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

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FXUS64 KFWD 211654

Area Forecast Discussion
National Weather Service Fort Worth TX
1154 AM CDT Fri Jul 21 2017

The latest water vapor loop clearly indicates a dominant ridge
over the central part of the CONUS, which will keep quiet weather
in place for the next 36 to 48 hours. The forecasts will consist
of VFR conditions and generally south winds around 10 KT. The
ridge will briefly break down on Sunday as an upper level
disturbance drops southeast through the Central Plains. This
feature may kick off a round of thunderstorms, which may affect
north Texas on Sunday afternoon. Though this would be beyond this
set of TAFs, it will be something to keep an eye on as we head
into the weekend.



.SHORT TERM... /Issued 331 AM CDT Fri Jul 21 2017/

/Today and Tonight/

An east-west oriented upper level ridge extended from eastern
Colorado across southern Kansas/Northern Oklahoma and southern
Missouri/northern Arkansas into the Tennessee Valley this morning.
This ridge will shift eastward during the next 24 hours. North
and Central Texas will remain under strong subsidence from this
ridge. Expect temperatures to climb into the mid 90s to 102
degrees this afternoon. Vertical mixing should allow dewpoints to
fall into the upper 60s along and west of I-35 with lower 70s
to the east. This will result in heat index values 104 to 108
degrees across most areas along and east of a Bowie to
Mineral Wells to Killeen line. Thus have expanded the Heat
Advisory a little farther west to include those areas. It will be
mostly sunny today with only some fair weather cumulus developing
mid to late morning. Winds will be southerly at 5 to 10 mph.

Expect another warm night tonight with overnight lows in the
mid to upper 70s, except some 80 degrees lows in the warmer areas
of the Metroplex.



.LONG TERM... /Issued 331 AM CDT Fri Jul 21 2017/

/Saturday through Thursday/

Saturday should be another hot and humid day across North and
Central TX. There will a gradual increase in rain chances for
some, which may result in a brief respite from the heat in the
form of afternoon showers and storms. Not all locations will see
rain, so it`s best not to be overly hopeful. Water vapor imagery
at this hour indicates a disorganized upper low across the
southeast CONUS. This feature is forecast to actually drift
westward as it undercuts H5 ridge on its southern flank. The
current state of the feature lowers confidence in widespread
rainfall on Saturday, so I`ve kept PoPs generally on the lower end
of the spectrum (closer to 20%) down across far southeastern zones.
In fact, it`s possible that this feature actually induces a bit
more in the way of mid-level subsidence across our region,
potentially resulting in mostly precipitation-free conditions
across the entire forecast area. I`ve kept rain in the forecast as
there`s not a large sign of synoptic scale subsidence in model
progs (likely because of its current disorganized state) and given
increasing PWATs, it`s likely the airmass will become
increasingly unstable and conducive for diurnal convection. Again,
the most likely area for this appears to be across the
southeastern zones. Organized storm modes appear unlikely given
that deep layer shear values are expected to generally be less
than 15 knots. However, DCAPE values of around 1000 J/kg and
inverted-v forecast profiles may foster a damaging downburst risk
with some of the more robust convection.

The increasing moisture combined with temperatures in the mid-90s
could yield heat index values up to near 105 degrees, so for now
will not make any changes to the extended portion of the Heat
Advisory. While typical, GFS forecast profiles indicate a fair
amount of mixing, especially near and west of the I-35 corridor.
If this comes to fruition, it`s possible some changes to the
current configuration of the Heat Advisory will be necessary.
Regardless, the next couple of shifts will look at this a bit

Sunday should offer better rain chances across a larger portion
of North and Central TX as additional low level moisture invades
from the east and southeast. Coarser models are in fair agreement
with developing what will likely be diurnally driven convection
again and I`ve kept the previous PoP forecast mostly unchanged
with only a slight increase in rain chances across far
southeastern zones. Forecast profiles don`t indicate that the
breadth of mixing will be quite as large as Saturday, but there is
still a signal that the environment could support convection
capable of strong to damaging downburst type. Otherwise, the hot
and humid conditions will continue and heat index values may once
again approach and/or exceed 105 degrees.

For Sunday night into Monday morning, our attention turns to the
north where a late season frontal boundary will sag southward
under the influence of a broad trough that is forecast to dig
southeastward through the Great Lakes region. Given the location
of the upper forcing and time of year, it`s unlikely that this
front will make it to the Red River. With the weakened state of
the upper ridge, there should be some storms clustered around the
frontal boundary. Low level wind fields may support some
maintenance of this cluster southward into North TX, so I`ve kept
just low rain chances north of I-20 on Sunday night.

The broad troughing should keep the western CONUS upper ridge at
bay, resulting in continued rain chances on Monday and Tuesday.
These rain chances will be highest across North TX, closer to the
proximity of the weak upper forcing. Should storms develop, they
will pose a gusty wind and heavy rain threat. Temperatures on
Monday and Tuesday will be just slightly lower, especially across
northern and northeastern zones where afternoon cumulus may keep
areas a bit more shaded.

Dangerous heat appears to return during the middle and end of
next week as the H5 ridge rebuilds back towards the east.
Afternoon high temperatures each day will climb back into the 90s
to near 102 for some parts of North and Central TX. Afternoon heat
index values could once again approach 105 degrees and this may
prompt the issuance of additional heat-related headlines.



Dallas-Ft. Worth   100  80  99  80  97 /   5   0   5   5  20
Waco               101  77  97  78  98 /   5   5  10   5  20
Paris               97  75  93  75  94 /   5   0  10   5  20
Denton              99  78  99  79  97 /   0   0   5   5  20
McKinney            97  76  95  77  96 /   5   0   5   5  20
Dallas              99  81  99  80  97 /   5   0   5   5  20
Terrell             97  76  95  76  96 /   5   0  10   5  20
Corsicana           98  77  93  77  96 /   5   0  10   5  20
Temple             101  76  97  76  98 /   5   5  10  10  20
Mineral Wells       99  76  99  76  98 /   0   0   5   5  20


Heat Advisory until 8 PM CDT Saturday for TXZ091>095-102>107-



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