WHAT ARE THE FORECAST MATRICES?


POINT FORECAST MATRICES (PFM) and AREA FORECAST MATRICES (AFM)

The PFM will display point forecast matrices for the following four points: Tri-Cities airport, Knoxville airport, Oak Ridge and Chattanooga airport. The AFM will display forecast matrices for each county in the Morristown forecast area. The matrices will display forecast weather parameters in 3, 6, and 12 hour intervals out to 7 days in the future. These two products will be routinely issued twice a day around 4 am and 4 pm LST and updated as needed.


HOW TO READ/INTERPRET THE FORECAST MATRICES
There are several forecast parameters which appear in the PFM and AFM products. Some of these values are forecast in 12 hour intervals while others are forecast in 3 and 6 hour intervals. Listed below is an example of a forecast matrix with a description of each of its parameters.

DATE             FRI 03/19/10            SAT 03/20/10            SUN 03/21/10
EDT 3HRLY 05 08 11 14 17 20 23 02 05 08 11 14 17 20 23 02 05 08 11 14 17 20
UTC 3HRLY 09 12 15 18 21 00 03 06 09 12 15 18 21 00 03 06 09 12 15 18 21 00

MAX/MIN 67 35 68 44 62
TEMP 49 61 65 58 49 44 40 38 51 62 67 61 55 51 48 45 53 59 61 56
DEWPT 33 31 31 32 33 33 33 34 36 38 39 39 40 41 42 42 42 42 41 40
RH 54 32 28 37 54 65 76 85 56 41 36 44 57 68 80 89 66 53 48 55
WIND DIR W NW W W S SE SE S SW SW W W S SE SE SE SE SE SE SE
WIND SPD 3 6 5 4 3 2 2 0 4 9 6 2 1 2 3 4 8 11 12 12
CLOUDS CL CL CL FW FW FW FW SC SC SC B1 B1 B1 B2 B2 B2 B2 OV OV OV
POP 12HR 0 0 10 20 60
QPF 12HR 0 0 0 0.01 0.10
RAIN SHWRS S C C L L C


DATE MON 03/22/10 TUE 03/23/10 WED 03/24/10 THU 03/25/10
EDT 6HRLY 02 08 14 20 02 08 14 20 02 08 14 20 02 08 14 20
UTC 6HRLY 06 12 18 00 06 12 18 00 06 12 18 00 06 12 18 00

MIN/MAX 40 50 34 56 35 66 42 65
TEMP 46 41 48 46 38 36 52 50 41 37 61 60 49 44 61 59
DEWPT 35 32 30 30 29 29 29 30 30 31 33 35 39 39 41 42
PWIND DIR SE W W W SW S S SW
WIND CHAR GN GN GN LT LT LT LT LT
AVG CLOUDS OV OV OV OV B2 B2 B1 B1 SC SC FW FW SC SC B1 B1
POP 12HR 50 30 20 10 5 5 10 30
RAIN S S
RAIN SHWRS C C C C C C

GENERAL FORMAT:

Below the product issuance time/date, the forecast date and time blocks will be displayed. The day of the week will be expressed by the standard 3-letter identifiers (SUN, MON, TUE, WED, THU, FRI, SAT) followed by the month, day and year (MM/DD/YY). The day and date labels are left justified above the 6:00 a.m. local time hour.

The following two lines provide the forecast times at 3 hour intervals (indicated by 3HRLY) out to 60 hours (2 ½ days) into the future. Listed on the far left of the time lines are the 3-letter time zone abbreviations (e.g., UTC, LST).

The second block is valid from 66 hours through Day 7. Once again, the local time zone is LST, but the forecast time intervals are at least 6 hours (indicated by 6HRLY). Some of the time intervals are 12 hours

Maximum/Minimum temperatures: MAX/MIN (or alternatively, MIN/MAX for afternoon issuance) is the forecast of maximum or minimum temperatures in degrees Fahrenheit (F). The nighttime MIN and daytime MAX may be displayed as single integer (e.g., -2, 8, 53, 102) , or as a range (e.g., 54 56 60) if the MAX/MIN temperatures are expected to vary across the area. This is illustrated in the AFM / PFM format documents. In AFM / PFM products, the middle number within the range is the representative single value for that area. MAX/MIN is included through Day 7.

The effective time periods for MaxT and MinT are set as follows:

    • MaxT
      1. o All but Alaska: 7 am to 7 pm LST
      2. o Alaska: 5 am to 8 pm LST
    • Min T
      1. o All but Alaska: 7 pm to 8 am LST
      2. o Alaska: 5 pm to 11 am LST

Note that due to a 3-hour minimum time resolution, this element is right justified in the column beneath the approximate ending time of the MAX/MIN period.

Maximum and minimum temperature values are determined by sampling the MaxT and MinT grids for each of the 14 day/night periods.

 

Temperature: TEMP is a snapshot of the expected temperature in degrees F valid at the indicated hour. The temperature is right justified in the column below the hour to which it refers..

+ Temperature is available at 3 hour projections through 60 hours, then 6-hour projections through Day 7.

 

Dewpoint: DEWPT is a snapshot of the expected dew point temperature in degrees F for the same time periods as its corresponding temperature forecast. DEWPT is located directly below the temperature line.

+ Dewpoint is available for the same time projections as temperature.

 

Relative Humidity: RH is a snapshot of the expected RH for the same time periods as its corresponding temperature and dew point forecast. The RH row is located directly below the “DEWPT” row.

+ RH is calculated from 3 hourly temperature and dewpoint grids

 

Wind Direction: WIND DIR is a snapshot of the expected wind direction forecast to occur at the indicated hour, using the 8 points of a compass (i.e., N, NE, E, SE, S, SW, W, NW).

If a calm wind is forecast, double zeros (00) will be listed in place of a wind direction. WIND DIR is located below the hour to which it refers. WIND DIR is available at 3-hour projections out to 60 hours. (Note special exception to wind direction for tropical cyclones below.)

In the 6HRLY block, PWIND DIR is the “predominant” wind direction for the area during the 12-hour period between 6:00 a.m. and 6:00 p.m., or 6:00 p.m. and 6:00 a.m. local time. PWIND DIR is available beyond 60 hours through Day 7.

 

Wind Speed: WIND SPD is a snapshot of the sustained wind speed in miles per hour (MPH) forecast to occur at the indicated hour. If a calm wind is forecast, double zeros (00) will be listed in place of a wind speed. (Note special exception to wind speed for tropical cyclones below). WIND SPD is valid at 3-hour projections out to 60 hours.

 

Wind Character: WIND CHAR codes are used beyond 60 hours through Day 7 of the forecast and denote the character of the wind for the 12-hour period between 6:00 a.m. and 6:00 p.m., or 6:00 p.m. and 6:00 a.m. WIND CHAR is comprised of range categories used in conjunction with deterministic wind speeds. Each range category is equated to a descriptive wind term, i.e., a “wind character” to best describe the MAXIMUM SUSTAINED wind speed during the period. . See below for the complete list of AFM / PFM wind categories.

Wind Character Codes Wind Character 12-hr Maximum Sustained Wind Speed
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/Damaging 40 mph

AFM/PFM Wind Character Codes.

 

Wind gust: A WIND GUST row will appear in the 3HRLY block whenever forecasted wind gusts exceed the sustained wind speed (WIND SPD) by at least 10 MPH. WIND GUST is a snapshot valid on the hour indicated at the top of the corresponding column. WIND GUST is a snapshot of gusts of wind occurring at the indicated hour and is available at 3-hour projections through 60 hours. (Note special exception to wind gust for hurricanes below.)

 

Cloud Cover: The CLOUDS category provides a snapshot of sky coverage during the indicated hour. CLOUDS is divided into five category codes ranging from clear to overcast . Each code represents an equivalent percentage of opaque sky cover in percent. CLOUDS parameter is included at 3-hour projections out to 60 hours. In the 6HOURLY section, AVG CLOUDS is valid for 6-hour intervals beyond 60 hours through Day 7 and denotes the average amount of all opaque clouds during the 6-hour period ending on the hour indicated at the top of the column. The complete cloud codes and sky cover definitions are shown in the table below.

AFM / PFM Sky Cover Code   

Predominant Sky Cover
(Opaque Cloud Coverage in Percent)

Equivalent Sky Cover Expressions
CL 0% to ≤ 5% SUNNY or CLEAR
FW > 5% and ≤ 25% SUNNY or MOSTLY CLEAR
SC > 25% and ≤ 50% MOSTLY SUNNY or PARTLY CLOUDY
B1 > 50% and ≤ 69% PARTLY SUNNY or MOSTLY CLOUDY
B2 > 69% and ≤ 87% MOSTLY CLOUDY or CONSIDERABLE CLOUDINESS
OV > 87% and 100% CLOUDY or OVERCAST

AFM / PFM Sky Cover Codes

 

12-hourly Probability of Precipitation (POP 12HR). Probability of Precipitation (POP), is defined as the likelihood, expressed as a percent, of a measurable precipitation event (1/100th of an inch) at any given point within the forecast area(s) covered by the AFM / PFM. The “12HR” refers to the 12-hour valid time ending at 6:00 a.m. or 6:00 p.m. local time (0600 or 1800). The POP 12HR values that may appear in the AFM / PFM are as follows: 0, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100. These values are right justified in the column beneath the hour defining the ending time of the valid period.

 

QPF 12HR. Quantitative precipitation forecast (QPF) represents the total amount of liquid precipitation, in inches, expected during a 12-hour period ending at 6:00 a.m., or 6:00 p.m. local time. The QPF is presented in locally defined ranges, (e.g., .10-.24), or single values. The QPF 12HR value is right justified in the column beneath the hour defining the ending time of the expected precipitation. QPF 12HR is included in the AFM / PFM out to 60 hours.

 

MAX QPF (optional). The value for MAX QPF is the estimated maximum amount of precipitation, in inches, during the12-hour period ending at 6:00 a.m. or 6:00 p.m. local time. This amount is presented as either a single value or a range, and is based upon a 75% confidence level of the QPF forecaster(s). MAX QPF is right justified below the hour defining the ending time of the expected precipitation, and is available out to 60 hours.

 

SNOW 12HR. The expected range of total snowfall accumulation (in whole inches) forecast to occur in the forecast area during a 12-hour period ending at 6:00 a.m. or 6:00 p.m. local time. SNOW 12HR will only appear during the locally defined winter period. The snow parameter contains 1 to 5 alphanumeric characters which are right justified in the column below the hour defining the ending time of the precipitation period. SNOW 12HR may appear as a one or two digit number (1, 4, 12), or as a specified range (2-4, 8-12). When no snowfall is forecast during the locally specified winter period, double zeros (00-00) will appear in the row. Snowfall that is not measurable (less than 0.1 inch of frozen precipitation) is referred to as a trace. A trace of snow is depicted by a “T.” SNOW 12HR is included out to 36 hours.

 

Precipitation Type (PTYPE) and Category: The AFM / PFM may list several types of precipitation. Precipitation types only appear if they are forecast to occur during the seven day forecast. Precipitation type codes are listed in the far left column of the AFM / PFM. For each type of precipitation that is forecast, an associated POP category is specified within the body of the product. During the first 60 hours, the POP category for the indicated precipitation type is a snapshot valid at the hour specified by the column header. Beyond 60 hours through Day 7, the POP category reflects the prevailing POP during the 6-hour period including and preceding the hour indicated at the top of the column. PTYPE and Category is available at 3-hr projections out to 60 hours, then for 6-hour periods beyond 60 hours through Day 7.

RAIN SHWRS Rain Showers
SPRINKLES Sprinkles
TSTMS Thunderstorms
DRIZZLE Drizzle
SNOW Snow, Snow Grains/Pellets
SNOWSHWRS Snow Showers
FLURRIES Snow Flurries
SLEET Ice Pellets
FRZG RAIN Freezing Rain
FRZG DRZL Freezing Drizzle

Sensible Weather Codes.

 

Probability of Precipitation and Areal Coverage codes appearing in the AFM / PFM are shown below, along with their equivalent POP or areal coverage in percent.

Code 

Qualifying Term
(Stratiform or Convective)

POP (%)
S Slight Chance (> 0 and  20%)
C Chance (30%-50%)
L Likely (60%-70%)
O Occasional/Periods of (80%-100%)
D None (80%-100%)
Code Areal Term (Convective) Areal Coverage (%)
IS Isolated ( 20%)
SC Scattered (30%-50%)
NM Numerous (60%-70%)
EC None (extensive coverage) (80%-100%)
Code Areal Term
(Non-Measurable Stratiform)
Areal Coverage (%)
PA Patchy (<25%)
AR Areas (25>50%)
WD Widespread (>50%)

POP and Areal Coverage Codes.

Precipitation categories are snapshots available at 3-hour projections out to 60 hours, then averaged over 6-hour intervals out to Day 7.

 

Obstructions to Visibility (OBVIS): If an OBVIS is predicted for the forecast area, a row labeled OBVIS will be listed underneath any forecast of precipitation. If no precipitation is forecast, then OBVIS will be listed under the row labeled CLOUDS. OBVIS is a snapshot available at 3-hour projections through 60 hours. The complete OBVIS code list and associated definitions are shown below.

AFM / PFM Code Obstruction to Visibility
F Fog
PF Patchy Fog
F+ Dense Fog
PF+ Patchy Dense Fog
H Haze
BS Blowing Snow
K Smoke
BD Blowing Dust
AF Volcanic Ashfall

Obstruction to Visibility Codes.

 

Wind Chill and Heat Index: Wind Chill and Heat Index are included seasonally based upon locally defined criteria. The decision on whether to include or exclude these parameters is determined by the local WFO criteria. Wind Chill and Heat Index are snapshots at the indicated hour and are forecast at 3-hour projections out to 60 hours.

 

MIN CHILL and MAX HEAT. When WIND CHILL or HEAT INDEX values appear in the AFM / PFM, a 6-hour minimum wind chill or maximum heat index may appear on the following row. These values indicate the minimum wind chill/ maximum heat index forecast to occur during the 6-hour period (inclusive of and preceding) the hour indicated at the top of the column. MIN CHILL and MAX HEAT, if included, will be at 6-hr intervals out to 60 hours.

 

WATCH, WARNING and ADVISORY. When Valid Time Event Codes (VTEC) as described in NWS Directive 10-1703 becomes available, long duration hazardous weather events will be included when a valid WATCH, WARNING and/or ADVISORY is issued by a WFO. The weather phenomena codes are decoded into plain language from VTEC and will appear as labels for additional rows at the bottom of the 3HRLY block. Within the text of the AFM / PFM, the VTEC codes for WATCH [A], WARNING [W], and ADVISORY [Y] will appear if the valid time of the event is in effect during the indicated hour. If valid, these codes will only appear during the first 60 hours.

 

Special Instructions for Tropical Cyclones. Due to the uncertainty in the location and intensity of tropical cyclones, special instructions will apply to AFM / PFM entries for wind speed, wind direction and wind gusts for various time periods as described in the following sections and the table below.

a. Zero to 24 Hours. If forecast winds for a specified land area meet or exceed hurricane force (i.e., 64 kts or 74 mph) within the first 24 hours, the AFM / PFM will portray wind direction (WIND DIR) to the 8 points of a compass, deterministic wind speed (WIND SPD), and deterministic wind gusts (WIND GUST) as shown in the table below.

b. Beyond 24 Hours. If the potential exists for winds in a specified land area to meet or exceed hurricane force beyond 24 hours, the AFM / PFM will portray wind direction, and wind speed using the code “HU” in lieu of the deterministic winds (beyond 24 to 60 hours), and in lieu of the predominant wind direction (PWIND DIR) and wind character codes (beyond 60 hours to 120 hours). The code “HG” will appear in the AFM / PFM to indicate hurricane force wind gusts are possible. HG will be portrayed in lieu of the deterministic wind gusts beyond 24 hours to 60 hours only (see table below). Both the HU and HG codes indicate hurricane force winds, or wind gusts respectively, could occur. Users should refer to the tropical cyclone center or local WFO for the latest details concerning the storm.

Forecast Period
(Hours)
Wind Direction
(8 pts. of Compass)
Sustained Wind Speed
(MPH)
Wind Gusts
(MPH)
0 to 24 N, NE, E, SE, S, SW,W, or NW Deterministic
(e.g., 74 mph, 95 mph)
Deterministic
(e.g., 115 mph)
> 24 to 60 HU
(variable & uncertain)
HU
(hurricane force possible)
HG
(hurricane force possible)
>60 to 120 (Day 5) HU
(variable & uncertain)
HU
(hurricane force possible)
N/A

AFM / PFM entries for Hurricanes.


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