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Quantitative Precipitation Forecasts (QPFs)
Forecasters at the WPC and its
predecessor organizations have been making Quantitative Precipitation
Forecasts since 1960. Quantitative Precipitation Forecasts, or QPFs, depi=
ct the
amount of liquid precipitation expected to fall in a defined period of
time. In the case of snow o=
r ice,
QPF represents the amount of liquid that will be measured when the precip=
itation
is melted. Precipitation am=
ounts
can vary significantly over short distances, especially when thunderstorms
occur, and for this reason QPFs issued by the WPC are defined as the expe=
cted
“areal average” (on a 32 x 32 km grid) in inches. Methods
for producing QPFs are similar to other meteorological forecasts. First, meteorologists analyze the
current state of the atmosphere.
Then they use model forecasts of pressure systems, fronts, jet str=
eam
intensity, etc., to form a conceptual model of how the weather will evolv=
e. The WPC has unique access to the=
full
suite of operational and ensemble model guidance from modeling centers in=
the
U.S., Canada, and Europe (the foreign models are global models, so they WPC
forecasters often engage in discussion with some of the 122 local National
Weather Service Forecast Offices, 12 River Forecast Centers, and other
national centers such as the Storm Prediction Center and National Hurrica=
ne
Center. In fact, per a 2005
agreement, the WPC provides the rainfall forecast (known as a rainfall
statement) that the National Hurricane Center inserts into each tropical
cyclone advisory it issues. The
WPC is also co-located w=
ith
NOAA’s National Environmental Satellite, Data, and Information Serv=
ices
(NESDIS) Synoptic Analysis Branch (SAB).=
The SAB provides information on satellite trends which helps refine
short range QPFs. Together,=
the SAB
and Day 1 QPF desk at the WPC are known as the National Precipitation
Prediction Unit (NPPU). This
collaborative process makes WPC forecasts generally more accurate than any
individual model, and consecutive versions of WPC forecasts do not differ=
as
much as consecutive model forecasts sometimes do. Technical
Description of QPFs: The QPF contours (isohyets) are drawn to encom=
pass
areal average amounts of 0.01, 0.25 inch, 0.50 inch, 1 inch, 1.50 inches,=
and
2.00 inches. Any values gre=
ater
than 2.00 inches are drawn in one-inch increments. It is
important to note the valid time period when viewing each product. Specifically, for the Day 1, 2, and 3 forecasts, QPFs are manually created for 6-hour periods=
and an accumulated 24-hour total QPF is also issued=
. For the Days 4/5 and Day 6/7 QPF<=
/span>, forecasters manually create a 48-hour accumulation=
of
areal average rainfall. Com=
puter
programs then take advantage of model forecasts of the timing of precipit=
ation
to break the WPC forecast down into 6-hourly QPFs. Forecasts issued on the day shift
correspond to periods ending at 0000Z (for issuance times between 1815Z a=
nd
2215Z), and those issued on the night shift correspond to periods ending =
at 1200Z
(for issuance times between 0615Z and 1015Z). Multi-day
Accumulation Products: Through simple addition of the various 24-hour and 48-hour Q=
PFs,
WPC is able to provide multi-day accumulation forecasts. Days 1-3, Days 1-5, and Days 1-7
accumulations are made available, and can be powerful planning tools for =
many
applications. |
![3D"http://www.wpc.ncep.noaa.gov/archives/qpf/20130531/fill_94qwbg_201=](3D"WPC_QPF_Product_Information_files/image002.jpg")
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Schedule
of QPF Issuance:
* Forecasts made on the night shifts are verified versus
observed rainfall. The vali=
d time
of 1200z to 1200z for QPFs issued by the night shift coincides with the
24-hour observation cycle for National Weather Service Cooperative Observ=
ers
and many other rainfall networks such as CoCoRaHS. All
Times shown are in Zulu (Z), or Greenwich Mean Time, and correspond to the
time in Greenwich, England, as seen on a 24-hour clock. |
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![3D"Excessive_cropped.GIF"](3D"WPC_QPF_Product_Information_files/image004.jpg")
<=
/span>
* Days 4/5 and 6/7 QPFs
are produced by the WPC medium range desk which issues a medium range
forecast discussion (PMDEPD). Text-based coding of the 24-hour QPF contours<=
span
style=3D'font-size:13.5pt;font-family:"Times New Roman","serif";mso-farea=
st-font-family:
"Times New Roman"'> Included at the en=
d of
the QPF Discussion for Days 1-3 is a bulletin that describes the shape of=
the
forecast isohyets using latitude and longitude pairs. The order in which the pairs appear is important as t=
his
gives the contour or isohyet a clockwise direction as the points are plot=
ted
to create the contour. The
forecast value associated with a given isohyet is valid in the enclosed a=
rea
to the right of the direction in which the contour is drawn. The 0.50” contour describe=
d in
the example below would represent an areal av=
erage
of 0.50” forecast over parts of northwestern Colorado and southern
Wyoming. Example: The first field is=
the
value of the contour; in this case the valu=
es are 0.25" or 0.50". The following fields =
are
the latitude/longitude pairs for the contour. The first three digits of t=
he
pair are the degrees of latitude (in tenths of degrees North latitude). T=
he
last three digits of the pair are the degrees of longitude (in tenths of
degrees West longitude). If t=
he
fourth digit is less than 5, a leading "1" is added to indicate
longitudes greater than or equal to 100° W. From the above message, the following table gives =
the
decoded lat/lon p=
airs:
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Tropical Rainfall Statement:
Per an agreement between the WPC and the NHC dat=
ed
March 13, 2005, the WPC is responsible for providing NHC with the wording f=
or rainfall
statements in all public advisories for tropical cyclones. The WPC provides rainfall statemen=
ts for
active tropical systems that threaten land masses in the Atlantic and easte=
rn
Pacific basins. The Tropical
Rainfall Statement describes specific islands or continental regions expect=
ed
to be impacted by heavy rain. The
statements include a range of expected areal average rainfall as well as the
maximum amounts expected at isolated locations.
QPF Forecast Verification:
6-hourly precipitation forecasts are verified
using a point (station) method while 24-hour forecasts are verified using a=
n areal method. Current graphs depicting WPC verificati=
on
scores are available on the WPC Verification =
page. For more details on the verification of WPC precipi=
tation
forecasts, read the article by Olson, Junker and Korty in Weather
and Forecasting.Volu=
me
10, 1995, pgs. 498-511