I wasn't criticizing your post, not meaning to. I just thought you had raised a good question.
Member Since: May 18, 2006 Posts: 19 Comments: 7072
17. Levi32
11:56 PM GMT on January 01, 2013
Thanks for the thought-provoking blog, Professor Grenci.

If I may ask a quick question:

You say that:

"The arrival of warm air at 200 mb over the center of the low increases the average mean temperature in the air column over the low. Thus, the average air density in the column decreases. In other words, the air column over the center of the low loses some weight, and, as a result, the surface pressures decreases".

How do we know that the air column has lost mass? A change in density alone cannot do that. After all, a heated balloon contains gas that is less dense than an unheated balloon, but the masses of the two balloons is the same.

We can probably assume that the warming around 200mb causes the air column to expand vertically, but how do we know whether this is just expansion, with mass being conserved, or that some air has been chopped off the top of the column, decreasing the total mass, and thus the surface pressure?

My first thought was that the warming 200mb air also expands horizontally, causing an outward mass flux and a drop in the mass of the column, but then that would mean there is divergence present, and we are assuming the divergence has ceased, right?
Member Since: November 24, 2005 Posts: 635 Comments: 26780
16. georgevandenberghe
3:29 PM GMT on December 31, 2012
What do you remember about the 1993 March superstorm? My memory of this is that it was an early medium range NWP success but almost busted because the storm developed 12H early, tracked about 100KM west of forecast and was therefore warmer with the major metro areas getting dry slotted after "only" a foot or so of snow fell. Forecast amounts were higher but to the public this verified as a much heralded blizzard. A band 100km to the NW of the cities got the epic 2-3' of dry powdery snow and did not get dry slotted.

I will state from time to time that I am not a top quality forecaster (I have other expertise) and defer to better ones when I really want to know what will happen.
Member Since: February 1, 2012 Posts: 19 Comments: 5436
15. Lee Grenci , Retired Senior Lecturer and Forecaster
3:09 PM GMT on December 31, 2012
Quoting georgevandenberghe:
The global forecast model of that time was a T126L28 spectral model.
Today it is T574L64. However T126L28 should have resolved the jet streak and handled the development. But I also remember this event
as a late warned (not till Friday night) event for the DC area. Friday morning looked like a substantial but not crippling snow event with crippling snow for tidewater VA to RIC. By the 18Z cycle Friday night, it
was clear we would get a crippling snow in the DC area.

As is my usual problem I was too busy doing other things that day and did
not do a thorough analysis.


I went on-air with only the 12Z runs at my disposal.
Member Since: October 24, 2012 Posts: 130 Comments: 1077
14. georgevandenberghe
3:06 PM GMT on December 31, 2012
The global forecast model of that time was a T126L28 spectral model.
Today it is T574L64. However T126L28 should have resolved the jet streak and handled the development. But I also remember this event
as a late warned (not till Friday night) event for the DC area. Friday morning looked like a substantial but not crippling snow event with crippling snow for tidewater VA to RIC. By the 18Z cycle Friday night, it
was clear we would get a crippling snow in the DC area.

As is my usual problem I was too busy doing other things that day and did
not do a thorough analysis.
Member Since: February 1, 2012 Posts: 19 Comments: 5436
13. Lee Grenci , Retired Senior Lecturer and Forecaster
2:53 PM GMT on December 31, 2012
Quoting georgevandenberghe:


I'll take this as a valid constructive criticism since at synoptic scales 0-3 days out I'm close to the blind model embracer. I do think conceptual models have a solid role in pedagogy.


A very interesting conversation. And I agree with you about the important role of conceptual models.

I remember being on-air (Weather World, PSU's 15-minute magazine show that airs on public television) the day before the Blizzard of 1996 (January 5). As I recall, the operational models on Friday before the Saturday-Sunday event (January 6-7) had the low far enough off the coast that it was a "flurry event" for central Pennsylvania. At any rate, I remember observing a 300-mb jet streak predicted to dive southeastward on the back side of the short-wave trough supporting the surface low. My conceptual model of such a pattern caused me to consider 500-mb heights backing along the Eastern Seaboard and the surface low coming farther inland.

So I went for a crippling snowfall in Southeast Pennsylvania (Weather World forecasters make forecasts for the entire state of Pennsylvania) and a "plowable" snowfall in central Pennsylvania. I did not predict numbers, given the great uncertainty. Still, I took specific criticism from the local TV stations who predicted flurries...they followed the models exactly.

In the aftermath, I received many congratulations for predicting the general tenor of the storm (the snowfall in SE Pennsylvania was indeed crippling), but the real credit went to my conceptual model taught to me by my instructors.

To this day, I treat forecast uncertainty very seriously. And I am a huge believer in multi-model ensemble forecasting, which addresses the issue of uncertainty.

I tell you this story not to tout my successful forecast of the Blizzard of 1996 (and, yes, I had my fair share of forecast busts over the years), but to show the importance of knowing the science of meteorology. I used to tell the story of the Blizzard of 1996 to my students in Meteo 411 in an attempt to convey to them the importance of knowing how the atmosphere really works (and, yes, even the mathematics of atmospheric science). And that's why I devote so much time explaining the science of weather forecasting.

Thanks again for the conversation.
Member Since: October 24, 2012 Posts: 130 Comments: 1077
12. georgevandenberghe
1:14 AM GMT on December 31, 2012
Quoting bappit:

I don't think this is caused by students' attitudes towards numerical models per se. I'm reminded of this comment from last blog post.



I suspect the numerical models are taking on the role of the rules of thumb (conceptual models).

I sometimes see amateur forecasters blindly embrace the conceptual models on Dr.M's blog. It is human nature for people to like simple answers that don't require thought. Things get twisted off when people who arrogantly embrace those conceptual models criticize people who blindly embrace the numerical models. It seems like the pot calling the kettle black--one unthinking person criticizing another unthinking person.


I'll take this as a valid constructive criticism since at synoptic scales 0-3 days out I'm close to the blind model embracer. I do think conceptual models have a solid role in pedagogy. By the way anyone who has an even halfway valid criticism of my posts should absolutely not hesitate to express it. I'll either post a defense or, more likely learn from it, or both I'm 54 though, and my 1987 masters is stale from lack of exercise in many areas. My actual field of work is HPC analysis and support and NWP has motivated my work for 25 years.

At meoscale (organized convective systems) scale we have a long way to go in NWP but I think those problems are tractable also.
Member Since: February 1, 2012 Posts: 19 Comments: 5436
11. Barefootontherocks
7:01 PM GMT on December 30, 2012
Thanks for the retro synoptics, Lee.
Easier for me to understand this way. I'm probably way behind and may never catch up with others here on the learning curve, but I much appreciate your blog efforts and the discussions.

Speaking of the "underlying science"...

bappit said, "A pet gripe of mine is giving the atmosphere wishes and wants as if the air has expectations and makes decisions."
Let's imagine... The atmosphere does have wishes and wants, the same expectations, wishes and wants described by the laws of physics we humans are subject to, even those laws that at this moment are beyond our conscious knowledge. Maybe not "decisions" but reactions to stimuli and even to geography. Then there are unexplained phenomena like Isaac landfalling on what humans view as the K storm 7-year anniversary. Was it to remind humans of our vulnerability? Or two violent tornadoes that simultaneously turn north and dissipate before reaching a river. Is that by chance or what? According to a guru at SPC whom I questioned about the latter, "It wasn't the River." Hmmm. Then what was it?

I'd just as soon mix some "magic" into science. But that's just me and everyone knows I'm a loon.
:)
Member Since: April 29, 2006 Posts: 174 Comments: 22109
10. bappit
6:25 PM GMT on December 30, 2012
Quoting 24hourprof:


I've been thinking more about what you wrote. Toward the end of my teaching career, I began to realize that there was a growing number of students who only wanted me to tell them the bottom line. They really weren't interested in learning how the atmosphere really works. And that discouraged me.

For better or worse, some students only wanted to look at the models, treating the models as if they were infallible and that the underlying science really didn't matter.

I don't think this is caused by students' attitudes towards numerical models per se. I'm reminded of this comment from last blog post.

Quoting georgevandenberghe:
I am still wondering why use the conceptual model at all if we have good NWP model outputs available for a situation (diagnosing vertical motion around a synoptic scale baroclinic disturbance) that the NWP models handle well?


I suspect the numerical models are taking on the role of the rules of thumb (conceptual models).

I sometimes see amateur forecasters blindly embrace the conceptual models on Dr.M's blog. It is human nature for people to like simple answers that don't require thought. Things get twisted off when people who arrogantly embrace those conceptual models criticize people who blindly embrace the numerical models. It seems like the pot calling the kettle black--one unthinking person criticizing another unthinking person.
Member Since: May 18, 2006 Posts: 19 Comments: 7072
9. bappit
6:04 PM GMT on December 30, 2012
I like this blog topic a lot. I like the examples. It is great to have something to sit back and think about. I like your blog. So, if you've read this far, you know that I'm working up to making a complaint. Oh, well. :)

A pet gripe of mine is giving the atmosphere wishes and wants as if the air has expectations and makes decisions. I'm sure that can make writing about fluid dynamics a lot easier, but it is sort of like saying things like that the sun goes to bed at night. Rank amateur that I am, I have questions about the apparent motions of the low relative to the upper air flow. The post describes it this way.

"So the low cuts back into the cold air, making it seem like an ill-advised move. But the low is actually moving back toward the warm pocket of air at 200 mb, banking on warm advection at this altitude to increase the mean column temperature and, thus, lower the mean column air density and weight."

Is the low moving back or is it the upper level flow overtaking the surface circulation? I imagine that the occlusion process slows down the surface low. Is that true?

If the surface low slows down, then is it that the sum of the upper air mass and low level air mass causes the lowest surface pressure to relocate further west? As long as the surface low is squarely under the divergent flow then does the low level air mass dominate the surface pressure distribution?

I can imagine both of these effects causing the apparent movements of the surface low relative to the upper level wind pattern. Am I off on a tangent?

Member Since: May 18, 2006 Posts: 19 Comments: 7072
8. georgevandenberghe
4:53 PM GMT on December 30, 2012
I commented in an earlier post that Arctic air in the southeast is often shallow (while remaining bitter cold during the day) and does not always show a solid signature at 850MB. At our latitudes though -20C at 850
is a good boundary. I think (and may be wrong) it's probably too cold in the midwest and south; that the arctic front will generally be found to the south of it. Otherwise I'm still looking for a good way to determine the differences. One salient one is that static stability is much higher in Arctic air and this has consequences for amplification of short (~1000km wavelength) waves going through it (they don't amplify or damp out in arctic air whereas they amplify in statically less stable polar air)

Sometimes I really miss the student life and environment.
Member Since: February 1, 2012 Posts: 19 Comments: 5436
7. Lee Grenci , Retired Senior Lecturer and Forecaster
4:23 PM GMT on December 30, 2012
Quoting georgevandenberghe:
I remember one time in a PSU synoptics lecture examining the 1978 February New England snowstorm. I made the tongue in cheek comment
"Ya know they just don't make them like they used to" and brought down the class.

The atmosphere has provided overwhelmingly numerous examples since then to show my comment, while humorous was wrong.

By the way I'm still waiting for some arctic or seasonably cold polar air
for the middle atlantic region and the newer GFS runs are weakening what I expected to be the first such outbreak this season 1/3-1/5 or so 2013.

"Several" is definitely not too many examples.


Many thanks for your support, George. Much appreciated.

I agree with you. Take a look at the short-range ensemble forecast of 850-mb heights (dark contours) and 850-mb temperatures (color fill) from the 09Z SREF run this morning (limited shelf life).

Scroll down and look at the ensemble mean forecast in the lower right corner. Run your cursor over the forecast times along the bottom of the page and watch the isotherm corresponding to minus 20 degrees Celsius (which I use to designate Arctic air) skirt the U.S.-Canadian border in time (focus your attention on the white color fill). Only northern New England gets into the "real" Arctic air (assuming my threshold at 850 mb), although it does get cold across portions of the northern border states.
Member Since: October 24, 2012 Posts: 130 Comments: 1077
6. Lee Grenci , Retired Senior Lecturer and Forecaster
4:11 PM GMT on December 30, 2012
Quoting Balwanz:
Most weather and climate blogs are at best offensively pedestrian. Yours address these topics at a level respectful of your readers. After so many other bloggers that I too soon abandoned, your writings are a breath of fresh air.
I thank you.


I've been thinking more about what you wrote. Toward the end of my teaching career, I began to realize that there was a growing number of students who only wanted me to tell them the bottom line. They really weren't interested in learning how the atmosphere really works. And that discouraged me.

For better or worse, some students only wanted to look at the models, treating the models as if they were infallible and that the underlying science really didn't matter.

The problem is that looking at isolated fields of computer output without regard to the overall big picture and a sound understanding of the basic science is kind of dangerous because computer models are imperfect. In order to make reasonable adjustments to computer-generated forecasts, forecasters must have a firm grasp on the big picture and scientific principles. Without these, forecasters are at the mercy of the models.

Yes, models are getting better, and more and more folks gravitate to their solutions without digging into the science. So you're correct in your observation that I want to give readers more credit for what they know. That doesn't mean that I drop sophisticated science on their laps without some explanation, but I do make assumptions that readers at WU have some working knowledge of meteorology and want to learn more about meteorology.

After all, that's why we're all here.

Thanks again for your kind words. I spend a great deal of time on these blogs, and it's encouraging to know that there are readers like you who appreciate what I'm trying to do here.

Lee

Member Since: October 24, 2012 Posts: 130 Comments: 1077
5. georgevandenberghe
4:03 PM GMT on December 30, 2012
I remember one time in a PSU synoptics lecture examining the 1978 February New England snowstorm. I made the tongue in cheek comment
"Ya know they just don't make them like they used to" and brought down the class.

The atmosphere has provided overwhelmingly numerous examples since then to show my comment, while humorous was wrong.

By the way I'm still waiting for some arctic or seasonably cold polar air
for the middle atlantic region and the newer GFS runs are weakening what I expected to be the first such outbreak this season 1/3-1/5 or so 2013.

"Several" is definitely not too many examples.
Member Since: February 1, 2012 Posts: 19 Comments: 5436
4. Lee Grenci , Retired Senior Lecturer and Forecaster
2:50 PM GMT on December 30, 2012
Yet another example...a deepening low south of Alaska's Aleutians in the Pacific. Here are the 18Z and 00Z surface analyses from OPC yesterday...the low deepened from 956 mb to 952 mb during the six-hour period.

Below is the GFS model analysis of 200-mb isotherms (dashed in degrees Celsius) and 200-mb streamlines (larger image). Again, there's warm air advection at 200 mb over the low, allowing the low to deepen after the process of occlusion got underway.


The GFS model analysis of 200-mb isotherms (dashed in degrees Celsius) and 200-mb streamlines at 18Z on December 29, 2012. Larger image. Courtesy of the Ocean Prediction Center.

Sorry for all the examples, but when I was teaching, I often showed several case studies of the same concept in order to drive home the concept. I hope my teaching method worked!

I'll shut up now.
Member Since: October 24, 2012 Posts: 130 Comments: 1077
3. Lee Grenci , Retired Senior Lecturer and Forecaster
1:59 PM GMT on December 30, 2012
Another great example last night. Compare the 06Z surface analysis (below) with the 00Z surface analysis. Note the rapid deepening of the low as it transitioned into the early stages of occlusion. By the way, I do not subscribe to the policy of naming winter storms, so I will not use their names in any of my blogs. Thanks for understanding.


The 06Z surface analysis on December 30, 2012. The low off the Northeast Coast was in the early stages of occlusion. Courtesy of HPC.

Now take a look at the 06Z GFS model analysis (below; larger image) and note the "warm pocket" at 200 mb (inside the closed isotherm designated by -42 degrees Celsius). Clearly the 200-mb streamlines blew from relatively high values of 200-mb temperatures to lower values of 200-mb temperatures. The resulting warm advection at 200 mb accounted, at least in part, for the rapid deepening of the low during the onset of early occlusion.


The GFS model analysis of 200-mb isotherms (dashed contours) and 200-mb streamlines (thin blue contours with arrows). Larger image. Note that the 200-mb streamlines blew from higher values of 200-mb temperatures (inside the closed isotherm designated by -42 degrees Celsius) toward lower 200-mb temperatures, producing warm advection at 200 mb. over the low off the Northeast Coast.
Member Since: October 24, 2012 Posts: 130 Comments: 1077
2. Lee Grenci , Retired Senior Lecturer and Forecaster
11:32 PM GMT on December 29, 2012
Quoting Balwanz:
Most weather and climate blogs are at best offensively pedestrian. Yours address these topics at a level respectful of your readers. After so many other bloggers that I too soon abandoned, your writings are a breath of fresh air.
I thank you.


You are most welcome. Much, much appreciated.
Member Since: October 24, 2012 Posts: 130 Comments: 1077
1. Balwanz
11:27 PM GMT on December 29, 2012
Most weather and climate blogs are at best offensively pedestrian. Yours address these topics at a level respectful of your readers. After so many other bloggers that I too soon abandoned, your writings are a breath of fresh air.
I thank you.
Member Since: November 23, 2011 Posts: 0 Comments: 9

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About 24hourprof

Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.