2006 NWS L3MTO PFS1 3p.PDF

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.

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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

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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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