GUIDE TO THE DETAILED COUNTY FORECAST MATRIX

INTRODUCTION

The National Weather Service Forecast Office in Peachtree City, GA is now providing detailed forecasts in a matrix format known as "Detailed County Forecast Matrices". The Detailed County Forecasts are routinely issued around 500 am and 400 pm local time, with updates issued as needed. This guide will explain how to read and interpret the new product.

WHAT IS THE DETAILED COUNTY FORECAST MATRIX?

The Detailed County Forecast Matrix displays forecast weather parameters in 3-hour, 6-hour and 12-hour intervals. These intervals, combined with a matrix format, create a detailed forecast of various weather parameters for each of the 96 counties in our "service area" or "county warning area" (CWA). In the Detailed County Forecast Matrix you will find 3-hourly and 12-hourly forecasts for up to 48 hours into the future. 6-hourly and 12-hourly forecasts are provided between 3 and 7 days into the future. The product is available via all NWS routine distribution channels, and is available on our website at

https://weather.gov/ffc/custom and https://weather.gov/ffc/afm

HOW TO READ AND INTERPRET THE DETAILED COUNTY FORECAST MATRIX PRODUCT

An example of the Detailed County Forecast Matrix product is displayed below. There are several forecast parameters which appear. Some of these values are displayed in 12-hour intervals while others are displayed in 6-hour or 3-hour intervals. At the top is the local time and date that the forecast was issued. In the case illustrated, the forecast was issued on Saturday February 8, and the time of issuance was 3:56 p.m. Eastern Standard Time (EST). The matrix format is below the time and date, and a forecast timeline is established in 3 hour increments for 48 hours (2 days) into the future. Listed on the far left of this timeline is the local time zone. In this example, EST is listed, which means the hours in the timeline are in Eastern Standard Time. Another timeline in "UTC" or Universal Coordinated Time (sometimes called "Z" time or "Zulu", or "GMT" - Greenwich Mean Time) is provided for reference. Whether using UTC or EST, the hours of the day use the 24-hour clock system, where "00" is "midnight" and "18" is 600 pm. The time lines are organized under a line called "DATE" which lays out the day of the week and the calendar date.

Note that the Detailed County Forecast Matrix product is organized into two blocks of data. The upper matrix includes forecast parameters for the first 48 hours out into the future, while the lower matrix includes data for days 3 through 7 into the future. The lower matrix can be identified by locating a second set of "timelines". The lower matrix is the "extended forecast".

447 PM EST THU JAN 18 2007

DATE                         FRI 01/19/07            SAT 01/20/07            SUN
UTC 3HRLY     20 23 02 05 08 11 14 17 20 23 02 05 08 11 14 17 20 23 02 05 08 11
EST 3HRLY     15 18 21 00 03 06 09 12 15 18 21 00 03 06 09 12 15 18 21 00 03 06

MIN/MAX                      33          50          27          50          33
TEMP             36 35 34 33 33 35 45 50 44 36 31 30 28 30 43 50 46 39 36 35 34
DEWPT            35 34 33 33 32 29 26 23 22 20 21 22 22 21 18 18 21 24 26 27 29
RH               96 96 96100 96 78 47 34 41 52 66 72 78 69 36 28 37 54 67 72 82
WIND DIR         NE  E NW NW NW NW NW NW NW NW NW  N NW NW  N NW  N  N NE  E  E
WIND SPD          4  3  5  6 10 12 14 16 17  9 10  8  8  9  9  8  6  3  2  3  5
WIND GUST                       24 27 28 29 19 20                              
CLOUDS           OV OV OV OV OV SC SC SC SC SC SC SC SC B1 B1 B1 B1 B2 B2 OV OV
POP 12HR                     30           5           5           5          10
QPF 12HR                      0           0           0           0        0.02
SNOW 12HR                 00-00       00-00       00-00                        
DRIZZLE          AR                                                            
RAIN              C                                                            
SPRINKLES            S  S                                                      
OBVIS             F PF PF PF PF                                                
WIND CHILL                27 25 26             23 22 20 22                   28
MIN CHILL        30    26    24    22          21    18    17          32    28


DATE           01/21/07  MON 01/22/07  TUE 01/23/07  WED 01/24/07  THU 01/25/07
UTC 6HRLY     17 23 05   11 17 23 05   11 17 23 05   11 17 23 05   11 17 23
EST 6HRLY     12 18 00   06 12 18 00   06 12 18 00   06 12 18 00   06 12 18

MAX/MIN          41      37    50      32    45      26    45      26    38
TEMP          38 40 38   37 46 45 35   33 41 40 29   26 39 40 29   27 34 35
DEWPT         32 36 37   36 36 35 33   30 27 25 23   21 20 19 21   24 23 22
PWIND DIR         E      NE    NW      NW    NW      NW    NW       W     W
WIND CHAR        GN      LT    LT      LT    GN      GN    GN      GN    BZ
AVG CLOUDS    OV OV B2   B2 B1 B1 B2   B2 B2 B2 B1   B1 SC SC B1   B1 SC SC
POP 12HR         70      70    50      30    20      20    10      10    10
RAIN                         C  C          S  S                            
RAIN SHWRS     L  L  C    C

FORECAST PARAMETERS

Taken line by line ... the upper matrix ...

1) MIN/MAX - The forecast maximum or minimum temperature. MIN/MAX is located near the ending time of each 12 hour period for which it is forecast out to 48 hours. The maximum temperatures are forecast from 7:00 a.m. to 7:00 p.m. Local Time. Minimum temperatures are forecast 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. In the example, a minimum (or "low") temperature of 32 degrees is forecast between 7:00 p.m. and 7:00 a.m. EST ending Sunday February 9, to be followed by a maximum (or "high") temperature of 50 degrees between 7:00 a.m. and 7:00 p.m. EST 2/9/03.

2) TEMP - The expected temperature at a specified time, in degrees Farenheit. The temperature is forecast in 3 hour intervals.

3) DEWPT - The expected dewpoint temperature at a specified time, in degrees Farenheit. The dewpoint temperature is forecast in 3 hour intervals.

4) RH - The relative humidity based on the expected temperature and dewpoint.

5) WIND DIR - The expected direction from which wind should blow at 3 hour intervals. The 8 point compass is used (e.g., W, NW, N. . . etc.) Dashes (- -) represent no wind direction forecast due to a calm wind.

6) WIND SPD - The expected average wind speed in miles per hour, during each three hour time period.

7) WIND GUST - The expected wind gust speed in miles per hour, during each three hour time period.

8) CLOUDS - The expected cloud cover during each 3-hour time period. The contractions used and their meanings are as follows:

CL = CLEAR SKIES (0-5% CLOUD COVER)
FW = SUNNY or MOSTLY CLEAR SKIES (6-25% CLOUD COVER)
SC = MOSTLY SUNNY (day) or PARTLY CLOUDY (night) SKIES (26-50% CLOUD COVER)
B1 = PARTLY SUNNY (day) or MOSTLY CLOUDY (night) SKIES (50-69% CLOUD COVER)
B2 = MOSTLY CLOUDY SKIES (70-87% CLOUD COVER)
OV = CLOUDY SKIES (87-100% CLOUD COVER)

9) POP 12HR - The probability of precipitation is for a 12-hour "daytime" period, or a 12-hour "nighttime" period. This percentage probability is listed toward the ending time of each period for which it applies. In the example, there is a 20% probability of precipitation during the day Sunday. There is an 80% probability of precipitation overnight Sunday night.

10) QPF 12HR - The amount of rainfall expected in each 12-hour period ("daytime"/"nighttime"). The values given are in inches, and may be in ranges. For example: ".01-.10" means between a hundredth and a tenth of an inch during the 12-hour period.

11) SNOW 12HR - The amount of snowfall expected in each 12-hour period ("daytime"/"nighttime"). The values given are in inches.

Additional lines, such as "RAIN", "WIND CHILL", and "MIN CHILL" in the example above, are included in the upper matrix if any of the following are in the forecast for that 48-hour period:

WIND CHILL = "How it feels" based on temperature and wind each 3 hours
MIN CHILL = Lowest wind chill over last 6 hour period
HEAT INDEX = "How it feels" based on temperature and relative humidity each 3 hours
MAX HEAT = Highest heat index over last 6 hour period
RAIN = Precipitation type for each 3 hour period is RAIN
RAIN SHWRS = Precipitation type for each 3 hour period is RAIN SHOWERS
SPRINKLES = Precipitation type for each 3 hour period is SPRINKLES
TSTMS = Precipitation type for each 3 hour period is THUNDERSTORMS
DRIZZLE = Precipitation type for each 3 hour period is DRIZZLE
SNOW = Precipitation type for each 3 hour period is SNOW
SNOW SHWRS = Precipitation type for each 3 hour period is SNOW SHOWERS
FLURRIES = Precipitation type for each 3 hour period is SNOW FLURRIES
SLEET = Precipitation type for each 3 hour period is ICE PELLETS
FRZNG RAIN = Precipitation type for each 3 hour period is FREEZING RAIN
FRZNG DRZL = Precipitation type for each 3 hour period is FREEZING DRIZZLE
 

EACH PRECIPITATION PARAMETER IS CLASSIFIED AS FOLLOWS...

IS = ISOLATED (10-20% COVERAGE)
S = SLIGHT (10-20% PROBABILITY)
SC = SCATTERED (30-50% COVERAGE)
C = CHANCE (30-50% PROBABILITY)
NM = NUMEROUS (60-70% COVERAGE)
L = LIKELY (60-70% PROBABILITY)
O = OCCASIONAL (80-100% PROBABILITY)
D = DEFINITE (80-100% PROBABILITY)
EC = EXTENSIVE COVERAGE (80-100% PROBABILITY)
AR = AREAS (10-20% PROBABILITY)
PA = PATCHY (30-50% PROBABILITY)
WD = WIDESPREAD (60-100% PROBABILITY)
 

OBSTRUCTIONS TO VISIBILITY ARE CLASSIFIED AS FOLLOWS...

F = FOG
PF = PATCHY FOG
F+ = DENSE FOG
PF+ = PATCHY DENSE FOG
 

Note: Some watch and warning information (Red Flag Warnings, Flood Watches/Warnings, etc) will be displayed as a line in the matrix. The A corresponds to watch while the W corresponds to warning.

Taken line by line ... the lower matrix or "Extended" Forecast ... (after the time lines)

1) MIN/MAX - The forecast maximum or minimum temperature. MN/MX is located near the ending time of each 12 hour period for which it is forecast. The maximum temperatures are forecast 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. In the example, a minimum (or "low") temperature of 32 degrees is forecast between 7:00 p.m. and 7:00 a.m. EST ending Wednesday February 12 (the morning low for Wednesday, which is the same thing as the "overnight" low for Tuesday Night). That minimum will be followed by a maximum (or "high") temperature of 54 degrees between 7:00 a.m. and 7:00 p.m. EST Wednesday 2/12/03.

2) TEMP - The expected temperature at the specified time, in degrees Farenheit. The temperature is forecast in 6 hour intervals.

3) DEWPT - The expected dewpoint temperature at the specified time, in degrees Farenheit. The dewpoint temperature is forecast in 6 hour intervals.

4) PWIND DIR - Primary wind direction for each 12 hour period

5) WIND CHAR - Wind characteristic for each 12 hour period, where
 

LT = LIGHT (< 8 MPH)
GN = GENTLE (8-14 MPH)
BZ = BREEZY (15-22 MPH)
WY = WINDY (23-30 MPH)
VW = VERY WINDY (31-39 MPH)
SD = STRONG (>40 MPH)
HF = HURRICANE (>=74 MPH)
 

6) AVG CLOUDS - Average cloud cover for each 12 hour period, where
 

CL = CLEAR SKIES (0-5% CLOUD COVER)
FW = SUNNY or MOSTLY CLEAR SKIES (6-25% CLOUD COVER)
SC = MOSTLY SUNNY (day) or PARTLY CLOUDY (night) SKIES (26-50% CLOUD COVER)
B1 = PARTLY SUNNY (day) or MOSTLY CLOUDY (night) SKIES (50-69% CLOUD COVER)
B2 = MOSTLY CLOUDY SKIES (70-87% CLOUD COVER)
OV = CLOUDY SKIES (87-100% CLOUD COVER)

7) POP 12HR - The probability of precipitation is for a 12-hour "daytime" period, or a 12-hour "nighttime" period. This percentage probability is listed toward the ending time of each period for which it applies. In the example, there is a 10% probability of precipitation during the day Friday. There is a 30% probability of precipitation Friday Night (overnight).

Additional lines, such as "RAIN", in the example above, are included in the lower matrix if any of the following are in the forecast in the extended period:

WIND CHILL = "How it feels" based on temperature and wind each 3 hours
MIN CHILL = Lowest wind chill over last 6 hour period
HEAT INDEX = "How it feels" based on temperature and relative humidity each 3 hours
MAX HEAT = Highest heat index over last 6 hour period
RAIN = Precipitation type for each 6 hour period is RAIN
RAIN SHWRS = Precipitation type for each 6 hour period is RAIN SHOWERS
SPRINKLES = Precipitation type for each 6 hour period is SPRINKLES
TSTMS = Precipitation type for each 6 hour period is THUNDERSTORMS
DRIZZLE = Precipitation type for each 6 hour period is DRIZZLE
SNOW = Precipitation type for each 6 hour period is SNOW
SNOW SHWRS = Precipitation type for each 6 hour period is SNOW SHOWERS
FLURRIES = Precipitation type for each 6 hour period is SNOW FLURRIES
SLEET = Precipitation type for each 6 hour period is ICE PELLETS
FRZNG RAIN = Precipitation type for each 6 hour period is FREEZING RAIN
FRZNG DRZL = Precipitation type for each 6 hour period is FREEZING DRIZZLE
 

EACH PRECIPITATION PARAMETER IS CLASSIFIED AS FOLLOWS...

IS = ISOLATED (10-20% COVERAGE)
S = SLIGHT (10-20% PROBABILITY)
SC = SCATTERED (30-50% COVERAGE)
C = CHANCE (30-50% PROBABILITY)
NM = NUMEROUS (60-70% COVERAGE)
L = LIKELY (60-70% PROBABILITY)
O = OCCASIONAL (80-100% PROBABILITY)
D = DEFINITE (80-100% PROBABILITY)
EC = EXTENSIVE COVERAGE (80-100% PROBABILITY)
AR = AREAS (10-20% PROBABILITY)
PA = PATCHY (30-50% PROBABILITY)
WD = WIDESPREAD (60-100% PROBABILITY)
 

OBSTRUCTIONS TO VISIBILITY ARE CLASSIFIED AS FOLLOWS...

F = FOG
PF = PATCHY FOG
F+ = DENSE FOG
PF+ = PATCHY DENSE FOG