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 AFM. An example of a complete AFM is also included.
WHAT IS THE POINT FORECAST MATRICES?
The Area Forecast Matrices (AFM) displays various forecasted weather parameters for a set of specific zones/counties 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 AFM is available to disseminators of National Weather Service products, and is available on the internet.
Currently, the AFM provides 3-hourly and 12-hourly forecasts up to 60 hours into the future. An extended portion of the AFM provides 6-hourly and 12-hourly forecasts 7 days into the future.
HOW TO READ AND INTERPRET THE AFM PRODUCT
An example of the AFM product is displayed below in Table 1. There are several forecast parameters which appear in the AFM. At the top of each product is the time and date that the AFM 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 AFM 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 AFM, 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.
Table 1. Sample AFM Product Output
AFM FORECAST PARAMETERS
Below are forecast parameters in the AFM.
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. This number is an average over the zone grouping in question.
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 zone group in question. The TEMP is generally forecasted 48 hours into the future.
3) DEWPT - The expected dewpoint temperature forecast for the same time period as its corresponding forecast temperature, forecast out to 48 hours.
4) RH - The relative humidity is based on the expected temperature and dewpoint. The RH is forecast for the same time as its corresponding temperature, and is forecast out to 48 hours.
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.
7) CLOUDS - The expected cloud cover forecast for every 3-hour time period out to 60 hours. Cloud cover is then forecast in 6 hour time periods out to day 7. The contractions used and their meanings are as follows:
940 AM CST MON MAR 24 2003
DATE MON 03/24/03 TUE 03/25/03 WED 03/26/03
UTC 3HRLY 09 12 15 18 21 00 03 06 09 12 15 18 21 00 03 06 09 12 15 18 21 00
CST 3HRLY 03 06 09 12 15 18 21 00 03 06 09 12 15 18 21 00 03 06 09 12 15 18
MX/MN 77 50 76 50 63
TEMP 57 66 76 74 67 60 53 51 59 67 75 73 67 61 55 50
DEWPT 50 48 48 49 49 49 49 50 51 51 50 50 50 51 52 49
RH 77 52 37 41 52 66 86 96 74 56 41 44 54 69 89 96
WIND DIR S SW SW S S S S S S SW S SW SW W NW NW
WIND SPD 3 5 6 6 6 6 5 6 10 14 14 11 8 5 6 8
CLOUDS FW FW FW FW FW FW FW FW SC SC BK BK BK BK BK BK BK SC SC SC
POP 12HR 0 0 10 60 40
QPF 12HR 0 0 0 0.38-0.77 0.01-0.15
SNOW 12HR MM MM MM
RAIN SHWRS L L C C
TSTMS L L C C
DATE THU 03/27/03 FRI 03/28/03 SAT 03/29/03 SUN 03/30/03
UTC 6HRLY 06 12 18 00 06 12 18 00 06 12 18 00 06 12 18 00 06 12
CST 6HRLY 00 06 12 18 00 06 12 18 00 06 12 18 00 06 12 18 00 06
MN/MX 41 69 50 72 48 58 35 59 35
CLOUDS SC FW FW FW FW SC SC BK BK BK BK BK BK SC SC SC SC SC
POP 12HR 0 0 0 0 40 50 30 0 0
RAIN SHWRS C C C
TSTMS C C C
RAIN C C
CL - Clear FW - Mostly Clear SC - Partly Cloudy
BK - Mostly Cloudy OV - Cloudy
8) POP 12HR- This parameter is forecast for all 7 days of the AFM. 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 AFM. A quantitative precipitation forecast, QPF 12HR lists the range of precipitation expected within the zone grouping 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 AFM may list several types of precipitation. Precipitation types are only shown in the AFM if they are forecast to occur at any point in the 7 day forecast, and are listed in the far left column of the AFM 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 AFM 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%) IS- Isolated (< 20%)
C - Chance (30%-50%) SC- Scattered (30%-50%)
L - Likely (60%-70%) NM - Numerous (60%-70%)
O - Occasional (80%-100%) D - Definite (80%-100%)
WP- Widespread (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
13) WIND CHILL and HEAT INDEX
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.
When the heat index is forecast to be 90 degrees F or higher, a row titled HEAT INDEX will be listed under any forecast precipitation types. Like Wind Chill, the Heat Index is forecast up to 48 hours into the future.