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

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FXUS64 KFWD 210920

Area Forecast Discussion
National Weather Service Fort Worth TX
420 AM CDT Sat Oct 21 2017

/Today and Tonight/

Breezy and warm conditions will occur today ahead of tonight`s
arrival of a potent upper level system and associated cold front.
Upper level troughing is still established over the region but the
main shortwave expected to track across the Southern Plains is
currently moving across the southern Rockies. Breezy south and
southeast winds have been occurring overnight and will continue
today and into tonight as the surface low moves across the Plains,
dragging a cold front south. H850-H700 winds will prevail from a
southwest direction today resulting in warming downslope flow.
Dense morning stratus clouds are expected to lift and partially
erode by midday allowing sunlight to contribute to a warming
factor. Some locations, in particular along and east of Interstate
35, may retain mostly cloudy skies for part of the afternoon, but
generally partly sunny skies are expected to help afternoon highs
reach the 80s across the region.

Mostly dry conditions are expected during the day, with a few
exceptions. Streamer showers have been developing across part of
the region early this morning, and isolated to scattered quick-
moving showers may continue through the late morning hours. With
southwest winds above the surface, the models are in good
agreement that a cap will keep a lid on most of the region today.
However, the cap is expected to weaken this afternoon in an area
south and east of a Cooper to Corsicana to Cameron line, and isolated
to scattered showers and storms may develop and/or move into this
area by mid to late afternoon. Instability of around 3000 J/kg and
25 kts of shear suggest a severe storm threat could exist in this
area, but lack of large scale lift may keep any activity below
severe limits. By this time, the upper level trough and associated
height falls are progged to still be too far west of the region.
The UCAR ensembles are the most aggressive with developing storms
in this area, but most other guidance is dry and/or appears to
keep the storms below strong/severe limits. However, the severe
storm threat cannot be discounted, and the main hazards would be
downburst winds and hail, but low level winds suggest a tornado
threat may exist, too. To reiterate, confidence in (severe) storms
affecting this area is low at this time, largely due to a lack of
better lift.

As we move into the evening hours, our attention turns to where
storms will be developing near a dryline to our west and the cold
front to the northwest. There isn`t too much guidance that
develops storms along the dryline, and the ones that do initiate
along the dryline develop very limited coverage. If storms develop
along the dryline, they are expected to develop outside of our
CWA but could move into our west and/or northwestern counties
after 7 pm. These storms would have a severe risk with them for
damaging winds, large hail and a tornado or two.

The storms along the cold front in Oklahoma may be a combination
of discrete and linear, but are expected to quickly merge into a
squall line that reaches North Texas around 10 pm. By this time,
the upper level trough will be encroaching into the region and is
expected to help maintain a line of storms along the front. As we
have been talking about for a few days, a threat for severe storms
will continue with the line of storms overnight, with the better
potential for severe storms mainly north of a Comanche to Athens
line. A threat for severe storms will also exist south of this
line, but we expect a weakening trend at some point as the line of
storms moves across Central Texas. Straight-line winds will be
the main hazard, but some hail is also possible. Low level wind
profiles continue to indicate a threat for embedded tornadoes is
possible as surface winds ahead of the line are expected to be
southeasterly. The front should be progressive enough to limit
flash flooding, but isolated instances of flooding may be possible
depending on the realized speed of the front. The line of storms
will take most of the night to cross our region.



As the ongoing convective complex separates further from the
forcing responsible for its initiation, it will also encounter
increasingly veered flow as it moves deeper into Central and East
Texas Sunday morning. Both of these factors should result in a
steady decline in intensity. The orientation of the upper flow
should limit the extent of trailing stratiform rain, and the
precipitation will likely come to an end in our CWA by midday.
In the system`s wake, breezy north winds will surge through North
and Central Texas. The sun will re-emerge, and temperatures are
still expected to recover into the 70s. But once the sun sets,
temperatures will plunge into the 50s Sunday evening.

The shortwave currently swinging through the southern Rockies is
already showing evidence of a PV anomaly, distinct from the main
upper trough. Guidance diverges somewhat on the extent this PV
will pool upstream of the exiting convective complex. However,
there is good agreement with respect to the associated jet
structure. The portion of the jet spilling into the Southern
Plains, providing energy for tonight`s event, is progged to weaken
considerably on Sunday, with a westerly streak through the
Northern Plains taking precedence. This increases confidence that
the southern impulse will be cut off from the polar flow. This
should result in low-level cyclogenesis over the Tennessee Valley
and a stalling of the front, but this shouldn`t occur until the
mid-level feature approaches the Mississippi Valley. Although the
operational NAM and a handful of GFS ensemble members suggest the
pinching off will occur within the Lone Star State, keeping
precipitation chances in our eastern zones through Monday, these
outliers appear to be too quick with this process. Our plate of
spaghetti is disconcertingly chaotic for being only 48 hours from
now, but we will maintain a dry forecast beyond Sunday`s MCS.

In either scenario, Texas should be dominated by northerly flow
aloft, allowing a reinforcing shot of cool air to arrive as early
as Monday afternoon. The cold advection doesn`t look particularly
strong; thus, sunshine and dry air may win out with high
temperatures reaching 80F in some locations Monday afternoon. The
main surge of cooler air will arrive on Tuesday, but highs should
still top 70F, an afternoon not unlike Sunday with breezy north
winds. The cool air`s effect will be most noticeable Wednesday
morning, the coolest morning of the upcoming workweek. Southwest
winds on Wednesday will hasten a warming trend that will continue
on Thursday.

The upper flow is expected to back late in the week in response
to a Gulf of Alaska low that will charge south out of the Canadian
Rockies. But with unseasonably cool air already plunging down the
High Plains ahead of this system, a cold front may still manage
to arrive late in the week. Despite a couple of days of southerly
flow preceding the front, the quality of moisture return will be
poor, but have introduced some low-end PoPs to accompany the
frontal passage. If the faster GFS solution pans out, Friday could
feature the coldest daytime temperatures so far this season. You
won`t need your heavy parka just yet since highs would still be
in the 60s. Or if the slower ECMWF is correct, the front may not
arrive until Saturday.



.AVIATION... /Issued 1159 PM CDT Fri Oct 20 2017/
MVFR ceilings have been spreading northward late this evening
and had moved into much of the Metroplex as of 0430z. Ceilings
may lower to IFR for a few hours at Waco so have placed a TEMPO
BKN007 between 09z and 13z. Southeast winds around 15 knots will
prevail and some gusts over 25 knots will be possible late
Saturday morning through the afternoon hours. Some isolated
showers/thunderstorms are possible during the day Saturday but
the cap is expected to keep this activity very limited, so
SHRA/TSRA has not been mentioned in the Metroplex TAFs. A line of
strong to severe thunderstorms are expected to develop ahead of a
strong cold front Saturday night. Thus have placed a VCTS in the
Metroplex TAFs starting at 05z. Have included a TEMPO 5SM TSRA
BKN025CB in the extended portion of the DFW TAF and shown a wind
shift to the north at around 17 knots at 09z Sunday.



Dallas-Ft. Worth    87  60  74  51  80 /  10 100  20   0   5
Waco                88  63  76  50  80 /  10  90  20   0   5
Paris               83  60  71  49  74 /  10 100  50  10   5
Denton              86  57  74  48  79 /  10 100  10   0   5
McKinney            85  59  73  49  78 /  10 100  20   5   5
Dallas              87  61  74  52  79 /  10 100  20   0   5
Terrell             86  61  74  48  79 /  10 100  40   5   5
Corsicana           86  64  73  50  77 /  20 100  40   5   5
Temple              87  62  76  49  80 /  10  90  30   0   5
Mineral Wells       86  55  73  47  80 /  10 100  10   0   5




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