GUIDE TO THE REVISED DIGITAL FORECAST (RDF)



INTRODUCTION

The National Weather Service is now providing detailed digital forecast data known as the Revised Digital Forecast (RDF). The RDF will be routinely issued around 4 am and 4 pm local time, with updates issued as needed. This guide will explain how to read and interpret the RDF with an example of a complete RDF included.

WHAT IS THE REVISED DIGITAL FORECAST?

The Revised Digital Forecast (RDF) displays forecasted weather parameters in 3-hour and 12-hour intervals. These intervals, combined with a matrix format, create a detailed forecast, allowing quick procurement of various weather parameter(s) of choice. Currently, the RDF provides 3-hourly and 12-hourly forecasts up to 60 hours into the future. An extended portion of the RDF provides 12-hourly forecasts up to 5 days into the future. The RDF is available to disseminators of National Weather Service products and will also be available on the internet.

HOW TO READ AND INTERPRET THE RDF PRODUCT

An example of the RDF product is displayed below in Table 1. There are several forecast parameters which appear in the RDF. Some of these values are displayed in 12 hour intervals while others are forecasted in 3 hour intervals. At the top of each RDF product is the local time and date that the RDF was issued. In the case of Table 1, the forecast was issued on Saturday April 15th, and the time of issuance was 4:06 p.m. Eastern Daylight Time (EDT). Below the time and date, a forecast timeline is provided in 3 hour increments for 60 hours (2 and one half days) into the future. Listed on the far left of this timeline is the local time zone that is used in the RDF. In this example, EDT is listed, which means the hours in the timeline are in Eastern Daylight Time. The forecast timeline is then broken down into 6 hour increments for 3 to 5 days into the future. Reference dates are located above each timeline.

Table 1. The Revised Digital Forecast.

406 PM EDT SAT APR 15 2000

                         SUN  04/16/00          MON  04/17/00     
EDT        15 18 21 00 03 06 09 12 15 18 21 00 03 06 09 12 15 18 21 00 03 06

POP 12HR                30          20          30          60          60
QPF 12HR           .01-.10           0     .01-.10     .10-.24           0
MAX QPF            .01-.10           0     .01-.10     .10-.24           0
SNOW 12HR            00-00       00-00       00-00
MN/MX                   51          76          54          70          53
TEMP     67 68 65 61 56 53 52 60 69 73 71 65 60 57 56 61 66 67
DEWPT    52 52 52 53 52 51 51 53 53 52 51 51 51 51 51 52 51 50
RH       59 57 63 75 86 93 96 78 57 48 49 60 72 80 83 72 58 54
WIND DIR  S SW SW SW  S  S -- SW  W  W  S SE SE SE  S  S SW  W
WIND SPD  2  2  2  2  2  2  0  2  5  5  2  2  2  2  5  8  8  8  5  5  5  5
CLOUDS   B2 OV OV OV B2 B2 B2 B1 B1 B1 B1 B1 B1 B1 B2 B2 B2 B2 B2 B2 B2 B2
RAIN SHWRS   C  C  C                          C  C  L  L  L  L  L  L  L  L
TSTMS                                               C  C  C  C

             \TUE 04/18/00\WED 04/19/00\THU 04/20/00
EDT        09 15 21 03 09  15 21 03 09  15 21 03 09

POP 12HR     40    30     20    20     30    30
MN/MX        68    54     76    55     79    54
CLOUDS       B2    B2     B1    B1     B2    B2
RAIN SHWRS    C     C                   C     C
TSTMS                                   C     C


RDF FORECAST PARAMETERS

1) POP 12HR - This parameter is forecasted for all 5 days of the RDF. The probability of precipitation is for a 12-hour period ending at 6:00 a.m. or 6:00 p.m. Local Time. This percentage probability is listed towards the ending time of each period out to 60 hours, then in the middle of each time period for days 3 through 5. In the example from Table 1, there is an 30% probability of precipitation between 6:00 p.m. EDT 4/15/00 and 6:00 a.m. EDT 4/16/00. Also from Table 1, there is a 60% probability of precipitation between 6:00 a.m. EDT and 6:00 p.m. EDT.

2) QPF 12HR - This parameter is forecasted out to 48 hours in the RDF. A quantitative precipitation forecast, QPF 12HR lists the average precipitation expected if precipitation falls. QPF 12HR is listed under POP 12HR, and is forecasted in 12 hour periods ending at 6:00 a.m. or 6:00 p.m. Local Time. QPF 12HR is located towards the ending time of each 12 hour period, and amounts are given in a range. For example, in Table 1, the QPF 12HR between 6:00 a.m. and 6:00 p.m. Monday 4/17/00 is 0.10 to 0.24 inch.

3) MAX QPF - The maximum amount of precipitation, in inches, for the forecast area during a 12-hour period ending at 6:00 a.m. or 6:00 p.m. Local Time. This amount is given in a range and can be identical to QPF 12HR in certain instances. MAX QPF is available up to 60 hours into the future, and is located towards the ending time of each 12 hour period for which it is forecast.

4) SNOW 12HR - The expected range of snowfall accumulation (in whole inches) during a 12-hour period ending at 6:00 a.m. or 6:00 p.m. Local Time. SNOW 12HR is available out to 36 hours, located near the ending time of each 12 hour period for which it is forecast.

5) MN/MX - An average minimum or maximum temperature during the nighttime or daytime hours, respectively. MN/MX is forecasted out to 5 days. MN/MX is located near the ending time of each 12 hour period for which it is forecasted out to 60 hours, then near the middle of each 12 hour period out to day 5. The maximum temperatures are forecasted from 7:00 a.m. to 7:00 p.m. Local Time. Minimum temperatures are forecasted from 7:00 p.m. to 7:00 a.m. Local Time, but on occasion the low temperature for the night may occur after 7:00 a.m. For example, in Table 1, a low of 51 degrees is forecasted between 7:00 p.m. 4/15/00 and 7:00 a.m. EDT 4/16/00, while the high temperature between 7:00 a.m. and 7:00 p.m. EDT Sunday April 16th is 76 degrees.

6) TEMP - The expected temperature at a specified time, in degrees Farenheit. Note that the forecasted temperature does not line up exactly with the timeline towards the top of the RDF. The temperature is forecasted in 3 hour intervals, for the earlier in-between hour listed in the RDF timeline. For example, in Table 1 for the time period between 6:00 a.m. and 9:00 a.m. (06 and 09) Sunday 4/16/00, the specified forecast temperature is for 7 a.m. EDT Sunday, at 52 degrees F. Also for 4/16/00, the forecast temperature for the time period between the hours of 3:00 and 6:00 p.m. EDT (15 and 18), is 73 degrees at 4 p.m EDT. TEMP is forecasted out to 48 hours.

7) DEWPT - The expected dewpoint temperature forecasted out to 48 hours.

8) RH - The relative humidity based on the expected temperature and dewpoint. RH is forecasted out to 48 hours.

9) WIND DIR - The expected wind direction during a three-hour time period in between two times listed on the timeline, using the 8 points of a compass (e.g., W, NW, N. . . etc.). For example, in Table 1, the forecast prevailing wind direction forecast between 9:00 and 12 noon EDT Sunday would be from the Southwest. Between 6:00 and 9:00 a.m. Sunday, the dashes (- -) represent no wind direction forecast due to a calm wind. WIND DIR is available out to 48 hours into the future.

10) WIND SPD - The expected average wind speed in miles per hour, during each three hour time period up to 60 hours into the future. For example, in Table 1, the forecast wind speed between 6 and 9 a.m. EDT (06 to 09) Monday is 5 mph.

11) CLOUDS - The expected cloud cover during each 3-hour time period, forecast out to 60 hours. Cloud cover is then forecast in 12 hour time periods out to day 5. The contractions used and their meanings are as follows (in tenths of cloud cover):

        CL - 0 Clear (Sunny), SC 1-4 Scattered (Mostly Sunny), B1 5-7 Broken (Partly Cloudy), B2 8-9 Broken 
        (Mostly Cloudy or Considerable Cloudiness), OV 10 Overcast (Cloudy)

12) PRECIPITATION - The RDF may list several types of precipitation. Precipitation types are only shown in the RDF if they are forecast to occur, and are listed in the far left column of the RDF under CLOUDS.

For each type of precipitation that is forecast, probability of precipitation is specified for 3-hour time periods out to 60 hours, then in 12 hour increments ending at 6 a.m. or 6 p.m. Local Time out to 5 days. For example, from Table 1, there is a chance of thunderstorms (TSTMS) between 9:00 a.m. and noon on Monday, April 17th. Rain showers are likely for that same time period. Between 6:00 and 9:00 a.m. Sunday, no precipitation is forecast, so no probability contraction is listed for either RAIN SHWRS or TSTMS under that time period. The types of precipitation that may be forecast in the RDF are listed below.

RAIN - Rain
RAIN SHWRS - Rain showers
TSTMS - Thunderstorms
DRIZZLE - Drizzle
SNOW - Snow
SNOW SHWRS - Snow showers
FLURRIES - Flurries
SLEET - Sleet (ice pellets)
FRZNG RAIN - Freezing rain
FRZNG DRZL - Freezing drizzle

The probability contractions and their meanings are as follows:

S - Slight Chance (20%), C - Chance (30%-50%), L - Likely (60%-70%), O - Occasional (80%-100%), D - Definite (80%-100%)

The RDF shown in Table 1 forecasts rain showers and thunderstorms during at least one time period out to 60 hours. Therefore, these 2 precipitation types are listed on the left hand column of the RDF. Freezing drizzle was not forecast for any time period, so it does not appear on the RDF example.