2006 NWS L3MTO PFS1 3p.PDF
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NATIONAL WEATHER SERVICE
AUGUST 2006
What is the Local 3-Month Temperature Outlook (L3MTO)?
The L3MTO product provides probabilistic local forecasts of 3-month average daily mean temperatures for thirteen
consecutive 3-month periods (January-February-March (JFM), February-March-April (FMA), etc.). Corresponding
lead times are from 0.5 months to 12.5 months. The L3MTO is available for approximately ten sites in each
National Weather Service (NWS) Weather Forecast Ofice (WFO) County Warning Area (CWA), or about 1150
stations nationwide. This product extends the Climate Prediction Center’s (CPC) national 3-Month Temperature
Outlook to speciic local sites, and is released simultaneously with the national product on the 3rd Thursday of
every month.
The L3MTO is available in several formats: tables, text discussions, and graphical outputs, such as Pie Charts,
Temperature Range Graphs, and Probability of Exceedance (POE) curves. A link to an Outlook Evaluation Tool is
also included, for a statistical evaluation of forecast accuracy.
What are the Beneits of the L3MTO?
1) Provides more spatial detail, or higher resolution outlooks
2) Includes multiple presentation formats to accommodate a wide range of user needs
3) Includes helpful resources for interpretation
4) Extends the national outlook to the local level, greatly enhancing the product’s exposure
and its value for local users
Where are the L3MTO and related features found?
The L3MTO can be accessed from all WFO web pages by selecting the “Climate Prediction” tab. The local
outlooks can also be reached from
http://www.weather.gov/climate/l3mto.php
either by clicking on the area of
interest on the national map or by using the search function below the map.
Every “Local Outlook” page features a map showing the stations for which forecasts are available in the WFO
area. The L3MTO Pie Chart displayed corresponds to the station shown in red on the map. To view the forecast
data in tabular form choose “Outlook Table” on the “Select Product” menu or click on the “Outlook Table” icon.
More complex versions of the L3MTO product, the Temperature Range Graph and the Probability of Exceedance
Plot, may also be viewed by using the “Select Product” menu or by clicking on the icons. A link to the Outlook
Evaluation Tool is provided below each of the graphical products. To view the forecast for another location, select
the station either by clicking on the map or by choosing it on the “Select Location” menu.
Each L3MTO page also includes a “Background Information” tab, which provides a lowchart overview of the
product components and links to supportive text and product description documents. The “Questions and Feedback”
tab links to a user survey. User feedback is always welcome and will be the main source inluencing future product
enhancements.
NATIONAL WEATHER SERVICE
Page 2
How is the L3MTO produced?
A method of statistical downscaling is used to translate the CPC national 3-Month Temperature Outlook into Local
3-Month Temperature Outlooks for about 1150 individual stations.
How to Interpret the L3MTO products?
The L3MTO is available in three graphical formats of increasing complexity. Pie Charts are the most basic, while
the Temperature Range Graphs and Probability of Exceedance (POE) curves offer more sophisticated ways to
view the outlooks. Each of the graphical formats is associated with helpful features, such as data tables and/or
interpretive text. All of the L3MTO products were developed using climate data from a 30-year reference period,
1971-2000.
L3MTO Pie Chart
The L3MTO Pie Chart is the simplest of the
product formats. It shows the expected chance for
the 3-month temperature to occur in each of three
categories: Above Normal, Near Normal, and Below
Normal. The larger the pie slice, the higher the
chance of occurrence. A legend, located to the right of
the pie chart, has three corresponding rectangles that
show the actual 1971-2000 temperature values used to
dei ne the forecast categories.
L3MTO Pie Chart
L3MTO Temperature Range Graph
The Temperature Range Graph shows the expected range of
the average 3-month temperature (in degrees Fahrenheit on
the y-axis) for each of the thirteen 3-month forecast periods
(x-axis). The expected temperature ranges can be viewed
for i ve coni dence intervals (or levels of expected chance):
99%, 95%, 90%, 75%, and 50%
L3MTO Temperature Range Graph
NATIONAL WEATHER SERVICE
Page 3
L3MTO Probability of Exceedance
The Probability of Exceedance (POE) Graph
provides the most detailed outlook information in
the L3MTO suite. It shows the expected chance
(y-axis, in percent) that the average 3-month daily
mean temperature will exceed (or be greater than) the
temperatures shown on the x-axis.
The graph contains two curves: a red curve,
representing the POE for the 1971-2000 reference
period, and a green curve, representing the forecasted
POE. The colored vertical lines represent the
observed average 3-month daily mean temperatures
for the past i ve years.
L3MTO Probability of Exceedance
Special Case: When There is No Forecast
Under certain conditions, the methods and tools employed in the forecast process cannot produce a reliable outlook
product. In these cases, there is effectively no forecast, and the “outlook” coincides with the climatological
information from the 1971-2000 reference period. For the Pie Chart, this case is indicated when all three slices
of the pie are shown to be about the same size. For the POE graph, the red and green curves overlap, and for the
Temperature Range Graph, the values shown are the same as those for the reference period. (This is analogous to
“EC” (equal chance) on the CPC national 3-Month Temperature Outlook map.) In all such cases, it is best to use
the 1971-2000 reference period climatology for guidance.
Where to i nd more information on the L3MTO?
Please contact your local Weather Forecast Ofi ce for more information on the L3MTO product and its
interpretation.
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