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5.4 USING PROBABILISTIC FORECAST GUIDANCE AND AN UPDATE TECHNIQUE TO GENERATE TERMINAL AERODROME FORECASTS 5.4
USING PROBABILISTIC FORECAST GUIDANCE AND AN UPDATE TECHNIQUE
TO GENERATE TERMINAL AERODROME FORECASTS


Mark G. Oberfield* and Matthew R. Peroutka
Meteorological Development Laboratory
Office of Science and Technology
National Weather Service, NOAA
Silver Spring, Maryland

Cynthia Abelman
Aviation Services Branch
Office of Climate, Water and Weather Services
National Weather Service, NOAA
Silver Spring, Maryland



1. INTRODUCTION

Since the early 1970s, the Meteorological De-
velopment Laboratory (MDL) has provided objec-
tive guidance for weather elements of importance
to aviation primarily to assist forecasters in creat-
ing Terminal Aerodrome Forecasts (TAFs). Since
that time, MDLs aviation guidance has steadily
evolved and expanded as computing power in-
creased. MDLs latest efforts have created short-
term statistical aviation guidance at hourly resolu-
tion.

In addition to developing statistical guidance
for aviation needs, MDL developed and currently
maintains the Aviation Forecast Preparation Sys-
tem (AvnFPS). As described by Peroutka et al.
(2005), this software assists aviation forecasters in
the National Weather Service (NWS) Weather
Forecast Offices (WFOs) in the preparation and
monitoring of their TAFs. AvnFPS software has
been deployed to all 122 WFOs as part of the
NWSs Advanced Weather Interactive Processing
System (AWIPS) (Seguin 2002).

This paper describes the design and imple-
mentation of a new algorithm within AvnFPS to
judiciously combine the latest Localized Aviation
MOS Program (LAMP) guidance with the current
TAF to create a new guidance TAF. This guid-
ance TAF will enhance forecast continuity by in-
corporating decisions already made by aviation
forecasters and, thus, be more readily acceptable
for use and dissemination.

2. LAMP

Overall description, development and imple-
mentation of the latest LAMP guidance are de-
scribed by Ghirardelli (2005) and briefly recounted
here.

LAMP guidance is objective statistical output
tailored to address the requirements of aviation
forecasters at the WFOs. LAMP uses surface ob-
servations, radar, lightning data, simple advective
models, and the latest GFS MOS as inputs to re-
gression equations to create a 25-h forecast at
hourly resolution. In addition to producing best
category forecasts of categorical elements, such
as ceiling heights and visibility, the probability of
occurrence of the individual categories is available
as well. These probabilistic data are not shown in
the LAMP guidance text bulletins, but can be
viewed in AvnFPS and play an important part in
our algorithm described below.

As of this writing, LAMP guidance is generated
12 times per day and will be produced every hour
by early 2009. Until now, LAMP development ef-
forts were focused mostly on creating guidance in
the early morning hours, reflecting the importance
of the 0600 and 1200 UTC TAF issuances in pi-
lots and airspace traffic managers' plans for the
day.

AvnFPS uses LAMP guidance as delivered to
AWIPS in the Binary Universal Format for data
Records (BUFR) messages. These messages are
received and decoded automatically in AWIPS and
* Corresponding authors address: Mark G. Ober-
field, NOAAs National Weather Service, Meteoro-
logical Development Laboratory, 1325 East-West
Highway, W/OST24, Silver Spring, MD 20910;
email: Mark.Oberfield@noaa.gov contain the additional hourly probability data on
two sets of ceiling height and visibility categories,
total sky cover category, probability of precipitation
occurring on the hour, and 2-h thunderstorm oc-
currence. Figure 1 shows the probability values
associated with various categorical elements in
the LAMP BUFR message as displayed in
AvnFPS. Two sets of categorical visibility data are
shown in Fig. 1. These are needed to describe
separately the dry (no precipitation occurring)
and the wet (precipitation occurring) conditions.
LAMP also provides ceiling height guidance for
dry and wet conditions as well. LAMP break-
points for ceiling height and visibility correspond
closely to aviation flight categories and are shown
in Table 1 and 2, respectively.

Table 1. LAMP Ceiling Height Category Definitions
Category
Ceiling Height
1
< 200 feet
2
200 400 feet
3
500 900 feet
4
1000 1900 feet
5
2000 3000 feet
6
3100 6500 feet
7
6600 12,000 feet
8
> 12,000 feet or unlimited

Table 2. LAMP Visibility Category Definitions

3. AVNFPS

AvnFPS is a heavily used tool within the NWS
for TAF preparation and dissemination (un-
published MDL survey, 2007). Besides assisting
in the aviation forecasters weather watch activi-
ties (NWS 2007), AvnFPS provides access to ob-
servations, guidance, and climatological datasets
to support TAF preparation. Observations include
surface observations, both METARs and SPECIs,
lightning strikes, and wind data obtained from air-
craft, profilers, and Doppler radars for detection of
non-convective low-level wind shears (LLWS) near
airports. Guidance includes direct model output
from the North American Mesoscale Model, ETA-,
NGM- and GFS-MOS, and LAMP for the WFOs
TAF sites.

AvnFPS can display MOS guidance in a table
or convert it into a TAF format. Not surprisingly
TAFs based solely on data contained within MOS
bulletins are not particularly good as the timing
and duration of events are artificially constrained
due to the bulletins relatively coarse 3-h resolu-
tion. LAMP guidance, with its 1-h resolution out to
25 hours, including additional elements useful for
aviation, can be transformed into a more usable
TAF. However, even with the increased temporal
resolution and the additional parameters, the TAF
based on LAMP guidancealthough a consider-
able improvement over those derived from tradi-
tional MOSstill has some drawbacks that limit its
usefulness. For example, sky condition, when not
clear, is a single cloud layer at fixed, predefined
heights. LAMP cannot forecast showers or thun-
derstorms in the vicinity, (VCSH, VCTS), nor can
it indicate build ups of convective clouds, i.e.,
CB modifier to a cloud layer. These missing
elements are important cues to pilots, particularly
to those licensed to fly under visual flight rules.
Aviation forecasters must add these elements
manually and this limits their acceptance of auto-
mated guidance TAFs in many weather situations.

4. TAF UPDATE TECHNIQUE

To overcome these limitations of creating
TAFs with only LAMP guidance, we used the cur-
rent, official TAF as an additional source of infor-
mation when generating a new TAF; in effect we
update the current forecast with LAMP guidance.
This technique has the advantage of incorporating
multiple cloud layers and other elements not avail-
able in LAMP guidance when the aviation fore-
caster judges they should be present in the TAF.
Also, the official TAF provides hints to the algo-
rithm as to how the new TAF should be summa-
rized into a concise form. Finally, this technique
introduces forecast continuity into the composition
process, provided the current TAF and LAMP
guidance do not disagree too much.

While LAMP guidance shows probability val-
ues for the various categories and binary ele-
ments, additional parameters are given in the bul-
letin to present the best category or a yes/no
decision to the forecaster. These parameters are
chosen in a manner that maximizes some meas-
ure of performance using either the threat or bias
scores (Wilks 2006).
Category
Visibility
1
< ½ mile
2
½ - < 1 miles
3
1 - < 2 miles
4
2 - < 3 miles
5
3 5 miles
6
6 miles
7
> 6 miles In deciding a yes/no decision or selecting a
best category, additional information, called
threshold values, in combination with the probabil-
ity values provided in the LAMP bulletin is re-
quired. These threshold values are created during
the development phase of each LAMP cycle and,
until now, have not been disseminated. These
threshold values provide a means to quantitatively
measure the amount of disagreement between
LAMP probabilistic guidance and the current TAF
and make adjustments accordingly for the new
guidance TAF.

To illustrate this and the use of thresholds in
determining best visibility category for dry condi-
tions in LAMP guidance, well demonstrate with
the values shown in Fig. 1 and redisplayed in Ta-
ble 3 along with the threshold values for the par-
ticular hour.

Table 3. Illustration of Best Category Selection.
1800 UTC LAMP Visibility Category Forecast
valid at 0900 UTC 31 August 2007 for Tri-
State Airport, Huntington, West Virginia,