Spring is around the corner!

[NWS Lubbock - Joe]

Hi all,
As we begin the transition from Winter to Spring, our weather is
changing. You've no doubt experienced the dust, wind, and fire back
on February 27th. That was the largest fire outbreak in about 5 years
here in West Texas and we're in for a bit more today. Granted, the
storm today will not be quite as bad as the last one, it still has the
potential to be a high-impact fire weather day.

So, as pilots, what does fire weather mean? In our region, fire
weather is generally comprised of relative humidity under 15 percent
and winds at the 20' height of 20 mph or more. When grasses are dry,
as they are right now, it is very easy to start a large fire when
these conditions exist. So, if you fly out of a grass strip, be sure
to stay out of the grass when doing your run-up. What else does it
mean? Well, for us normally aspirated pilots, it means that if we
braved taking off in fire weather conditions, we probably will get a
rough ride. Usually, to get fire weather conditions, there is
typically no substantive temperature inversion aloft and very
efficient mixing occurs in the vertical. So, the temperature will
probably decrease dry adiabatically (close to 9.8 degrees C/km) until
above 10k ft. Its a nice cool off, but with no inversion to prevent
mixing, the turbulence will be noticeable.

As the title mentions, Spring is not far away. As a matter of fact,
some of the models are showing the dryline possibly becoming active
with thunderstorms as early as this Sunday east of Interstate 27. The
dryline is most definitely something to be aware of when you fly. On
the west side, its possible to get the aforementioned fire weather.
On the east side, thunderstorms. In the early morning, fog is often
prevalent just east of the dryline as well.

If flying during the late evening through daybreak hours, one extra
feature which may be of interest is the low level jet. Usually, as an
upper low approaches from the west, a fairly narrow jet of winds
(usually southerly at 30-45 kts) forms after dark, peaking around
midnight. If you happen to be heading north out of here overnight, it
may save you time to make use of this feature. This feature usually
shows up around 5000 MSL here. However, use caution as this is a
stream for moist air advection and clouds may exist in its vicinity.
To ask about the location of the low level jet and ascertain its cloud
risk, simply give a call to the National Weather Service forecast
office and we'll be glad to tell you what it looks like. As we have
several forecasters in the rotation, just say "I'm a pilot flying from
KXXX to KXXX tonight. How strong and where will the low level jet be
tonight and are clouds expected in its vicinity?"

POP QUIZ TIME:

1. At what height are the airfield winds measured? (like an AWOS or
ASOS)

A. 10 meters or about 30'. Why? Simply because it is the winds
encountered when beginning the transition to landing flare.

2. How does the ASOS know that thunderstorms exist in the vicinity of
an airport?

A: Well, the logical answer might be some sort of sensor on-site.
Well, good try, but the real answer is different. The ASOS (Automated
surface observation system) receives data from the National Lightning
Detection Network, a corporate run network in the USA. Lightning
information is either automatically triggered from this information or
via a contract weather observer, such as we have at Lubbock's Preston
Smith International. There are two ASOS units in the Lubbock county
warning area and are maintained by our forecast office technicians.
At KLBB and KCDS. The observation at Plainview is an AWOS (Automated
Weather Observing System.) AWOS units are usually state maintained
devices.

3. In the spring, we often have warm fronts that push north across the
area ahead of a dryline. What are some of the most prolific aviation
weather hazards near these boundaries?

A: Warm fronts are notorious for low ceilings, poor visibility, and
significant icing just north of the warm front boundary.


THE AVIATION FORECAST DISCUSSION

As we enter the transition season, it remains as important as ever to
be aware of the weather conditions along your route of flight. One of
the best ways to better understand forecast uncertainties is by
viewing the aviation forecast discussion (AFD). This product is free
form and allows forecasters to communicate potential flight issues
that cannot be coded in the TAF. You can find the AFD here
http://aviationweather.gov/products/afd/


AND FINALLY...

I encouraged members to submit their questions about any aspect of
aviation weather and we have our first one.

Q: Referring to the attached ADDS Icing Chart, when the weather
depicted is speckled (i.e. not a solid color but instead white and
blue or white and purple, etc.), what does that indicate?
Reference: http://aviationweather.gov/adds/icing/frzg_nav.php


A: From the Aviation Weather group in Kansas City

"Hatched regions (if present) represent areas with multiple freezing
levels"

Indeed, our plotting code tests for multiple different cases:
- freezing from the surface all of the way up through the column
- freezing at the surface but warm (above freezing) higher in the
column
- warm at the surface, freezing above, then warm again above

In any of those case, it will be more helpful to look at the
conditions
at each individual altitude, as presented in the Icing plots (CIP/
FIP),
since a single Freezing Level Plot can't capture the altitudes of
every
freezing/warming level.

Fly safely and keep the questions coming!
Joe Jurecka
Aviation Program Manager - NWS WFO Lubbock