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Area forecast discussion...updated 
National Weather Service Fort Worth Texas
1230 PM CST Sat Dec 3 2016

Aviation...
/18z tafs/

Intermittent light rain will persist throughout the afternoon as a
steady stream of showers continues spreading north across our taf
sites. The heartier echoes are deeper showers, benefiting from
decent mid-level lapse rates. However, the instability aloft is
wholly inadequate for thunder; thus, we've removed the mention in
the Waco taf. The vigorous 850mb flow will be focused into
southeast Texas where cape values will continue to support
embedded thunder. The Cedar Creek cornerpost is northwest of the
best instability and should remain fully open this afternoon.

The swath of precipitation impacting our taf sites will gradually
shift east late in the day. Northeast flow within the boundary
layer this time of year typically results in deteriorating
ceilings/visibility, and LIFR conditions are expected late
tonight. As the stratus deck supersaturates and descends, drizzle
will likely prevail. Conditions will slowly improve to MVFR by
Sunday afternoon, which will be a respite from the rainfall.

25

&&

Update...
a chilly and damp/rainy day will continue across the region this
afternoon. The best isentropic lift continues to shift east of the
region and is taking our first wave of rain with it. However,
scattered areas of light rain and showers will continue across
the region as some isentropic lift continues, in particular along
the 290-295k surfaces. Later this afternoon, another wave of more
widespread rain will likely move across our south and southeast
counties as indicated by hi-res guidance. This is separate from
the next round of rain expected tonight. Rainfall intensities this
afternoon are expected to be light to moderate, and rainfall
amounts are expected to be less than half an inch. Decided to
remove all mention of thunder this afternoon, even though an
isolated thunderstorm may occur later this afternoon in our
southeast counties.

Temperatures this afternoon will remain in the 40s to near 50
degrees under overcast skies. East and northeast winds between
5-15 mph will make it feel slightly cooler. Overall the forecast
was in good shape and only minor adjustments were needed.

Jldunn

&&

Previous discussion... /issued 339 am CST Sat Dec 3 2016/
morning water vapor imagery shows that our upper low which will
bring several rounds of rain to the area has become cutoff and
continues to dig southeastward into northern Mexico. In addition,
a large plume of mid/upper level moisture can be seen
overspreading the region within a strong upper jet axis.
Increasingly diffluent flow aloft in advance of this upper low
will continue to support widespread rainfall across much of the
Southern Plains over the next 3 days.

Our first of 4 rounds of rain is departing to the northeast early
this morning, but our second wave is not far behind. A large swath
of showers and storms has developed across south/central Texas
overnight which will continue expanding northward over the next 12
hours. All of north and central Texas is expected to receive rainfall
today, especially throughout the morning hours. The development of
this rain swath is due to incredibly strong warm
advection/isentropic ascent above an inversion in the lowest
~200mb. Ascent is being aided due to our position beneath the
right entrance region of a 120kt 300mb jet located over the
Central Plains where strong upper-level divergence is occurring.
Elevated instability remains limited, and thunder becomes
increasingly unlikely the farther north your location. Have
restricted a mention of thunderstorms to along/south of I-20
throughout the day where at least a few hundred j/kg of MUCAPE is
available. The most intense rainfall is still expected to be
across our southeastern counties, roughly southeast of a Killeen
to Athens line; this area could receive in the neighborhood of
3-4" of rain through early this afternoon. At this time, do not
have much concern regarding flooding issues as antecedent
abnormally dry/drought conditions have been in place, so 3-4" of
rain over a 6-12 hr period should not pose much of a threat other
than some ponding in the typical low-lying areas. For these
reasons, and after inter-office collaboration, did not feel a
Flash Flood Watch was warranted in our forecast area. After this
batch of rain begins weakening and shifting to the east later this
afternoon (due to the strongest warm advection shifting to the
east), a lull in the rain activity should occur during the evening
hours.

Also of note for today, our high temperatures have already
occurred (at midnight) and will continue falling or remain nearly
steady in the mid 40s to around 50. Increasing east/northeast
winds at 10-15 mph coupled with cooling temperatures and
widespread rain will make for a nasty day for any outdoor
activities that may still be scheduled.

The third batch of rain should begin developing late tonight and
last into the early hours on Sunday morning. Most of this
activity will be across our central Texas counties where the better
moisture content will be located as a northern stream shortwave
swings through the area causing some weak ascent. Some parts of
North Texas may actually remain dry during this time frame. As the
shortwave departs to the east Sunday morning, we may actually
become weakly subsident, and the plume of mid-level moisture
should start to shift east of the area. As a result, most of North
Texas may actually be dry for most of Sunday while showers remain
possible across our far southern counties where an overrunning
setup remains in place in advance of the approaching cutoff low
moving northeastward out of Mexico.

The 4th and final wave of showers and thunderstorms will occur
late Sunday night through Monday as the upper low ejects
northeastward directly across our forecast area. A surface coastal
low is expected to develop across south Texas as the low encounters
increased baroclinicity. This should be some of the strongest
forcing out of the entire event, and with cooling mid-level
temperatures, it should be the best chance for thunderstorms area-
wide rather than more stratiform rain. Have continued likely pops
area-wide on Monday with a mention of thunder as well. Depending
on the rainfall totals on Saturday/Sunday, we'll need to keep an
eye on a low flooding potential for monday's activity as many
areas will now have saturated soils.

While the rain will come to an end Monday night, that doesn't
mean we're done with an active weather pattern. A shortwave will
drive a weak cold front into the area on Tuesday, so despite only
partly cloudy skies, temperatures will struggle to rebound to much
warmer than the mid 50s to low 60s due to weak cold advection.
This front will also keep the low-level moisture shunted to the
south of the area.

A much stronger front is still expected to sweep through the
region sometime Wednesday afternoon or Wednesday night, but models
still disagree on the timing at this point. Without much moisture
available, am not expecting much rain with this front, although
will keep a mention of showers east of I-35 in advance of the
front where some weak isentropic ascent and steep mid-level lapse
rates will be in place. Regardless of the cold front's arrival
time, Thursday will be the coldest day of the season so far with
lows starting out mostly in the 20s and strong cold advection
keeping high temperatures in the upper 30s and low 40s during the
day. In addition, it will be quite windy with this front due to
strong surface pressure rises, and wind chill values in the teens
will be likely for some locations on Thursday morning. Cold
temperatures will continue through the end of the week with
surface high pressure continuing to build southward through the
plains. Have undercut guidance on temperatures by a few degrees as
models generally struggle with the "extreme" temperature changes
and events that differ vastly from seasonal normals. A slight
warming trend should occur heading into next weekend.

-Stalley

&&

Preliminary point temps/pops...
Dallas-ft. Worth 49 45 53 47 51 / 100 60 40 60 70
Waco 50 46 52 47 53 / 100 80 50 80 70
Paris 48 44 51 44 50 / 100 50 30 50 80
Denton 48 45 53 46 51 / 100 60 30 60 70
McKinney 48 45 53 46 50 / 100 60 30 60 70
Dallas 49 46 53 47 51 / 100 60 40 60 70
Terrell 49 46 53 47 52 / 100 60 40 70 70
Corsicana 51 46 53 47 52 / 100 80 50 80 70
Temple 49 46 51 47 54 / 100 80 60 80 70
Mineral Wells 48 45 54 45 52 / 100 60 40 60 70

&&

Forward watches/warnings/advisories...
none.
&&

$$

25/82

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