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afdfwd

Area forecast discussion...updated
National Weather Service Fort Worth Texas
723 PM CDT Sat Oct 21 2017



Aviation...
concerns...line of strong to severe storms expected to affect the
metroplex taf sites 05-07z...and Waco 08-10z.

A cold front extended southeast of a khbr-kfls-kmus line as of
00z while a dryline extended southwest from kpfs to south of kcgs
and khob. Scattered to numerous showers and thunderstorms will
continue ahead of this front. As this front continues to move
southeast a full line is expected to develop and there will be
scattered showers and thunderstorms will occur ahead of the front.
Expect the line of storms is forecast to reach the metroplex taf
sites 05-07z. These storms will have the potential to be severe
with wind gusts of 50 knots and hail around 1 inch. In the tafs,
have included 31030g40kt for the 05-07z period with the
thunderstorms. Conditions will improve to VFR behind the front
but there will be northerly winds 20 to 25 knots. Wind speeds will
gradually decrease mid to late afternoon with speeds lowering to
around 12 knots by 00z Monday.

A similar situation is expected at Waco. MVFR ceilings are
expected to spread into Waco around 06z. The line of storms ahead
of the front should move through 08z to 10z with some strong to
severe storms possible. VFR conditions are expected after 11z with
north winds around 20 knots with some gusts of 25 to 30 knots.

58

&&



Short term... /issued 300 PM CDT Sat Oct 21 2017/
/tonight and tomorrow/

The focus of the short-term forecast lies on the much-anticipated
cold front and squall line arriving during the overnight hours.
Strong or severe storms are expected to affect most of the
forecast area with the highest severe threat north of I-20. While
damaging winds will be the main threat with the squall line, hail
and a couple brief spin-up tornadoes are also possible given the
environmental conditions discussed below.

An unseasonably warm and humid airmass is in place across north
and central Texas today with temperatures climbing to near-record
values across parts of central Texas this afternoon. These
conditions have contributed to ample instability across most of
the area. However, a Stout cap remains in place which will
continue to suppress deep convection through the next several
hours. Ascent is still relatively weak with the upper trough
displaced well to the northwest at this time. As a result, only
the weak lift from warm advection has been able to generate
isolated showers across areas east of I-35 so far this afternoon.
An isolated storm is possible in this area through the late
afternoon or early evening hours when instability will be
maximized, although the lack of vigorous ascent should keep
activity fairly tame.

By 6-7pm, we should see a marked increase in convective activity
through southwest OK and the Texas Panhandle region along the
southward-advancing cold front with aid from an intense shortwave
trough. Additional cells may develop a bit farther south along
the dryline and near the dryline/front intersection around
Abilene. Initial development will consist primarily of discrete
cells, many of which will be capable of becoming severe, although
this activity will likely remain 50+ miles west/northwest of our
forecast area. Convection should grow upscale rather quickly as
cold pools congeal, especially given that storm motion and deep-
layer shear vectors are nearly parallel to the frontal boundary. A
qlcs should develop by late evening, prior to any convection
arriving in our forecast area from the northwest.

By 9-10pm, the squall line should be moving into our northwest
counties along the fast-moving front. As the strong shortwave
approaches, lapse rates will steepen and the cap will lift/cool.
This will allow for continued surface-based convection during the
overnight hours which means all modes of severe weather will be
possible. The main concern will be the potential for damaging
straight line winds, especially associated with any bowing
segments within the line. Some severe winds in excess of 60 mph
are likely within the forecast area overnight. Given the steep
lapse rates, stronger cells within the line will also have the
potential to produce severe hail as well. In addition, widely-
curved hodographs are expected to be supportive of a qlcs
tornado threat with 20-30 kts of 0-1 km shear in place. The
northern flanks of any bowing segments will need to be monitored
closely for tornadic potential, especially north of I-20 where the
qlcs may be oriented slightly more perpendicular to the frontal
boundary and where the most vigorous lift will occur from the
shortwave trough.

At this point, there does not seem to be much of a potential for
discrete cells to develop ahead of the front. This would further
limit the tornado/hail risk if discrete cells are not present. It
appears most of the forcing will be positioned immediately along
the front which will be surging rather quickly to the southeast.
Since storm motions should be roughly parallel to the front, any
cells ahead of the front will quickly become engulfed by the line.
That being said, some embedded supercell structures would be able
to survive within the qlcs given the strong deep-layer shear. If
any cells do manage to sustain themselves ahead of the qlcs (which
again, appears unlikely), they could pose a high-end severe
threat.

The front/squall line should affect the dfw metroplex around
midnight or later, the Waco/Temple area by 3-4am, and should be
clearing the forecast area to the southeast around daybreak.
Lingering stratiform rain may persist through mid-morning across
our southeastern zones before drier Post-frontal air scours out
remaining moisture, leaving a pleasant fall Sunday with near/below
normal temperatures and clearing skies. North winds will remain
breezy through the afternoon at 10-20 mph.

-Stalley

&&

Long term... /issued 300 PM CDT Sat Oct 21 2017/
/Sunday night through Saturday/

The cold front and precipitation will have moved through the area
by Sunday night leaving drying conditions and pleasant
temperatures headed into early Monday morning. High pressure will
settle into the Southern Plains on Monday with diminishing winds
and temperatures in the lower 80s during the afternoon. A strong
upper trough will continue to amplify over the eastern half of the
U.S. Late Monday into Tuesday. As it does, a reinforcing cold
front will move through North Texas Monday night. North winds will
increase through the night and it will become gusty during the day
Tuesday with cooler air filtering into the region. Highs on
Tuesday will only manage the lower 70s. A brief warmup will occur
on Wednesday and Thursday with highs in the 80s, then a stronger
cold front will move through on Friday. At this time, it appears
that the Friday cold front will have limited moisture to work
with, so we'll maintain some very low pops. There are also some
timing differences among the guidance, but it appears that it'll
be noticeably cooler for next weekend.

Dunn

&&

Preliminary point temps/pops...
Dallas-ft. Worth 60 76 51 82 54 / 100 5 0 0 0
Waco 61 77 49 82 52 / 90 10 0 0 0
Paris 59 70 49 77 50 / 100 10 5 5 5
Denton 57 74 49 82 51 / 100 5 0 0 0
McKinney 58 73 49 80 52 / 100 10 0 0 0
Dallas 60 77 52 82 54 / 100 10 0 0 0
Terrell 60 74 48 81 51 / 100 10 0 0 0
Corsicana 62 73 49 79 52 / 100 20 0 0 0
Temple 62 77 49 82 52 / 90 10 0 0 0
Mineral Wells 57 73 47 83 49 / 100 5 0 0 0

&&

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

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