WEATHER FORECAST
1. Persistent Forecast
(1). Today rains, tomorrow also rains.
(2). Best method to forecast weather for the lead time of 6
hours or shorter.
2. Climatological Forecast
(1). Statistic average of weather elements (temperature,
pressure, rain) for a given day, month, season, or year from the
historical weather records.
A. The normal High temperature on September 10 at Northridge is
82 oF.
B. There is no rain in July and August at Northridge: 99%
accurate (it rained in 1995).
C. The average precipitation at downtown Los Angeles is 14.5Ó.
(2). Weather singularity
A. Weather event that repeats year after year on approximately
the same day or days.
B. It always rains or is overcast on November 14 to 16 over
southern California.
3. Steady State (Trend ) Forecast
The surface storm center travels in the same direction and speed
within the forecast period.
(1). Direction
A. Same direction as the previous direction
If a storm travels toward southeast today, it will continue
moving in the same direction until tomorrow.
B. Surface wind direction
If the surface wind is west wind, the storm will travel
toward east.
C. Normal path
(A). A storm will travel in the direction averaged for
30 years.
(B). Need a normal path map.
D. 500-mb contour
A storm travels in the same direction as a 500 mb contour
runs.
(2). Speed
A. Surface wind speed.
B. half of the 500-mb wind speed.
C. normal speed: 600 mpd (miles per day) for a cyclone.
D. Long-wave: sluggish speed.
4. Synoptic Forecast (Analogue Forecast)
(1). The prior weather event associated with a given synoptic
weather type (pattern) in the past serves as a guide for the forecast
of the future weather event in case the similar synoptic type occurs.
(2). Synoptic weather types
A. Classification of weather types based on the positions of
High and Low , upper-air flow, and prevailing storm track.
B. Weather elements are predicted from the weather types.
C. Example: Hawaiian Synoptic weather types.
5. Computer Weather Forecast (numerical weather prediction, NWP).
(1). Mathematical models of several equations that describe how
pressure, temperature, moisture, air density, and wind will change
with time at each grid point of different atmospheric levels.
(2). not physical models (wind tunnel, for example).
(3). NWP models do not represent the real atmosphere. They
retain the most important aspects of the atmosphere.
(4). NWP processes
A. Data observations
B. Analysis of data at grid points of different layers.
C. Initialization
D. model run (10-minute interval)
E. Use the model outputs as the initial data and run model to
obtain the next 10 minuteÕs forecast.
F. Repeat the same procedures until the desired forecast
lengths are reached, 6-hour, 12-hour, 24-hour, 36- hour,
48-hour, 72-hour, 96-hour, etc.
(5). Models
A. Barotropic model
(A). Assume no warm or cold air advection (contours are
parallel to isotherms).
(B). Grid spacing: 200 km
B. Baroclinic model
(A). There is warm or cold air advection (contours
intersect isotherms).
(B). Grid spacing: 200 km.
C. Limited-Area Fine Mesh (LFM) model
(A). Grid spacing: 130 km.
(B). A type of baroclinic model.
(C). 7 layers (Sigma coordinate).
(D). Ceased since 1994.
D. Spectrum model
(A). Operated by NMC since 1980.
(B). Describe the atmosphere using a set of equations
with wavelike characteristics rather than a set of
discrete numbers associated with grid points.
(C). Resolution is defined by number of waves used
rather than the size of grid points.
E. Nested Grid Model (NGM)
(A). 16 layers.
F. Middle Range Forecast System (MRF)
(A). 18 layers.
G. Eta Model
(A). 38 layers.
(B). Eta coordinate.
(6). Dynamic system: A dynamic system is one whose evolution from an
initial state can be described by one or more mathematical
equations.
(7). Ensemble forecasting:
A. A numerical model generates several forecasts, each based
on a slightly different set of initial conditions.
B. The forecast is consiered reliable if the several forecasts
are consistent.
(8). Model comparison:
A. Forecasts are generated by several different models.
B. The forecast is reliable when model froecasts all agree.
6. Probability Forecast
(1). PoP (probability of precipitation) forecast.
(2). 60% chance of rain:
A. Any random place in the forecast area will receive a
measurable amount of rainfall of 0.01 inches or more in either
6-hour or 12-hour interval.
B. 6 of 10 forecast rainy days have rains.
C. Confidence of forecast
(3). Derived from MOS (Model Output Statistics) package
A. Model Output
Weather elements predicted from numerical weather prediction
models (NWPs).
B. Statistics (Multiple Regression Equation)
(A). PoP as a dependent variable (predictand).
A binary predictand (rain = 1, no rain = 0)
(B). Independent variables (predictors)
a. Predicted weather elements from NWP output.
b. Analysis or observed weather elements for
example rainfall amount prior to the time (00Z or
12Z) of data input into NWP models.
c. Other variables: Cos (day), Climatological
frequency of rain at a given station, etc.
d. Dummy or binary variables having values of 0 or 1
are used. For example, relative humidity of 70% or
higher is converted to a value of 1 while that lower
than 70% is converted to a value of 0.
(C). Forecasts are based on regionalized equations and
are valid for periods ending from 12 to 60 hours after
00Z or 12Z.
(D). The most important predictors at Los Angeles are
LFM forecasts of mean relative humidity and precipitation.
7. National Centers for Environmental Prediction (NCEP)
(1). A branch of the National Weather Service (NWS) which is a
division of the NOAA (the National Oceanic and Atmospheric
Administration).
(2). Formerly called National Meteorological Center (NMC).
(3). Responsible for preparing weather maps and MOS package for
daily meteorological reports for the entire nation.
8. Weather Service Forecast Office (WSFO)
(1). Responsible for weather service of a large region (one
state, half of a large state, or two small states).
(2). California
A. San Francisco WSFO: Northern California.
B. Oxnard WSFO: Southern California.
9. Weather Service Office (WSO)
(1). Metropolitan area: Lancaster WS0.
(2). Use MOS package as a guideline for weather forecast.
10. Professional Meteorologist
Certified by the American Meteorological Society.
11. Announcer
Read weather reports issued by the NWS.