Decoding the Point Forecast Matrices
An increasing number of National Weather Service offices are now providing detailed digital forecast data products known as the Point Forecast Matrices (PFM) and the Area Forecast Matrices (AFM). This document will explain how to read and interpret the PFM. An example of a complete PFM is also included.
WHAT IS THE POINT FORECAST MATRICES?
The Point Forecast Matrices (PFM) displays various forecasted weather parameters at a specific point or geographic location in 3-hour and 6-hour intervals. These intervals combined with a matrix format create a detailed forecast, allowing quick procurement of forecast parameters. The PFM is available to disseminators of National Weather Service products,
Currently, the PFM provides 3-hourly and 12-hourly forecasts up to 60 hours into the future. An extended portion of the PFM provides 6-hourly and 12-hourly forecasts 7 days into the future.
HOW TO READ AND INTERPRET THE PFM PRODUCT
An example of the PFM product is displayed below in Table 1. There are several forecast parameters which appear in the PFM. At the top of each product is the time and date that the PFM was issued. In the case of Table 1, the forecast was issued on Monday, March 24th, and the issuance time was 9:40 a.m. Central Standard Time (CST). Below the time and date, two forecast time lines are provided in 3 hour increments for 60 hours (2 and one half days) into the future. Listed on the far left of the first time line is UTC (the abbreviation for Universal Coordinated Time). Below that line and more familiar to most users, the local time zone that the PFM is created in is displayed. In this example, CST is listed, which means the hours in the time line are in Central Standard Time. Note that in the bottom segment of the PFM, there are similar time lines displayed. This forecast time line is broken down into 6 hour increments, providing an extended forecast 3 to 7 days into the future. Reference dates are located above each time line.
940 AM CST MON MAR 24 2003Table 1. Sample PFM Product Output
PFM FORECAST PARAMETERS
Below are forecast parameters in the PFM.
1) MX/MN - A forecast of maximum or minimum temperatures during the daytime or nighttime hours, respectively. The maximum temperatures are forecast from 6:00 a.m. to 6:00 p.m. Local Time. Minimum temperatures are forecast from 6:00 p.m. to 6:00 a.m. Local Time. The overnight lows and daytime highs are displayed as a single number for the specific point or geographic location.
MX/MN is forecast out to 7 days. MX/MN is located near the ending time of each 12 hour period forecasted out to day 7.
2) TEMP - The temperature is forecast in 3 hour intervals. It is an average temperature within the specific geographic location for the time in question. The TEMP is generally forecasted 48 hours into the future.
5) WIND DIR - The expected wind direction forecast for the corresponding time using the 8 points of a compass (e.g., W, NW, N. . . etc.). WIND DIR is available out to 48 hours into the future.
6) WIND SPD - The expected average wind speed in miles per hour for the time in question.
SC - Partly Cloudy
BK - Mostly Cloudy OV - Cloudy
8) POP 12HR- This parameter is forecast for all 7 days of the PFM. 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 day 7.
9)QPF 12HR- This parameter is forecast out to 60 hours in the PFM. A quantitative precipitation forecast, QPF 12HR lists the average precipitation expected if precipitation falls. QPF 12HR is listed under POP 12HR, and is forecast 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.
10) SNOW 12HR- Seasonal element. 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.
11) PRECIPITATION- The PFM may list several types of precipitation. Precipitation types are only shown in the PFM if they are forecast to occur at any point in the 7 day forecast, and are listed in the far left column of the RDF underneath CLOUDS.
For each type of precipitation that is forecast, a probability of precipitation is specified for 3-hour time periods out to 60 hours, then in 6 hour increments ending at 6 a.m. or 6 p.m. Local Time out to 7 days. The types of precipitation that may be forecast in the PFM are listed below.
RAIN SHWRS- Rain showers
SNOW SHWRS- Snow showers
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%)
12) OBVIS- An obstruction to visibility. If this is forecast at any point out to 48 hours, a row titled OBVIS will be listed underneath any forecast precipitation. If no precipitation is forecast, then OBVIS will be listed under the row titled QPF 12HR. OBVIS is forecasted in 3 hour intervals. The contractions used and their meanings are as follows:
F - Fog PF - Patchy Fog F+ - Dense Fog PF+ - Patchy Dense Fog
H - Haze BS - Blowing Snow K - Smoke BD - Blowing Dust
When the Wind Chill Index is forecast to be 20 degrees F or lower with a 5 mph or greater wind speed, a row titled WIND CHILL will be listed under any forecast precipitation types, OBVIS or QPF 12HR. The Wind Chill Index is forecast out to 48 hours.