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Terminal forecasts:
Reading between the lines 
Some terminal aerodrome forecasts (TAFs) are of a
higher quality than others. That is, some forecasters describe the
expected weather better than other forecasters while still adhering to
the strict NWS guidelines for TAFs.
Forecasters at the local weather
forecast offices (WFOs) issue these point forecasts four times a day.
TAFs are perhaps one of the most difficult forecasts any meteorologist
must make. Their primary goal, however, is to construct the
forecast in a way that is simple and easy for the pilot to understand.
It also must provide guidance that is representative of the
expected weather event. In other words, can a pilot clearly
envision what the weather will look like strictly based on the terminal
forecast?
Let's look at an example of a TAF that
could have been much better. Take a look at the following terminal
forecast for Lake Charles, Louisiana (KLCH). This TAF was
issued by the Lake Charles WFO on April 17, 2010 at
1143 UTC.
KLCH 171143Z 1712/1812 VRB03KT 5SM BR SCT010
TEMPO 1712/1714 1/2SM FG VV010
FM171500 13010G18KT P6SM VCSH SCT035
BKN150=
This is a
forecast that describes a marginal VFR (MVFR) condition (5SM BR SCT010) that
is temporarily
interrupted by a low IFR (LIFR) weather event (1/2SM FG VV010).
Keep in mind that a forecast for 5SM and 1/2SM is teetering
right on the very edge of a VFR and very low IFR (VLIFR) event,
respectively - VFR is greater than 5SM visibility and VLIFR is less
than 1/2SM visibility. This means that the pilot could expect
to see the weather vary from nearly VFR (based on the prevailing
conditions) to nearly VLIFR (based on the TEMPO conditions) all within
the two
hour period from 1200 UTC through 1400 UTC. This particular
forecast did not accurately describe the weather that was most likely
to occur on this April morning.
The first point of confusion is that FG
(fog) is used by the forecaster as the obstruction to visibility.
For some pilots that may paint an image of a low stratus
cloud temporarily blanketing the terminal area such as shown in the
image above for a radiation fog event at the Napa County airport.
Click here to view a larger image.
When, in fact, the FG (fog) descriptor is simply a
manifestation of the 1/2SM visibility forecast. In other
words, the NWS directives require that FG be used anytime there is a
forecast of 1/2SM or less regardless of whether a true cloud exists
near the surface. In this case there was no such stratus
cloud blanketing the surface.
Instead, the weather was expected to
improve after 1200 UTC even though it would begin the morning as a LIFR
event. The meteorologist could have prepared this forecast
differently to provide better guidance to the pilot. So what
triggered the forecaster to produce such a pessimistic forecast?
All TAFs begin with prevailing
conditions. It is not surprising that the first couple of
hours of the forecast should look remarkably like the latest surface
observation (METAR). In this case, at 1059 UTC, the latest
surface observation (below) for Lake Charles before the TAF was issued
included a surface visibility of 3/4SM, BR (mist) and a VV010 (vertical
visibility of 1,000 feet). This METAR likely influenced the
forecasters decision to include VV010 in the TEMPO group.
KLCH 171059Z 00000KT 3/4SM BR VV010 14/13 A3002 RMK AO2
As the sun was just beginning to rise, the
terminal area was likely extremely hazy, but not foggy per se -
although, forward visibility was still very poor. The
forecaster decided to take the visibility forecast down to 1/2SM
instead of sticking with a 3/4SM forecast with mist which would have
been more representative of the conditions at the Lake Charles airport
since the weather conditions were expected to improve from a LIFR
flight category to a VFR flight category very quickly with time.
KLCH 171143Z 1712/1812 00000KT
3/4SM BR VV010
FM171330
VRB03KT 3SM BR
CLR
FM171500 13010G18KT P6SM VCSH SCT035 BKN150=
From a pilot's perspective, the TAF above shows
the expected trends in the weather without any significant confusion.
This TAF implies the weather starts out as LIFR with a steady
and rapid improvement in visibility and ceiling with time. As
seen (below) by
the subsequent METARs for Lake Charles, the weather did improve quickly
and by 1400 UTC,
the sky was essentially clear bordering on a VFR flight category.
KLCH 171153Z 00000KT 3/4SM BR VV010 14/13 A3003 RMK AO2
KLCH
171234Z 00000KT 2SM BR CLR 16/15 A3003 RMK AO2
KLCH
171253Z 00000KT 2 1/2SM BR CLR 17/16 A3004 RMK AO2
KLCH
171317Z 05003KT 3SM BR CLR 18/17 A3004 RMK AO2
KLCH
171353Z 00000KT 5SM BR CLR 19/17 A3005 RMK AO2
You might be thinking that anyone can build a
good forecast after the fact. True, but a
meteorologist has an obligation to forecast the most likely event.
While a VV010 with a 1/2SM visibility is meteorologically
possible, it is also highly unlikely. In most cases where the
visibility was 1/2SM or less with a vertical visibility of 1,000 feet,
there were other events taking place such as heavy snow (+SN), blowing
snow (BLSN), blowing dust (BLDU) or heavy rain and thunderstorms
(+TSRA). Purely based on climatology the occurrence of a
visibility less than 3/4SM and a VV010 with just FG as the obstruction
to visibility is extremely small. In the end, the forecaster
had no justification for taking the visibility down to 1/2SM especially
when the forecaster clearly felt the conditions would quickly improve.
The purpose of this discussion wasn't to be
nitpicky about a forecast, but to understand how a small difference to
a forecast can provide the pilot with much better expectations and less
confusion.
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