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Frequently Asked Questions about WPC Surface Analyses


  1. What changes have been made to the WPC North American analysis?

  2. Why has WPC reduced the size of the North American surface analysis area?

  3. How much data over the oceans does the analyst have available for the preparation of the North American Surface analysis?

  4. What if I need an analysis covering an area larger than the North American chart?

  5. Why are surface analyses prepared at three different centers?

  6. How do the three NCEP analysis centers coordinate their analyses?

  7. What are the areas of responsibilities for the three analysis centers?

  8. How are the analyses prepared at the three centers combined to make the analysis products available on the web?

  9. What surface analyses are available from NCEP?

  10. Why are the OPC and NHC analyses issued so much later than the WPC analysis?

  11. Why does the WPC issue an analysis every three hours while the OPC and NHC issue analyses only every six hours?

  12. Where can I find the surface analyses from the three NCEP centers?

  13. Why do the analyses from the various centers look different?

  14. How can I contact the Weather Prediction Center?


What changes have been made to the WPC North American analysis?
There are two changes to the North American analysis.  The domain of the chart has been reduced by eliminating a portion of the analysis over the Atlantic and Pacific Oceans. The analysis over this smaller domain is issued approximately 15-30 minutes earlier than analyses prepared before June 3, 2003. These changes only apply to the North American analysis.  The other charts available from the WPC web site, including the CONUS area charts have not changed.

Why has WPC reduced the size of the North American surface analysis area?
The previous, larger domain for the North American surface analysis was designed for a time when the chart was distributed by facsimile and only one chart could be sent for each analysis time. Most users of NWS products were able to receive only the WPC analysis and it needed to cover a wide area to meet the needs of all users.  The analysis domain and issuance time were a compromise between the need for a wide area of coverage and a timely analysis. NCEP now has three centers preparing surface analysis charts over much of the Northern Hemisphere and all are available to all users.   The oceanic areas eliminated from the WPC analysis are better served by the Ocean Prediction Center (OPC) and the National Hurricane Center (NHC).  Their later deadlines allow more observational data to be incorporated into OPCs' and NHCs' analyses, resulting in a more accurate product.

How much data over the oceans does the analyst have available for the preparation of the North American surface analysis?
The WPC analyst relies heavily on satellite imagery for the placement of fronts and pressure centers over the oceans.  Very few ship observations are available by the time the WPC North American surface analysis is issued.  The National Hurricane Center and the Marine Prediction Center provide later analyses that take into account a fuller set of observations.

What if I need an analysis covering an area larger than the North American chart?
NCEP plans to provide a composite analysis that covers the entire domain of the analyses produced by the three NCEP centers producing surface analyses.

Why are surface analyses prepared at three different centers?
Each of the three NCEP Service Centers producing a surface analysis has expertise in a different area.  The Ocean Prediction Center (OPC) concentrates on marine weather, the National Hurricane Center (NHC) is the source of expertise in tropical weather, and the Weather Prediction Center (WPC) focuses on weather over North America.  Each center brings its unique skills to the problem of surface analysis.  The preparation of the surface analysis is an important part of the forecast process.  Each center prepares the surface analysis over its own area of responsibility as the first step in the forecast process.

How do the three NCEP analysis centers coordinate their analyses?
Until recently, each NCEP center prepared its own surface analysis independent of the other centers.  Now that each center prepares its analysis on a computer workstation, they can exchange files and each center can see the analyses prepared by the other centers before they are released.  If centers disagree on a feature, they can coordinate via chat software or telephone.  Through this collaboration process, the three centers arrive at a consistent analysis.

What are the areas of responsibilities for the three analysis centers?
The Weather Prediction Center is responsible for the analysis over North America north of 30N.  The NHC is responsible for the area south of 30N including oceans and land areas.  The Ocean Prediction Center is responsible for the Atlantic and Pacific Oceans north of 30N.

How are the analyses prepared at the three centers combined to make the analysis products available on the web?
After the three centers have coordinated, each sends their completed analysis to the other two centers.  Each center then electronically “clips” the analyses from the other two centers into its analysis.  A meteorologist then manually blends the analyses along the borders to ensure a smooth fit.  This process is designed so that each center brings its particular expertise to the analysis process and each center can issue an analysis over an area beyond its area of responsibility without any duplication of effort.

What surface analyses are available from NCEP?
The following surface analysis charts are available from NCEP:
Time after synoptic hours (e.g. 00 UTC) Surface Analysis Chart
1 hour WPC Operational North American analysis (every 3 hours)
2 1/2 hours NHC Operational Tropical analysis (every 6 hours)
3 hours MPC Operational Marine analysis (every 6 hours)
3 1/2 hours NCEP Final analysis (every 6 hours)

Why are the OPC and NHC analyses issued so much later than the WPC analysis?
The WPC analysis area is mostly over land and is covered by a dense network of observations available within minutes of observation time. These observations are augmented by radar and satellite imagery.  The analyst can begin the analysis within minutes of the observation time.

The Ocean Prediction Center and the National Hurricane Center cover large areas of ocean.  There are much fewer observations and it may take several hours for these observation to reach NCEP.  Furthermore, these observations are taken by volunteer observers often under difficult conditions and are subject to quality control.  There is no radar imagery over most of these ocean areas.  Satellite data is used extensively over the oceans, but some of the processed satellite data is  not available until hours after observation time. For these reasons, the analysts cannot begin their analyses over the oceans until much later than the WPC analysts.

Why does the WPC issue an analysis every three hours while the OPC and NHC issue analyses only every six hours?
The dense network of observations, radar, and satellite imagery that form the basis for the Weather Prediction Center (WPC) analyses are available every hour.  Observations are much more limited over the oceans, where, by international agreement, observations are only required to be taken every six hours.

Because of the rich set of observational data over North America, the WPC can analyze relatively small scale features, such as dry lines and outflow boundaries.  In order to track these meso-scale features, we need to have frequent analyses.  The data over the oceans do not support that kind of detail.  The analyses prepared by the National Hurricane Center and the Ocean Prediction Center depict primarily synoptic scale systems that are adequately tracked at six hour intervals.

Where can I find the surface analyses from the three  NCEP centers?
Analyses over North America from the Weather Prediction Center:
/html/sfc2.shtml

Analyses over the Tropics from the National Hurricane Center:
http://www.nhc.noaa.gov/forecast.shtml

Analyses from the Atlantic and Pacific Oceans from the Ocean prediction Center:
http://www.mpc.ncep.noaa.gov

Why do the analyses from the various centers look different?
Each center developed their surface analysis for a different set of customers.  While the three NCEP centers collaborate to produce one NCEP analysis, the depiction of these weather features will vary from center to center.

The Weather Prediction Center produces a detailed analysis over a data rich area.  Most users receive the analysis in electronic format.  For these reasons, the WPC analysis is more detailed than those produced by the other two centers and the graphics use finer lines.

The National Hurricane Center (NHC) and the Ocean Prediction Center (OPC) provide analyses of synoptic scale features over areas with less data.  Their analyses are distributed primarily by lower resolution radio facsimile which dictates thicker lines and less detail. The NHC and OPC also include forecast positions for some weather features on their analyses as an aid to shipping.

How can I contact the Weather Prediction Center?
For questions or comments regarding our web site, contact our webmasters.
Click here if you have questions about our analysis or forecast products.
 


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Page last modified: Tuesday, 05-Mar-2013 13:08:42 UTC