Meteorology: Chapter 13: Weather Forecasting

How much does weather forecasting save for American households per year versus the amount spent by the U.S. government?

$30 billion versus $5 billion.

The dawn of numerical weather forecasting. The father of modern meteorology, Vilhelm Bjerknes, suggested in the early 1900s that we can't yet forecast tomorrow's weather because meteorologists didn't yet possess the two basic ingredients for an accurate forecast. What were they?

(1) Initial conditions; and (2) primitive equations.

There are four types of weather forecasting. What are they?

(1) Persistence forecasting; (2) Steady-state or trend forecasting; (3) Analogue method; (4) Climatological forecast.

Numerical modeling has gotten vastly better over the years. It uses a four step process. What is that process.

(1) Weather observations; (2) Data Assimilation; (3) Forecast Model Integration; and (4) forecast tweaking and broadcasting.

What is and advantage of spectral models?

Computers can compute waves more efficiently than data at lots of points on a map. Spectral models, because they can run on a computer faster than grid point models, have been used to make longer range and more global forecasts than grid point models.

The RUC was run every hour, as is its replacement, the RAPID REFRESH model (or "RAP").

However, these modelsarelimitedo short-term forecasts, a few days at the most.

Why do forecasts still go wrong: Reason #1: Imperfect data. Model results are only as good as the data in its ____________ ____________. Sometimes meteorological features still evade detection, especially over _________.

Initial conditions. Oceans.

What does analog mean?

of or relating to a device or process in which data is represented by physical quantities that change continuously

Why can grid-point models blow up a weather model when it is integrated?

Because the distance between adjacent grid points (known grid-spacing) is important, even if small, and the time between measurements at a grid point is important, even if small. The closer the grid-spacing, the smaller time lapses are needed or else the model will blow up.

What does "empirical" mean?

Based on, concerned with, or verifiable by observation or experience rather than theory or pure logic.

What is numerical modeling's greatest weaknesses?

(1) getting exact readings at all places at the same time regrind the conservation of momentum, mass, energy, and moisture, combined with the Ideal Gas Law from chemistry; (2) most importantly, the atmosphere is a fluid that is always everywhere in time and space. In the model, the finite-difference calculations are only made HERE and THERE and NOW and EARLIER. This latter is called the "strobe effect;" you only get to see certain things and not others even with numerical modeling.

Medium Range Forecast Models: SPECTRAL MODELS. What are they?

A class of numerical weather forecast models that divides the atmosphere in terms of waves rather than grid points. Spectral models taken the whole atmosphere at once and interprets its motions as the WIGGLING of WAVES!

What are grid point models?

A class of numerical weather forecast models that divides the atmosphere into grids. A way of assimilating data collected.

WEATHER FORECASTING TYPE #2: Climatology forecast.

A forecast based upon the climatological statistics of a region. A climatology forecast for a particular DATE does not differ much from one year to the next. A climatology forecast is simply a prediction that the weather you will have will be the same as the long-term average of weather for that day or month. The most obvious climatology forecast for the Northern Hemisphere middle latitudes is 'cold in December, warm in July."

WEATHER FORECASTING TYPE #1: What is persistence forecasting?

A prediction of future weather based on current weather and weather will be the same several hours from now. In other words, the weather you are having now will be the weather you have later.

What is the grid?

A set of orderly arranged points on which variables are analyzed or predicted in a numerical weather forecast model.

What is a "model?"

A simplified but relatively accurate approximation of reality. Today's weather forecasts are computed by numerical models that approximate the behavior of the actual atmosphere.

What is chaos theory?

A theory that demonstrates that in a complex system such as the atmosphere, small differences at the beginning of an event can lead to large differences later. Chaos does not mean that everything is a mess. Instead, it means that the atmosphere -- both in real life and in a computer model -- may react VERY differently to initial conditions that are only SLIGHTLY different. This property of the atmosphere was uncovered by MET meteorologist Ed Lorenz in one of the most celebrated discoveries of 20th century science.

What is the MRF?

A type of spectral numerical weather forecast model that is used for medium to long range forecasts across the globe. It was the longer-range version of the AVN. It is now known as the GFS.

What are gravity waves?

A wave propagated in a fluid through the effects of gravity. An alternating small-scale pattern of high and low pressure maintained with the help of gravity. Gravity waves are sometimes visible when the rising air in the crests in the waves becomes saturated and forms parallel lines of clouds.

Weather Forecasting Type #3: Trend Forecasting. What is it?

A weather forecast that assumes that tomorrow's weather will change as a result of approaching weather systems that, BY ASSUMPTION, are not themselves changing in speed, direction or intensity.

Weather Forecasting Type #4: The analog forecast. What is it?

An empirical method of forecasting that uses past weather events that resemble the current conditions to create forecasts for the days ahead.

Short-range forecast models: The latest advance in the U.S. numerical weather prediction is the Weather Research and Forecasting models, or "WRF," (pronounced "worf."), which is based on the "Eta" model. What is different about WRF?

As theme implies, WRF is designed to allow both forecasters and researchers to use the same basic computer code to create a suite of different models tailored to different situations. Yet again, this was a grid-point model.

The Spectral Model was developed at NMC in 1980. Its forecasts were especially useful for transcontinental airline flights, hence the nickname what?

Aviation Model (AVN). AVN is known worldwide as the name for the first major spectral model. The AVN is a spectral numerical weather forecast model that is used for short to medium range global forecasts.

Why does chaos theory stand in the way of long-term accurate weather predictions?

Because we don't know the atmospheric conditions perfectly at any time, chaos means that the resemblance between a models' forecast and reality will be less and less with each passing day. Meteorologists believe that a TWO WEEK forecast is close to the ETERNAL limit for a forecast done Richardson's way. No amount of computer improvements, parameterization advances or complaining will change this limit.

Who pioneered trend forecasting?

Ben Franklin! Clouds ruined an eclipse for him in Philadelphia, but his brother in Boston saw it clearly, The clouds did not reach Boston for many hours later. Franklin discovered that "nor'easter" cyclones frequently moved up the East Coast, a trend that could be used to forecast the weather.

Where are the jet streams and describe them and their location.

Both the Northern and Southern hemispheres have jet streams, although the jet streams in the north are more forceful. Each hemisphere has two primary jet streams — a polar and a subtropical. The polar jet streams form between the latitudes of 50 and 60 degrees north and south of the equator, and the subtropical jet stream is closer to the equator and takes shape at latitudes of 20 to 30 degrees.

What are weather types as pertains to the analog forecaster?

Categories of large-scale weather patterns that were used to classify weather conditions for use in analog forecasts.

High pressure systems follow what type of front?

Cold front.

The jobs of interpolation (the process of creating an evenly spaced data set from irregularly spaced observations) and data initialization (balancing out the data to remove any eccentricities) are COLLECTIVELY called what?

DATA ASSIMILATION! For those who like food more than SUV's, this step can be summarized metaphorically as meteorologists use data assimilation to "cook" the raw data they feed into the numerical model so that the forecast doesn't get poisoned. It cleans up the numerical information received of the "initial conditions."

What is El Nino?

El Niño-Southern Oscillation (ENSO) is an irregularly periodical variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean, affecting much of the tropics and subtropics. The warming phase is known as El Niño and the cooling phase as La Niña.

No single numerical weather prediction model imperfect. Each one is a little different, with strengths and weaknesses. In the 21st century, weather forecasting us increasingly using the concept of ensemble forecasting. What is it?

Ensemble forecasting is a method of weather forecasting that uses the results of chaos theory to assess the amount of confidence that should be placed in a forecast. A forecast model is run repeatedly with slightly different initial conditions. If the resulting forecasts, the "ensemble," agree closely, then confidence in the forecast is high. If the ensemble exhibits a wide range of different forecasts, then confidence in the forecast is low.

Why do forecasts still go wrong: Reason #3: Chaos. Explain.

Even if you have the world's fastest computer ever, you could still not do a one-month accurate forecast in advance. Brute-force numerical forecasting, with extremely fine resolution, has its limits. Even if you are just a LITTLE BIT off on the initial conditions, the extended forecast will be totally wrong. This is called "sensitive dependence on initial conditions" and is a hallmark of what's popularly known as chaos theory.

What is an upper atmosphere / jet stream blocking pattern?

Every so often, a dome of upper-level high pressure sits in place for a few days, sometimes as long as several weeks. A major block can produce seemingly endless stretches of blazing heat or bitter cold. It also blocks the typical eastward flow of the polar jet stream (thus the label "blocking") and throws storm systems far from their usual tracks because the in-place highs are blocking their movement so they go someplace else. Along those displaced paths, the storms can generate successive bouts of heavy rain or snow. By the time it dissipates, a major block may leave behind a whole stack of broken weather records and an array of disastrous consequences. For example, relentless high pressure over Russia led to unprecedented summer heat in 2010, with estimates of more than 10,000 people killed either directly or indirectly. Toward the south side of the Russian block, unusually strong monsoonal flow sent vast amounts of moisture into Pakistan, leading to catastrophic flooding. And when huge bubbles of high pressure popped up in and near Greenland over the last two winters, cities from Washington, D.C., to London found themselves grappling with heavy snow even as parts of the Arctic experienced periods of record mildness.

AVN and MRF were consolidated in 2002 and renamed what? How many days can this new system can make how many day forecasts for the whole globe?

Global Forecast System (GFS). It can make 16 day forecasts for the globe. It is a spectral numerical weather forecast model that makes 16-day forecasts for the globe. The GFS is a consolidation of the AVN and MRF models.

How can ensemble forecasting try to reduce the effects of chaos theory?

If there a numerical forecast for a certain day, there is not guaranteed that the data used by the model for the initial conditions were measured precisely and accurately. For example, radiosonde measurements of temperature have an error range of about 0.6 degrees Celsius. So one can make a different forecast using slightly different initial conditions that are within the error range of the observing instruments. Then make yet another forecast with yet another set of slightly different initial conditions. Repeat for many different sets of initial conditions. Then compare all of the different forecasts, which re called the "ensemble." In most or all of the different forecasts agree, then there isa high degree of confidence that there prediction will become reality. If the different forecasts give wildly different results, then there is lower confidence in whatever forecast is eventually chosen. Today ensemble forecasts are made at most of the world's forecasting centers. NCEP runs a global ensemble based on the GFS model four times a day with 20 different forecast runs apiece.

How does the NCEP take all the billions of pieces of input and integrate them into a forecast using the formulas created.

In 2013, the NWS installed new supercomputers at NCEP capable of making 146 trillion calculations PER SECOND. It would take an ordinary person 5 million years to make as many calculations as the new NCEP supercomputers can perform in ONE SECOND.

Where does a 24-hour persistence forecast work pretty well? And why?

In Hawaii or southern Florida. These locations are rarely under the direct influence of a strong jet stream. Also, when upper level winds are "stuck" in a "blocking" pattern for several weeks (for example, during a drought or the Flood of 1993 in the midwestern United States, a persistence forecast may be fairly accurate for days or even weeks at a time.

What are Rossby waves?

In meteorology, they are large horizontal atmospheric undulations that are associated with the polar-front jet stream and separates cold polar air from warm tropical air. These waves are named for Carl-Gustaf Arvid Rossby, who first identified them and explained their movement. Rossby waves are formed when polar air moves toward the Equator while tropical air is moving poleward. Because of the temperature difference between the Equator and the poles due to differences in the amounts of solar radiation received, heat tends to flow from low to high latitudes; this is accomplished, in part, by these air movements. Rossby waves are a dominant component of the Ferrel circulation. The tropical air carries heat poleward, and the polar air absorbs heat as it moves toward the Equator. The existence of these waves explains the low-pressure cells (cyclones) and high-pressure cells (anticyclones) that are important in producing the weather of the middle and higher latitudes.

What is the analog effort of "pattern recognition" used for?

In short-term forecasting, an analog method called "pattern recognition" is still used by weather forecasters to supplement today's computerized methods. Pattern recognition is used to identify the "fingerprints" of past weather events that match those of today and then used past maps to make the forecast. In the end, however, the complexities of weather, like human personalities, defy simple explanations.

The NUMERICAL WEATHER PREDICTION PROCESS: STEP FOUR: FORECAST TWEAKING AND BROADCASTING.

In this last step, some of the other forecasting methods sneak in the back door of the modern forecasting process. They are used to make small improvements on the numerical forecast. The best-known of these approaches is MODEL OUTPUT STATISTICS, or MOS (pronounced "moss"), that is used by the NCEP.

What is the NGM?

It is the Nested Grid Model created in the 1980s. It was a grid-point model.

What is the downside to fine resolution in a grid point model?

It means more points at which the model must make forecasts. A forecast model with very fine resolution takes a very long time to compute, even on a supercomputer. Even worse, the forecast can "blow up" if the ratio of grid spacing versus time resolution is too small, even if the original data were silky-smooth.

What is the Northeast Snowfall Impact Scale (NESIS)

It rates and ranks snowstorms since the late 1800s based on the storms' meteorological severity as well as their societal impact.

Short-range forecast models: In the early 1980s, the Nested Grid Model (NGM) was implemented at NMC as an improvement to the LFM. Its name refers to the FINER INNER GRID over the U.S. that is inside or "nested" within a more coarse-resolution grid while it had a coarser grid outside of the United States. Why a coarser grid outside of the U.S.?

It saved computer time.

How long does trend forecasting work for? Why?

It works will only for a period of several hours. Large scale weather systems such as cyclones don't usually change much over a given place over several hours, but we have learned that they have definite life cycles and can change dramatically from one day to the next -- in size, speed, and intensity from one day to the next. Therefore, the accuracy of trend forecasts declines quickly when they remade for longer than a few hours ahead.

What is the jet stream?

Jet streams are like rivers of wind high above in the atmosphere. These slim strips of strong winds have a huge influence on climate, as they can push air masses around and affect weather patterns. The jet streams on Earth typically run from west to east, and their width is relatively narrow compared to their length. Jet streams are typically active at 20,000 feet (6,100 meters) to 50,000 feet (9,144 meters), or about 7 miles (11 kilometers) above the surface and travel in what is known as the troposphere of Earth's multi-layered atmosphere. While they are fairly narrow, they cover a wide latitude running north to south and often travel a very winding path; at times they can even fade away or break off into smaller "rivers" of air that merge again "downstream."

What does NAM and RUC stand for? How often were they run?

North American Mesoscale (NAM) was run every six hours; and Rapid Update Scale (RUC) was run every hour. These are grid-point models.

How does the jet stream affect the weather?

Jets streams play a key role in determining the weather because they usually separate colder air and warmer air. Jet streams generally push air masses around, moving weather systems to new areas and even causing them to stall if they have moved too far away. While they are typically used as one of the factors in predicting weather, jet streams don't generally follow a straight path — the patterns are called peaks and troughs — so they can shift, causing some to point at the poor forecasting skills of meteorologists. Climatologists say that changes in the jet streams are closely tied to global warming, especially the polar jet streams, because there is a great deal of evidence that the North and South poles are warming faster than the remainder of the planet. When the jets streams are warmer, their ups and downs become more extreme, bringing different types of weather to areas that are not accustomed to climate variations. If the jet stream dips south, for example, it takes the colder air masses with it. Jet streams also have an impact on air travel and are used to determine flight patterns. An airplane can travel much faster, and save fuel, by getting "sucked up" in the jet stream. That can also cause a bumpy flight, because the jet stream is sometimes unpredictable and can cause sudden movement, even when the weather looks calm and clear.

You should know LFM, NGM, NAM, RUC, RAP, WRF, UKMET, AVN, MRF, GFS, ECMWF are nicknames of modern numerical forecast models. You need to know each of them Statistical relationships known collectively as MOS help turn forecast model predictions into realistic forecasts for particular locations based on climatological experience.

LFM: An early grid point numerical weather forecast model (developed in the early 1970s) that was used for short-range weather forecasts across America. NGM: A gripping numerical weather forecast model that was used to make short-range weather forecasts for the United States, starting in the early 1980s. Eta: A grid point numerical weather forecast model that issued for short-range forecasts over the United States. Now called NAM. Developed in the 1990s. NAM: The North American Mesoscale model. A gridpoitn numerical weather forecast model that is used for short-range forecasts over the United States. Formerly known as the Eta model. Developed in the 1990s. RUC: A grid point numerical weather forecast model that was used to make short-term forecasts for the United States. Developed in the 1990s. RAP: Abbreviation for the Rapid Refresh Model, a grid point numerical weather forecast model used operationally in the United States. It replaced the RUC in 2012. WRF: Weather Research and Forecasting models, fully developed in the first decade of the 2000s. Based on the Eta model , WRF allows forecasters and researchers to use the same basic computer code to create a different suite of computer models tailored to different situations. UKMET: A grid point numerical weather forecast model developed by the UK Meteorological Office that is used for global short- and medium-range forecasts. AVN: A SPECTRAL numerical weather forecast model that is used for short-to medium-range global forecasts. MRF: A type of SPECTRAL numerical weather forecast model that is used for medium to long range forecasts across the globe. It was the longer-range version of the AVN. It is now known as the GFS. GFS: a SPECTRAL numerical weather forecast model that makes 16-day forecasts for the globe. The GFS is a consolidation of the AVN and MRF type of models. ECMWF: A spectral numerical weather forecast model developed by the european Centre for Medium-Range Weather Forecasts that is used for a wide range of forecasts. MOS: A series of statistical relationships between a particular model's predictions and observed weather variables at a particular location, yes to adjust the raw model output.

Who was the first person to try to do numerical modeling seriously.

Lewis Fry Richardson, during and after World War II, but he failed miserably, but his notes were later resurrected once modern technology made gathering and processing of information much more do-able. If Vilhelm Bjerknes was the father of Meteorology, Richardson is the father of weather forecasting. He created the first numerical forecast model. Remember this. Richardson once said "the forecasting scheme is complicated because the atmosphere is complicated."

Short range forecast models: The first truly modern numerical forecast model was called what?

Limited Area Fine Mesh Model (or LFM for short). Meteorologists at NCEP's forerunner, the National Meteorological Center (NMC) developed it in the early 1970s. It was for the United States, not Europe. It was a grid-point model.

What does LFM stand for?

Limited Area Fine Mesh Model.

What is MOS?

MODEL OUTPUT STATISTICS. It takes the numerical weather prediction and adds certain past and present weather variables at a given location. These complex statistical formulas are used to tweak the "raw" model output. It is basically a scientific version of experience, with MOS saying "Ah, I recall that the model makes too warm a forecast in these situations, so fix the forecast so it is a little cooler than what the model said.

"Dew on the grass, no rain will come to pass." What does this mean and is it true?

MOSTLY TRUE. Dew occurs when the surface, ie. the grass, has cooled and this tends to happen under clear skies at night when the heat radiates from the ground. If the clear skies remain then of course there will be no rain, but if a weather system moves in during the day then a change in the weather can follow.

Trend forecasting for only the next several hours is called what? (know this!)

NOWCASTING. Modern nowcasting combines trends with high-resolution numerical forecast model information.

How can an analog forecaster sort through so many maps and information quickly enough to make a forecast?

One approach, borrowed from personality testing in psychology, is to categorize the weather into a small number of WEATHER TYPES. If today's weather has a "type A personality," then you can create a multiway forecast based on how the weather evolved in type A cases. If today's weather isn't type A, then you try to match it to one of the other six main types and hope for a perfect match. This "weather typing" was used widely in the US from 1935 until 1950.

What are parameterizations?

Portions of numerical weather prediction models that are devoted to the approximation of phenomena that the model cannot calculate precisely. Smallest-scale phenomena are often the most daunting to understand. Forecast models have to parameterize of "fudge" all the small-scale phenomena listed previously. Worse yet, the atmosphere's interactions with other spheres, such as the ocean or the land, also have to be approximated -- usually very poorly. This seemingly trivial part of modeling turns out to be a critical area for forecast improvement today. Computer speed increase will help, especially in regard to precipitation.

What does RAP stand for?

Rapid Refresh model. This is yet another grid-point model.

Medium Range Forecasts: For forecasts of a week or so into the future, NCEP has relied on a slightly different forecast technique. So far, all the models we have looked at have been GRIDPOINT MODELS that chop the atmosphere into boxes. But the atmosphere can be described COMPLETELY in terms different kinds of waves such as what two types?

Rossby waves and gravity waves.

STEP ONE: WEATHER OBSERVATIONS. How is it done today?

Satellites, radiosondes, surface observations and super-computing. Airlines also provide forecasters with up to 100,000 observations per day of mostly upper-tropospheric data along major flight paths. Remote sensing sources, including satellites, now provide 99.9% of all date used for weather forecast models. Instead of a few weather balloon reports in 1910, today's forecast models ingest about 2 BILLION observations per day.

What has made the most difference in obtaining accurate initial conditions via data assimilation?

Successful assimilation of satellite information into data assimilation. This has led to marked improvement in weather forecasts in regions that suffer from a lack of surface or radiosonde observations, such as the Southern Hemisphere.

The observational network to assess initial conditions includes what types of devices? (Name four).

Surface stations (see picture), radiosondes, satellites and airplanes.

What are these analog statistical weather typing making long-distance relationships called?

TELECONNECTIONS.

"If in the sky you see cliffs and towers, it won't be long before there is a shower." What does this mean and is it true?

TRUE. As a general rule of thumb, the more vertical clouds appear the more unsettled the air is and consequently the less calm the weather will be.

"If woolly fleeces bestow the heavenly way, be sure no rain will come today." What does this mean and is this true?

TRUE. Scattered cumulus clouds that appear like fluffy sheep are a sign of settled weather. They are known as 'fair-weather clouds'.

What does a trend forecast assume?

That although the weather features are moving they are not otherwise changing (namely, they are "steady state") in speed, size intensity, and direction of movement.

What is the key -- but sometimes flawed -- assumption of analog forecasting?

That history repeats itself, meteorologically speaking.

The U.S.'s GFS is pretty cool, but today the best model is run by the European Center for Medium-Range Weather Forecasts or (ECMWF). What type of model is it.

The ECMWF model is a type of spectral model but with more waves (and therefore better resolution) than the GFS and pays special attention to data assimilation. It can make forecasts as long as 10 days in advance (which is short than the MRF model but is more accurate).

Describe the LFM.

The LFM was, as its name advertised, LIMITED! It made forecasts only for North America. Its longest forecasts for48 hours from the present and its grid spacing was a very coarse 160 kilometers (100 miles). The LFM, like a nearsighted person, could only "see" larger details and not the finer details of weather, even features several hundred miles across. It is no longer used, but was the forerunner or later models.

A special version of the AVN, made global forecasts up to 15 days in advance. What was this called?

The MRF.

Other than a finer inner grid over the U.S., what else was it doing?

The NGM model of the early 1980s also focused more attention on the jet-stream winds; the LFM's ability to 'see" in the vertical wasps fuzzy as in the horizontal. More and smoother data were incorporated into the model calculations. The NGM was discontinued in March 2009!

What is the NCEP?

The National Centers for Environmental Prediction, which receives the weather-related numerical data from the World Meteorological Organization and then uses such information for U.S. forecast models.

STEP TWO: WEATHER OBSERVATIONS: Who is the vast data-collection process for numerical modeling overseen by?

The World Meteorological Organization and then relayed world-wide. In the United States, the National Centers for Environmental Prediction (NCEP) receive the data for use in their forecast models.

What are Bjerknes' initial conditions?

The current conditions of the atmosphere over a wide area. The values of atmospheric variables (temperature, dew point, etc.) over a wide area at the starting time of a numerical weather forecast.

What is grid spacing?

The distance between one grid point and another in a numerical model.

What does a climatological forecast depend on?

The fact that the weather, although changeable, is strongly determined by the tilt of the Earth and the global energy budget. One favorite climatological forecast is percent chance that it will snow on Christmas.

What are Bjerknes' primitive equations?

The nearly exact physical laws of the atmosphere that, together with the initial atmospheric conditions of a given area, can be used to create a numerical forecast for a later time. They are the mathematical calculation of the atmosphere's future conditions, using the physical laws that govern the atmosphere's changes. Bjerknes admitted that this was a "problem of HUGE dimensions... that the calculations would require a preposterously long time."

What is critical for this step of integration to succeed?

The physical distance between the interpolated data points becomes very important. The smaller the grid spacing, the easier it is for the model to "see," or "resolve", small-scale phenomenon. This is called good, or fine, resolution. Wide spacing between grid-points is called poor, or COARSE, resolution.

What is the grid point?

The point in the middle of a grid in a grid point model.

Grid-point models of the atmosphere can get fussy when the data in the initial conditions are not obtained exactly at the location of the grid points. What is interpolation?

The process of creating an evenly spaced data set from irregularly spaced observations. A part of the numerical weather forecast process in which data from irregularly spaced observation sites is mathematically adjusted onto a regularly spaced grid for use in the numerical forecast model.

What affects the jet stream?

The seasons of the year, location of low and high pressure systems and air temperature all affect when and where a jet stream travels. Jet streams form a border between hot and cold air. Because air temperature influences jet streams, they are more active in the winter when there are wider ranges of temperatures between the competing Arctic and tropic air masses. Temperature also influences the velocity of the jet stream. The greater the difference in air temperature, the faster the jet stream, which can reach speeds of up to 250 mph (402 kph) or greater, but average about 110 mph (177 kph).

What is resolution?

The spacing between grid points in a numerical weather forecast model. "Fine" resolution results from close spacing of grid points; "coarse" resolution results from wide spacing of grid points. Fine resolution allows a model to "see" smaller-scale phenomena more accurately than does coarse resolution.

What is "numerical modeling?"

The technique approximating tough real world problems with numbers. The simulation of fluid motions, such as in the atmosphere, using mathematical approximation of the equations describing the fluid. These complex computations usually require the use a powerful computer. The numerical formulas used are called a "model" just as a realistic approximation of a trains is called a "model railroad."

What is a good rule of thumb about the limits of modern meteorology?

The weather skill extends 1 day further into the future every decade. In February 2010, the ECMWF model broke the 10-day barrier for skillful forecasts for the very first time. It has shown an improvement in skillful forecast range of 1 day for every 10 years. However, precipitation forecasts are harder because of the spotty nature of rain and snow.

Why are "primitive equations" called "primitive?"

They are called "primitive" because they are so complicated that they cannot be solved with algebra and pencil and paper.

Researchers have returned to weather typing. How and why?

They have returned via sophisticated statistical approaches. Today we know that certain weather patterns in widely separated locations -- for example Alaska and the East Coast of the Untied States -- vary in relation to one another in a predictable way during global phenomenon such as El Nino - Southern Oscillation.

Short-range forecast models: In the 1990s, the U.S. Government scientist began using two new models, the "Eta," now called the North American Mesoscale or "NAM." and the Rapid Update Scale (RUC). What did these models do?

They incorporated continued improvements in resolution in all three dimensions.

Folklore forecast: "When spiders' webs in the air do fly, the spell will soon be very dry."

This forecast seems to be well founded. Spiders want to catch insects in their webs, and insects will often breed after a rain. However, spiders don't want to have their webs covered with water; a sopping-web spider web is easily detected, making it a lousy trap for insects. So spiders spin webs at the end of a period of rain as the air dries out -- for example after a cold frontal passage in spring. HIGH PRESSURE SYSTEMS follow cold fronts and often lead to a spell of dry weather.

Folklore forecasting: If a ring forms around the moon, twill rain or snow soon? What does this mean and is it accurate?

This is often but not always relatively accurate and also applies for halos around the sun. High ice-crystal clouds such as cirrostratus often occur in advance of a mid-latitude cyclone's warm front. Cirrostratus clouds, in turn, cause halos. So the halo may be the first sign of an approaching warm front and cyclone and, thus, bad weather, especially at night. The saying is not always accurate because not all warm fronts produce extensive regions of high clouds, and not all high clouds are the result of a warm front. But it often works.

Folklore forecasting: There is a saying "Red Sky at night, sailor's delight. Red sky at morning, sailor take warning." What does this mean and is it correct?

This saying is fairly accurate. Red sky at night: Sailor's delight -- A clear western sky at sunset allows the Sun to shine through the atmosphere, its light reddening as a result of Rayleigh scattering and then reflecting off clouds in the Eastern sky. Clouds to the east usually move away as the weather in the mid-latitudes usually moves from west to east under the influence of jet stream winds. While your weather tomorrow will likely be from tonight's clear Western sky. Red sky at morning, sailors take warning: Red skies in morning means that Rayleigh scattering is bouncing off of storm clouds that are approaching from the west. Meaning a cloudy day and maybe rain or storms.

The NUMERICAL WEATHER PREDICTION PROCESS: STEP THREE: FORECAST MODEL INTEGRATION. What is it?

This step puts together the observed data and the model's formulas, which are approximations to the actual primitive equations describing the atmosphere. In other words, once the data is "cooked," the model's formulas "ingest" it, and the real forecast begins. Millions of arithmetic calculations may be made to get forecasts for atmospheric variables at a later times. Then these forecasts are used as the new initial conditions for a forecast at still a later time. The whole process piggybacks on itself and marches forward in time. Mathematicians call this INTEGRATION, and it is the same concept as integration in calculus.

"Mackerel sky and mare's tails make tall ships carry low sails."

This weather proverb originates from a nautical background when different cloud types were used to determine whether sails needed to be lowered. Also referred to as just a 'mackerel sky', it is associated with altocumulus clouds while 'mare's tails' refer to cirrus clouds. Both could develop before the instance of a storm which would lead to the lowering of the ships sails. Altocumulus clouds appear when there is a certain level of moisture in the air suggesting rainfall is approaching. The term "mackerel sky" comes from the clouds resemblance to the scales of the mackerel.

What are the two variables that data initialization weather forecasting mean to address.

To balance data regarding wind and pressure.

The NUMERICAL WEATHER PREDICTION PROCESS: STEP TWO: DATA ASSIMILATION. What is this?

To do their work, most numerical models look at the atmosphere as a series of boxes. In the middle of each boxes a point for which the model actually calculates weather variables and makes forecasts. The result of this three-dimensional boxing-up of the atmosphere is called THE GRID.

What is the "analog forecastor's" job?

To locate the date in history when the weather is a nearly perfect match, or ANALOG, to today's weather. Then the analog forecast for tomorrow is simple. Given what is happening today, whatever happened in the "matching" next day's weather away will be the weather for tomorrow. The forecast for the day after tomorrow is whatever happened in the second day after the analog, and so forth. Analog forecasting, therefore, requires many years of weather maps and an efficient way to compare one map with another.

What is the analog effort of "teleconnections" used for?

To make general forecasts months into the future.

Why do forecasts still go wrong: Reason #2: Faulty "Vision" and "Fudges." Explain.

Today's forecasts are a tradeoff between horizontal resolution and the length of the forecast because fine resolution means lots of points at which to make calculations which mean lots of computer time. So even today's models are still not able to "see" many small-scale phenomena, from thunderstorms to turbulence, not to mention clouds, raindrops, and snowflakes. To COMPENSATE for this fuzzy vision of models, the computer code includes code includes crude approximations of what's not being seen. These are called parameterizations.

Observations are likely to include little gusts and swirls that do not reflect the large-scale weather. This can distort the model if theg just or swirl shows up at a grid-point. Putting this "bumpy" data into a forecast model is a lot like driving an SUV with no shock absorbers over a bumpy road. The variables predicted by the forecast model, just like the SUV, can bounce around out of control. As a result, the model, like the SUV, can "crash," and become useless. So, what to do?

Today, meteorologists know that a crucial step in forecasting into take the unbalanced "shocks" out of the data. This is called DATA INITIALIZATION and can be thought of as the mathematical equivalent of shock absorbers. The goal of data initialization is essentially to achieve balance the data -- in this case, TO MUTUALLY BALANCE WIND AND PRESSURE -- before those data enter the model.

Who is the father of modern meteorology?

Vilhelm Bjerknes. He proposed the procedure now known as numerical weather prediction. He suggested that weather prognosis (forecasting) should be considered as an initial value problem of mathematical physics, and could be carried out by integrating the governing equations forward in time, starting from the observed, initial state of the atmosphere. In short, Bjerknes discovered that with enough information about the current state of the atmosphere, scientists can use math formulas to predict future weather patterns. Unfortunately, due to the lack of processing facilities (computers), the calculations he envisioned were not feasible at that time. Nevertheless, Bjerknes considered weather forecasting as the principal objective of meteorological research, trusting that adequate observations and computer facilities would emerge in the future. He also came up with the idea of the Norwegian Cyclone Model, with his son.

So how does ensemble forecasting work at its best?

When many different sophisticated models reach the same result despite their differences in how the atmosphere is represented in their computer code, meteorologists have high confidence in them. Supercomputing, baby!

Where is a 24-hour persistence forecast horrible?

Where the jet stream is strong. In these regions, growing low pressure systems cause rapidly changing weather. The Northeastern United States is one place where a persistence forecast is not very good for more than a few hours.

What is the accuracy of a persistence forecast dependent upon?

Where you are, the type of upper-troposphere winds that exist over your location, and how far in advance of a forecast you want.

Are there other jet streams than the main four: northern polar, northern subtropical, southern polar, southern subtropical.

While the polar and subtropical jet streams are the best known and most studied, other jet streams can form when wind speeds are above 58 mph (93.3 kph) in the upper atmosphere at about 6 miles (9.6 kilometers) to 9 miles (14.5 kilometers) above the surface. The term is often misued, even by meteorologists giving the weather forecast who sometimes, for the sake of simplicity, call all strong upper-atmosphere winds jet streams.