|
Verify those observations -
when METARs lie 
It is easy for a pilot to casually accept a
surface
observation (METAR) at face value. While it is important to
be
especially careful trusting automated surface observations (AUTO), it
is just as critical to verify the integrity of ALL surface observations
you may use. Depending on the METAR and the actual weather
environment, verifying the integrity of the observation is often
trivial.
One way to verify the integrity of a surface observation is
through the use of a "buddy check." In other words, examine
the
surface observations around the same general vicinity. Are
all of
the observations consistent? Or is there one that stands out
as
being different? In this example (above) from Shreveport in
northwest Louisiana there are three observations in close proximity to
include, Shreveport Regional Airport (KSHV), Barksdale Air Force Base
(KBAD) and Shreveport Downtown Airport (KDTN). Based on the
precipitation type symbols (click here
to see a mapping of METAR code to graphical symbols) Barksdale Air
Force Base is reporting light freezing rain (-FZRA) and
Shreveport
Downtown Airport is reporting an unknown precipitation type (UP).
However, Shreveport Regional Airport is reporting light snow
(-SN). While all of the observations are reporting a
consistent
surface temperature of 30°F, is it really snowing at Shreveport
Regional Airport?
While there may be
a valid meteorological reason for this inconsistency, in some instances
the observation (in part) may be erroneous. In this case, an
observation of light snow at Shreveport Regional is a bit inconsistent
with the other two reports. When a METAR reports UP
or unknown precipitation, the precipitation is likely to be ice pellets
(sleet), freezing drizzle, light freezing rain or some kind of a
precipitation mixture. This matches more closely to the
report of
freezing rain from Barksdale Air Force Base.
In the case of freezing rain, freezing drizzle or
ice
pellets being reported at the surface, there is likely a fairly
significant temperature inversion in the atmosphere above.
One of
the best tools to view the temperature profile aloft is a thermodynamic
diagram.
At 1200 UTC, a radiosonde (weather balloon) was
launched
from the NWS Weather Forecast Office in Shreveport. When this
data from the radiosonde is depicted graphically on a thermodynamic
diagram such as a Skew-T log (p) diagram, it is called a temperature
sounding. Click here
to view a complete Skew-T log (p) diagram for Shreveport. The
Skew-T log (p) diagram for Shreveport (above) confirms that a
significant temperature inversion was indeed present with temperatures
significantly above 0°C from 2,000 feet MSL through 10,000 feet MSL.
With a layer nearly 8,000 feet thick consisting
of
temperatures above freezing,
snow (especially light snow) falling into this warm layer (called a
warm nose) cannot possibly survive to the surface. Given the
other two surface observations and the temperature sounding, the report
of light snow is definitely erroneous.
Want
to learn more about reading a Skew-T log (p) diagram?
We are now offering the Introduction to the Skew-T log (p) Diagram
premium workshop for only $99.
This is $30 off the regular price of this workshop.
If you
want to learn more about this diagram, here's your chance to get it at
the lowest price available. If you are still not convinced, click here to see a 15 minute
preview of this premium workshop.
Don't
delay, get online access to this 2 hour and 15 minute premium workshop
right now for only $99! We'll send you the program on CD-ROM
for
an additional $10 shipping and handling. Regular members of
AvWxWorkshops.com receive a 10%
discount and Elite members receive a 20% discount off of this premium
workshop. Click here to purchase.
|
