Many folks have asked me what a "Norlun Trough" is, and if I can explain more about this phenomenon. Yesterday, I linked to a previous post in which I'd examined the Norlun Trough, but I'd love to take a moment to expand upon what's coming together in this particular circumstance.
First, what is a "Norlun Instability Trough?" A trough is a weak disturbance in the atmosphere - in this case, near the surface - marked by lower surface barometric pressure that results from converging wind direction and speed. Surface convergence of air results in rising air (afterall, air converging at the surface must go either up or down, and clearly can't go into the ground), and rising air produces clouds and precipitation. Hence, troughs can focus locally heavier bands of precipitation. It was noted by operational forecasters Steve NOgueiRa and Weir LUNdstedt (Nor-Lun) that these troughs, when inverted (extending northwest from a surface low) and beneath a middle and upper level atmospheric cold pool, could result in very strong, localized bands of precipitation. On occasion, these narrow but intense bands of snow could produce in excess of one foot of snow, and largely were missed by computer guidance. Mr. Norgueira and Lundstedt both worked to find methods of forecasting these events, even when computer modeling missed them. Since their initial study in 1992, computer modeling has improved significantly, and these events often are projected in advance, though placement and intensity continues to be a challenge, especially in more complex setups.
Where does the "mid-level" and "upper-level" instability come from? Instability in the atmosphere results, most often, from cold air above warm. Warm air rises, so warmer air parcels beneath a colder air layer become buoyant, or "unstable," meaning they exhibit a tendency to rise. Cold air aloft accompanies storms in the upper levels of the atmosphere, and the cold pool with this particular event can be seen in this projected temperature forecast at the 500 mb pressure level (approximately 22,000 feet in altitude). Note temperatures as cold as -38 Celsius at the core of the cold pool. As this cold pool drifts east across the Northeast United States, this will increase the "instability" by cooling the layer above relatively warm near-surface air.
How do the instability and surface trough interact? Putting the upper level and near-surface philosophy together, we generate a scenario in which the surface trough of converging air results in rising air, and because of the cold pool aloft, this rising air is unstable, meaning it will continue rising, and at a swift rate. This results in a corridor of fast upward vertical motion along the trough, meaning healthy cloud development and therefore enhanced precipitation.
Where do Norlun Troughs usually set up? Inverted troughs with instability most often setup along the coast of Maine - especially the Mid-Coast of Maine. The farther southwest from that location one is, the less likely a significant Norlun Trough event usually is, unless topography (terrain) helps to enhance precipitation rates, as hills/mountains can assist in forcing air upward. For instance, inverted troughs draped across Southeast New York State often can produce little fanfare in New York City, but several inches in the Catskills of New York.
Why is Norlun Trough position sometimes difficult to forecast? The advancement of atmospheric modeling technology in the 2000s has vastly improved the ability to forecast location of Norlun Troughs. There are a few issues that still are significant obstacles to the operational forecaster, however, most noteably total precipitation amount (QPF forecast) and placement/development of Norlun troughs when more than one inverted trough is involved.
In this instance, two inverted troughs develop - one in Northern NJ that drifts into Connecticut Thursday night to Friday evening, and another off the Eastern MA coast that extends into the Mid-Coast of Maine Friday night into Saturday (see surface pressure/temperature/wind forecast below, and see if you can find the well defined trough near CT, and developing trough off the coast Saturday morning). Even highest-resolution guidance, which is the strongest tool to predict localized events, are strikingly varied in their solutions, from a very heavy snow event in Central New Jersey, to a weak event at best in Connecticut. Interestingly, there is excellent agreement on the most significant event occurring right where it should - the coast of Maine, with the second trough development.
I'm skeptical of forecasts for a substantial event in New York City to New Jersey, simply because we so often see such forecasts end up incorrect, with only the Catskills and areas northwest of Poughkeepsie, NY, seeing substantial accumulation in New York, though the hills of Connecticut can often enhance snowfall amounts, as well. One wildcard is if a small-scale "mesolow" storm center can develop along the inverted trough near Long Island Sound, which would increase Connecticut snowfall, and is the reason I've highlighted Connecticut as one area of locally maximized precipitation, particularly for New Haven, Middlesex and New London Counties, northwest to Hartford County.
The more significant event along the coast of Maine with the second trough would occur Friday night through Saturday, focused just northeast of Casco Bay. In the transition from one event to the other, the upper level cold pool mentioned earlier, will drift over New England, and this will mean increased instability, resulting in snowbursts migrating from southwest to northeast Friday and Friday night, producing varied snowfall amounts from a dusting to three inches. Amounts should gradually increase beyond those values through far Northeast MA, Southeast NH and Maine, until reaching the axis of heaviest precipitation, which could be several inches. Keep in mind that part of the reasoning for a broader accumulation is the presence of a northeast wind Friday into Friday night for many areas, which blows off the ocean and should provide at least some snow for many areas, regardless of Norlun Trough placement.
What is an Omega Block Pattern?
The Omega Block resembles the Greek letter Omega. The image below shows an example of this blocking pattern. Air over the Southwest U.S. in forced to the north into Canada and then back south into the Southeast U.S. by the huge high-pressure ridge in the center of the country.
The high pressure covers such a broad latitude that the west to east air flow has difficulty going around the high. The region under the omega block experiences dry weather and light wind for an extended period of time while rain and clouds are common in association with the two troughs on either side of the omega block. Omega blocks make forecasting easier since you can pinpoint areas that will be dominated by dry or rainy weather for several days. The right side of the omega block will have below normal temperatures (due to CAA) while the region to the left will have above normal temperatures (due to WAA) in this case.
What is El Nino and how does it effect our Weather?
El Niño (Spanish name for the male child), initially referred to a weak, warm current appearing annually around Christmas time along the coast of Ecuador and Peru and lasting only a few weeks to a month or more. Every three to seven years, an El Niño event may last for many months, having significant economic and atmospheric consequences worldwide. During the past forty years, ten of these major El Niño events have been recorded, the worst of which occurred in 1997-1998. Previous to this, the El Niño event in 1982-1983 was the strongest. Some of the El Niño events have persisted more than one year.
In the tropical Pacific, trade winds generally drive the surface waters westward. The surface water becomes progressively warmer going westward because of its longer exposure to solar heating. El Niño is observed when the easterly trade winds weaken, allowing warmer waters of the western Pacific to migrate eastward and eventually reach the South American Coast (shown in orange). The cool nutrient-rich sea water normally found along the coast of Peru is replaced by warmer water depleted of nutrients, resulting in a dramatic reduction in marine fish and plant life.
The warm water pushes heat and moisture into the atmosphere, which disrupts the usual paths of the winter jet streams. At one time, El Niño was thought to affect only South America's Pacific Coast, bringing flooding rains to Peru and ruining the anchovy fishery. Now we know El Niño can do very strange things to the world's weather for a year or even longer.
In some areas, El Niño means fairly predictable weather. For instance, it is almost sure to cause droughts in Northeastern Brazil, eastern Australia and southern Africa, produce floods and mudslides in Ecuador, quiet the hurricane season in the Atlantic Ocean, delay the Indian monsoon, and bring copious winter rains to southern California.
In Canada, El Niño's impacts are considered marginal, showing up most clearly during winter-time in western Canada. But, El Niño is both good and bad news for Canadians. For example, in British Columbia schools of hungry mackerel riding the El Niño wave may devour young sockeye stock. To Prairie farmers, anxious to see soil moisture replenished, El Niño's usually snow-free winter is not welcome news for those who enjoy skiing and skating. On the other hand, an El Niño year also correlates with a wetter spring and a warmer summer - making for good crop weather. When El Niño occurs, it seems to have something for everyone.
Computer models used to forecast hurricanes
The behavior of the atmosphere is governed by physical laws which can be expressed as mathematical equations. These equations represent how atmospheric quantities such as temperature, wind speed and direction, humidity, etc., will change from their initial current values (at the present time). If we can solve these equations, we will have a forecast. We can do this by sub-dividing the atmosphere into a 3-D grid of points and solving these equations at each point. These models have three main sources of error:
1) Initialization. We have an imperfect description of what the atmosphere is doing right now, due to lack of data (particularly over the oceans). When the model starts, is has an incorrect picture of the initial state of the atmosphere, so will always generate a forecast that is imperfect.
2) Resolution. Models are run on 3-D grids that cover the entire globe. Each grid point represents of piece of atmosphere perhaps 40 km on a side. Thus, processes smaller than that (such as thunderstorms) are not handled well, and must be "parameterized". This means we make up parameters (fudge factors) that do a good job giving the right forecast most of the time. Obviously, the fudge factors aren't going to work for all situations.
3) Basic understanding. Our basic understanding of the physics governing the atmosphere is imperfect, so the equations we're using aren't quite right.
Types of hurricane forecasting models
The best hurricane forecasting models we have are "global" models, that solve the mathematical equations governing the behavior of the atmosphere at every point on the globe. Models that solve these equations are called "dynamical" models. The four best hurricane forecast models--GFDL, GFS, UKMET, and NOGAPS--are all global dynamical models. These models take several hours to run on the world's most advanced supercomputers. There are also dynamical models that cover just a portion of the globe. These are less useful, unless the hurricane happens to start out inside the domain the model covers and stay there. Hurricanes moving from outside the model domain into the model domain are not well handled. An example of this kind of model is the NAM model covering North America and the surrounding waters, run by the National Weather Service (NWS). Another type of hurricane model is a statistical model. These models do not try to solve mathematical equations on a grid. The advantage of these statistical models is that they are fast to run and can provide output in a few minutes. There are also hybrid statistical/dynamical models, and simple trajectory models.
A summary of the top four models:
GFDL: The NWS/Geophysical Fluid Dynamics Laboratory model. This model has been the best overall performing model the past three years. An upgrade to the model this spring has produced a 10-12% improvement in track forecasts and a 30% improvement in intensity forecasts when re-run on hurricanes from the 2005 season. It is the only global model that provides specific intensity forecasts of hurricanes. The raw data is available for users who want to create their own plots of the data, but the GFDL group does not make graphics of their model available on the web. wunderground.com plans to make these graphics available later this season. GFDL graphics are available at several universities, including Penn State.
GFS: The Global Forecast System model run by the NWS. Excellent graphics are available on the web from the National Center for Environmental Prediction. I like their long surface pressure/precipitation loops. Wunderground.com also has GFS plots. I like the Tropical Atlantic imagery. If you select "Shear" from the "level" menu, then click on "Add a Map", you'll get plots of the wind shear that I talk so much about.
UKMET: The United Kingdom Met Office model. Data from this model is restricted from being redistributed according to international argreement, and graphics from the UKMET are difficult to find on the web. Only paying subscribers are supposed to have access to the data.
NOGAPS: The U.S. Navy's Navy Operational Global Prediction Center System. Graphics are available at the Navy web site.
Two other global models are worth looking at, but usually don't do as well as the "big four" above:
The Canadian GEM model.
The European Center for Medium-Range Weather Forecasting (ECMWF) model. This is the premier global model in the world for medium range weather forecasting in the mid-latitudes; the 4-day ECMWF forecasts are as good as the 3-day GFS forecasts. However, the ECMWF does not do as good a job developing and tracking hurricanes. Output from this model is also restricted via international agreement, and is available on the web only at the ECMWF web site.
In 2007, a new model called the Hurricane Weather Research Forecast models (HWRF) is scheduled to replace the GFDL. The HWRF is still in Beta testing, and is not available to the public. The NHC also has several other experimental models that show promise that the public does not get to see.
The BAMM model (Beta and advection model, medium layer) is included on wunderground.com's computer model page. The BAMM is a simple trajectory model that is very fast to run, and did the best of any individual model at 3-5 day track forecasts in 2005. Since this model is always available, we have included it along with the "big four". In general, one should not trust the BAMM model for the 1-2 day time period when output from "the big four" are available. "The big four" are generally not available for tropical disturbances, and for these situations we post plots of a number of other non-global models such as the LBAR, A98E, etc. All of these models are described in detail on NHC's web site.
Model performance in 2005
The National Hurricane Center issues annual verification reports comparing model performance to the official NHC forecast. The 2005 report found that the official NHC forecast was usually the best forecast, but was closely matched by taking an average of the "big four" models to come up with a consensus forecast. There are several techniques used to come up with these consensus model forecasts. The three best techniques are called the GUNA, CONU, and Florida State University Superensemble (FSSE). The FSSE model was developed by FSU with funding from the private company Weather Predict, and is not available to the public. The performance of the "big four", official forecast, and consensus models are plotted below. Among individual track models, the GFDL did the best at 1-2 day forecasts, and the UKMET and BAMM (not shown in the plot) did very well at 4-5 day forecasts. For intensity, the SHIPS model (which we post in the lower right corner of the wunderground.com computer model forecast image) was the best performer. The SHIPS model is run using input from the GFS model.
What is the Difference between a Tropical Cyclone - Extratropical Cyclone and Subtropical Cyclone?
Tropical, subtropical, extratropical?
It is often difficult to tell from looking at forecast model data this time of year whether a low that is expected to develop near the U.S. coast will be tropical, subtropical, or extratropical. The difference is important, since tropical systems have the potential to quickly grow into hurricanes, while extratropical or subtropical storms do not. So, here's a quick meteorology lesson on the differences. We talk about three main types of large-scale storms (also called cyclones):
Tropical cyclones. These include tropical depressions, tropical storms, and hurricanes (which are called typhoons the Western Pacific). Tropical cyclones have warm air at their core, and derive their energy from the "latent heat" released when water vapor that has evaporated from warm ocean waters condenses into liquid water. Tropical cyclones form only over waters of at least 80 F (26 C). One does not find warm fronts or cold fronts associated with a tropical cyclone. Tropical cyclones regularly become extratropical cyclones when they get close enough to the pole to get caught up in a front.
Extratropical cyclones. These include blizzards, Nor'easters, and the ordinary low pressure systems that give the continents at mid-latitudes much of their precipitation. Extratropical cyclones have cold air at their core, and derive their energy from the release of potential energy when cold and warm air masses interact. These storms always have one or more fronts connected to them, and can occur over land or ocean. In winter, extratropical cyclones over water can grow as strong as a Category 3 hurricane.
Subtropical cyclones. These storms occur over the oceans, and are a mix between a tropical cyclone and an extratropical cyclone. Subtropical cyclones get their energy from latent heat like tropical cyclones, and from potential energy of contrasting air masses, like extratropical cyclones. A subtropical cyclone typically has an exposed center of circulation, with very heavy thunderstorm activity in a band removed at least 100 miles from the center of circulation. The difference between a subtropical storm and a tropical storm is not that important as far as the winds they can generate. It is common for an extratropical cyclone to form over cold waters, move Equatorward over warmer waters, and gradually acquire a warm core and enough deep thunderstorm activity to be classified as a subtropical storm. Eventually, many of these will become full-fledged tropical storms if the deep thunderstorm activity can move all the way to the center, and the core becomes warm from the surface to the upper atmosphere. Subtropical cyclones very rarely attain hurricane strength.
NAO - I use it a lot, what does it mean?
Pressure patterns in North Atlantic and across the Polar Regions are critical to the severity of the winters across the hemisphere. The two indices used to measure these pressure patterns are the North Atlantic Oscillation and the Arctic Oscillation.
The North Atlantic Oscillation involves a flip-flop in relative strength of pressure systems north to south in the North Atlantic. Normally on average in the North Atlantic in winter, low pressure is found in the north near Iceland (called the Icelandic low) with high pressure to the south off of Portugal or the Azores (called the Azores High).
At times, the Icelandic low and/or the Azores high become especially strong resulting in a very fast jet stream flow across the Atlantic and into Western Europe. This drains cold air off of North America and often results in above normal temperatures especially in the southeastern states. The air is warmed and moistened over the relatively warm Atlantic waters then the westerly flow off the North Atlantic into Europe carries this mild, moist, maritime air inland resulting in a cloudy, wet and mild conditions there.
Occasionally, this pattern flip-flops with high pressure developing or moving into the far Northern Atlantic (often settling temporarily into a position near Greenland, where it is called the Greenland block). Meanwhile, relative low pressure develops to the south displacing the Azores high. This negative phase of the North Atlantic Oscillation often results in harsh winter weather over eastern North America and in Europe. This is because like a rock in a stream, this high pressure retards the passage of cold air off of North America (thus the term blocking). The cold air pools and expands well to the south over the eastern United States. The storm track too is pushed south of normal resulting often in snowstorms for the eastern metropolitan areas.
What does MB mean?
Those numbers followed by "mb" means the pressure in millibars. At the surface, the average pressure is about 1013mb. Pressure is the weight of the atmosphere in a column of air. The circular lines on pressure maps are called isobars. "iso" means same. So every point along a continuous line is the same pressure. When you see a map showing, let's say, 500 mb stuff... you are looking at the air aloft. You are seeing what's going on in the atmosphere where the pressure is equal to 500 millibars. We don't typically measure altitude in meters or feet in meteorology; we typically go by pressure. Pressure decreases rapidly with height... somewhere on the order of 1 mb for every 10 vertical feet.
When you see map showing 800mb contours or weather or whatever, you're seeing what's going on at that level of pressure. It's important to understand what is going on aloft as well as the surface to make an accurate forecast and study the atmosphere. Weather is 3-dimensional and we aren't limited to only the x,y plane but rather the x,y,z plane. 800mb isn't the same altitude everywhere in the world, by the way. The pressure at my house and your house are probably quite different right now, therefore the altitude at which the pressure equals 800mb above your house and above my house are also quite different right now.
Wind circulates almost perpendicular to the isobars around high and low pressure systems. In the north hemisphere, wind moves clockwise around high pressure systems, and counter clockwise around low pressure systems. The directions are reversed in the south hemisphere. So, in the northern hemisphere, if you spot a low pressure system, generally one can conclude the winds around this system are flowing counter-clockwise around the core, or from north to west to south to east, with a slight angle towards the center of the storm. High pressure systems rotate in the opposite direction, with a slight angle outward from the center. That is, air flows into low pressure systems (like water down a drain) and it flows out of high pressure systems (like water out a hose). As previously mentioned, the deeper the low (the lower it's millibar reading) the stronger the storm and the stronger the winds are around it. So a low pressure at 956 mbs is much stronger than one at 980 mbs, and has much stronger winds associated with it. Surf forecasters like to review surface pressure analysis images to confirm the existence of storms initially viewed in satellite images. The storm's location is plotted and it's distance from shore calculated. Though a satellite image indicates that a storm has potentially formed, a surface level pressure map helps confirm its presence at the ocean's surface.
The Primary Low Pressure is Weakening and a Secondary is Forming Off the Coast, What Does all This Mean?
Could you explain why and how many low pressure systems "fill" as they approach the western slopes of the Appalachians and "transfer" energy to new lows along the East Coast? it happens so frequently and the mountains are not all that high (it seems that the majority of the circulation is above any surface influence when compared to the Rockies). Does it just happen because there is a coastal front waiting that gets charged/ventilated (rising air) and spun as the original storm approaches from the west?
First, consider the drawing above:
On the picture, there's a cross section from the Ohio Valley to the East Coast. An easterly flow of cold and dense low level air is trapped in and east of the Appalachians. Warm air from the west and southwest reaches the west slopes of the Appalachians and simply rides up and over this stubborn cold dome. There are 4 L's across the top of the picture showing the progression of the upper air low pressure area. Each L is labeled with a number, and the table below the picture shows what effect it has on surface pressure falls as it reaches each location. As it moves into the Ohio Valley, air pressure readings fall beneath and ahead of it. More air is being carried away in the upper atmosphere than flows into the storm at the surface. There are two main contributors to the falling pressure: the approach of the upper air storm...and the increase in warm air east of the storm. As that air warms, it expands, and so the pressure drops. This happens quite readily west of the Appalachians and over the relatively warm waters offshore...but it is is different over and east of the Appalachians. There, the approach of the upper air trough still induces pressure falls. However, since the lowest part of the airmass in this area does not warm... or warms very slowly... the pressure does not fall as much there.
So, what does all this mean? As the low pressure area moves through the Ohio Valley, it maintains its strength or gets stronger. However, as the upper level energy passes the Ohio Valley and moves over the Appalachains, the pressure is still low over the Ohio Valley but doesn't drop too much over the Appalachians. This makes it appear that the storm center west of the mountains is barely holding its own. Then, as the upper air support moves toward the warmer waters offshore, the low level warming contribution to pressure falls increases again. Eventually, you will see one weakening low pressure center west of the mountains and another one strengthening off the East Coast. As the main part of the upper air support moves away from the Ohio Valley and toward the coastal waters, the pressures west of the mountains start to rise while they collapse just offshore.In the end, this all makes it look like the western storm fell apart at the expense of the eastern one. However, if the mountains were not there and the low level temperature fields were more uniform, it would have looked more simple. The pressures would fall as the upper air support approaches, then rise after it leaves. You would see the low pressure area simply move from the Ohio Valley right across the absent mountains, and then over to and then off the coast. Instead, the stubborn cold air east of the mountains makes it appear the western storm weakened (when the truth is the center was unable to translate eastward over the cold air), and then it appears that the coastal storm took over. And from a practical standpoint, it does appear that the energy was handed off from one storm to other, when in fact the low level cold air over and east of the Appalachians prevented the pressures from dropping as low as they did in the Ohio Valley or off then coast. This makes it appear that the storm jumped.
What is Virga?
Virga is any form or precipitation that doesn’t reach the ground. There could be rain virga or snow virga. But in either case, the precipitation evaporates somewhere on the journey from clouds toward earth. Virga, which is spelled v-i-r-g-a, is pretty common and you’ve probably seen it but didn’t know it had a special name. Mostly in the summer, virga can be seen falling away in streaks from the bottom of one of those puffy gray and white cumulus clouds on a crisp afternoon. It looks like a torn drape or a curtain hanging from the cloud, but only down about halfway to the ground below. Sometimes the air thousands of feet above the ground is moist enough to produce clouds and rain at the same time that the air closer to the ground is as dry as a bone. So when rain falls in these conditions it evaporates on its freefall to earth. So virga is technically virgin precipitation and it’s easy to spot its wispy form seemingly hanging in the air beneath its parent cloud.
VIRGA ON RADAR:
When the lower troposphere is dry, light precipitation echoes on radar (commonly coded as shades of light green) will not reach the surface. The precipitation has to either be moderate to heavy (commonly coded as shades of dark green, yellow and red) or occur for several hours in order to reach the surface because of the dry initial conditions. Virga is common on radar in winter when light snow or rain is detected aloft but the lower troposphere is initially dry. The leading edge of a rain or snow band will often be virga on radar.