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Area forecast discussion
National Weather Service Fort Worth Texas
1142 PM CDT Fri may 26 2017

/6z tafs/

No major changes to the latest forecast across the region. MVFR
cigs have developed across southeast Texas and are spreading into
central and North Texas at this hour. This trend should continue
into the early morning hours. MVFR cigs should prevail with
southerly flow around 15 kt through mid/late morning before
lifting and scattering shortly after midday.

Main concerns through the remainder of the taf period will be the
potential for late afternoon/evening thunderstorms and overnight
convection along/ahead of a cold front. There is still a low
chance for thunderstorms Saturday afternoon mainly northwest of
the major airports, but if storms do develop, they could quickly
become severe and cause significant disruptions to air traffic.
The initial storms should track off to the northeast. Later
Saturday night, there is a continued threat for additional
thunderstorms to develop along and ahead of a cold front across
southeastern Oklahoma. Given the amount of instability across
North Texas, these storms will have some potential to organize
into a mature complex of storms and March into the region. This
would be most likely to occur in the 3-7 am timeframe early Sunday
morning. A wind shift to the north would accompany the line of

For now, given the overall low probabilities of convection
tomorrow afternoon/evening, we'll leave the mention out of the
current taf. If the signal remains strong for the overnight
thunderstorm activity, then we'll have to add a mention of
convection in the latter parts of the tafs for all sites with the
next issuance.



the overall going forecast remains in good shape this evening, but
made a few updates to short-term temperature, dewpoint, and sky
trends based on latest observations. A relatively expansive deck
of mid and upper cloud cover has overspread the region in tandem
with an upper-level jet streak nosing into Oklahoma and upped sky
cover to account for this before additional low stratus--currently
consolidating across the mid and Upper Texas coast--surges across
the region later tonight.

Regional radar mosaic imagery shows some showers and storms trying
to develop across portions of northeastern Oklahoma at this hour,
and this activity seems to be encouraged by waa/isentropic ascent
clearly evident along the 310 Theta surface, and some degree of
ongoing convergence near a surface boundary. It appears any warm
advection of significance should remain relegated north of the Red
River overnight, and as a result we'll maintain a precip-free
forecast overnight.

Tomorrow morning will feature plentiful cloud cover across much of
the region, but this is expected to gradually thin and lift
through the morning and afternoon. A heat advisory remains in
effect from 2-7 PM tomorrow as heat index values climb towards 105
degrees. The strength of the capping inversion present tomorrow
morning, and the level of diabatic heating we achieve will play a
crucial role in the potential for convective development during
the late afternoon/evening. Specifics regarding the conditional
severe threat for Saturday are discussed in greater detail from
this afternoon's discussion.



Previous discussion... /issued 409 PM CDT Fri may 26 2017/
hot and humid conditions will persist for the next
day or so across most of north and central Texas as south breezes
continue to transport moisture northward. There are some heat
related headlines for Saturday given unseasonable warmth and above
normal moisture. The increase in moisture will also lead to a
substantial buildup of instability, which when combined with
adequate amounts of deep layer shear will promote a risk for
isolated severe thunderstorms on Saturday afternoon. This risk,
however, is highly conditional and this will be discussed further
below. The better risk for showers and thunderstorms appears to be
late Saturday night into the day on Sunday as a slow-moving cold
front slides southward. There will also be a risk for strong to
severe thunderstorms along the front as it moves through north and
central Texas. The boundary will stall out down across our central
Texas counties during the day on Sunday. Beyond Sunday, the motion of
the front is a little unknown , but it should remain close enough
such that additional showers and thunderstorms will be possible
through just about the entire work week.

Surface analysis this afternoon indicated that the dryline was
continuing to slowly push eastward through the Big Country. On its
heels, a precipitation free frontal boundary was also sliding
towards southwestern Oklahoma and western North Texas. These features
will retreat towards the west and north as an upper trough across
The Four Corners region lifts out of the Great Basin and into the
Central Plains.

For tonight---another mild and humid night is anticipated as low
level moisture continues to lift northward. Nocturnal breezes
shouldn't be quite as strong as the previous night, but sustained
speeds of around 10 to 20 miles per hour can still be expected. Overnight
conditions will be quite balmy with most areas only falling into
the 70s. Another swath of stratus will lift northward, most likely
encompassing much of north and central Texas by daybreak Saturday

For Saturday---morning stratus will gradually dissipate beneath
the late-may sun. Dewpoints are forecast to be a few degrees
higher than Friday, mainly due to additional time within the
warm/moist advection regime. A Stout capping inversion, as
evidenced on the the 12 UTC Friday morning sounding will likely
remain in place and limit any type of deep vertical mixing to the
east of the dryline. With temperatures expected to climb into the
mid 90s and dewpoints in the mid 70s, heat index values will
likely climb towards advisory level criteria. In fact, several
sites as of 3 PM Friday are already nearing these levels. With the
numerous outdoor activities likely ongoing on Saturday and
potential for impacts, decided a small heat advisory for the dfw
metroplex and western portions of North Texas was prudent. As noted
yesterday, advisory or not---individuals outside should adhere to
heat safety procedures: stay hydrated with lots of water, take
frequent breaks in shaded regions if outdoors and most
importantly, never leave children or pets in vehicles!

With regards to the severe weather threat---the dryline will
likely be slower to mix eastward on Saturday given the increased
low level moisture depth. At this time, feel that operational NAM
and European model (ecmwf) solutions have the best handle on the dryline position.
The deep mixing characteristics of the GFS appear a tad overdone
and this model even advertises a dryline bulge along the U.S.
Highway 287 corridor with resultant convection. The ttu-WRF also
initiates convection, but like the GFS, it appears to have the
dryline much too far to the east and generates sufficient
convergence necessary to overcome the very impressive capping
inversion. While outliers, we will have to monitor trends in other
model guidance that remains silent (06 UTC and 12 UTC namnest and
latest runs of the hrrrx). In addition, any storms that can
develop will become severe very quickly with giant hail and
damaging winds being the main hazards. The larger scale
environment isn't overly conducive for tornadoes, however, any
backing of surface winds due to any sort of storm scale or
boundary would change the game. Dramatically. This would not be
too dissimilar from what happened about 20 years ago across
central Texas. That in mind, we still feel that this potential is
still on the low end of the spectrum with regards to hazards.

The best window for initial severe thunderstorm development
appears to be after 21 UTC for areas near and just east of a
Stephenville to Graham to Wichita Falls line, but west of a Paris
to McKinney to Granbury line. Between the 2100-0000 UTC time
frame, the coverage should be very isolated (around or less than
20% coverage). If the dryline mixing is closer to that of the
GFS/ttu WRF solution, the coverage could be greater. Towards the
evening hours, different convective allowing models continue to
produce convection along and ahead of the dryline. Aloft, there
doesn't seem to be any real focus for this and the only plausible
explanation would be that the combination of daytime heating and
abundant low level moisture are sufficient to break the cap. At
this time, it appears unlikely, but we will advertise just low
chance pops for areas near and northwest of a Killeen to Meridian
to Cooper line. There may be a slightly better opportunity for
convection along the Red River as the cold front slowly encroaches
from the north. Again, the main thunderstorm hazards will be
extremely large hail and damaging winds.

Rain chances increase through the evening hours as the
aformentioned cold front slowly slides southward towards north and
central Texas. I've not made too many significant alterations to the
inherited forecast as there still remains a high degree of
uncertainty as to the coverage of any convection. The namnest
offers an intriguing solution and develops a large amount of
convection along the front. If surface based convection can
develop, it should in theory slide south and eastward both in
accordance with the conceptual model a forward propagating mesoscale convective system and
perhaps more importantly towards the axis of greatest instability
which will be in place across portions of north and East Texas. This
is somewhat conditional on convection developing, however, and
other hi-res guidance offers varying solutions. For now, will have
the best chances for storms across northeastern zones where there
may be a glancing blow of some upper level forcing that will more
than likely slide in across the Ozarks. If a complex of storms
does indeed develop, it'll likely be severe with potentially
damaging winds and large hail. The complex of storms at this time
is not expected to progress much past the I-20 corridors as the
onset of nocturnal stabilization occurs. If a complex of storms
plows through all of north and central TX, there's a possibility
that instability will be limited for sunday's storms.

For Sunday---the cold front will likely slow across portions of
central Texas. If the mesoscale convective system does not materialize, there will be
sufficient instability and wind shear for strong to severe storms
across central Texas. Main hazards here would be damaging winds and
large hail. Farther north towards the I-20 corridor, there may be
enough ascent along the 925-850mb baroclinic zone which could
result in elevated convection which would more than likely produce
large hail. The front should sweep southward through the
remainder of central Texas Sunday evening into early Monday morning.

For next week---the active weather pattern looks to remain, but
there's some uncertainty with regards to exact placement of
highest rain chances. Unfortunately, will have to broadbrush/lean
on model blends during the extended portion of the forecast.
Fortunately, it does appear that temperatures will be quite lower
than the past several days compliments of cloud cover and rain



Preliminary point temps/pops...
Dallas-ft. Worth 78 94 75 83 64 / 5 20 40 50 10
Waco 78 93 76 87 65 / 5 10 20 50 60
Paris 76 87 70 80 61 / 10 10 60 50 10
Denton 76 94 74 82 59 / 5 20 40 40 10
McKinney 76 91 75 81 61 / 10 20 40 50 10
Dallas 78 93 76 83 65 / 5 10 40 50 10
Terrell 77 90 74 83 65 / 10 10 40 50 30
Corsicana 77 91 75 86 66 / 5 10 30 50 60
Temple 77 92 75 88 65 / 5 10 20 50 60
Mineral Wells 74 96 74 82 61 / 5 20 30 40 10


Forward watches/warnings/advisories...
heat advisory from 2 PM to 7 PM CDT Saturday for txz091-092-

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