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Area forecast discussion...updated
National Weather Service Fort Worth Texas
619 am CDT Mon Apr 24 2017

/12z tafs/

VFR with no concerns this taf cycle outside of some stratus that
could skim Waco Regional Airport after 12z Tuesday. For now,
confidence is too low to insert into forecast. Some scattered to
broken cirrostratus will like occur today along and south of I-20
from a disturbance moving out of northeast Mexico. No rainfall is
expected. SW winds 5-10 knots will increasing to S 10 to 15 knots
with gusts to 20 knots possible by afternoon and continuing at
times tonight.



Previous discussion... /issued 417 am CDT Apr 24 2017/
quiet and very warm conditions are forecast over the next couple of
days with little to no precipitation expected. Warm conditions are
expected again on Wednesday with a low rain chances, mainly across
the eastern third of north and central Texas as a cold front slides
through the area. The cold front will usher in cooler and breezier
conditions with most areas near and west of I-35 remaining
precipitation-free in its wake. Southerly warm and moist flow at the
surface will intensify through the latter half of the week and into
the weekend. While low level humidity will increase, southwesterly
winds aloft will ensure a Stout cap is in place which will limit the
areal extent of precipitation chances. Current thinking is that the
best chances for a higher coverage of showers and storms won't occur
until the weekend.

Monday and Tuesday should be precipitation-free, despite the
sharpening of the dryline out across western zones. While this
boundary will be present, southwesterly flow aloft will result in
the development of a formidable cap remain in place. If convection
can breach the cap and maintain itself, it will have the potential
to become severe. This scenario appears highly unlikely at this
time. Monday will be mostly sunny with a few passing high clouds,
especially across southern zones. Temperatures will climb into
the upper 70s to mid 80s on Monday afternoon. The dryline will
become more well defined on Tuesday and should slowly progress
towards the I-35 corridor. In the wake of the dryline,
southwesterly winds should promote downslope warming and allow
temperatures to climb to above normal levels with readings in the
mid 90s. To the east, high temperatures are also expected to be
above normal with many areas in the mid to upper 80s. If the
dryline mixes a bit further to the east, it's possible that some
locations along the I-35 corridor may near or break record high
temperatures. There may be an low-end fire weather threat across
areas west of I-35 as forecast soundings are indicative of a
classic dryline passage with steep lapse rates and unidirectional
wind shear. Both of these factor will likely Foster some momentum
Transfer and breezy southwesterly winds. Overall, live vegetation
and generally poor energy release component's (ercs) should
mitigate a more appreciable fire weather threat. That being said,
the breezy conditions may support fire spread and a low-end
grassfire danger statement may be warranted on Tuesday. The
dryline should mix back towards the east during the overnight
hours into Wednesday morning as moisture sloshes back up along the
higher terrain back towards the west as a low level jet of around
35-40 knots develops.

On Wednesday morning, there could be some streamer showers across
far eastern zones, but model guidance seems to frequently over do
the amount of precipitation. For now, will show a 15-20 pop out
across eastern zones where low level moisture is expected to be most
abundant and we could see the development of a few light returns
beneath the strong temperature inversion. Models are in fair
agreement with the synoptic pattern regarding wednesday's rain
potential. An upper trough should dive south and eastward out of the
4-corner's region. As this occurs, low level moisture should lift
northward quickly out of the Gulf of Mexico. The quality of moisture
should be somewhat poor given the recent cold front intrusion and
the return flow in most guidance appears to be overdone, with the
GFS being the most aggressive. The European model (ecmwf)/consmos output appear to be
the most reasonable, so will trend dewpoints and pop chances towards
these solution. That in mind, a sharp dryline should still progress
towards the east with a fast moving cold front diving southward out
of Oklahoma. There is some discrepancies between timing of frontal passage
with the 00 UTC NAM being the fast outlier as opposed to the 00 UTC
GFS/00 UTC European model (ecmwf) solution. The 18 UTC ttu-WRF seems to be more in
line with the GFS/European model (ecmwf) and given the higher resolution of this
model, went more in line with the GFS/European model (ecmwf) /ttu-WRF consensus.
Forecast soundings indicate that a strong cap will be in place until
the late afternoon hours and thus, the front should pass through the
western 2/3rds of the forecast area without much fanfare (outside of
some breezy conditions). The one area that we will need to monitor
for possible convective development will be along the front/dryline
interface up along the Red River. Interestingly enough, model output
remains scant on precip chances here, but we will have to monitor
trends as in there could be sufficient mesoscale ascent here for
some convection. Farther to the east, the front will pass through at
a time in which the cap should begin to weaken courtesy of daytime
heating and large scale ascent as 70 to 80 meter heights overspread
the area.

While this is a pretty potent upper trough forecast to move through
on Wednesday, there's too much uncertainty with the depth and
quality of the moisture to warrant anything higher than a 30-40%
pop. The strongest synoptic scale forcing for ascent (and thus rain
chances) is expected across northeastern and eastern zones.
Instability and shear parameters do support a conditional risk
(conditional if storms develop) for strong to severe thunderstorms
with the main risks being large hail given the steep lapse rates and
strong/damaging thunderstorm wind gusts given the likely high based
nature of convection and amount of mid-level dry air. The tornado
risk looks to be limited as low level winds will veer slightly. The
gradual upscale growth into more of a linear complex should also
limit the tornado potential.

In the wake of the front, lower temperatures and breezy conditions
are expected as 12mb/6 hr surface pressure rises overspread areas
north of the I-20 corridor. Temperatures will fall down into the 70s
and 80s with dry low level air spilling southward. This dry air will
likely seep all the way down to the Texas Gulf Coast which will have
important ramifications for thursday's precipitation chances. Winds
will shift back towards the east and southeast quickly on Thursday
as the next upper trough develops out towards the west. Initially,
models advertised that the cold front would stall out across
portions of central TX, but recent runs, which have a deeper trough,
now push the front as far south as the Upper Texas coast. Given the
amount of dry air forecast to spill southward in the wake of the
front, it appears unlikely that sufficient moisture will lift back
towards the north in time for appreciable rain chances on Thursday.
As a result, i'll show a dry forecast for all of the area through
Thursday. Low level moisture will return quickly on Friday ahead of
the next upper trough.

As expected, models have slowed the eastward progression of the
upper trough as it should continue to dig southward through Arizona
and New Mexico. The warm front will lift northward, and while there
may be some ascent atop the frontal surface, it appears that the
best focus will be to the north of the Red River late Thursday into
Friday. With strong capping expected per forecast soundings and
little in the way of forcing across north and central TX, rain
chances should generally be below 20 percent, mainly along the Red
River zones. Late Friday afternoon, the dryline does appear to
sharpen somewhat, beneath the southwest flow aloft. While there is
the potential that a subtle wave could ripple through, it appears
pretty unlikely at this time and so for now, will show a 10-15 pop
mainly out across western zones and along the Red River. If a storm
can develop, it's probable that they will be strong to severe as
shear and instability parameters will support supercellular storm
structures. At this time, it's too early to tell whether or not
enough forcing for ascent will be in place to breach the cap across
north TX, so for now, stay tuned for updates.

The long term forecast remains challenging and diverse model
solutions result in low confidence in timing and placement of pops.
As a result, significant alterations were not made to this portion
of the forecast from the previous issuance. Saturday and into Sunday
at this time appear to be the better chances for more widespread
showers and thunderstorms. Given the forecast shear and instability
profiles a few of these thunderstorms could be strong to severe. The
00 UTC European model (ecmwf) appears a bit more progressive with the upper trough
and slice the dryline quickly through north and central Texas through
the day on Saturday. The 00 UTC GFS and to some extent, the 00 UTC
Canadian are a tad slower and keep the dryline back towards the west
before advancing it eastward through the day on Sunday. At this
time, am inclined to lean more on the GFS/Canadian solution given
model trends and the greater likelihood that the upper trough will
continue to dig southward, thus slowing its eastward progression.
With that being said, I broad-brushed pops with values in the 20-40%
range given the uncertainty in timing on Saturday and Sunday. We'll
have to watch the weekend carefully as robust values of instability
and shear could mean the potential for a severe weather episode
across the region.



Preliminary point temps/pops...
Dallas-ft. Worth 80 62 89 67 77 / 0 0 10 10 10
Waco 80 59 88 70 85 / 5 5 10 10 10
Paris 78 56 83 65 72 / 0 0 10 20 40
Denton 79 58 89 64 74 / 0 0 10 10 10
McKinney 79 59 85 66 75 / 0 0 10 10 20
Dallas 80 63 87 68 79 / 0 0 10 10 10
Terrell 79 59 85 67 78 / 0 0 10 10 30
Corsicana 80 59 86 68 84 / 0 0 10 10 30
Temple 80 60 87 70 88 / 5 5 10 10 10
Mineral Wells 82 58 94 63 76 / 5 5 5 10 5


Forward watches/warnings/advisories...

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