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Frequently Asked Questions (FAQ)


FAQs related to...

1. Precipitation Frequency (PF)

2. Precipitation Frequency Data Server (PFDS)

3. Probable Maximum Precipitation (PMP)

4. General Precipitation Questions



1. PRECIPITATION FREQUENCY (PF)


1.1 How do I obtain a hard copy of NOAA Atlas 14?

1.2 When will updated precipitation frequency estimates be available as part of NOAA Atlas 14 in my state?

1.3 The new precipitation frequency estimates differ from some of the other resources that I have, which are more accurate? Which should I use?

1.4 Considering the great deal uncertainty associated with computing 500- and 1000-year precipitation frequency events, why have you decided to continue publishing them?

1.5 How do you address impacts of climate change on precipitation frequency estimates?

1.6 Are the annual maximum series data that was used to calculate precipitation frequency estimates available, and if yes where can I find it?

1.7 Rainfall intensity or depth is sometimes expressed as a function of parameters such as: time of concentration, duration, frequency, etc. Where can I find these equations and related coefficients on the PFDS?

1.8 When using NOAA Atlas 2 or TP-40, how do I determine estimated depth-area-reduction factors for areas larger than 400 square miles?

1.9 When using NOAA Atlas 2, how do I determine estimated depth-area-reduction factors for durations less than 30-minutes or greater than 24-hours?



1.1 How do I obtain a hard copy of NOAA Atlas 14?

Through the President's e-gov initiative, NOAA Atlas 14 is a completely electronic document available for download from the PF Documents web page.


1.2 When will updated precipitation frequency estimates be available as part of NOAA Atlas 14 in my state?

We are currently working on precipitation frequency estimates as a part of NOAA Atlas 14 for various parts of the United States. The states that have been published and those that are in progress are shown in the opening map of the precipitation frequency data server at http://hdsc.nws.noaa.gov/hdsc/pfds/index.html. Details on progress and schedules for these projects can be found in our Quarterly Progress Report on our Homepage under "Current Projects". We believe updated precipitation frequency estimates are long overdue for the remainder of the U.S. not already included in NOAA Atlas 14. Funding for new projects come from contributions from each of the states with additional funding from NOAA's Climate Program and from The National Weather Service Office of Hydrology. If your state is not listed, then funding is not yet available for it. We have been working with the Federal Highway Administration (FHWA) using their "pooled funding" mechanism. That mechanism allows a variety of Federal, State and local agencies to pool funds for particular projects. Check the FHWA Pooled Fund page for our proposal to extend NOAA Atlas 14 to the rest of the country. The proposal divides the country into project areas for the purposes of funding, doing the work, and publication as volumes of NOAA Atlas 14. Since our biggest impediment to moving forward is the funding, we're hoping that agencies such as USACE, NRCS, FEMA, EPA, DOT/FHWA and state DOTs and DNRs will be able to provide support when they see the work broken down into smaller sized funding chunks. These agencies are often more influenced when they hear of the need directly from actual users.

Until updated estimates are available, the current standards can be found by clicking on your state of interest on the PF Documents page.

Feel free to get in touch if you want to discuss this further.

Geoff Bonnin
Chief, Hydrologic Science and Modeling Branch
Office of Hydrologic Development
NOAA's National Weather Service
301-713-0640 x103
Geoffrey.Bonnin@noaa.gov


1.3 The new precipitation frequency estimates differ from some of the other resources that I have, which are more accurate? Which should I use?

You may want to consult your client or the governing regulatory agency with regards to which precipitation frequency resource to use. Our precipitation frequency estimates have been endorsed by the Federal Advisory Committee on Water Information's (ACWI) Subcommittee on Hydrology and are de-facto national standards.


1.4 Considering the great deal uncertainty associated with computing 500- and 1000-year precipitation frequency events, why have you decided to continue publishing them?

We found that there is enough demand for the estimates for us to continue to publish them. However, we do intend to expand upon our discussion of the high uncertainties of these estimates and also provide more statistical uncertainty information. 1000-yr_responses.pdf document provides the responses to an email soliciting opinions from our users on the uses and understanding of 500-year and 1,000-year estimates.


1.5 How do you address impacts of climate change on precipitation frequency estimates?

Precipitation frequency analysis methods used in NOAA Atlas 14 volumes are based on the assumption of stationary climate. We test the assumption by applying various statistical tests to see if statistically significant trends are present in the annual maximum series (AMS) of observations used in our frequency analysis. So far, tests have shown very little observable or geographically consistent temporal change in these data. Regardless, we continue to follow pertinent research. At this stage, the impact of potential changes in climate on precipitation frequency estimates is uncertain. The differences among different climate model projections are large with respect to the expected changes in extreme precipitation frequencies and magnitudes and so there is a lot of uncertainty about these potential trends. Further research is also needed to determine how to adjust precipitation frequency estimates if that is found necessary. Currently, very limited research has been published that addresses that issue. Regardless, it is expected that traditional methods, assuming a stationary climate, will continue to be an important baseline for NOAA Atlas 14 precipitation frequency estimates.


1.6 Are the annual maximum series that were used to calculate precipitation frequency estimates available, and if yes where can I find the data?

The final quality controlled annual maximum series (AMS) data from which the precipitation frequency estimates were prepared are available for states that are included in NOAA Atlas 14 volumes from our Precipitation Frequency Data Server web site. They are posted under Time Series Data. The data also includes basic station information (latitude, longitude, elevation in feet) as well as period of record


1.7 Rainfall intensity or depth is sometimes expressed as a function of duration and frequency. Where can I find these equations and related coefficients on the PFDS?

We don't supply those equations or coefficients for use with the new NOAA Atlas 14. An alternative to summarizing the estimates using the traditional equations, is to make use of the grids of estimates available from our site. We have spatial grids of the estimates for each duration frequency pair over the NOAA 14 domain. The grids are freely available for download and are in a variety of common formats.


1.8 When using NOAA Atlas 2 or TP-40, how do I determine estimated depth-area-reduction factors for areas larger than those available?

We don't recommend extrapolation of the curves much beyond the limits. The basic assumption behind areal reduction factors is that there is dependence between the point and areal values. The correlation between point estimates reduces as they get farther apart until the values become independent and so the dependence relation between a point and an area breaks down as well.


1.9 When using NOAA Atlas 2, how do I determine estimated depth-area-reduction factors for durations less than 30-minutes or greater than 24-hours?

We recognize that there is a number of engineering applications that require estimates at shorter durations, but the data used create the curves in Figure 14 of NOAA Atlas 2 does not confidently support these very short or very long durations. Therefore, we don't recommend extrapolation of the curves much beyond the interval for which they were determined.



2. PRECIPITATION FREQUENCY DATA SERVER (PFDS)


2.1 The PFDS web site can't be found.

2.2 When I try to load a PFDS web page it says it is either missing or I am not authorized to view it. It says HTTP Error 403 Forbidden. What am I doing wrong?

2.3 The state-specific page won't load.

2.4 After downloading and uncompressing a NOAA Atlas 14 ArcInfo ASCII grid I end up with a *.ver file, but ArcGIS 9.x does not recognize it when I try converting it to an ESRI Binary Grid using the Spatial Analyst Extension. Additionally, my decompression software still recognizes the *.ver file as a compressed file, is it?

2.5 Could you help me interpret and understand the PFDS and NOAA Atlas 14 products?


2.1 The PFDS web site can't be found.

If http://hdsc.nws.noaa.gov/hdsc/pfds/ does not work, try http://dipper.nws.noaa.gov/hdsc/pfds/.


2.2 When I try to load a PFDS web page it says it is either missing or I am not authorized to view it. It says HTTP Error 403 Forbidden. What am I doing wrong?

This is usually caused by your web browser loading an old web page that has been stored in its cache memory. Try returning to the page you tried to load, and force the page to re-load by holding down the CTRL key while you reload/refresh. This will force the page in your computers cache memory to be over-written with the new web page. If the new web page still does not load or work properly, please report it to us.


2.3 The state-specific page won't load.

The page may require more cache memory to load this page than is available. All browsers have a way to clear the memory cache, so you may want to try this.


2.4 After downloading and uncompressing a NOAA Atlas 14 ArcInfo ASCII grid I end up with a *.ver file, but ArcGIS 9.x does not recognize it when I try converting it to an ESRI Binary Grid using the Spatial Analyst Extension. Additionally, my decompression software still recognizes the *.ver file as a compressed file.

Some decompression software wrongly recognizes *.ver file as a compressed file. It is ASCII text file that could be directly imported in ArcView. Older versions of Spatial Analyst recognize any file extension so file renaming is not necessary. However, before using Spatial Analyst in ArcGIS 9.x, you must rename the *.ver file to *.asc or *.txt.

2.5 Could you help me interpret and understand the PFDS and NOAA Atlas 14 products?

For help with using the Precipitation Frequency Data Server (PFDS) or with interpretation of PFDS products, please see Volume 6 or higher of NOAA Atlas 14 or send an e-mail with your question to HDSC.questions@noaa.gov.



3. PROBABLE MAXIMUM PRECIPITATION (PMP)


3.1 I do not see any information on current PMP projects in your Quarterly Progress Report. Are you currently working on updating PMP estimates anywhere in the USA?

National Weather Service has provided PMP guidance and studies since the late 1940s at the request of various federal agencies and with funding provided by those agencies. In recent years that funding has diminished and gradually ceased. As a result we are unable to continue our PMP activities. However, we will continue to provide copies of related documents on our site.



4. GENERAL PRECIPITATION QUESTIONS


4.1 Where can I find historical rainfall information?

Beyond NOAA Atlas 14 annual maximum series data provided at Time Series page, this site does not have or provide historical weather and climate information. You can obtain historical weather and climate information from:


4.2 I do not see my question listed here. Is there another related FAQ list I could access?

NOAA's National Weather Service maintains a general weather/climate FAQ list here:
http://www.nws.noaa.gov/pa/faq.php.

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Page last modified: May 16, 2013
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