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

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FXUS64 KFWD 211716 AAB
AFDFWD

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
1216 PM CDT Fri Apr 21 2017

.UPDATE...
Enhanced by storms in southeastern Oklahoma a few hours ago, an
outflow boundary/front is currently moving into our northwest
counties. This boundary will be important to track during the
afternoon as it may be a focus for convection. With northeast
winds behind the boundary and cool temperatures in the 50s, this
front should continue to move south for at least a few more hours,
and then could stall around mid afternoon. Some cloud clearing
has been occurring across North and Central Texas this morning,
but immediately ahead of the boundary dense cloud cover continues
to develop. This may limit the amount of instability available
over the next few hours, and it may take until later this
afternoon until we are able to erode the cap.

By late afternoon, if not before, we expect storms to develop
along the front, most likely in our northwestern counties, with a
southeastward advancement with time. There is a risk for severe
storms producing large hail, damaging winds, and isolated
tornadoes, mainly along and north of the Interstate 20 corridor
late this afternoon through the evening hours. The threat for
severe storms will lower as the storms move south overnight into
Central Texas.

For the update, made some adjustments to PoPs through tonight, and
was a little forgiving on the areal extent during each time block due
to uncertainty in where the front will be located. For the
evening period, did slide the highest PoPs east based on the
latest guidance. Adjusted high temperatures down in our far
northwest counties where the cool air is already arriving, and
also adjusted the wind grids to reflect a faster arrival and
potential movement of the front/outflow boundary.

JLDunn

&&

.AVIATION...
/ISSUED 733 AM CDT Fri Apr 21 2017/
Several challenges exist beginning with cig trends this morning,
then timing of potential scattered-numerous strong to severe
storms late this afternoon and through the evening hours.

All sites are starting MVFR this morning, however, a brief hour or
so of IFR may exist at DFW airports through 14Z. Waco was already
above 2000 feet, but DFW sites will likely see lower MVFR cigs
until about mid-morning, with VFR conditions at all airports by
midday or at the latest, 18Z. The rapid lifting of cigs is likely
two-fold due to increasing lift of isentropic surfaces from the
approaching upper trough and warmer temps with more rapid mixing
than what has occurred the past two days.

As for convection, a synoptic surface frontal boundary was draped
from the Lubbock, TX, area east-northeast into OK. This front was
expected to slowly mix northward, but how far is in question with
elevated storms currently occurring from the TX Panhandle into
Central/Northern OK. The position of this front and where storms
initiate later today geographically will likely determine the best
window for storms into North TX either side of 00Z Saturday and
Waco later in the evening or overnight hours. For now, have split
the difference between the more progressive 3km NAMNest model and
the slower TTU WRF. The HRRRx is somewhat of a compromise of the
other solutions and I will introduce categorical convective
chances across DFW airports between 23Z- 01Z, and Waco before or
possibly at midnight.

Otherwise, a cold front will progress with the upper trough and
likely into the DFW Metro mid-late evening, then through Waco
during the overnight hours. With more uncertainty on FROPA at
Waco, decided to leave potential FROPA timing out of this
morning`s TAFs with likely better confidence for later forecasts
to predict.

South winds 10-15 knots with a few gusts 20-25 knots will occur
through the afternoon hours. A diminishing and veering of winds to
southwest or west will likely occur for a brief hour or two
tonight ahead of the advancing cold front. Behind the cold front,
north-northwest winds 15-20 mph with occasional higher gusts are
expected before daybreak Saturday.

05

&&

.PREV DISCUSSION... /ISSUED 436 AM CDT Fri Apr 21 2017/
After a generally quiet week weather-wise, widespread rain
chances and the potential for severe weather return to the
forecast for today (Friday) and will linger through the morning
hours on Saturday. Breezy north winds are expected once
precipitation clears on Saturday with below normal overnight
temperatures Saturday night into Sunday morning. Temperatures
should moderate through the rest of the weekend and into the first
half of the new work week with precipitation-free conditions. The
middle to end of next week appears active with a chance for
showers and thunderstorms.

The immediate short term wx concern will be the area of
convection across portions of western North TX along a stalling
frontal boundary at this hour. Overall radar presentation
indicates that this convection is becoming a bit more disorganized
as it heads into a more unfavorable airmass for convective
longevity. As a result, will confine PoPs to mainly the Red River
with generally nil rain chances everywhere else.

A general increase in cloudiness is expected today as low level
moisture streams northward in response to the main upper low that
is forecast to swing southeastward towards the OK and KS border.
The resultant cyclogenesis across the panhandle should ensure that
the stationary front currently across the Red River Valley should
lift rapidly to the north. This is where the short term forecast
becomes a bit more complicated as there are potential caveats that
could keep this boundary further south. The first wave of
convection continues to push eastward as advertised across east
central Oklahoma. As the upper low approaches, increasing forcing
for ascent in the form of strong thermal advection and
subsequent isentropic ascent will likely lead to the additional
development of numerous showers and thunderstorms further to the
west. Much of the hi-resolution model guidance advertises this
uptick in convection quite well and regional radars indicate that
the second wave of convection is developing across the OK/TX
panhandles. A bulk of this activity should remain to the north of
North TX, but it`s possible that a few storms may skirt the
immediate Red River zones today, and I`ve included a mention of
low PoPs/kept lower temperatures here to account for this. More
importantly, however, is that this convection may serve to hinder
the frontal boundary`s northward progression. If the boundary is
able to sink back farther to the south, this could be a catalyst
for additional convective development early this afternoon. If
storms occur here, they would more than likely become severe with
large hail, damaging winds and perhaps a small area near the
boundary (or any outflow boundaries) may be favorable for a
tornado or two. With that in mind, this scenario does not appear
very likely as the strong height falls forecast to occur across
the OK/KS border and resultant surface cyclogenesis should help
lift the boundary northward, even in the presence of convection.
That said, we will monitor the position of the boundary closely.

Later this afternoon, the strong shortwave will continue to eject
eastward into the Ozarks and will likely send the cold front
quickly to the south. With the airmass across North TX being
conditionally unstable (forecast soundings indicate a cap will be
in place), sufficient mesoscale ascent along the front should be
enough to lift parcels to through the cap (and/or weaken the cap)
and up to their LFCs for rapid ascent into the free atmosphere.
Instability values are progged to climb into the 1500-2000 J/kg
range with deep layer shear values on the order of 40 knots. These
values would suggest that storms that develop initially will have
supercellular characteristics, capable of large hail and damaging
winds. As noted above, there may be a brief window for a tornado
or two should storms interact with any boundaries. This potential
is low, but will bear monitoring. With deep layer shear vectors
oriented parallel to the cold front, it`s likely that activity
will morph into a squall line, quickly. This should mostly
relegate the severe hazards to damaging winds and large hail. Most
model output progresses the front quickly towards the south and
this may favor elevated convection as updrafts initially rooted in
the boundary layer become undercut by the cooler, denser airmass.
Should this occur, the severe hazard scale would tip more towards
large hail than damaging winds with a diminishing threat (though
non-zero) for tornadoes. The progressive nature of the front
should also mean that prolonged periods of training echoes should
be at a minimum, but we won`t write off the possibility of some
isolated flooding, especially in urbanized areas of North and
Central TX.

Showers and storms should continue to push southward through
Central TX during the overnight hours. The severe weather threat
should wane here for a couple of reasons. 1) Convection will
likely be removed from the best upper level support and thus deep
layer shear decreases with southward extent and 2) the time of
day isn`t overly conducive for widespread severe weather. With
those two factors in mind, will keep the mention of severe wx out
of southeastern and southern zones for now. Should the convective
line be more robust than currently thought, a severe wx threat may
exist even further to the south.

The main weather highlights for Saturday morning and Saturday
afternoon will be decreasing rain chances in the morning, followed
by breezy conditions post-FROPA. Surface pressure rises should
help create breezy northerly winds of 15 to 20 MPH. Some gusts to
near 30 MPH are also expected, especially for areas that clear and
are able to mix some of the higher momentum aloft down to the
surface. At this time, sustained wind speeds should remain below
wind advisory status and I don`t anticipate one being needed at
this time. That being said, there are likely numerous outdoor
festivities ongoing this weekend and those with high profile
objects or sensitivity to the wind should take precautions to
secure any loose articles. Cooler conditions are forecast on
Saturday with most areas in the 60s and 70s for high temps.

Sunday through Wednesday appears quite dry with moderating
temperatures each day. Sunday morning will likely be the coolest
morning we`ve experienced in a while, with low temperatures in
the 40s. Some upper 30s may even be possible along the Red River,
but at this time it appears that winds may stay elevated enough to
preclude this. We will evaluate this with the afternoon and next
overnight package, however. Temperatures will be near normal
starting off next week with mostly dry conditions.

The middle to end of next week, however, appears that it`ll be
quite active, as one would expect for this time of year. There is
a general consensus in model output that low level moisture will
facilitate healthy amounts of instability. The quasi-zonal flow
aloft should ensure that deep layer shear values will promote
organized storm structures. While it`s a little too early to delve
into the specifics, it does appear that a risk for strong to
severe storms will accompany any shortwave troughs that ripple
through the flow aloft.

24-Bain



&&


.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    84  54  68  50  74 /  30  80  20   0   0
Waco                84  56  69  47  74 /   5  60  30   0   0
Paris               82  53  64  44  71 /  20  80  20   0   0
Denton              82  52  67  44  73 /  40  70  20   0   0
McKinney            84  54  67  46  73 /  40  80  20   0   0
Dallas              86  55  69  50  75 /  30  80  20   0   0
Terrell             83  56  68  47  74 /  10  80  20   5   0
Corsicana           85  58  69  46  74 /   5  60  30   5   0
Temple              86  57  69  49  74 /   5  50  30   0   0
Mineral Wells       85  52  68  42  74 /  30  60  20   0   0

&&

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

$$

26/82


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