NTIM I sent today - QPF NGM MOS guidance

• To: l.miller.ucar(rec), f.ostby(rec), r.sheets(rec),
• Subject: NTIM I sent today - QPF NGM MOS guidance
• From: C.ALEX
• Date: Thu, 14 Oct 1993 09:51:59 -0600

Posted: Wed, Oct 13, 1993   3:28 PM EDT              Msg: JGJD-5937-5194
        l.uccellini(rec)
Subj:   NTIM I sent today - QPF NGM MOS guidance      
I sent the following NTIM this aftn:

WSHPNSNMC
NOUS41 KWBC DDHHMM

NATIONAL TECHNICAL INFORMATION MESSAGE 93-41
NATIONAL WEATHER SERVICE HEADQUARTERS
310 PM EDT WED OCT 13 1993

TO:         ALL NWS OFFICES, DOMESTIC DATA SERVICE SUBSCRIBERS,
            FAA/WEATHER MESSAGE SWITCHING CENTER, NATIONAL DATA
            BUOY CENTER

FROM:       MARY M. GLACKIN
            CHIEF, SERVICES DEVELOPMENT BRANCH

SUBJECT:    IMPLEMENTATION OF NGM-BASED MOS QPF FORECASTS

(THE FOLLOWING INFORMATION WAS PROVIDED BY MARK ANTOLIK OF THE
TECHNIQUES DEVELOPMENT LABORATORY.)

Effective on or about the 1200 UTC cycle on October 27, 1993,
Nested Grid Model (NGM)-based MOS quantitative precipitation
forecast (QPF) guidance will be added to the NGM MOS forecast
bulletin.  The NGM MOS guidance is distributed under the
WMO header FOUS14 KWBC and the AFOS category FWC.  Users are
referred to Technical Procedures Bulletin (TPB) No. 408 for a
description of the complete FOUS14/FWC message, including the new
QPF guidance.

NOTE:  The addition of this line to the NGM-based MOS message
completes the implementation of this product.  In other words,
all of the lines are now available in the NGM-based MOS message.

The QPF forecasts will appear in the FOUS14 KWBC/FWC message on a
line labeled "QPF".  This line provides categorical forecasts of
equivalent-liquid precipitation amounts valid for 6- and 12-h
periods ending at specified projections.  The precipitation
amount forecasts are displayed beneath the time of the day
ending the valid period, with the 6-h and 12-h forecasts
separated by a slash.  The 6-h period forecast projections are
12, 18, 24, 30, 36, 42, 48, 54, and 60 hours after 0000 or 
1200 UTC.  The 12-h period forecast projections are 24, 36, 48,
and 60 hours after either cycle time.  The precipitation
categories for the 6-h periods are indicated by the following:

0 = no measurable precipitation;
1 = 0.01 to 0.09 inches;
2 = 0.10 to 0.24 inches;
3 = 0.25 to 0.49 inches;
4 = 0.50 to 0.99 inches;
5 = 1.00 inches or more.

The categories of precipitation amount for 12-h periods are:

0 = no measurable precipitation;
1 = 0.01 to 0.09 inches;
2 = 0.10 to 0.24 inches;
3 = 0.25 to 0.49 inches;
4 = 0.50 to 0.99 inches;
5 =  1.00 to 1.99 inches;
6 = 2.00 inches or more.

The character "9" denotes missing values.  The QPF will be
available on the message year-round for stations in the
contiguous United States.  However, forecasts of the heaviest
categorical amount (category 5 for the 6-h periods and category 6
for the 12-h periods) will not be made for sites near the Great
Lakes, on the High Plains, and throughout most of the western
United States except for the northwest Pacific coast.  In
addition,  forecasts of category 5 (1.00 inch or more) for 12-h
periods will be unavailable in the mountainous western United
States.

The categorical forecasts are prepared objectively from MOS
probabilistic forecasts of precipitation amounts equal to or
exceeding the lower bound of each of the above categories. 
Equations that produce the probability forecasts have been
developed for each forecast projection, both forecast cycles
(0000 and 1200 UTC), and each of 19 regions in the contiguous
United States.  These probabilities are not, however, displayed
in the NGM MOS message.  Instead, the categorical forecast which
appears is determined by comparing the forecast probabilities to
a set of pre-determined "threshold" probabilities which also were
derived for each forecast projection, cycle, and region.  The
algorithm that selects the categorical forecasts from the
ensemble of probabilities was designed to maximize the threat
scores for the rarest events without unduly increasing the number
of false alarms or excessively under- or over-forecasting
individual categories.

In tests on independent data, the NGM MOS precipitation amount
guidance was found to be superior to the raw NGM grid point
precipitation fields.  Note, too, that the NGM MOS QPF guidance
is prepared independently of the probability of precipitation
(POP) guidance.  Consequently, inconsistencies between the QPF
and POP guidance may occur, that is, the QPF may indicate
measurable amounts while the POP guidance indicates a very small
probability of precipitation.  Inconsistencies between the 6- and
12-h QPF are also possible.  At this time, no post-processing
algorithm has been implemented to eliminate these types of
inconsistencies.

A TPB which specifically describes the new NGM MOS QPF guidance
will be prepared in the near future and distributed to TPB
subscribers when it becomes available.  If you have any questions
concerning this new guidance or the message format, call 
Mark Antolik at 301-713-1065 or Paul Dallavalle at 301-763-8151.

END
SENT - W/OM23