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

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FXUS64 KFWD 240508 AAC
AFDFWD

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
1208 AM CDT Thu Aug 24 2017

.AVIATION...
/06Z TAFs/

The bulk of the showers and thunderstorms Wednesday afternoon were
along a surface boundary within Central Texas. The extent was
sufficient to send outflow as far north as the Dallas/Fort Worth
Metroplex by midnight. Spotty showers will accompany the decaying
boundary in vicinity of the Metroplex TAF sites during the early
morning hours. This outflow will temporarily disrupt the surface
wind field, but easterly surface flow will otherwise prevail.

Veering winds 925-850mb will become southeasterly early this
morning, upglide above the shallow surface layer that may result
in additional nocturnal showers into the morning push. The
activity around midnight yielded a few lightning strikes, but the
limited depth of the convection should preclude thunder.
Saturating east winds near the top of the descending frontal
boundary will likely result in some MVFR stratus, but with the
outflow disrupting this process, VFR conditions may dominate. The
06Z TAFs will TEMPO MVFR, but this will need to be reassessed
early this morning.

Despite the milder temperatures, abundant moisture and August
surface heating will result in disorganized showers and
thunderstorms Thursday afternoon, mainly west of the I-35
corridor. The best instability will be well west of the TAF sites,
and the TAFs will maintain VCSH for the afternoon hours as a
result.

25

&&

.UPDATE... /Issued 915 PM CDT Wed Aug 23 2017/
A cluster of showers and storms containing localized, very heavy
rainfall have cold-pooled south out of the CWA other than of the
Centerville/Palestine areas that weren`t affected by earlier
convection. A diffuse surface front remains draped from along a
Goldthwaite, Hillsboro, Waco line with outflow boundaries moving
concentrically in all directions across the area. Will maintain
mid range chances south of I-20 due to these outflow boundaries
and the insanely high, very tropical PWAT values well over 2
inches across much of the area. Some isolated activity was moving
southwest toward Young Co, but overall, areas north of I-20 to the
Red River Valley were quiet.

The very moisture-rich PWAT environment will remain entrenched
across the area overnight. Despite the diffuse surface front being
across our Central TX counties, models do indicate a very subtle
shortwave disturbance dropping south over the top of the weak
NM/West TX upper ridge across our western counties overnight.
SREF/NAM models are very aggressive with scattered-numerous
showers with some embedded storms re-developing after midnight,
while the TTU/NSSL WRF and HRRRx models are less enthusiastic and
only show at best scattered coverage. This may be due to low level
east- southeast inflow in the boundary layer being weak and only
10-15 knots overnight. I had considered toning POPs down into
slight chance to mid range chances overnight, but considering the
very rich moist environment in place, have decided it would not
add any value to the forecast and confidence is just not there to
remove it with the shortwave dropping south across the west CWA
overnight. I will be interested to see the 00Z runs of the WRF
models to see if they increase their coverage of convection.

Lastly, any convection tonight will be slow-moving and mean layer
flow in the lowest 10-15 kft remains weak. Look for similar
trends to what was seen across Central TX earlier this afternoon
with propagation-dominated movement from outflow. In addition, the
atmosphere will provide an environment for rainfall rates to be
very proficient with locally heavy rainfall and flooding possible
across the western half of the CWA overnight. Some heavy rainfall
mention was added to the forecast for this potential.

05/

&&

.SHORT TERM... /Issued 355 PM CDT Wed Aug 23 2017/
/This afternoon through Thursday/

This morning`s convection pushed a pseudo-front/outflow boundary
into our southern counties this afternoon. North of the boundary,
generally north winds, mostly cloudy skies and cooler temperatures
prevail. In the visible satellite imagery, a remnant MCV appears
to be moving across Grayson Co. Convection is now forming near
the MCV and also continues to develop in Central Texas near the
front/outflow. Additional convection is also forming between these
two features as heating occurs.

The front/outflow that pushed through the region is shallow in
nature. Beginning this evening, the winds above this cool layer
will turn to the east and southeast providing weak, moist ascent.
Scattered showers and isolated thunderstorms are expected to
continue to develop through the evening and overnight hours.
Highest rain chances are forecast to be across the western half of
the CWA (along and west of Interstate 35/35W) near the location of
a weak low pressure within the 925-700 mb layer. Lightning strikes
and brief heavy downpours are possible as PWATs continue to rise
to near 2 inches as sampled on the 18Z FWD special sounding in
support of TD Harvey. If a concentration of convection occurs in
one or more locations to the west of I-35/35W overnight and
Thursday morning minor flooding may become a threat.

With east winds above the surface continuing into Thursday, and
the weak low remaining in our western counties according to the
models, will continue likely rain chances across most of the CWA
during the day. The rain will likely be scattered in nature, but
pockets of more continuous rain coverage may occur. The main
hazards will again be lightning with isolated storms and heavy
downpours.

JLDunn

&&

.LONG TERM... /Issued 355 PM CDT Wed Aug 23 2017/

/Thursday Night through Monday/

A majority of the long term forecast will be dominated by the
potential impacts from Harvey, most likely late Friday and in
into Sunday. To be clear--- impacts are expected to be confined
primarily for areas near and south of a Goldthwaite to Killeen to
Centerville line. Impacts will mainly be in the form of heavy rain
with wind gusts to near 40 mph. Areas north of this line will
likely see minimal impacts based on the latest forecasts.

For Thursday Evening---Generally easterly flow will continue to
overspread the southern portion of North TX and just about all of
Central TX. Model PoPs seem to be on the high side given the large
amount of tropospheric moisture with nearly 2" PWAT values being
advected in from upstream. While forcing will generally be on the
low side, the moist adiabatic profile suggests that even weak
ascent will result in the generation of some convection. Some of
this forcing could be in the form of synoptic scale ascent in the
form of the jet entrance forcing, so I`ll paint a swath of 20 to
30 PoP up across northern zones.

For Friday---Lingering showers and thunderstorms will likely
continue, especially across northwestern and northern zones in
response to the aformentioned forcing. Farther south---Harvey is
expected to inch northward through the day and will likely make
landfall along the TX coast between Corpus Christi and
Brownsville. This will place North and Central TX along the
northern periphery of the storm track. Conceptually, sinking
motion would tend to ensue, but given the amount of low level
moisture still present in model forecast RH fields, I can`t rule
out a few showers or storms. While it`s also possible that we get
a some ill-defined spiral rain-bands, I feel that most of the
concentrated rain and isolated storm activity will remain south of
our Central TX counties through most of the day on Friday.

For Saturday---The general model consensus (minus the NAM)
appears to bring Harvey northward through the TX Coastal Plain
Saturday morning, albeit at a slower pace. Thereafter, the general
consensus is that Harvey will drift northward, but stall
somewhere across the TX Coastal Plain or eastern Hill Country.
However, it`s important to note that run-to-run consistency
between model suites will be highly variable given the early
stages of Harvey`s development. Users of model forecasts are urged
to exercise caution, when interpreting output. Even with a more
southern track---model low level wind fields (specifically at
925mb) spread 30 to perhaps 40 knot winds northward towards
southern and southeastern zones during this time. There could be a
few near-tropical storm force wind gusts on Saturday mainly
across far southern portions of Central TX, but right now,
confidence to go with any official tropical products is a bit on
the low side right now. The highest rainfall totals at this time
appear to be across southern and southeast zones. For Saturday,
it`s probable that areas along and southeast of an Athens to
Temple line will see 1 to 2" of rain as a 24 hour total on
Saturday. Again--- it should be noted that rainfall totals will
likely change with time as we get a better handle on the track of
Harvey. Rainfall totals through Monday are expected to range
between less than a half inch up across northwest zones to near 4
inches down across far southeastern/southern zones.

For Sunday and beyond---parts of the area could be still feeling
some influence from Harvey, so I`ve elected to leave just low
chances for rain down across southern and southeastern zones. As
noted above, this forecast will likely change, but given that
there is a signal from the model guidance that Harvey (or its
remains) will depart the region, I`ve hedged towards a
conservative forecast.

24-Bain

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    75  86  72  90  72 /  60  50  10  20  20
Waco                75  90  73  91  73 /  50  50  30  20  20
Paris               70  85  68  86  70 /  20  20  10  10  10
Denton              72  85  70  88  70 /  50  40  10  20  20
McKinney            70  85  68  89  70 /  40  40  10  10  10
Dallas              75  86  73  90  73 /  60  50  10  20  10
Terrell             73  87  71  90  71 /  50  40  10  10  10
Corsicana           76  89  72  90  73 /  50  50  20  20  20
Temple              74  91  73  91  72 /  60  50  30  30  30
Mineral Wells       71  85  69  89  69 /  60  50  10  20  20

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

25/05


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