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

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364
FXUS64 KFWD 240343 AAC
AFDFWD

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
943 PM CST Fri Feb 23 2018


.UPDATE...
This is a short update discussion to convey that the weather set-
up for tomorrow still looks the same as it did earlier today.
Around or slightly before daybreak, we expect an increase in
storm coverage with numerous to scattered storms spreading
northeast across the region. Eventually, some of these storms will
form into a line that moves east, but storms will continue ahead
of the line through the afternoon hours. The initial storms will
be elevated with a threat for large hail and heavy rain. By the
afternoon hours, any storms east of Interstate 35/35E will have
the potential to become surface based as the warm front will have
lifted north into that area. A threat for severe storms will
continue with the line of storms, but storms ahead of the line
will likely also have a tornadic potential owing to the nicely
curved low level hodographs with near 30 kts of 0-1 km shear.

Most of the area within the Dense Fog Advisory is no longer
reporting dense fog, but visibilities in many places are still
below 1 mile. The potential for dense fog to redevelop exists
after midnight, especially if this round of light rain lifts north
as suggested by hi-res models. Therefore, will leave the Dense Fog
Advisory in effect as is for now. It currently expires at 4 am.

JLDunn

&&


.UPDATE... /Issued 746 PM CST Fri Feb 23 2018/
This update focuses on the near-term hours, and we will continue
to assess the weather set-up for the late night and early morning
Saturday hours. However, at this time, changes to the overall
storm trends and severe weather threat do not appear different
than what is represented in the ongoing forecast. For the most
part, expect showers across part of the region through at least
midnight, and then an increase in showers and storms closer to
daybreak. Some of the storms may become severe producing mainly
hail, but gusty winds may also be a threat.

Foggy conditions continue to expand across the region and have
recently expanded the Dense Fog Advisory to cover almost all of
the region. Variances in the dense fog will likely occur,
especially where light rain occurs, but a return to dense fog is
likely after the rain passes. The warm front is not expected to
make much progress north, at least through midnight, and we will
reassess the dense fog potential throughout the night.

Otherwise, just updated hourly trends and added the dense fog
wording to the zones. Temperatures across most of the region will
rise through the night, even with the warm front still to the
south.

JLDunn

&&

.AVIATION... /Issued 619 PM CST Fri Feb 23 2018/
/00Z TAFs/

Metroplex Airports: LIFR conditions are expected to persist across
the Metroplex for most, if not all, of the night due to a
combination of low clouds and fog, and the fog will be dense at
times. We are monitoring the northward advancement of a warm front
to the southeast. The latest thinking is the warm front will
remain south of the Metroplex keeping LIFR prevailing all night;
however, if the northward movement is faster, conditions may
improve at some time overnight.

Light and variable winds for much of the night will become
easterly and then southerly Saturday morning. However, a strong
southerly low level jet of 35-45 kts will prevail just above the
surface. Some showers will move across the Metroplex this evening,
and then a lull in activity is expected before showers and storms
ramp up again close to daybreak. Some of the storms in the
morning may be severe with a threat for large hail and strong
winds. A line of thunderstorms will move east across the region,
affecting the DFW Metroplex between 14-17Z. After the storms
clear, the ceilings will clear and west winds will prevail through
the end of the TAF period.

Waco Airport: Conditions around Waco are currently better than
locations to the north, but a return to LIFR conditions due to low
cigs and fog is expected tonight. There is a chance KACT may
improve to VFR conditions this evening as the skies are nearly
clear not too far east of the airport, and will amend the TAF if
needed for these trends. Light south winds are expected to prevail
through the night.

Showers and thunderstorms will increase early Saturday morning and
may affect the airport through 19Z.  Some of the storms in the
morning may be severe with a threat for large hail and strong
winds. A line of thunderstorms is expected to move through
between 17-19Z, and then dry conditions will prevail through the
end of the valid TAF period. Westerly winds will occur behind the
line of storms and conditions will improve to VFR.

JLDunn

&&

.SHORT TERM... /Issued 334 PM CST Fri Feb 23 2018/
/Tonight and Saturday/

The primary weather highlights in the short term forecast
period will be the continued rain chances and consequent severe
weather and flood risks. A this time, it appears that there could
be two rounds of severe weather across North and Central TX. One
in the morning and another in the afternoon. With these rounds of
storms, the risk for flooding will continue and we`ve kept the
Flood Watch running through 18 UTC Saturday. While activity will
move swiftly from west to east, antecedent conditions mean that
it won`t take too much to result in additional flood problems.
Rain chances should rapidly diminish late tomorrow afternoon as a
Pacific front/dryline sweeps through the area.

For tonight---Showers and thunderstorms should persist across the
Big Country and along our western most Red River counties as a
conveyor of strong isentropic ascent along the 310K theta surface
continues. Severe weather isn`t anticipated with this wave of
activity this evening, but heavy rain could result in some minor
flood problems. The moist regime and generally light winds at the
surface should facilitate some fog development. Given the areal
extent of low dewpoint depressions, I`ve included fog/drizzle
across all of North and Central TX with some slight improvements
across southeastern zones as the warm front lift northward.

Low level flow will continue to ramp up across the area with a
nice theta-e plume setting up across the Big Country. Convective
allowing models do differ a bit on the exact placement of the most
widespread activity. For now, I`ll side with the CAMs that have
the highest convective coverage (70 to 80 percent) conceptually
where it should be---along the tip of the theta-e plume. I can`t
completely discount the HRRR solution, which advertises widespread
convection down across the east and southeast, so I`ll carry a 50
PoP here.

The first round of severe storms could take place during the pre-
dawn hours on Saturday between 0800-1200 UTC. Deep layer shear
will be around 60-70 knots with MUCAPE values approaching 1100
J/kg. With steep mid-level lapse rates (almost 8 C/km in some
spots) and the elevated nature of storms---there should be an
appreciable hail threat (perhaps up to the size of golfball). The
most likely area for severe storms during the pre-dawn hours will
be near and northwest of a Comanche to D/FW to Gainesville line.

Simultaneously, a surface warm front will be lifting northward out
of the Brazos River Valley as low level winds continue to
increase. Winds across East and Central TX may increase through
the morning hours such that any fog or drizzle dissipates. Where
this boundary stalls will be paramount to the areal extent of the
warm sector and thus the severe weather potential later in the day.
While winds may shift to the south even north of this boundary,
it appears probable that the true warm front will stall near and
just south of the I-20 corridor and be oriented such that area
near just west of I-35 remain on the cool side of this boundary.
If GFS and ECMWF solutions are to be believed, however, it`s
possible that this boundary could lift as far north as the Red
River. For now, I`m a little hesitant to go with the coarser
guidance and I`ll keep the boundary a bit farther to the south per
the NAM and some of the other higher resolution data sets.

The second round of severe storms will likely be somewhat of a
continuation of remnant morning convection. What appears quite
plausible is that as early morning convection marches eastward,
it`ll become surface based, allowing it to realize an environment
characterized by 1000-1500 J/kg of CAPE, deep layer shear values
on the order of 40-50 knots and low level SRH in the 200-400 m2/s2
range. Should storms realize this type of environment, all facets
of severe weather will be at play. Initially, a mixed mode of
supercellular structures and line segments are expected with the
dominant convective morphology becoming linear in nature through
the afternoon. If the mode remains more supercellular OR
convection develops ahead of the line---the risk for large hail
and tornadoes may persist well into the afternoon. The most likely
area for severe weather will be near and east of I-35 and south
of I-30. If the warm front lifts farther to the north, a greater
portion of East TX could be under the risk for severe weather.

In addition to a severe weather risk, extraordinary moisture, per
forecast 1.25`-1.50" precipitable water values will support a
continued threat for heavy rain. While storms motions will likely
be quite fast, the antecedent conditions suggest that any bout of
heavy rain will only aggravate saturated soils and swollen creeks,
small streams and rivers. With that in mind, we`ve elected to
continue to Flood Watch through midday Saturday. It`s possible
that parts of the Flood Watch may need to be extended a little
later, but we will defer to later shifts to handle those details.
Convection should gradually come to an end as a Pacific
front/dryline sweeps eastward through the region.

24-Bain

&&

.LONG TERM... /Issued 334 PM CST Fri Feb 23 2018/
/Saturday Night through Thursday/

We`ll see mild temperatures and a chance to dry out (at least for
a few days) the remainder of the weekend and Monday, as the
shortwave energy and final round of convection and locally heavy
rainfall moves east away from the area and toward the
Ohio/Tennessee Valleys. A weak cold front will nudge into the area
late Sunday and Sunday night, but be devoid of any substantial
cooler air. Highs both Sunday and Monday will be mostly in the
60s, though a few areas west of U.S. 281 where stronger insolation
will be present may see highs nudge just above 70 degrees.

As the surface high pressure ridge centered northeast of our area
shifts east Monday night into Tuesday, south winds will transport
Gulf moisture back northward. Southwest flow aloft will intensify
in advance of yet another upper level low organizing over Arizona
and California. Low level warm advection increases in advance of
a lead shortwave ejecting northeastward from northern Mexico later
Tuesday and Tuesday night for scattered showers and a few
thunderstorms, especially east of I-35. A prominent elevated mixed
layer (capping inversion) should keep activity below severe
limits and possibly even elevated, though any stronger activity
Tuesday night could put out small hail and localized heavier
downpours. That said, amounts across eastern North and Central
Texas should remain generally under an inch, so any flooding
threat appears minimal at this time.

As upstream upper level energy across the Eastern Pacific plunges
southeast toward the Pacific Northwest, the California and
Arizona closed low will open up and progressively move east over
the Southern Rockies and possibly over the Southern High Plains by
late Wednesday. This is where things get a little murky, however,
as the GFS is much more progressive with this southern stream
energy than the slower Canadian and European models and their
deeper solution. Either way, a surface trough or dryline looks to
move east either into, or just to the western edge of our CWA by
early Wednesday evening. The associated forcing from both the
surface and upper level features, along with increasing warm
advection should at least promote higher end convective chances by
Wednesday afternoon and night. However, with track and timing
uncertainties, I felt it best to keep rain probabilities in the
mid-high chance category across the eastern two-thirds of the CWA
until timing and environmental uncertainties can be ironed out by
future upper-air sampling and model output. Either way, it does
appear some severe weather and/or localized flooding is possible
during the mid-week period.

This mid-week system will lift away from the area on Thursday,
allowing a cold front to move through and dry the entire area out
once again. As with the previous cold front late this weekend, the
bulk of the colder air remains well northeast of the area with
just a slight cool down into the 60s.

05/

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    48  63  38  69  41 /  70  80   5   0   0
Waco                55  69  39  67  37 /  70  80   5   5   0
Paris               54  66  38  64  40 /  60  80  10   5   5
Denton              43  64  34  68  36 /  70  80   0   0   0
McKinney            50  61  36  66  37 /  60  80  10   0   0
Dallas              48  63  39  68  41 /  70  80   5   0   0
Terrell             55  67  38  67  40 /  50  80  10   5   5
Corsicana           54  69  40  68  41 /  50  80  20  10   5
Temple              55  72  40  67  40 /  60  80   5  20   5
Mineral Wells       39  66  31  69  33 /  70  80   0   0   0

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 4 AM CST Saturday for TXZ092>095-
102>107-117>123-130>135-141>147-156>161.

Flood Watch through Saturday morning for TXZ091>095-101>107-
116>123-130>134.

&&

$$

82/30



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