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

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897
FXUS64 KFWD 272115
AFDFWD

Area Forecast Discussion
National Weather Service Fort Worth TX
415 PM CDT Sat May 27 2017

.DISCUSSION...
The forecast today is quite complicated with regards to both heat
and severe weather headlines. Fortunately, the oppressive
conditions being experienced by a good portion of North and
Central TX should be relegated to the remainder of this afternoon.
The severe weather risk, however, looks to persist into the day
on Sunday.

Hot and very humid conditions will continue this afternoon and as
of 3 PM, many areas near and just west of the I-35 corridor are
near or just below Heat Advisory criteria. As clouds slowly thin,
temperatures will continue to climb into the mid-90s. Farther
south, the advisory may be a bit more on the marginal side,
depending on the rate of clearing, but at this time, still expect
at least a few sites on the western periphery of the advisory to
experience a couple of hours with heat index values of around 105
degrees. As we`ve stressed this week, it will remain hot and very
humid and people who spend prolonged periods of time outside
without either adequate shade or adequate hydration (water), may
experience heat exhaustion or heat stroke. Fortunately, it looks
like a reprieve from the hot conditions is on tap for a good
portion of the area starting tonight.

Early afternoon surface analysis indicated that a sharp dryline
was slowly mixing eastward from near a Lawton, OK, to Abilene, TX,
line. The dryline will continue its slow trek to the east this
afternoon before retreating back towards the west under quiescent
conditions during the early evening hours. A cold front was noted
across northwest Oklahoma and will slowly move towards the south,
eventually overtaking the dryline. The dryline, but more
importantly the cold front will be the main players for the
severe weather threat across North and Central TX over the next
24-36 hours.

For this afternoon, just about all convective allowing models are
silent for much of North and Central TX this afternoon which fits
the conceptual model for an environment characterized by strong
capping and little in the way of synoptic scale ascent. On the
smaller scale, surface winds are veered well ahead of the actual
dryline along what appears to be a broad surface trough. This
trough extends from near a Bowie to Decatur to Granbury line and
we will have to keep an eye on this surface trough as it could
provide just enough convergence to assist what should likely be
buoyant surface parcels.

The environment this afternoon will be characterized by extreme
instability values with some forecast model soundings indicating
nearly 6,000 J/kg of CAPE. While these values are quite
extraordinary, it`s plausible that little to none of it will be
realized compliments of a very strong capping inversion as sampled
by the 12 UTC FWD sounding. If a thunderstorm can manage to
develop and sustain itself, it will become severe very quickly
with a high likelihood of giant hail (perhaps near 3" in diameter
or greater), destructive straight line winds (70-80 MPH) and a
tornado or two. While the surface wind fields aren`t necessarily
overly conducive for tornadogenesis, the impressive instability
values, particularly in the low levels, will more than compensate
for the lack of strong low level shear. While the tornado hazard
IS the most conditional risk, it will be closely monitored should
storms develop as there is a potential (again, given the extreme
instability) for a robust tornado or two.

The atmosphere will be most prime for convective initiation after
about 6-7 P.M. Some of the latest suite of hi-res guidance
initiates convection across southwestern Oklahoma. This appears
plausible as surface analysis revealed a nice bulge in the dryline
across this region. Given the environment, some of these models
do take this activity east and eventually southeastward into
extreme portions of North TX. Again, this seems like a plausible
solution, but given the uncertainty as to whether or not storms
will even develop across OK, I`ve elected to only advertise 20-40
PoPs through the late afternoon into early evening hours. Storm
chances after 00 UTC should begin to increase rapidly at this
time and I do expect some sort of severe storm cluster over
southern Oklahoma. Some hi-res guidance actually depicts that this
complex will grow upscale rapidly and slide southeastward through
our northernmost row of counties against the Red River. Should
this occur, the main hazards would be large, damaging hail and
destructive straight-line winds. This complex may decay as the
onset of nocturnal cooling occurs. However, its cessation may be
tied more to the start of the low level jet, which is forecast to
become quite strong, a little later in the evening. If this
complex can survive through then, it may have some additional
forcing which would promote a greater longevity.

A potentially second, but more probable round of thunderstorms is
expected near or just after midnight as the cold front slowly
slides southward. If this line of thunderstorms can remain rooted
in the boundary layer and develop a strong cold pool, it has the
potential to morph into a forward propagating MCS as there will be
an abundance of instability as well as assistance from a 30-45
knot 925mb jet. Similar to the first round, the main hazards would
be destructive straight line winds and very large hail. The most
likely areas to be impacted would be locales east of I-35 and
north of I-20. If two rounds of convection do develop, there could
be some isolated instances of flooding or even low-end flash
flooding. At this time, I do not feel compelled to go with any
sort of short-fused/small areal coverage FFA given 1) the
antecedent conditions across North TX do not support it as most
areas missed out on the heavy rain last week and 2) the speed of
both rounds of convection should be swift enough to mitigate any
widespread concerns. We will keep an eye on areas along the Red
River, especially across more urbanized areas where flash flooding
would be more likely. All convection should be efficient at
producing brief bursts of heavy rain, however, given the high
amount of moisture in place.

Close to sunrise, there`s a great deal of uncertainty as to how
quickly this potential line of convection decays. Some guidance
suggests that this complex will plow through just about the
entire eastern half of the forecast area, while some guidance
indicates a quick and painless death of the MCS as it nears I-20.
For now, will confine PoPs to far eastern zones with slightly
lower PoPs elsewhere. The cold front will continue to slowly
trudge southward with breezy northerly winds in its wake. This
feature looks to be quite slow given that there isn`t a ton of
strong cold advection southward.

The slow progression of the front means that it`ll likely not
clear our far southern and southeastern counties by Sunday night.
While it`s likely to be mostly cloudy through the day, surface
moisture will be quite high. The mid-level flow isn`t overly
impressive, but steep lapse rates aloft coupled with modest shear
values may support multicell storm clusters capable of some
damaging downburst winds and potentially large hail. There will be
a heavy rain threat as well, again, given the high moisture
content in the atmosphere. If the front slides to the south a
little quicker than currently anticipated, it`s quite possible
that the severe weather threat also shifts to the south. For now,
we will have to simply monitor the front`s motion through the
overnight hours to get a better handle on this.

For Memorial Day, it appears that areas generally along and north
of I-20 will be mostly dry. Unfortunately, areas farther to the
south will be at greatest risk for showers and thunderstorms. At
this time, the severe weather risk is a little unclear, but
forecast soundings do not generally support widespread severe
storms on Memorial Day, but we will keep an eye on this, given
the multitude of outdoor activities.

For next week, the threat for showers and thunderstorms will
continue as low level moisture lingers across the area.
Unfortunately, there still does not appear to be any meaningful
signs of strong ascent for most days. As a result, PoPs were
broad-brushed just about each day. Towards the end of next week,
it appears that a slow moving upper low may be more of a focus for
a more concentrated area of showers and thunderstorms.

24-Bain

&&

.AVIATION...
/ISSUED 106 PM CDT Sat May 27 2017/
/18Z TAFs/

Concerns...Ceiling trends this afternoon and overnight. Low
chances for convection through the valid TAF period.

A dense layer of clouds continues to cover the TAF sites with MVFR
conditions still occurring at KACT due to cigs and light fog. The
Metroplex airports have just recently improved above 3 kft. Expect
conditions to continue to slowly improve this afternoon with
scattered-broken VFR conditions at all the sites after 21Z. Breezy
south and south-southwest winds will continue with sustained
speeds of 10-20 kts and gusts up to 25 kts. Severe thunderstorms
may form along the dryline west to northwest of the DFW airports
after 23Z, affecting air traffic in these areas through the
evening hours. Depending on where storms break the cap and
develop, there`s a small chance they could track into the DFW area
this evening. However, at this time, the chances of this scenario
happening and coverage of storms is too low to mention in the
TAF.

Overnight trends are a little more uncertain as a complex of
storms and a cold front move into the region. Latest guidance
suggests the bulk of the storm complex will remain east of the DFW
TAF sites, and have decided to pare back to VCSH in a smaller time
window from 09-13Z. MVFR cigs are expected to accompany the front
as it moves through the Metroplex around 11-12Z and prevail
behind the front into at least the early evening hours with
northwest to north winds of 10-15 kts.

There is uncertainty regarding if MVFR cigs will spread north
into KACT ahead of the front overnight. Strong southwest winds
above the surface may prevent widespread coverage of stratus
overnight in North and Central Texas. Showers and thunderstorms
may also occur in the KACT area as the front approaches a few
hours later. Will include a VCSH mention starting at 15Z for KACT,
but the threat for storms may extend later into the afternoon
hours depending on the speed of the front. MVFR cigs are expected
to prevail behind the front into at least the early evening hours
with northwest to north winds of 10-15 kts.

JLDunn


&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    75  85  65  86  66 /  40  40  20  10   5
Waco                76  87  65  83  66 /  10  50  50  40  20
Paris               71  82  62  84  62 /  80  50  10  10   5
Denton              72  84  60  86  61 /  50  40  10  10   5
McKinney            74  83  62  85  62 /  60  50  10  10   5
Dallas              76  85  66  87  67 /  40  40  20  10   5
Terrell             74  83  66  85  64 /  50  50  30  20  10
Corsicana           74  85  67  83  67 /  30  40  50  40  20
Temple              75  88  66  82  68 /  10  50  60  40  30
Mineral Wells       72  84  62  87  60 /  30  20  10  10   5

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 7 PM CDT this evening for TXZ091>093-101>104-
116>120-130>133-143>145-156>159.

&&

$$

82/24



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