Area Forecast Discussion
Issued by NWS Dallas/Fort Worth, TX

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FXUS64 KFWD 120923
AFDFWD

Area Forecast Discussion
National Weather Service Fort Worth TX
423 AM CDT Sat Aug 12 2017

.DISCUSSION...
Two separate impulses rotating around the northwest periphery of a
mid-level ridge are responsible for the widespread convection
across West Texas early this morning. The two resulting convective
complexes have essentially joined forces, effectively becoming one
shortwave as they approach North Texas today. The extent of the
activity and its reach as far south as the Trans-Pecos should help
to deamplify the ridging across the southern part of the state,
allowing the westerlies to slip farther south into North Texas.

The outflow boundary that invaded North Texas Friday morning was
the focus for renewed thunderstorm development Friday afternoon.
With the radar quiet and near-calm winds within the nocturnal
boundary layer, it is difficult to locate the boundary during the
predawn hours. However, it remains well defined in VWP data (GRK-
FWS-TLX), and after the sun rises this morning, it should become
easier to identify in the surface data. Although the low-level
flow will remain fairly light today, the increased westerly flow
above the boundary may enhance the warm/moist advection on the
south side. In addition, rain-cooled air and outflow winds on the
north side would only further clarify the discontinuity. The
resulting moisture pooling along the boundary will increase heat
index values even if cloud cover shaves a few degrees off
afternoon temperatures within the northern extent of the heat
advisory. It will also give a bump to the already adequate
surface-based instability.

A combination of daytime heating and the approaching forcing aloft
should aid in the initiation of showers and thunderstorms along
the boundary later today. Despite the quality of the surface
moisture and its contribution to enhancing the instability,
meager inflow and marginal lapse rates aloft should limit the
precipitation efficiency. However, the slow movement of cells may
increase rainfall amounts, and the resulting precip loading could
result in strong winds in an environment that would otherwise not
be particularly conducive to downbursts. In addition, the backed
flow in vicinity of the boundary may enhance low-level rotation,
though the flow aloft will not be supportive of deep mesocyclones.
But for cells near the boundary, the mechanics in the sub-cloud
layer could produce a brief landspout, particularly when an
updraft is still in its developmental stage. Even with dew points
in the 70s, the summer heat will mean storm bases will be in
excess of 3kft AGL, which is not as favorable for tornadogenesis.
The severe threat will be enhanced for cells that are able to
both move sufficiently eastward while maintaining their proximity
to the boundary. The increasing westerly flow aloft may aid in
this potential, but more disorganized convection on both sides of
the boundary could disrupt this process.

Even with extensive activity this afternoon and evening, there may
be no respite as additional forcing for ascent arrives tonight.
The flow suggests the resulting nocturnal MCS would impact areas
along and north of the I-20 corridor, but there remains some
disagreement among available mesoscale guidance about the
evolution of the system. The most likely area to be impacted by
significant rainfall through Sunday morning still appears to be
our northeastern zones. While these areas have had nearly a week
to dry out from their last heavy rain event, if this complex
follows significant rainfall this afternoon/evening, there may be
a concern for flooding. The day shift will have a better handle on
this potential and the need/placement of any flood watch.

The unsettled westerlies will maintain some rain chances mainly
north of the I-20 corridor early in the upcoming workweek, but
subtropical ridging looks to take hold thereafter. While
climatology supports this scenario, this August has so far not
played out according to plan. Long-range dynamical output keeps
the westerlies unseasonally close to North Texas, but ensemble
members for mid-level heights look much more like summer. As a
result, will taper off rain chances on Tuesday and maintain a
rain-free forecast thereafter.

25

&&

.AVIATION... /Issued 1154 PM CDT Fri Aug 11 2017/
A frontal boundary remains draped across North Texas from north
of KSWW to KGDJ to KF44. Winds were generally northeast to east at
5 to 8 knots to the north of the boundary and light south to
southwest to its south. Showers and thunderstorms will continue to
wane during the next few hours. The frontal boundary is expected
to drift back northward during the day Saturday with winds
becoming southeasterly at 7 to 9 knots in the Metroplex during the
afternoon. Showers and thunderstorms are expected to develop by
mid afternoon /20z/ along and north of the I-20 corridor. Some
strong to severe storms are likely. Have placed VCTS in the
Metroplex TAFs for the 20z-01z period. Additional showers and
thunderstorms will be possible during the evening through the
overnight hours, thus have kept either VCSH or VCTS in the TAFs
through the period.

For KACT, showers and thunderstorms are expected expected to
remain to the north of the airport. VFR conditions should prevail
through the period with south winds 6 to 8 knots becoming
southeast during the afternoon.

58

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    95  77  94  77  95 /  40  70  40  30  20
Waco                98  78  98  78  98 /  20  20  10  10  10
Paris               90  71  85  72  89 /  30  80  70  50  40
Denton              93  76  91  76  94 /  50  70  50  40  20
McKinney            94  75  90  76  93 /  50  80  50  40  30
Dallas              96  78  93  78  95 /  40  70  40  30  20
Terrell             95  75  92  77  94 /  40  70  50  30  20
Corsicana           95  77  96  77  95 /  30  40  20  20  20
Temple              99  77  99  77  97 /  10  10   5   5  10
Mineral Wells       93  76  95  75  96 /  40  50  30  30  20

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 6 PM CDT this evening for TXZ118>122-133>135-
145>148-158>162-174-175.

&&

$$

25/82



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