Point Forecast Matrices*
INTRODUCTION
The National Weather Service Forecast Office in Austin/San Antonio,
TX is now providing detailed, point-specific forecasts in a matrix
format known as "Point Forecast Matrices". The Point Forecast Matrices
are routinely issued around 400 am and 400 pm local time, with updates
issued as needed. This guide will explain how to read and interpret
the new product. The Point Forecast Matrices product will be considered
"experimental" until at least July 2003.
WHAT ARE THE POINT FORECAST MATRICES?
The Point 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 various locations in our "service area" or "county
warning area" (CWA), including Austin Bergstrom International
Airport and San Antonio International Aiport. In the Point 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 http://www.srh.noaa.gov/data/EWX/PFMEWX.
HOW TO READ AND INTERPRET THE POINT FORECAST MATRICES PRODUCT
An example of the Point Forecast Matrices 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. Near the top is the local time and
date that the forecast was issued. In the case illustrated, the forecast
was issued on Saturday May 3, and the time of issuance was 4:45 AM
local time. The matrix format is below the time and date, and a forecast
timeline is established in 3 hour increments for 60 hours into the
future. Listed on the far left of this timeline is the local time
zone. In this example, CDT is listed, which means the hours in the
timeline are in Central 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. 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 Point Forecast Matrices product is organized into two
blocks of data. The upper matrix includes forecast parameters for
the first 60 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".
FORECAST PARAMETERS
The following Point Forecast Matrix parameters are listed in order
of their appearance within the product. Other elements such
as wind chill and heat index are seasonal as defined by the local
office. Plans are in place for Watch, Warning, and Advisory information
to appear as additional rows in the matrices by the end of 2003.
a.MX/MN. (Alternatively labeled as MN/MX for afternoon
issuance)
A forecast of maximum or minimum temperatures in degrees Fahrenheit
(F). The MX temperature is valid for the period 7:00 a.m. through
7:00 p.m. local standard time, and MN is valid for 7:00 p.m. through
8:00 a.m. local standard time (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 MX/MN period). The
nighttime MN and daytime MX are displayed as a single integer comprising
3 character spaces or less (e.g., -2, 8, 53, 102). In the example
above, MN temperature forecast for Sunday morning 5/4/03 is 73 degrees
Fahrenheit. MX/MN is forecast out through Day 7.
b.TEMP. TEMP is a snapshot of the expected temperature
in degrees F valid at the specified point at the indicated hour.
The temperature is right justified in the column below the hour to
which it refers. In the example above, the forecasted temperature
for 6:00 p.m. CDT Sunday 5/4/03 is 93 degrees F. TEMP is forecast
at 3-hour intervals through 60 hours, then 6-hour intervals through
Day 7.
c.DEWPT. 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.
d.RH. The 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. In the example above, the RH at
6:00 p.m. (1800 hours CDT) on 5/4/03 is based on a temperature
of 93 degrees F and a dew point of 69 degrees F is 45%. RH is
available at 3-hour intervals through 60 hours.
e. WIND DIR. 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). In
the example above, the forecasted prevailing wind direction, i.e.,
the direction from which the wind is blowing, at 6:00 p.m. CDT Sunday
5/4/03 is South (S). 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 intervals out to 60 hours. (Note special
exception to wind direction for hurricanes in section below.)
In the 6HRLY block, PWIND DIR is the predominant wind
direction at the point 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.
f.WIND SPD. WIND SPD is a snapshot of the sustained wind
speed in miles per hour (MPH) forecast to occur at the indicated hour
through 60 hours. In the example above, the forecasted wind
speed at 6:00 p.m. (1800 hours) CDT Tuesday 5/4/03 is 11 MPH.
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 hurricanes in section below.) WIND SPD is valid at 3-hour
intervals out to 60 hours.
WIND CHAR codes are used beyond 60 hours through Day 7 of
the forecast and denote the character of the wind for the specified
point 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. 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. In the example above, on Tuesday 5/6/03 during the
12- hour period between 6:00 a.m. and 6:00 p.m., the code LT indicates
that light winds are expected though the period. See below for
more...
g. CLOUDS. 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 in 3-hour time intervals out to
60 hours. In the 6HOURLY section, AVG CLOUDS is valid for 6-hour
intervals beyond 60 hours through Day 7 and indicates the average
amount of all clouds during the 6-hour period ending on the hour indicated
at the top of the column. The complete cloud codes and equivalent
sky cover definitions are shown below...
h. 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 the specific location.
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 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. In the example above, there
is a 20% chance of precipitation during the 12-hour period between
6:00 a.m. and 6:00 p.m. CDT Tuesday 5/6/03. POP 12HR is forecast
through Day 7.
i. QPF 12HR. This parameter, quantitative precipitation
forecast (QPF) represents the total amount of liquid precipitation,
in inches, expected at the specific point 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.
In the example above, the QPF 12HR indicates that a total of 0 inches
of precipitation is expected sometime during the period beginning
at 6:00 p.m. CDT Saturday 5/3/03 and ending by 6:00 a.m. CDT Sunday
5/4/03. QPF 12HR is forecast out to 60 hours.
j. SNOW 12HR. The expected range of total snowfall accumulation
(in whole inches) forecast to occur at the specific point 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 forecast out to 36 hours.
k.PRECIPITATION TYPE AND CATEGORY. Precipitation types
only appear if they are forecast to occur at the specified point during
the seven day forecast. Precipitation type codes are listed
in the far left column of the product. For each type of
precipitation that is forecast, an associated POP category is specified
within the body of the product. The POP expression for the indicated
precipitation type is valid at the hour specified by the column header.
Precipitation type expressions are forecast at 3-hr time intervals
out to 60 hours, then 6-hour time intervals beyond 60 hours through
Day 7. The types of precipitation that may be forecast in the
product are...
Probability of precipitation codes and their equivalent POP
definitions are shown below....
l. OBVIS. If an obstruction to visibility (OBVIS) is forecast
at the specific point, 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.
m. WIND CHILL and HEAT INDEX. Wind Chill and Heat Index
are included seasonally based upon locally defined criteria.
WIND CHILL and HEAT INDEX are snapshots at the indicated hour and
are forecast at 3-hour intervals out to 60 hours.
n. MIN CHILL and MAX HEAT. When WIND CHILL or HEAT INDEX
values appear, 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 are included at 6-hr intervals
out to 60 hours.
o. WATCH, WARNING and ADVISORY. When Valid Time Event
Codes (VTEC) as described become available, long duration hazardous
weather events will be included when a valid WATCH, WARNING and/or
ADVISORY is issued. 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 matrix, the VTEC codes for WATCH [A], WARNING [W], and ADVISORY
[Y] are applied to the 3-hour time spans which most nearly correspond
to the actual valid times of the event. That is to say, if the
valid time falls within any portion of a particular 3- hour
time window, the code will be applied to the entire time span.
If forecast, these codes will only appear during the first 60 hours.
p. Special Instructions for Hurricanes. Due to the uncertainty
in the location and intensity of tropical cyclones, special instructions
will apply to entries for wind speed, wind direction and wind gusts
for various time periods...
Forecast Period (Hours) | Wind Direction | Sustained Wind Speed |
0 to 24 | N, NE, S, etc. | Deterministic (e.g., 74mph, 95 mph, etc.) |
> 24 to 60 | HU (variable and uncertain) | HG (hurricane force possible) |
> 60 to 120 (Day 5) | HU (variable and uncertain) | N/A |
*More information on the Point Forecast Matrices will be made available
later this summer when the product will no longer be "experimental".
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