Chapter 27, Forecasts 27-53
Table 27-14. Select Constant Pressure Levels from the GFS Model
Constant Pressure
Level
Approximate Altitude
(MSL)
925 mb 2,500
850 mb 5,000
700 mb 10,000
500 mb 18,000
300 mb 30,000
250 mb 34,000
200 mb 39,000
27.12 Freezing Level Forecast Graphics
The freezing level forecast graphics provide an initial analysis and forecasts at specified times into the
future. The forecasts are based on output from NWS computer models. They supplement the forecast
freezing level information contained in the icing AIRMETs.
The freezing level is the lowest altitude in the atmosphere over a given location at which the air temperature
reaches 0°C. This altitude is also known as the height of the 0 °C constant-temperature surface. A freezing
level forecast graphic shows the height of the 0°C constant-temperature surface.
The initial analysis and forecast graphics are updated hourly. The colors represent the height in hundreds
of feet above MSL of the lowest freezing level. Regions with white indicate the surface and the entire depth
of the atmosphere are below freezing. Hatched or spotted regions (if present) represent areas where the
surface temperature is below freezing with multiple freezing levels aloft.
More information on the freezing level forecast graphics is available on the AWC’s website.
27.13 Forecast Icing Product (FIP)
The NWS produces the Forecast Icing Product which is derived from NWS computer model data with no
forecaster modifications. The FIP provides the same suite of products as the CIP ( see Section 25.5),
describing the icing environment in the future and being solely NWP model based. Information on the
graphics is determined from NWP model output; observational data, including WSR -88D; satellite,
PIREPs, and surface weather reports; and lightning network data.
FIPs contain a heavy intensity level. Heavy icing is defined as the accretion of ¼ inch of ice on the airfoil
in < 15 minutes. This is a relative value and the use of which should take into account the airframe and the
level of icing protection provided by the aircraft. The ultimate safety fac tor is the vigilance demonstrated
by the pilot in potential icing situations.
FIPs will continue to evolve over the coming years with increased model resolutions, additional horizontal
layers, and improvements to the algorithms and/or data sets used to produce the products. Along with these
improvements may come a change in references to the product update version. Users can find additional
information on these products and any changes on the AWC’s “Icing” web page.
Chapter 27, Forecasts 27-54
The FIP suite as it appears on the AWC’s website consists of three graphics, including:
• Icing Probability;
• Icing Severity; and
• Icing Severity plus SLD.
The FIPs are generated for select altitudes from 1,000 ft MSL to FL300. FIPs are available at select forecast
times through 18 hours.
The FIPs can be viewed at single altitudes and FLs or as a composite of all altitudes from 1,000 ft MSL to
FL300, which is referred to as the “maximum” or “max.”
The FIP should be used in conjunction with the report and forecast information contained in an AIRMET
and SIGMET.
27.13.1 Icing Probability
The Icing Probability product displays the probability of icing at any level of intensity. Probabilities range
from 0 percent (no icing expected) to 85 percent or greater (nearly certain icing). The product is available
in single altitudes (e.g., 3,000 ft MSL) or a composite of all altitudes from 1,000 ft MSL to FL300.
27.13.2 Icing Severity
The Icing Severity product depicts the icing intensity likelihood at locations where the Icing Probability
product depicts possible icing. Icing intensity is displayed using icing intensity categories: trace, light,
moderate, and heavy. The product is available in single altitudes (e.g., 17,000 ft MSL) or a composite of
all altitudes from 1,000 ft MSL to FL300 (i.e., max level).
27.13.3 Icing Severity Plus SLD
The Icing Severity plus SLD product depicts the intensity of icing expected as well as locations where a
threat for SLD exists. The product is available in single altitudes (e.g., 3,000 ft MSL) or a composite of all
altitudes from 1,000 ft MSL to FL300 (i.e., max levelError! Reference source not found.).
SLD is defined as supercooled water droplets larger than 50 micrometers in diameter. These size droplets
include freezing drizzle and/or freezing rain aloft.
Icing intensity is displayed using icing intensity categories: trace, light, moderate, and heavy.
27.14 Graphical Turbulence Guidance (GTG)
The NWS produces a turbulence product that is derived from airborne turbulence observations and NWS
model data with no forecaster modifications. This product is GTG.
GTG computes the results from more than 10 turbulence algorithms , then compares the results of each
algorithm with turbulence observations from both PIREPs and AMDAR data to determine how well each
algorithm matches reported turbulence conditions from these sources. GTG then weighs the results of this
comparison to produce a single turbulence forecast. Note that the success of GTG is proportional to the
number of PIREPs and AMDAR reports available to verify the algorithms. This means the accuracy of
GTG imp roves during daylight hours and where there is more traffic making PIREPs and sending of
AMDAR data. GTG produces its forecasts every hour. Currently, GTG has separate forecasts for each hour
through the first three hours, followed by forecasts at three-hour intervals through 18 hours. GTG forecasts
are available at select altitudes from 1,000 ft MSL through FL450. GTG forecasts are also scaled to three
ICAO weight class sizes for aircraft, with light-sized aircraft being less than 15,500 lb, heavy-sized aircraft
being more than 300,000 lb, and medium-sized in between.
Chapter 27, Forecasts 27-55
GTG does not specifically predict turbulence associated with convective clouds or small-scale local terrain
features, but it does predict turbulence associated with upper-level clear and mountain wave sources.
GTG provides three depictions of turbulence:
• CAT,
• MWT, and
• Combined Turbulence (the Combined GTG product depicts the higher of CAT values and MW T
values at any given point).
This turbulence product will continue to evolve over the coming years with increased model resolutions,
additional horizontal layers, and improvements to the algorithms and/or data sets used to produce the
product. Users can find additional information on these products and any changes on the AWC’s
“Turbulence” web page.
The GTG product suite is issued and updated every hour by the AWC and is available on the AWC ’s
website and other sources.
27.15 Cloud Tops
The Cloud Tops product is one of the products transmitted through the FIS-B. This product uses HRRR
model data, which currently provides a one- and two-hour forecast of the altitude of cloud tops and the
cloud amounts. The FIS-B data source receives the cloud tops data from this model. The HRRR model data
is updated hourly and the transmission interval occurs every 15 minutes.
This product is only currently available for the CONUS.
27.16 Localized Aviation Model Output Statistics (MOS) Program (LAMP)
The NWS has a long history of developing and using statistical analysis of historical and model weather
data to produce forecast guidance for forecasters, which is known as MOS.
The LAMP system was developed to provide aviation forecast guidance. LAMP is designed to frequently
update the central MOS product suite primarily by incorporating the most recent observational data. The
guidance is available at over 2,000 stations in the CONUS, Alaska, Hawaii, and Puerto Rico. The products
are updated hourly and valid over a 25-hour period.
The LAMP products are derived from a statistical model program that provides specific -point forecast
guidance for select weather elements (e.g., precipitation, temperature, wind, visibility, ceiling height, sky
cover). LAMP aviation weather products are provided in both graphical and coded text format and are
currently generated for more than 2,000 airports in the CONUS, Alaska, Hawaii, and Puerto Rico.
The LAMP product may be used for destination forecast planning by the General Aviation (GA)
community. Additionally, while LAMP is one product that can be used for destination forecast planning, it
isn’t the sole means by which someone can conduct destination forecast planning.
27.16.1 Alaska Aviation Guidance (AAG) Weather Product
The AAG is a completely automated product designed to provide a short -term projection of weather
conditions at select locations based off the LAMP. The goal of this product is to provide additional aviation
guidance to Alaska airports that have AWOS or ASOS observations, but do not have TAFs.
The AAG is a decoded plain language forecast valid for six hours and updated hourly.
Refer to the FAA’s Information for Operators (InFO) 20002 , Use of the Experimental Alaskan Aviation
Guidance (AAG) Weather Product, dated March 25, 2020, for additional information and use of the AAG.
Chapter 27, Forecasts 27-56
27.16.1.1 AAG Example
Guidance for: PXXX (Someplace, AK) issued at 0900 UTC 12 Jun 2019
Forecast period: 0900 to 1000 UTC 12 June 2019
Forecast type: FROM: standard forecast or significant change
Winds: from the E (90 degrees) at 21 MPH (18 knots; 9.3 m/s) gusting to 28 MPH
(24 knots; 12.3 m/s)
Visibility: 2.00 SM (3.22 km)
Ceiling: 1500 feet AGL
Clouds: overcast cloud deck at 1500 feet AGL
Weather: -RA BR (light rain, mist)
27.17 Additional Products for Convection
This section will describe the following additional thunderstorm forecast products produced by the NWS
that are of interest to aviation users:
• Convective Outlook,
• TCF, and
• ECFP.
Note: The National Convective Weather Forecast (NCWF) was retired in 2018.
27.17.1 Convective Outlook (AC)
The NWS SPC issues narrative and graphical Convective Outlooks (AC) to provide the CONUS NWS
WFOs, the public, the media, and emergency managers with the potential for severe (tornado, wind gusts
50 kt or greater, or hail with diameter one inch or greater) and non-severe (general) convection and specific
severe weather threats during the following eight days. The Convective Outlook defines areas of marginal
risk (MRGL), slight risk ( SLGT), enhanced risk ( ENH), moderate risk ( MDT), or high risk ( HIGH) of
severe weather based on a percentage probability, which varies for time periods from one day to three days,
and then two probabilistic thresholds for days four through eight. The day one, day two, and day three
Convective Outlooks also depict areas of general thunderstorms ( TSTM). The outlooks in graphical (see
Figure 27-28) and text formats are available on the SPC’s website. See Figure 27-29 for the legend.
Chapter 27, Forecasts 27-57
Figure 27-28. Day 1 Categorical Convective Outlook Graphic Example
Figure 27-29. Categorical Outlook Legend for Days 1-3 Convective Outlook Graphic Example
27.17.2 Traffic Flow Management (TFM) Convective Forecast (TCF)
The TCF is a high -confidence graphical representation of forecasted convection meeting specific criteria
of coverage, intensity, and echo top height. The TCF graphics are produced every two hours and are valid
at four, six, and eight hours after issuance time.
Areas of convection in the TCF include any area of convective cells meeting the following criteria (at a
minimum):
1. Composite radar reflectivity of at least 40 dBZ;
2. Echo tops at or above FL250;
3. Coverage (criteria 1 and 2) of at least 25 percent of the polygon area; and
4. Forecaster confidence of at least 50 percent (high) that criteria 1, 2, and 3 will be met.
Lines of convection in the TCF include any lines of convective cells meeting the following criteria (at a
minimum):
1. Composite radar reflectivity of at least 40 dBZ having a length of at least 100 NM;
2. Linear coverage of 75 percent or greater;
Chapter 27, Forecasts 27-58
3. Echo tops at or above FL250; and
4. Forecaster confidence of at least 50 percent (high) that criteria 1, 2, and 3 will be met.
All four of the threshold criteria listed above for both areas and lines of convection are necessary for
inclusion in the TCF. This is defined as the minimum TCF criteria. The TCF does not include a forecast for
all convection. If the convection does not meet the threshold criteria, it is not included in the TCF.
The TCF domain is the FIR covering the CONUS and adjacent coastal waters. It also includes the Canadian
airspace south of a line from Thunder Bay, Ontario, to Quebec City, Quebec.
From March 1 through October 31, the TCF is collaboratively produced by meteorologists at the AWC in
Kansas City, MO, and embedded at the FAA ATCSCC in Warrenton, VA; at the CWSU embedded at the
FAA’s ARTCC; at various airlines; and by other authorized participants. Automated routines will continue
to make the TCF available from November 1 through February 28.
The TCF is issued 24 hours a day, seven days a week at 30 minutes prior to the indicated issuance time.
The issuance time supports the FAA’s Strategic Planning Webinar, which occurs 15 minutes following odd
hours Eastern Time. The Canadian portion of the forecast is available from April 1 through September 30.
However, NAV CANADA may request the issuance of each forecast as early as March 1 and as late as
October 31. All available Canadian forecasts are incorporated into the TCF. During times the forecasts are
not available for Canadian airspace, the TCF graphics will be annotated with “No Canadian TCF. ” The
graphical representation is subject to annual revision.
The AWC also produces an Extended TCF that provides TCFs from 10 to 30 hours at two-hour increments.
The TCF and Extended TCF is used by air traffic management decisionmakers in support of convective
weather mitigation strategies within the NAS. It is designed to meet the needs of TFM decisionmakers at
the FAA’s ATCSCC, the FAA’s ARTCC TMUs, and airline and corporate flight operations centers (FOC).
Figure 27-30 shows an example of a TCF.
Figure 27-30. TCF Example
Chapter 27, Forecasts 27-59
27.17.3 Extended Convective Forecast Product (ECFP)
The ECFP planning tool (see Figure 27-31) is a graphical representation of the forecast probability of
thunderstorms and is intended to support the long -range planning for convective constraints in the NAS.
The product identifies graphically where thunderstorms are expected through the next 72 hours over the
CONUS. Although the ECFP uses TCF -style graphics to facilitate ease of interpretation, the ECFP does
not forecast TCF criteria.
The development of the ECFP planning tool was a response to FAA and industry needs in planning for
weather hazards, specifically convection, one to three days in advance. To meet these planning needs, the
ECFP provides traffic planners and collaborators a quick look at where the probability of convection is
greatest. By utilizing TCF -style graphics, users familiar with the TCF can easily determine where traffic
constraints are most likely to occur over the next three days.
The ECFP is an automated forecast product issued by the AWC. It is issued four times a day at
approximately 0100, 0700, 1300, and 1900 UTC.
Figure 27-31. ECFP Example
27.17.4 Watch Notification Messages
The NWS SPC issues severe weather Watch Notification Messages to provide an area threat alert for the
aviation meteorology community to forecast organized severe thunderstorms that may produce tornadoes,
large hail, and/or convective damaging winds within the CONUS.
Chapter 27, Forecasts 27-60
The SPC issues three types of Watch Notification Messages:
• Aviation Watch Notification Message (SAW),
• Public Severe Thunderstorm Watch Notification Message, and
• Public Tornado Watch Notification Message.
The SAW was formerly known as the Alert Severe Weather Watch Bulletin (AWW), as well as the Severe
Weather Forecast Alert. The NWS no longer uses these titles or acronym for this product. The NWS uses
the acronym SAW for the Aviation Watch Notification Message but retains “AWW” in the product header
for processing by weather data systems. The NWS uses the acronym AWW for their Airport Weather
Warning product, which is a completely different product from the SAW (see Section 27.17.4.1).
The Severe Thunderstorm and Tornado Watch Notification Messages were formerly known as the Severe
Weather Watch Bulletins (WW). The NWS no longer uses that title or acronym for this product but retains
“WW” in the product header for processing by weather data systems.
It is important to note the difference between a Severe Thunderstorm (or Tornado) Watch and a Severe
Thunderstorm (or Tornado) Warning. A “watch” means severe weather is possible during the watch valid
time, while a “warning” means that severe weather has been observed or is expected within the hour. Only
the SPC issues Severe Thunderstorm and Tornado Watches, while only NWS WFOs issue Severe
Thunderstorm and Tornado Warnings.
27.17.4.1 Aviation Watch Notification Message (SAW)
The SPC issues the SAW to provide an area threat alert for the aviation meteorology community to forecast
organized severe thunderstorms that may produce tornadoes, large hail, and/or convective damaging winds
as indicated in Public Watch Notification Messages.
The SAW product is an approximation of the area in a watch. For the official area covered by a watch, see
the corresponding Public Watch product. To illustrate, Figure 27-32 is an example of the Aviation Watch
(polygon) compared to the Public Watch (shaded). Also, the SAW is easier to communicate verbally over
the radio and telephone than reciting the entire Public Watch product.