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Area forecast discussion 
National Weather Service Fort Worth Texas
1245 PM CDT Thursday Aug 21 2014

Aviation...
the last batch of straocu will continue to lift above 4000 feet as
it moves north of the metroplex taf sites. We expect few to
scattered cumulus to linger through the afternoon and dissipate with
the loss of surface heating.

A 25 to 30 knot low level jet will develop late this evening and
will bring another round of stratus to The Hill Country and
portions of North Texas. It looks like the low level jet will be a
bit more southwesterly Friday morning than it was this morning and
this should keep most of the low clouds east/southeast of the
metroplex taf sites. We will still carry scattered low clouds at
the metropolitan terminals from 14 to 17z. Waco will have a better chance
at seeing MVFR ceilings between 11 and 17z.

A southerly wind will prevail through Friday morning at speeds
between 12 and 17 knots this afternoon and between 10 and 15 knots
tonight/Friday morning.

79

&&

Update...
low clouds will continue to scatter out across the region as
drier air mixes down through the boundary layer. A partly cloudy
and hot afternoon is still expected with afternoon highs from the
upper 90s to around 101.

The current forecast is in good shape so no update is needed at
this time.

79

&&

Previous discussion... /issued 542 am CDT Thursday Aug 21 2014/
remarkably Stout low level jet for this time of year (35-40kts) has
resulted in widespread (albeit thin) stratus along the Balcones
Escarpment. A fibrous deck has invaded southern portions of the
County Warning Area early this morning...and a few fragments will likely reach the
I-20 corridor after daybreak. As these strong winds mix with the
surface layer later this morning...wind gusts will pick up again.
Sunny skies will push temperatures back up into the upper 90s to
near 100 degrees...but the wind will help take an edge off the
heat. With the best moisture flux into Louisiana today...expect
the western extent of the sea breeze activity this afternoon will
be in far southeast Texas.

The troughing in the western states is prognosticated to be a bit weaker
with the main upper low taking a sharp left turn into the prairie
provinces late in the weekend. As a result...the middle-level ridge
in the southeast will be able to build farther west the previously
prognosticated. The ridge will be neatly stacked vertically above the
boundary layer...from 10kft above mean sea level to around 35kft...which will
assure subsidence will dominate into next week. However...its
intensity is still rather Ordinary for August...and its placement
east of Texas should prevent temperatures from soaring into the
triple digits. In addition...breezy conditions will continue the
remainder of the week...which will prevent optimal heating of the
surface layer. This will reduce high temperatures 1-2 degrees from
what would otherwise be expected...meaning that achieving triple-
digit highs will be a little more difficult. A nightly low level jet will
mean repeated stratus intrusions...primarily into central Texas.
These morning clouds will be short-lived and will do little to
prevent hot afternoons. Heat index values will easily top 100 but
should remain below 105.

A piece of upper vorticity is still prognosticated to eject from the
TUTT south of Brownsville...and the southerly flow beneath it will
guide it northward over the Lone Star state. The polar jet will
effectively shear it...but the troughiness in the upper reaches
of the troposphere will linger through the weekend. Even so...the
feature will quickly become an open wave and should be unable to
disrupt the deep subsidence beneath it.

As the upper low enters Canada...the flow aloft into the Central
Plains will diminish considerably. A corresponding weakening of
the persistent Lee trough will mean lighter surface winds across
north and central Texas. This will bring an end to the morning
stratus and will make the heat feel a little more oppressive...
but it may also allow the sea breeze to reawaken.

The GFS and European model (ecmwf) solutions diverge next week...with the European model (ecmwf)
developing a cut-off low and the GFS far more progressive. The
Canadian is a compromise between the two...but leans toward the
GFS in that it keeps the flow unimpeded. Despite considerable
spread among the GFS ensemble members...the consensus favors a
frontal passage late next week. Have introduced north winds and
slight precipitation chances on Thursday next week...but these will not
appear in the public forecast until the afternoon package today.

25




&&
fire weather...
hot and rain-free conditions will persist through the weekend. A
broad upper high will be centered across the western Gulf states
and will result in highs between the middle 90s and 102 degrees
across the area. Hottest high temperatures will be across the
far western counties where deeper mixing will result in lower
afternoon humidity values each day between 20 and 30 percent.
However...20 foot winds will remain sustained between 10 to 15 miles per hour
with occasional gusts exceeding 20 miles per hour. A weak surface low near
the western half of North Texas should keep winds from becoming
too strong. This area will need to be watched closely in the
coming days for a high fire danger...particularly those areas
northwest of a Gainesville...Decatur...Comanche line. Fuel moisture
levels at both possum kingdom Reservoir and Miller/S creek RAWS
observations were already below 10...indicating some drying of
fine fuels across the area.

Otherwise...areas further to the east may see occasional higher
gusts but will also see Richer Gulf moisture and afternoon
humidity values remaining above critical levels. The higher
humidity across the eastern areas should hold high temperatures
into middle to upper 90s. The fire danger should remain low into early
next week...as wind speeds remain below 15 miles per hour.

05

&&

Climate...
late August frontal passages are remarkably common. Although normal
high temperatures (based on dfw data 1981-2010) gradually decrease
after August 17 at a steady rate...the average temperatures tell a
different story.

Daily normals are created using a harmonic fit...a method employed
to dampen outliers. The result assures a smooth transition from one
day to the next...but this smoothing removes intricacies in the data
that can reveal more abrupt seasonal transitions. For example...much
of the decrease in high temperatures during late August occurs in a
2-day span between August 26 and 28.

The data below are based on the Dallas/Fort Worth climate record
1981-2010.

Average normal
high / low high / low

August 26 97.4 / 75.4 95 / 74
August 28 93.5 / 73.4 94 / 74

The average high temperature (1981-2010) actually increases between
August 17 and 26. The sudden 3.9-degree drop in just 2 days (august
26-28) is thus very significant.

These are not simply artifacts of a 30-year data set. Using the
entire climate record (which began in September 1898) yields a
decrease in the average high temperature between August 17 and 26
(1.2 degrees) that is still less than the 2-day span August 26-28
(1.4 degrees). And yes...when incorporating the entire climate
record of more than a century of data...the time frame of the
significant drop remains August 26-28.

What does this mean?

It means that historical frontal passages cluster around these
dates...showing a tendency for the first significant late Summer
front to occur at this time. It looks like 2014 may follow suit. The
GFS projects a frontal passage next week on August 28.

Does this mean fall has arrived?

Not quite yet...as high temperatures will likely be in the 90s even
if the cold front arrives. Normal (and average) high temperatures
are still in the 90s in early September.

Another significant drop in average temperatures occurs around the
autumnal equinox...which is only a month away. (The below are based
on dfw data 1981-2010.)

Average normal
high / low high / low

September 20 88.1 / 65.8 87 / 66
September 24 83.1 / 62.6 85 / 64

Perhaps there is something to the first day of fall.

&&

Preliminary point temps/pops...
Dallas-ft. Worth, Texas 99 78 99 78 100 / 0 0 0 0 0
Waco, Texas 100 76 100 76 100 / 0 0 0 0 0
Paris, Texas 97 74 97 74 98 / 0 0 0 0 0
Denton, Texas 99 77 99 75 100 / 0 0 0 0 0
McKinney, Texas 97 75 98 74 99 / 0 0 0 0 0
Dallas, Texas 99 78 99 79 99 / 0 0 0 0 0
Terrell, Texas 97 75 98 75 98 / 0 0 0 0 0
Corsicana, Texas 98 75 98 75 98 / 0 0 0 0 0
Temple, Texas 99 76 98 74 98 / 0 0 0 0 0
Mineral Wells, Texas 99 73 99 73 101 / 0 0 0 0 0

&&

Forward watches/warnings/advisories...
none.

&&

$$

79/79

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