Crawfish Tales

A Quarterly Publication of the National Weather Service Lower Mississippi River Forecast Center
Slidell, Louisiana

David Reed, Hydrologist In Charge Vol. 2 No. 4, Summer 1999 Ethan A. Jolly, Editor


Features Precipitation Data from ASOS QPF/MAP Verification AWIPS Update USGS Rating Curve Project


From the HIC

Hurricane season has been quiet so far and we hope that trend continues. During this time, the NWS and LMRFC are preparing to enter the new millennium. The NWS has completed the installation of all AWIPS systems and expects to begin commissioning them starting in January, 2000. LMRFC expects to perform all operations on AWIPS and use AWIPS exclusively for forecast operations in October of this year. Along with this effort, LMRFC has continued to expand forecast services and provide timely and accurate forecasts to our users.
- Dave Reed


Precipitation Data from ASOS

In the mid 1980's, the NWS, the Federal Aviation Administration (FAA), and the Department of the Navy entered an agreement for the development, production, and deployment of Automated Surface Observing Systems (ASOS). In 1994, the U.S. Air Force joined the program.

As of early July, 1999, over 800 ASOS have been commissioned across the United States. The primary function of ASOS is to provide minute by minute observations, and generate Aviation Routine Weather Reports (METAR) and Aviation Selected Special Weather Reports(SPECI). One of the main functions of ASOS is to measure precipitation. Accurate liquid-equivalent precipitation measurements are essential for hydrological, flood forecasting, and agricultural interest.

ASOS uses a pivoting dual chamber tipping bucket to measure precipitation. The bucket tips when one chamber is filled with .01 inch of liquid precipitation. ASOS Tipping BucketThe tipping bucket has a precipitation accumulation range of 0 to 10.0 inches per hour, a resolution of .01 inch, and a accuracy of +-.02 inch or 4% of the hourly total (whichever is greater). Data are measured and recorded for each minute.

METAR is the primary observation code used in the United States for reporting surface meteorological data. Coded and/or plain language information which elaborates on the data of the report may be appended. SPECI are unscheduled reports which are taken between hourly METAR observations when certain criteria have been observed. SPECI contain all the data found in a METAR plus additional plain language information which elaborates on the data in the body of the report. Within the NWS the message is identified by their MTR AFOS/AWIPS header (CCCMTRXXX). CCC is the originating node, MTR is the designation for the surface report, and XXX is the ASOS location identifier. The precipitation accumulation algorithm uses each one minute precipitation to generate cumulative precipitation remarks/message in METAR code. These include: (1) METAR hourly message remark (Prrrr):the amount of liquid equivalent precipitation, in hundredths of an inch, which has occurred since the last scheduled hourly METAR message (ex. P0018). If any one minute accumulations are missing or the sensor is inoperative, then the remark is omitted and "PNO" is appended to the remarks section. If there has been no precipitation since the last hourly METAR observation, the remark will not be reported. If only a trace of precipitation has occurred, the remark will be reported as "P0000". (2) METAR 3 and 6 hourly reports (6RRRR): the amount of precipitation which has accumulated in the last 3 or 6 hours. Three

hourly reports are reported in the 03, 09, 15, and 21 UTC METAR reports. Six hourly amounts are reported in the 00, 06, 12, and 18 UTC METAR reports. If any of the hourly precipitation remarks are missing, the 6RRRR remark is encoded as 6////. (3) METAR-24 Hour reports (7RRRR):the amounts of liquid precipitation which has occurred over the last 24 hours. This remark is reported in the 12 UTC METAR observation as long as there has been at least .01 inch of precipitation. The precipitation accumulation reported in the 7RRRR is a running total of all the hourlies, so if any of the hourlies are missing the 7RRRR is reported as 7////. (4) SPECI precipitation remark (Prrrr): cumulative liquid precipitation amount since the last hourly METAR observation.

Two SHEF messages are generated by ASOS. One is a 15 minute precipitation criteria message generated when the current 15 minute period accumulation exceeds the local threshold, and continues until the accumulation for the current interval falls below the termination threshold. The precipitation accumulations are for the four most recent 15-minute periods (ending at H00, H+15, H+30, and H+45). For NWS sites, the message is disseminated in a AFOS/AWIPS product CCCRR6XXX in ".E" SHEF format. These products go through the NWS network to the NWS Telecommunications Gateway (NWSTG) and the System Monitoring and Coordination Center (SMCC) where they are distributed to other NWS offices. The data is compiled on the daily Service Records Retention System (SRRS) tape for the National Climate Data Center (NCDC). If any 1-minute precipitation calculations are missing, the SHEF 15 minute message will report missing "M" for the entire 15- minute period. The second SHEF message is a hourly precipitation ending at H+00 in ".A" SHEF format and are distributed in the same manner as the SHEF 15-minute product. Within the NWS the message is identified by their RR7 AFOS/AWIPS header (CCCRR7XXX). If any 1-minute accumulations are missing, the SHEF Hourly Precipitation message will report missing (PPH M). All SHEF data from the FAA sites are captured at the FAA WMSC (WMSCR in the future) and passed across the Gateway to the NWS as collectives. The collectives are identified by the header NMCRR7NKA and contain both types of messages (RR6, RR7).

All ASOS locations are able to generate and transmit daily and monthly weather summaries, which includes precipitation data, but only selected NWS ASOS locations transmits the data through the network.

Some limitations of the ASOS tipping bucket has been found. During high rainfall events the tipping bucket cannot keep up with the water flow, which results in under-reporting accumulations. ASOS software corrects for the bias to under report precipitation amounts, but during extreme heavy rainfall events (> 10 inches per hour), the tipping bucket may still under report the total rainfall. During freezing conditions, the use of heat to melt snow and prevent gage icing can result in evaporation or sublimation, especially during light freezing rain or snow events around 32 F.

References
- ASOS User Guide
- ASOS Implementation Status
(http://www.nws.noaa.gov/asos/implst.html)


QPF/MAP Verification

In the mid 90's, Weather Forecast Offices (WFO)s started generating Quantitative Precipitation Forecasts (QPF)s for the LMRFC. The 24-hour QPF was prepared with each mornings forecast package for individual river sub-basins from 12 UTC to 12 UTC in 4 to 6 hour increments. To help determine the accuracy of the QPF, the Hydrometeorological Analysis and Support (HAS) group at the LMRFC developed a verification program. The verification program uses one month of archived QPF from each individual WFO. The program compares the daily WFO QPF to the Mean Areal Precipitation (MAP) generated by the NWS River Forecast System (NWSRFS). The MAP is computed by using precipitation gages located near and in the river basin using a Thiessen method within NWSRFS.

The verification program generates statistical information for each WFO's river sub-basin. The statistics used in the program are: root mean

square error, standard deviation, skewness, and wet/dry bias. Once the monthly statistics are generated, a copy of the results are sent by mail to each WFO QPF focal point for their review.

Initially, the verification program was setup to run statistics on QPF issued once per day. QPF forecast are now generated twice a day with updates as needed. Future enhancements to the verification program will include using both issuances of the QPF for verification.

MAP has also evolved since the inception of the verification program. MAP can now be derived from precipitation estimates from the WSR-88D radar. The areal precipitation estimates from the radar, referred to as MAPX, could possibly provide better estimates of mean precipitation over an area. Future enhancements to the verification program will allow for using MAP values generated by both the Thiessen method and the MAPX radar grid method.


AWIPS Update

Late this summer or early fall, AWIPS Build 4.2 will be installed at the LMRFC. This build will bring us one step closer to making AWIPS the "official" system of the LMRFC. This build will give us limited functionality in issuing our local products across the AWIPS local network which is one of the first steps needed to eliminating AFOS. AFOS is our 25 year old communication system.

A component of this build will be the upgrading of LDAD's (Local Data Acquisition and Dissemination). The LDAD server is the sole local external link to and from the AWIPS system at the LMRFC. With LDAD we will be able to interrogate automated river/rainfall gauges and store the data on our AWIPS database. We will also be able to distribute products to our external users using this system, which eventually will be the only wayof communication to them, with the elimination of AFOS.

As of May 1999, all local weather service offices in the LMRFC's area of responsibility have had AWIPS installed. With AWIPS, all offices will have the same database format as the LMRFC. This will allow for easy

AWIPS Workstation
information exchanges between weather service offices and the LMRFC of river/rainfall/station gauge information. Also all offices will have one universal robust SHEF decoder. This will eliminate any deficiencies of previous decoders and allow offices to decode all valid SHEF encoded information.

At the end of July 1999 all offices in the National Weather Service will have had AWIPS installed at their site. Official commissioning of AWIPS nationally is expected to begin in January 2000.


USGS Rating Curve Project

The USGS is teaming up with the LMRFC to implement a program which would enable the NWS's River Forecast Centers to update their rating curves (stage-discharge relationships) semi-automatically on weekly basis. This will ensure that the RFCs are using the most current rating curve in order to accurately simulate real world conditions.

Although this project is still in the development stages, the basics are as follows:
1. Each week the USGS in each state kicks off a process which extracts the latest expanded rating tables and primstat output for all modeled points in the specific RFCs area. The primstat out put consists of datum corrections to account for gage house shifting and three point shifts to account for shifting and settling of the stream bed. This

information is then placed into a public FTP directory for the RFC to download.

2. Once downloaded, the RFC will run a program that will apply the datum corrections and three point shifts to the expanded rating table to come up with the latest stage-discharge relationship. This new stage-discharge relationship will pop up in a editor window along with the current stage-discharge relationship so the Hydrologist can compare the two for quality control. Once the Hydrologist is satisfied, the new stage-discharge relationship will be automatically placed into NWSRFS for use in the RFCs model.

The LMRFC is currently working with the USGS in Missouri as a test. Once the test phase is complete, the process will be expanded to all of the USGS offices that the LMRFC works with.


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