OVERVIEW:

We will discuss the elements of a weather forecast, the overall methodology of weather forecasting, and forecasting techniques that do not involve centrally-produced prognostic and forecast products.

OUTLINE:

1. What is a forecast?

1.1. A forecast is a carefully formatted, detailed statement of the meteorologist's expectations for specific weather elements at one or more points in time -- usually up to 24 hours in the future.

1.1.1. A prognosis is an estimate of the future state of the atmosphere in terms of airmasses, fronts, pressure systems, troughs, ridges, jets, moisture, etc. You need a good prognosis to make a good forecast.

1.1.2. An analysis/prognosis discussion is the forecaster's detailed meteorological explanation of the causes of the weather occurring in the present, and how these causes will change in the future.

1.1.3. An outlook is much more general (i.e. non-specific) than a forecast. Outlooks may extend several days to several months into the future, and become progressively more general as the time period of interest gets further and further into the future.

1.2. A forecast typically includes the expected state of the following weather elements:

1.2.1. Surface wind direction, speed, and character. (Character means the instantaneous values of gusts, as opposed to speed, which is the one- or two-minute average.)

1.2.2. Sky condition: The amount, type, and height of clouds. In other words, a description of the future state of the dome-shaped region of the sky above the horizon.

1.2.3. Visibility: How far the human eye can see along the horizon circle. "Prevailing visibility" is defined as the greatest visibility over one half or more of the horizon circle.

1.2.4. Precipitation and/or surface-based obscurations and other phenomena.

1.2.4.1. Precipitation includes rain (RA), snow (SN), showers (RASH/SNSH), drizzle (DZ), freezing precipitation (FZRA/FZDZ), hail (GR), etc.

1.2.4.2. Surface-based obscurations include mist (BR), fog (FG), haze (HZ), and smoke.

1.2.4.3. Other phenomena include thunderstorms (TS), tornadoes, funnel clouds, water spouts, etc.

1.2.5. Surface air temperature and dew point (or humidity).

1.2.6. Surface atmospheric pressure.

1.2.7. Hazards to flight. These include icing and turbulence.

2.1. Figure out what is causing the weather right now.

2.1.1. Scan the latest surface chart; the 850 mb, 700 mb, 500 mb, and 300 mb upper-air (UA) charts; and the latest satellite picture to create a careful mental picture of the synoptic (continental-scale) pattern of highs, lows, ridges, troughs, jets, fronts, and moisture fields.

2.1.2. Use the latest radar summary to nail down areas of precipitation.

2.1.3. Use the latest local skew-t to determine the airmass's stability, locations of inversions, and vertical strata of cloudiness.

2.1.4. Compare your synopsis to the latest surface observations at your station and in surrounding cities and airfields.

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It doesn't hurt to look out the window!
Is it raining? Snowing? What kind of clouds do you see? What do the clouds suggest about the vertical motion in the atmosphere? Which way are the clouds moving?

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2.2. Next, consider the weather trends over the last 24 hours. How has the local observation evolved? Is the pressure falling or rising? Are clouds increasing or decreasing? Are the cloud bases getting higher or lower? What's happening with the dewpoint? Has the wind direction shifted recently? Is it much warmer or colder now than it was 24 hours ago? How do these changes relate to the changes in the historical configuration of synoptic features in your area?

2.3. Next, figure out how the forces causing the weather right now are going to change over the next 24 hours.

2.3.1. Look at your data (latest surface, UA series, satellite, and radar) and think first about the next few hours. Is there anything obvious staring back at you? Is there a front about to move past your location? Is there a low nearby? Is there a thunderstorm in the next town?

2.3.2. Next, consider the longer view -- out to 24 hours in the future. Recalling your synopsis of the major features (troughs, ridges, etc.), ask yourself which way and how fast will each of these players go. (See below for a few methods of making these estimations.)

2.3.2.1. Look at the latest skew-t. How will the pattern of horizontal temperature advection (see UA charts) change the airmass profile? Will cold air advection (CAA) aloft make it more unstable than it is now? Will warm air advection (WAA) aloft make it more stable than it is now? Do you expect the horizontal moisture advection (see UA charts) to add moisture or subtract moisture from your skew-t's dew point profile?

2.3.2.2. What's happening with the regional radar loop? Is there an area of precipitation advancing toward you? If it's raining/snowing now, can you make out a clearing line approaching you? The radar summary is a concrete observation of what the atmosphere is producing now -- it is a very useful tool.

2.3.2.3 Ask yourself how the future configuration of synoptic features (and the altered airmass profile) will change what the observer sees from the ground at your location. In particular, imagine the future state of:

2.3.2.3.1. Atmospheric pressure, temperature, and vapor content.

2.3.2.3.2. Type and amount of cloudiness.

2.3.2.3.3. Type and intensity of precipitation, and other types of weather phenomena (fog, etc.).

2.3.2.3.4. Wind direction, speed, and character.

2.4. For all forecast periods, consider local effects. Will the synoptic-scale arrangement of pressure systems, etc. that you expect in the future lead to the set-up of sea or lake breezes? Land breezes? Mountain or valley breezes? Are there other local effects that may come into play?

3.1. One of the major challenges in weather forecasting is estimating the future locations of major meteorological features. The process begins with at least three complete sets of analyses (UA charts, corresponding surface charts, and skew-t's) -- the most recent set, the set from 12 hours prior, and the set from 24 hours prior. Using these charts and a few techniques, the forecaster should make a mental "prognosis" of the future locations of features affecting the forecast in his or her area of responsibility.

3.2. After completing standard analyses on the UA package (850/700/500/300), the 12-hour old position of major features should be traced onto the corresponding chart using a bright yellow marker. The 24-hour old position should be traced with a bright orange marker. (One skew-t should be used each day, with the 0000 GMT profile plotted in blue, and the 1200 GMT profile plotted in red.)

3.3. The first and most commonly used technique for determining the future positions of various features is by extrapolating from their current positions using their historical rates and directions of movement. This is called using continuity. This method applies to fronts, troughs, lows, ridges, highs, jets, areas of moisture, and many other large-scale features.

3.3.1. For example: If a trough has moved 500 miles toward the east over the last 24 hours, it will probably move another 500 miles toward the east over the next 24 hours.

3.3.2. Continuity is limited by its inability to accurately estimate accelerations in the motion of systems, its inability to estimate changes in the relative strength of systems, and its inability to predict the formation of new systems.

3.4. A more sophisticated set of techniques is required for estimating accelerations, changes in the strength of pressure systems, and system formation. These lead to generally more accurate results for all synoptic prognoses. Each analysis yield a different class of information. Some of these are:

3.4.1. 300 millibar constant-pressure chart. 300 millibars is the clearest place to find the longwave (barotropic) pattern. Storms (baroclinic systems) generally track along the longwave pattern, deepen (grow more intense) in the bases of longwave troughs, and fill (grow weaker) in the crests of longwave ridges. To estimate the movement and evolution of baroclinic systems:

3.4.1.1. Speed: Use 20 to 40 percent of the jet stream speed associated with the system.

3.4.1.2. Direction: Baroclinic systems generally follow the longwave pattern traced out by the jet stream, i.e., the storm track. Very intense baroclinic systems often alter the storm track.

3.4.1.3. Regions of strong directional difluence: Deepening of lows/weakening of highs.

3.4.1.4. Regions of strong directional confluence: Filling of lows/building of highs.

3.4.1.5. Height falls/rises indicate both the deepening/filling of systems, and direction toward which the systems are moving. For example, lows will usually track toward the adjacent region of maximum height falls.

3.4.2. 500 millibar constant-pressure chart. 500 millibars is the clearest place to analyze mid-tropospheric baroclinic wave dynamics, or put another way, the upper-tropospheric support for strong dynamic pressure systems that you find in lower regions. To estimate the movement and evolution of baroclinic systems:

3.4.2.1. Speed: Use 12 and 24 hour continuity of trough, ridge, low and high positions. Are the features speeding up? Slowing down?

3.4.2.2. Direction: Troughs and lows move toward region of maximum height falls; ridges and highs move toward region of maximum height rises. Features track with direction of 300 millibar winds. Continuity is also useful for tracking direction of movement.

3.4.2.3. Temperature advection:

3.4.2.3.1. Cold air advection (CAA) => Destabilization of airmass => Height falls and increasing moisture.

3.4.2.3.2. Warm air advection (WAA) => Stabilization of airmass => Height rises and decreasing moisture.

3.4.3. 850 millibar constant-pressure chart. Dynamic lows will track toward the region of maximum warm air advection, which usually corresponds to regions of maximum height falls. Dynamic highs will track toward the region of maximum cold air advection, which usually corresponds to regions of maximum height rises. Note historical trends in the central height of lows and highs for indications of strengthening or weakening.

3.4.4. Surface chart. Dynamic lows will track toward the region of maximum warm air advection, which usually corresponds to regions of maximum pressure falls. Dynamic highs will track toward the region of maximum cold air advection, which usually corresponds to regions of maximum pressure rises. Note historical trends in the central pressure of lows and highs for indications of strengthening or weakening.

LAB:

Divide up into groups of three. Using no more than one hour of time, each group should jointly:

1. Review the pre-analyzed UA package, surface charts, and skew-t, as well as the available GOES photographs and radar summaries.

2. Using continuity and basic meteorological principles, forecast the following weather elements for Portsmouth, New Hampshire for (a) 12 hours after the latest chart valid time and (b) 24 hours after the latest chart valid time:

2.1. Surface wind speed [knots] and direction [degrees true].

2.2. 500 mb wind speed [knots] and direction [degrees true].

2.3. Surface air temperature [degrees C] and sea-level pressure [millibars].

2.4. The amount (none, partly cloudly, mostly cloudy, or overcast) and type (cumuliform or stratiform) of clouds at 850 mbs, 700 mbs, and 500 mbs.

2.5. Whether or not precipitation will be occurring.

HOMEWORK:

1. Read Lutgens and Tarbuck chapter 12.

2. Read Quayle and Steadman, 1998.

3. Read Ball, 2000, and skim Orlove et al., 2000.

4. Study notes and labs from meetings 1 through 8.