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

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293
FXUS64 KFWD 181646 AAA
AFDFWD

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
1046 AM CST Sun Feb 18 2018


.UPDATE...
An area of light to moderate rain associated with a weak upper
level disturbance continues to move in from the west and should
reach the I-35 corridor by early afternoon before accelerating off
to the east late afternoon and this evening. Have increased POPs
to likely from the DFW Metroplex southwestward where the heavier
RADAR echoes are headed. Lingering areas of fog and low clouds will
mix out with the onset of precipitation. Otherwise, high temperatures
were lowered by a few degrees with this update based on recent
trends.

30

&&


.AVIATION... /Issued 636 AM CST Sun Feb 18 2018/
Areas of fog developed overnight across parts of North and
Central Texas. The fog has become widespread across Central Texas
where some patchy dense fog has formed. KACT has bounced around
from 1/4SM to 2 1/2SM. Expect LIFR to IFR conditions to prevail
at KACT through 17z with an improvement to MVFR through 06z
Monday with ceilings lowering back into the IFR after 06z.

The Metroplex TAF sites have been more of a challenge where none
of the TAF sites were reporting fog as of 12z, but sites such as
KFWS and KJWY were reporting 1/2 to 1 1/4SM. Expect mostly MVFR
conditions to prevail today but through 16z some IFR conditions
are possible with ceilings 600-800 feet and light fog of 3SM or
less possible. Ceilings should lower back into the IFR category
toward 12z Monday.

Showers and isolated thunderstorms that are associated with a
shortwave have develop over Southwest Texas between KMRF and KSJT.
This activity will move northeast and approach the I-35 corridor
including all of the TAF sites by 17z. Have just indicated VCSH
for now but will further refine the TAFs at 15z and may have to
include 3SM -SHRA and add VCTS for the 18-21z period.

Light, mostly easterly, winds at 12z will become south or south
southeast at 12 to 14 knots by 17z. South winds around 15 knots
are expected to prevail overnight tonight.



58

&&

.SHORT TERM... /Issued 400 AM CST Sun Feb 18 2018/
/Today and Tonight/

Areas of mostly light fog have been developing overnight along
and east of the I-35 corridor. With light winds and small dewpoint
depressions across the forecast area, additional development is
expected as we approach daybreak. Have included a mention of
patchy or areas of fog area wide. Although some dense fog is
possible, confidence in areal coverage is not high enough at this
time to issue a Dense Fog Advisory. We will continue to monitor
and, if needed, will issue one for parts of the forecast toward
daybreak.

Radars are already showing that some showers have developed over
Southwest Texas. As a weak shortwave lifts northeast, expect
showers to spread into the southwestern zones after daybreak today,
and across the rest of the forecast area during the day. As
instability increases, isolated thunderstorms will be possible
this afternoon and tonight. Rainfall amounts through 6 am Monday
will average less than 1/4 inch. Winds will be light and variable
at daybreak but as a surface low deepens over eastern Colorado,
winds will become southerly at 10 to 15 mph by late morning and
some 15 to 20 mph winds are expected this afternoon with some
gusts over 25 mph likely along and west of the I-35 corridor.
Southerly winds of 10 to 20 mph will prevail tonight. Highs today
will be mostly in the 60s, but range from the upper 50s northeast
to the lower 70s northwest. Lows tonight will be in the mid 50s to
lower 60s.

58

&&

.LONG TERM... /Issued 400 AM CST Sun Feb 18 2018/
/Monday through Friday/

The main concern in the long term forecast period will be the
active weather pattern. There will be a good potential for heavy
rainfall, especially east of I-35 and north of I-20 during the
early to mid-week time frame. The potential for severe weather is
quite questionable (really becoming doubtful) as stout capping and
an unfavorable shear-instability combination won`t be overly
conducive to widespread severe thunderstorms. We will, however,
continue to monitor this very conditional risk. As if that`s not
enough---there are some subtle hints at the potential for a light
winter mix across the far northwest early Wednesday. Significant
impacts are unlikely given that recent temperatures have been well
above normal. In addition, any winter precipitation, should it
fall, will be light!

For Monday---As noted above, stout capping is expected to be in
place across much of North and Central TX thanks to strong
southwesterly flow aloft. Higher resolution model guidance such as
the NAM, NAMNEST and TTU WRF all indicate that the moist boundary
layer will be trapped beneath this layer of warm air aloft. The
GFS attempts to convect along the dryline, but given that the main
trigger would be WAA and/or meager dryline convergence, this
appears unlikely. Output from the NAMNEST and TTU WRF indicate
that it`s most probable that the only meaningful precipitation on
Monday and Monday afternoon will come from streamer showers and
this makes sense given the strong capping. I`ve lowered PoPs
during the morning and afternoon hours due to the capping and
insufficient lift. Bottom line, severe weather prospects are
looking quite low.

Rain chances will increase just a smidge through the evening
hours as low level theta-e advection increases. While instability
will increase some, it`ll still most likely remain pretty limited
with peak values near 500 J/kg. Deep layer shear will certainly be
plentiful, however, the narrow instability profile suggests that
updrafts may be sheared apart, further limiting the severe weather
potential Monday evening. Forecast precipitable water values will
increase closer to 1.5". This is WELL above normal for North and
Central TX and suggests that showers and thunderstorms that
develop will be very efficient at producing moderate to heavy
rain.

For Tuesday---The threat for widespread precipitation increases
during the pre-dawn hours on Tuesday out across the Big Country as
large scale ascent ahead of a broad trough overspreads the area.
I`ll go ahead and maintain likely PoPs across the Big Country
during this time, but it`s more likely that rain won`t begin until
closer to sunrise. A deep plume of low level moisture, with
origins from the Caribbean, will continue to be transported
towards the north. Broad troughing, with likely embedded
perturbations, will inch closer to North and Central TX through
the day and this should provide ample lift for widespread
convection.

The strongest lift will likely overspread the area midday Tuesday
and I`ve painted widespread 80% rain chances across a majority of
the area with a small sliver of 90% rain chances north of I-20
and generally east of I-35 along the tight theta-e gradient. PWAT
values will remain near or above 1.5" further supporting a very
moist atmosphere. Forecast soundings indicate a moist adiabatic
profile and generally unidirectional wind shear, both hallmarks of
an environment favorable for warm-rain processes. The
undirectional wind profile will also promote the training of
convective cells and this may result in some flooding. The degree
of flooding remains somewhat uncertain given the recent dry
weather. However, we are dealing with a pretty extraordinarily
moist airmass, with near record PWAT values likely to be observed
by area RAOBs.

After coordination with surrounding offices as well as the
Weather Prediction Center (WPC), it`s agreed that the most
favorable area for the heaviest rainfall will be east of I-35 and
north of I-20. When all is said and done, rainfall amounts could
tally as great as 4-5 inches in some spots. This could result in
flooding of low lying/poor drainage areas as well as some of the
smaller tributaries across North and East TX. Elsewhere, a general
1-4 inches of rainfall will be possible, though a minor flood
threat may surface here. While rain chances are high, there is
still some uncertainty regarding the position of the front. Model
consensus is better compared to 24 hours ago with the placement of
the front, however, so the uncertainty is waning a bit. If the
front is to move a bit slower, that could certainly result in
additional rainfall over a longer period of time. If the front is
faster, it could mean a quicker end to rain chances. For now, have
not made too many drastic changes with regards to sensible
elements most impacted by the front (winds, temps and PoPs).

For Wednesday---By Wednesday morning, the frontal boundary should
be near a Goldthwaite to Killeen/Temple to Palestine line. Strong
isentropic ascent atop of this feature, however, will mean that
rain chances will extend well behind the surface boundary. Again,
with the high atmospheric moisture content, there will be a
potential for some heavy rain. The best chance for the heaviest
rain on Wednesday will be across Central and East TX.

Shallow colder air will filter in down across northwestern zones
and temps will flirt with 32 degrees across western North TX. For
now, I`m going to leave winter weather out of the official
forecast for a couple of reasons. 1) There is a high degree of
uncertainty with regards to how far back the precipitation shield
will extend and 2) it`s possible that the air aloft is so warm
that quantity of supercooled drops or ice is lacking for freezing
drizzle/rain or sleet. We will keep an eye on this.

For Thursday and beyond---Generally low rain chances will continue
through the rest of the week and into the first part of the
weekend. There are some significant model discrepancies, however,
that only lends to a low confidence PoP forecast. The GFS shunts a
great deal of dry air southward, while the ECMWF is very
aggressive with the return flow. By Friday and Saturday, there`s a
bit more agreement among the two main long range models that low
level moisture will have continued to stream northward. WAA should
result in some isolated to widely scattered shower activity before
another frontal boundary slides southward late Saturday and into
the pre-dawn hours on Sunday.

24-Bain

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    63  61  75  65  69 /  60  30  40  60  90
Waco                66  63  77  66  71 /  50  20  20  40  80
Paris               58  56  71  63  69 /  40  30  30  50  90
Denton              62  61  76  64  69 /  60  40  40  60  90
McKinney            61  60  74  64  68 /  50  40  40  50  90
Dallas              63  61  75  65  69 /  60  30  40  50  90
Terrell             64  61  74  64  70 /  50  30  30  50  90
Corsicana           65  62  76  66  71 /  40  20  20  40  80
Temple              66  63  78  66  71 /  30  20  20  40  80
Mineral Wells       65  60  81  63  71 /  50  40  30  70  80

&&

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

$$

30/79



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