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

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580
FXUS64 KFWD 290301
AFDFWD

Area Forecast Discussion
National Weather Service Fort Worth TX
1001 PM CDT Fri Apr 28 2017

.UPDATE...
Main update for this evening was to decrease areal coverage of
rain chances through midnight. Also made a few cosmetic changes to
PoPs for Saturday morning and increased wind speeds based on
latest guidance. Otherwise, the remainder of the forecast remains
in good shape.

Quiet weather conditions were observed late this evening across
North and Central TX. While the 00 UTC FWD RAOB did reveal that
capping had largely diminished, the lack of any sort of lift or
trigger prevented the development of deep moist convection. A
large and potent upper trough continues to churn out across
southern AZ and will slowly nudge eastward over the next 48 hours.
For tonight, main change to the forecast was to limit precip
chances to areas along the Red River as a ribbon of weak ascent
lifts out towards the north in the southwesterly flow aloft. With
this upper air pattern, there`s bound to be a few perturbations in
the flow and with steep lapse rates in place, even modest ascent
could trigger some elevated convection. Some hi-res model guidance
do develop a large complex of storms across western OK and
adjacent portions of TX and pushes this eastward. This will be
coincident with an increasing 50-55 knot low level jet, as already
evidenced by an increase in surface winds/stratus across the
upper TX coast. While the strongest conveyor of winds should
remain east of the convection, it`s not out of the realm of
possibility that some of this activity may backbuild southward
into portions of North TX as good flow will still exist farther
to the west. Should back building of this complex occur, it`s
likely that storms will be elevated and pose mainly a large hail
risk.

The longevity of this convection could play a role in additional
thunderstorm development on Saturday for parts of North TX. If a
very robust line of convection does backbuild and continues
through much of Saturday morning, the afternoon severe weather
risk would in theory be hampered by lack of diabatic heating and
instability for some. Any subsequent outflow boundary could also
be a focus for additional convective development elsewhere,
however. For now, we will monitor this potential and should have a
better idea by Saturday morning. The forecast for Saturday
afternoon still looks to be on track with numerous showers and
thunderstorms developing along the I-35 corridor and marching
eastward. Adequate amounts of shear and instability will support a
risk for strong to severe storms capable of large hail and
damaging winds. Overall, the unidirectional wind shear would
generally mitigate a widespread tornado potential, but the low
level wind field will need to be monitored. In addition, any
outflow boundaries could be a source of higher storm relative
helicity sufficient for transient low level mesocyclones.

We will also monitor the potential for perhaps a second round of
post-frontal/elevated convection. While the front will slice
eastward through the afternoon and early evening hours, much of
the strong forcing for ascent will lag behind. There should be
some elevated instability remaining, even after convection
associated with the front slides eastward. With lift impinging on
this area, there may still be a risk for hail with this post-
frontal/elevated convection. Heavy rain will be possible with all
convection and if later guidance keys in on this second batch of
storms, we may need to consider a Flash Flood Watch for parts of
North TX with the overnight package.

The other change to the forecast was to increase wind speeds
tonight as most model guidance has a strong signal with a stout
low level jet developing. While the time of day would preclude
highly efficient mixing, there should be enough momentum transfer
for sustained winds of 15 to 20 MPH with gusts to near 30 MPH,
especially for areas along and east of I-35. I toyed with the
idea of going with a short-duration wind advisory, but given the
likely sub-optimal mixing (due to time of day), I`ll hold off on
this.

The remainder of the forecast is in good shape and updated
products have been transmitted.


24-Bain

&&

.AVIATION...
/ISSUED 710 PM CDT Fri Apr 28 2017/
VFR conditions are expected through the evening hours at the area
TAF sites. MVFR ceilings will spread north tonight on a 50-knot
low level jet. Ceilings around 1500 feet should move into Waco
around 05-06z and into the Metroplex around 06-07z. Surface winds
of 15 to 20 knots will prevail. There is a low chance of showers
and thunderstorms before 12z, but the chances are not high enough
to mention at this time.

MVFR ceilings should prevail through 18z with some scattered
showers and maybe isolated thunderstorms. For now, have just
placed VCSH starting at 16z. Although the cap should remain in
place, we will have steep lapse rates, so if we get some elevated
thunderstorms, large hail will be a possibility. Southerly winds
of 15 to 25 knots will prevail.
winds
Thunderstorm chances will increase during the afternoon as a cold
front and upper level disturbance approach. Have placed a TEMPO
TSRA for the 18 to 22z period in the Metroplex TAFs. Some of the
storms may be severe with large hail and strong gusty winds
possible. The front is expected to move through the Metroplex by
22z with rain likely and thunderstorm chances continuing behind
the front. Winds behind the front will be northwest at 15 to 20
knots.

58


&&



.PREV DISCUSSION... /ISSUED 409 PM CDT Fri Apr 28 2017/

Water vapor imagery shows a large upper trough over the western
U.S. this afternoon. This system is expected to move east through
tomorrow night bringing increasing chances for showers and
thunderstorms to North Texas along with an increased threat for
severe weather.

At the surface this afternoon, an area of low pressure is located
near Big Spring with a dryline extending southward from there. A
warm front is located just north of the Red River. South of the
warm front, the atmosphere has become quite unstable with surface
dewpoints now in the upper 60s and lower 70s across North Texas.
In the presence of very steep lapse rates associated with a deep
elevated mixed layer, this high near-surface moisture results in
instability values between 2500-3500 J/kg. While this is an
impressive amount of instability, it remains mostly capped, as
indicated by unimpressive visible satellite data and visual
observations of the cumulus field. This strong inhibition should
remain in place through early evening resulting in little or no
chances for thunderstorms across North Texas.

By later this evening as stronger forcing for ascent spreads into
the Southern Plains from the approaching upper trough, low level
southerly flow will strengthen. A 50 kt low level jet will develop
and be centered right over North Texas with low level warm
advection maximized right near the Red River. The combination of
increasing ascent and the presence of the surface frontal boundary
should be sufficient to weaken the strong capping and allow deep
convection to develop. Most of this activity should be north of
the Red River across much of Oklahoma, however there will be low
chances for our Red River counties through late tonight. The
timing of this development would likely be 8-10 pm or so. Given
the amount of instability available, there would be a threat for
severe weather including very large hail and damaging winds. Low
level shear will also be sufficient to support a tornado threat,
although the better environment will be north of the Red River
closer to the actual frontal boundary.

A large area of thunderstorms are expected to be ongoing during
the overnight hours across Oklahoma and northwest Texas. These
storms will approach North Texas from the northwest in the form of
a weakening squall line as the stronger low level jet shifts to
the northeast. This ongoing convection is likely to move through
our western counties during the morning hours and may pose a
severe hail threat, so will have some higher PoPs through midday
across the western counties.

The evolution of the afternoon severe threat across North Texas on
Saturday will be somewhat dependent upon where any outflow
boundaries set up and the position of the synoptic cold front.
With the strong upper trough still to the west, it does appear
that much of North Texas will have time to destabilize by midday
with low level wind fields recovering behind any morning activity.
The main question for tomorrow is where exactly the new vigorous
convection develops along the cold front. Right now, it appears
that this will occur very near the I-35 corridor in the 1-3 pm
timeframe. Wind fields will be unidirectional by this time which
should help limit the tornado potential, but ample instability
will support a very large hail threat. The highest PoPs and best
severe weather threat will generally be along and east of I-35
through the evening hours with our eastern counties having the
most time to fully destabilize. The severe thunderstorms will move
east with the cold front and should clear our CWA during the late
evening hours.

The upper trough will actually not move through North Texas until
Saturday night and there will be sufficient moisture for continued
elevated convection across much of the region. In fact, PoPs may
actually be higher across the region Saturday night as the trough
moves through than with the deep convection along the front. There
will be a continued hail threat with this overnight activity.

Conditions improve across the region on Sunday with northwest
winds and clearing skies. Additional rain/storm chances arrive mid
week with another upper trough moving through the Plains.

Dunn



&&


.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    74  82  54  67  49 /  20  70  60  10   0
Waco                75  84  55  70  47 /  20  50  60  10   0
Paris               72  81  56  63  46 /  20  80 100  20   0
Denton              72  79  51  65  45 /  20  70  50  10   0
McKinney            73  82  53  65  46 /  20  80  70  10   0
Dallas              75  83  54  67  50 /  20  70  60  10   0
Terrell             74  83  54  67  47 /  20  80  90  10   0
Corsicana           75  84  55  69  49 /  20  70  80  10   0
Temple              74  85  56  71  48 /  20  40  60  10   0
Mineral Wells       70  78  47  67  44 /  20  40  50   5   0

&&

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

$$

58/24



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