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

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FXUS64 KFWD 211824 AAA

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
124 PM CDT Sun May 21 2017

18 UTC TAF Cycle

Concerns---Late afternoon/early evening convection at Waco.
Another round of convection overnight and onset of MVFR stratus at
all terminals. IFR ceilings probable at Waco.

For the Metroplex TAF sites---The ascent that resulted in
convection near and just south of the terminals this morning has
subsided and slightly drier air will slide southward briefly this
afternoon. This dry air should largely mitigate the potential for
widespread convection this afternoon at Metroplex TAFs, though a
few showers may graze the outer edges of the 10nm range ring at
GKY and DAL. Otherwise, this afternoon should be quiet with just
mid and upper level cloudiness along with east winds.

During the overnight hours into daytime hours on Monday, there are
a couple of aviation concerns for the Metroplex TAFs.

1) The first concern will be a re-development of widespread
convection in response to a shortwave trough lifting out of
southwest TX late tonight into Monday morning. Initially, the
highest concentration should be to the west of the TAF sites with
a gradual re-generation of showers and storms eastward with time.
Due to their elevated nature, they will pose a frequent lightning
and hail threat. For now, have placed VCTS between the 08-12 UTC
time frame, but adjustments to this will likely be needed as
additional hi-resolution model guidance arrives.

2) The second concern will be the development of MVFR stratus. As
low level flow just above the surface intensifies, moisture should
surge northward and overspread most North TX terminals with cigs
around FL015 around the mid-morning time frame. The low level
moisture will remain in place through the remainder of the morning
and perhaps into the afternoon.

3) The third concern, which is specifically for the DFW extended,
will be the potential for a complex of convection to develop
upstream up near the Bowie cornerpost and slide southward towards
the Metroplex terminals. Given that this is at the tail end of
TAF period, I`ve elected to go with a mention of VCSH around 2100
UTC on Monday afternoon. Depending on the amount of instability
preceding this cluster of expected convection, it`s possible that
VCTS will be needed, but confidence is not high enough to include
this in the TAF at this time.

For the Waco TAF site--There are a couple of aviation weather
challenges that exist at the Waco TAF site. A large cluster of
convection continues to develop to the southwest and is making
slow progress towards the north. At this time, there is some
uncertainty as to whether or not this activity maintains itself
and makes it into the Waco TAF site. For now, will carry a mention
of VCSH for the mid to late afternoon time frame to account for
the possibility of some impacts to the terminal. This activity
should largely wane with the loss of daytime heating. Re-
development is expected near midnight as the shortwave across
southwest TX lifts northward and spreads northward. With this in
mind, will go with several hours of VCTS at the TAF site based on
latest model guidance. Towards sunrise, MVFR and IFR stratus will
develop and lift northward, impacting the TAF site. For now,
confidence isn`t high enough to prevail IFR, so will advertise
temporary IFR cigs for a majority of the morning.



Some quick adjustments to the forecast for this afternoon, as our
morning round of strong to severe storms winds down for the time
being. I have moved all low-mid convective chances south of I-20,
as the 850mb front slowly shifts southward and stalls over
Central TX by this evening. Progressive shortwave energy was
streaming over this boundary, with the shallow surface front now
well to the south near the I-10 corridor over South Central Texas.

The next significant shortwave we await is over Northern Mexico
and far West TX and will arrive tonight. The question will be
whether the reprieve awaits the lift from this feature, or if more
spotty convection forms over our Central TX counties later this
afternoon near the 850mb front and any resultant outflow
boundaries. High resolution models are differing on this scenario
with the 3km NAM wanting to pop some discrete storms by late
afternoon ahead of the 850mb front, while the HRRR/HRRRx is dry
and waits for the approach of the stronger shortwave to our
southwest to arrive. The TTU WRF is indifferent to either
scenario, but leans more toward a longer quiet period until this
evening into early Monday morning over Central TX with lift from
the well-defined shortwave over West TX.

As noted above, will keep 20-40% chances over Central TX, but
with the caveat that the models aren`t handling the boundaries and
mid level energy all that well, thus much uncertainty exists.



.PREV DISCUSSION... /ISSUED 352 AM CDT Sun May 21 2017/
Surface analysis early this morning places an ill-defined quasi-
stationary front more or less along the 70 degree isodrosotherm
which stretches from near a Del Rio to Austin to Nacogdoches. To
the north of this feature, dewpoints gradually fall into the 50s
across roughly the northwestern half of the CWA. Aloft, the
effective 850 mb front is quite a bit farther north--laid out
almost right along the I-20 corridor--and this is resulting in
MUCAPE values in excess of 1000 J/kg to its south. Modest
isentropic upglide above this feature along the 300-310 K theta
surfaces appear to be the primary culprit in the re-development of
isolated to widely scattered showers and storms overnight. The
combination of sufficient elevated instability and effective deep
layer shear values in the 30-40 kt range are supporting some
occasionally strong pulse/multicell storms.

As the 850 front continues to slowly push southward today, the
main thunderstorm chances should likewise gradually sink south of
the I-20/30 corridors this afternoon. Based on the rather modest
nature of the warm advection today, think the Hrrrx forecasted
convective coverage is a bit overzealous, and have favored chance
PoPs across our southern counties. Given the aformentioned
kinematic profile, the more rambunctious (short-lived) updrafts
could be capable of some marginally severe hail, and perhaps some
gusty winds with the help of some diabatic warming today. High
temperatures today are tricky given the splotchy precipitation and
cloud coverage, but in general, temperatures will warm into the
upper 70s to near 80 degrees or so.

Later this afternoon and evening, an uptick in convection is
anticipated across the Big Bend of Texas as a shortwave--presently
shifting eastward across Sonora, Mexico--emerges into the Trans-
Pecos region. Based on this feature`s track, it appears the bulk
of thunderstorm activity should occur across the Hill Country
tonight. There may be enough of a northward push, however, to
drive decent precipitation coverage potentially as far north as
the Metroplex late tonight, but for now, we`ll confine the likely
PoPs to south and east of a Lampasas to Athens line. While
effective shear values will be on the upswing through this period,
increasingly moist-adiabatic thermodynamic profiles will support
only limited instability. As a result, the most we`d expect from
these overnight showers/storms would be some small hail.

As this shortwave exits the region Monday morning, we may actually
be left with a fairly dry day on Monday in the shortwave`s
subsident wake. As a result, opted to trim PoPs slightly across
the region, but these may still be a bit overdone. The next
feature of interest will be a potential southeastward-advancing
convective complex that coarse and high-resolution guidance alike
is keying in on for the Monday evening and overnight period.
Synoptically, it appears this feature will have its origins across
the Texas/Oklahoma Panhandles in response to a sharp mid-level
perturbation swinging into the Great Plains Monday afternoon.
With bulk shear vectors oriented roughly parallel to the
initiating boundary (a cold front), it`s not surprising that CAMs
quickly develop an upscale-growing MCS. Investigation of upstream
forecast soundings reveal the presence of at least some mid-level
dry air which may support cold pool development, lending at least
some credibility to the CAM-depicted surging convective complex.
With this in mind, we`ve bumped PoPs up across the southwestern
half of the CWA Monday night. The severe potential with this
system is in question at this point given both the limited time
for moisture to return across the Concho Valley/Big Country and
a very unimpressive wind field below 700 mb (15 kts or less). For
now, won`t rule out the possibility of this system delivering a
strong/gusty wind threat to our western counties Monday evening
and into the early overnight hours.

We`ll hang onto low-grade PoPs on Tuesday before a reinforcing
cold front drives through the region during the afternoon. This
will result in a dry stretch of weather from Wednesday and into
the upcoming weekend. Moisture will rapidly return to the region
Thursday night and into Friday, but along with this moisture will
come the advection of a stout EML plume off the Mexican high
terrain. This will effectively curtail any threat of
thunderstorms until Sunday as a cold front and dryline impinge
upon the region from the north and west.



Dallas-Ft. Worth    81  63  79  62  79 /  20  30  30  40  20
Waco                78  63  78  61  81 /  30  50  50  50  30
Paris               80  58  75  59  75 /  20  20  40  30  30
Denton              81  58  78  60  77 /  20  20  40  40  20
McKinney            81  59  77  59  77 /  20  20  40  30  20
Dallas              81  64  78  63  79 /  20  30  30  40  20
Terrell             77  61  75  61  78 /  30  40  40  40  30
Corsicana           77  63  77  62  80 /  40  50  50  40  30
Temple              78  63  78  61  81 /  40  60  60  50  30
Mineral Wells       81  58  78  59  76 /  20  20  30  50  20




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