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

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FXUS64 KFWD 181739

Area Forecast Discussion
National Weather Service Fort Worth TX
1239 PM CDT Fri Aug 18 2017

Main aviation concern is convective potential this afternoon.

An outflow boundary has ignited scattered thunderstorms just NW
of the DFW Metroplex as of 17z. With ample moisture and
instability positioned along the I-20 corridor, expect new
development to continue in vicinity of DFW area TAF sites through
the early afternoon. Initiation will be aided by large-scale
ascent as well as lift from the approaching boundary. With
coverage and longevity of convection at the TAF sites still in
question, am still reluctant to TEMPO any TS for now, but will
handle with AMDs if it becomes necessary. There could also be a
brief wind shift to the NW associated with the boundary`s passage,
but flow behind it isn`t terribly strong. Otherwise, southerly
winds around 10 kts or less and VFR conditions will prevail
into Saturday. Waco is expected to remain rain-free through this
time frame.



.SHORT TERM... /Issued 339 AM CDT Fri Aug 18 2017/
/Today and Tonight/

Late evening rainfall across part of the region yesterday has led
to a relatively cool start to the morning today. Temperatures are
generally in the low 70s across the region under mostly cloudy
skies. Main concern for today will again be the chance for
scattered showers and thunderstorms. We`re watching a cluster of
thunderstorms across northwest Oklahoma which will make some
southeast progress through the early morning hours. Most of this
convection will remain to our north, but it is likely that an
outflow boundary will make it into our area sometime later this
morning. Increasing convergence along the boundary as we heat up
later this morning into early afternoon should help generate at
least some scattered showers and thunderstorms. GOES-16
precipitable water images indicate a pocket of relatively drier
air currently over south-central TX. The model guidance seems to
have a good handle on this and spreads it north through midday
into the vicinity of Waco. This results in a pool of higher
surface to 850mb moisture along the I-20 corridor north to the Red
River. This will be the most likely location for scattered
showers and thunderstorms through this afternoon. Any afternoon
convection should diminish this evening.

Later tonight, we`ll have to continue to watch to our north for
overnight convection. The model guidance has been rather
inconsistent with whether or not another round of convection will
develop across central/southern Oklahoma and possibly affect our
northern counties. With fairly weak flow aloft, organized
convection appears unlikely, although it will remain unstable and
uncapped well into the late evening. We`ll keep some low PoPs
across the northern couple of rows of counties through the late
overnight hours.



.LONG TERM... /Issued 339 AM CDT Fri Aug 18 2017/
/Saturday through Thursday/

As mentioned above, Saturday morning may start out with a decaying
complex of thunderstorms pressing south of the Red River. While
most coarse-resolution model guidance indicates little in the way
of QPF across our northernmost counties during this time, several
past runs of the NAM and a handful of members from the NCAR
ensemble indicate this as a potential. Given the level of moisture
and instability present to our north, any subtle influence from
forcing aloft would allow for at least some storm development,
sustained across the Red River by 20-25 kt southwesterly 925 mb
winds. This rich inflow will be waning and veering during the
Saturday morning timeframe, so any lingering activity would likely
be quickly diminishing in coverage through the morning hours.

During the afternoon, high pressure will attempt to build back
west across the region. However, latest indications are that the
main area of subsidence will remain displaced mainly across
Central Texas and across the Mid-Texas Coast. With 1500-2500 J/kg
of MLCAPE, bolstered by afternoon heating and near 70-75 degree
dewpoints on the northern fringes of the better subsidence, it
appears at least isolated convection will be possible Saturday
afternoon mainly across the northeastern 2/3rds of the CWA. Since
any storms would be diurnally-driven, opted simply to introduce
very low (10-15%) chances given a lack of more appreciable
forcing. Deeply mixed inverted-V profiles with cloud bases near 8
kft mean any more robust pulse storms would be capable of
delivering a downburst wind threat.

Low storm chances will linger across our eastern counties on
Sunday and Monday afternoons, amidst the zone of best moisture
and associated instability.

By Tuesday and into the end of the week, it appears North and
Central Texas will come under a squeeze play from from an incoming
TUTT out of the Gulf of Mexico, and the southwestern fringes of a
cold front sliding south out of Kansas. The GFS continues to be
the laggard with this front, hanging it up across central Oklahoma
where it washes out by week`s end, while the ECMWF and Canadian
both drive the front surprisingly far south--all the way to the
Gulf Coast. With the main upper forcing displaced well to the
north and east across the Great Lakes and Ontario, it seems
unlikely this front would have enough forward momentum to blast
all the way through our region, at least without the help of a
well-developed MCS and associated cold pool. Either way, it
appears at least low thunderstorm chances are warranted across the
forecast area Tuesday through Friday, but no single day appears to
be a washout.

Regarding heat index values: while hot conditions are forecast
across the region (still near climatological norms), it appears
that sufficient mixing will take place into next week--especially
across our hottest areas in Central Texas--to limit both the areal
coverage and duration of 105+ heat index values. As a result,
while pockets of 105-108 degree values are forecast (mainly
Saturday), we do not think the coverage and duration is sufficient
to warrant a Heat Advisory at this time. We will, however,
continue to assess the latest model guidance and observational
trends on a shift-by-shift basis as we go through the next few



Dallas-Ft. Worth    97  78  98  79  98 /  40  10  10  10  10
Waco                98  77 100  77  99 /  10  10  10  10  10
Paris               93  74  95  75  94 /  30  20  20  10  20
Denton              96  76  97  77  97 /  50  20  20  10  10
McKinney            95  76  96  77  96 /  50  20  20  10  10
Dallas              97  79  98  80  98 /  40  10  10  10  10
Terrell             96  73  97  76  97 /  30  10  10  10  10
Corsicana           97  77  98  77  97 /  20  10  10  10  10
Temple              99  75 100  76  99 /   5   5  10  10  10
Mineral Wells       95  74  97  75  98 /  50  10  10  10  10




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