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Area forecast discussion 
National Weather Service Fort Worth Texas
232 PM CDT sun Apr 26 2015

Discussion...

With the focus of the forecast on our short term severe weather
evolution...will keep the discussion with regards to the remainder
of the forecast brief. The discussion regarding the evolution of
severe storms today and tonight remains mostly valid still at 2
PM...so left the early afternoon update appended below.

Changes from the previous update for today...the 18z subjective
surface analysis indicated surface cyclogenesis was occurring
farther north than what models were indicating this morning. If
the surface low does not develop farther south this
afternoon...the primary ingredients for discrete supercell
thunderstorms may move north towards the dfw metroplex this
evening. The surface low is expected to deepen through 00z and the
strongest forcing and combination of ingredients for supercells
will exist at that time as a result. The best window for discrete
supercell development within the County Warning Area...east of the surface low
still appears to be on track for the 5 PM to 10 PM time frame.
Will continue to watch the evolution of the surface low closely
this afternoon because if it sets up farther north...the biggest
threat for discrete supercells may move away from central Texas
and towards the dfw area. Will have to just see what happens at
this point. Another Tornado Watch is likely to be issued late this
afternoon for much of north and central Texas. While there may not
be many storms...the storms that do develop are expected to pose a
very high threat of severe weather. Conditions still look
favorable for very large hail...damaging winds and tornadoes
wherever supercells develop.

A squall line still looks likely to develop across far southwest
Oklahoma or across northwest Texas this evening. This squall line
still looks like it will be very organized and pose a widespread
threat for damaging winds as it moves southeast across North Texas
late this evening and overnight.

The threat for flooding appears to be unchanged at this time. Will
continue to monitor the threat for training storms through
tonight...however confidence is not high enough to issue a Flash
Flood Watch at this time.

Monday...thunderstorms are very likely across north and central
Texas Monday afternoon...however the Mode and overall threat for
severe weather is very uncertain at this time. If a large squall
line/derecho moves southeast across North Texas overnight...it may
leave a strong outflow boundary that will significantly alter the
thermodynamic environment for Monday. Until we know what happens
with storms through tonight...it will be difficult to identify the
exact threat for severe storms on Monday. There will be strong
lift as the upper trough pulls east over the Southern Plains...and
there will be moisture...so overall probability of precipitation are quite high in the 50
to 70 percent chance range. There will probably be a sufficient
amount of wind shear to support organized storms tomorrow
afternoon...however conditions near the surface may be cool and
cloud cover may inhibit surface based instability for much of the
day tomorrow. At this time...think that the locations with the
best chance for severe storms tomorrow afternoon will tend to be
south of Interstate 20 as the overnight squall line is most likely
along and north of I-20. We will obviously have to reevaluate the
thermodynamic environment tomorrow morning after the squall line
has moved east of the County Warning Area.

Tuesday...the upper trough/low is expected to be over North
Texas...centered over the Red River. There is a chance of seeing
some storms right in the middle of the upper low as the cold air
aloft will help destabilize the environment...however skies will
likely be overcast...so surface based storms appear to be very
unlikely at this time. Scattered elevated showers and
thunderstorms are likely near the center of the upper low...so
have the highest probability of precipitation near the Red River...and lowest probability of precipitation across
central Texas on Tuesday.

Wednesday through Friday...upper level ridging is expected to
result in a warming trend under mostly clear skies for the
remainder of the week.

Cavanaugh

&&

Previous update /issued at 1239 PM/...

Mesoscale forecast update regarding the forecast for severe
storms across north and central Texas today...

12z upper air analysis showed an intense upper level trough
located over the southern Arizona and New Mexico border. 500 mb
height falls of 6 to 12 dm were observed downstream of this upper
trough...and the magnitude of these height falls is indicative of
strong large scale forcing for ascent to the east. At the 700 mb
level...a plume of steep middle-level lapse rates was observed...with
a temperature difference in the 700 to 500 mb layer of 22 degree c
located southwest of north and central Texas. To the southwest of
this plume of enhanced lapse rates...a 50 knots middle-level jet core
was observed over northern Mexico. Closer to the surface...at the
850 mb level...deep moisture was only observed over southwest
Texas with a dryline subjectively analyzed near the Texas/OK
border...extending southwest over west central Texas and into
northern Mexico about 60 miles west of del Rio. A 15z surface
analysis observed rich moisture in place along the Texas Gulf Coast
with middle 70s dew points observed west and southwest of Houston.

For today...nearly all short range model guidance indicates that a
surface low pressure system will intensify rapidly this afternoon
over west central Texas. Most guidance shows surface pressure
falling at a rate of 2-3 mb per hour leading up to 00z. At
15z...the lowest surface pressure over the southern High Plains
was around 1002 mb...most guidance indicates the surface low's
pressure will fall to 993 to 995 mb by 00z. That magnitude of
pressure fall associated with the surface low is fairly
intense...and should result in a convergence of parameters that
will support a localized severe weather outbreak across the
region this afternoon and evening. One of the key forecast
challenges today will be determining where the surface low moves
as the relative threats for supercells and tornadoes will vary
depending on the exact position of this low later this afternoon.

The 12z NAM and 16z hrrr indicate the surface low will be located
just south of Interstate 20 and just north of San Angelo. If this
position for the surface low is correct...the threat for discrete
supercells would be maximized to the east of the surface
low...where a triple point would likely set up. Supercells would
be most likely to develop near this triple point...and then move
east in the relatively strong southwesterly flow aloft. Storms
appear to be very likely in a supercell storm Mode if they develop
in this triple point by 00z as cape is forecast to be on the order
of 3000 j/kg with deep layer shear of 50 kts in the 0-6 km above ground level
layer. The shape of the 0-3 km hodograph is expected to be
clockwise looping...and dynamically more favorable for right
moving supercells. As a result...expect supercell storm motion to
be almost due east late this afternoon and early this evening.

Right moving supercells look most likely south of Interstate 20
based on the forecast position of the surface low. This places
much of central Texas in the primary threat corridor for discrete
supercells. The environment is supportive of well-organized
supercells capable of producing very large hail...damaging wind
gusts and tornadoes. The residents of central Texas are encouraged
to pay very close attention to the weather this afternoon and
evening as a result. Of course if the surface low is farther north
..the primary threat corridor for discrete supercells will move
north with it. Away from this primary threat corridor...it is
certainly possible for supercells to develop just about anywhere
in the warm air advection regime indicated by the clockwise
looping low-level hodograph. This lift/forcing is not as strong
and focused...so it is simply more difficult to know if...when
and where a supercell may develop in this warm air advection
regime.

After 00z...the very strong forcing for ascent associated with the
upper trough over southern Arizona and New Mexico is expected to
lift out over the Southern Plains. The latest model guidance is
converging on a solution that suggests a very well organized
squall line will develop near Childress...and move southeast
towards North Texas this evening. There are hints in the model
mass fields that this squall line could potentially be very well
organized...and may pose a widespread damaging wind threat as it
moves southeast across North Texas overnight. If this squall line
does indeed organize...it may persist long enough and produce
enough straight line wind damage to qualify as a derecho by the
time it exits the County Warning Area to the east well after midnight. The primary
time this squall line/Bow-echo is expected to move over North
Texas is from 10 PM to 3 am. The existence of this squall line is
far from 100 percent certain...but it will probably organize based
on the consistent signal in this morning's high resolution model
guidance. The primary threats within this squall line would be
damaging straight line winds. Assuming the squall line moves
through the area after sunset as currently forecast...this may
reduce the potential for high-end damaging winds. Regardless...
wind damage would still be likely as this squall like would
probably be very well organized. Large hail and a brief tornado
may also occur in this line...assuming it develops.

Last but not least...we will also be monitoring for heavy rainfall
and flash flooding potential this afternoon and evening. If a warm
front sets up along the Interstate 20 corridor...scattered
thunderstorms could develop along this boundary and train over
areas to the northeast. This may evolve into another localized
flash flooding event north of Interstate 20 and generally east of
Interstate 35...very similar to what happened on Friday. Will hold
off on a Flash Flood Watch for now as confidence in this taking
place is not high at this time. If it occurs...these locations
have already received quite a bit of rainfall on Friday...so
flooding would tend to occur quickly if storms begin to train over
this region through tonight.

Will continue to make short term forecast updates for convection
Mode through this evening. Expect storms will start developing
near the surface low and evolve into discrete supercells by/after
4 PM/21z.

Cavanaugh

&&

Aviation...
/issued 1256 PM CDT sun Apr 26 2015/
the next 24 hours will be a very active period for the north and
central Texas taf sites with conditions favorable for numerous
showers and thunderstorms...some of which will be severe.

Convection is ongoing southwest of metroplex taf sites and just
to the west of Waco. This activity is elevated...and based around
10kft. Steep lapse rates through the column are likely
contributing to turbulence in this area as well. Lightning will be
the primary concern as this activity continues to push towards the
metroplex this afternoon.

Later this afternoon and evening...severe thunderstorms are
expected to develop along the dryline as surface based instability
increases due to strong low-level moisture advection. It appears
the greatest threat for severe storms near the metroplex will
occur after 02z. High resolution guidance is keying in on an area
along a stalled frontal boundary which would be favorable for a
strong linear convective system to develop and ride the boundary
eastward toward the metroplex taf sites around midnight. This
solution is far from certain...but there is decent agreement from
short term guidance which lends credence to this scenario. The
main threat with this solution would be very strong winds...but
all types of severe weather would still be possible in this
favorable environment.

The line of storms being advertised along the stationary boundary
will likely be too far north to impact the Waco taf site.
However... Waco will be threatened by discrete supercells
developing late this afternoon and early evening along and ahead
of the dryline. Again...all types of severe weather will be
possible late this afternoon and evening in central Texas.

After storms clear out early on Monday morning...some borderline
IFR/MVFR ceilings will be possible in the wake of the widespread
convection. Have kept taf sites on the low end of MVFR for now
since there is much uncertainty on the Post-convective environment
tomorrow.

Stalley

&&

Preliminary point temps/pops...
Dallas-ft. Worth, Texas 63 69 53 63 48 / 70 60 60 20 5
Waco, Texas 64 76 54 65 46 / 60 60 40 10 0
Paris, Texas 56 66 52 63 45 / 70 70 70 30 5
Denton, Texas 62 66 52 63 45 / 70 60 60 20 5
McKinney, Texas 60 66 52 63 46 / 70 70 70 20 5
Dallas, Texas 63 69 53 63 50 / 70 60 60 20 5
Terrell, Texas 61 70 54 63 48 / 60 70 70 20 5
Corsicana, Texas 64 73 54 64 48 / 60 60 60 20 0
Temple, Texas 64 81 55 66 48 / 60 30 40 10 0
Mineral Wells, Texas 61 71 53 63 46 / 70 60 40 10 0

&&

Forward watches/warnings/advisories...
none.

&&

$$

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