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

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FXUS64 KFWD 181724 AAA

Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
1224 PM CDT Thu May 18 2017

Generally quiet conditions late this morning/early this afternoon
are expected to yield to scattered convection during the mid-afternoon
hours. The 12 UTC FWD RAOB indicated a stout capping inversion
across much of North and Central TX. Beneath this inversion,
however, was a good plume of low to mid 70 degree dewpoint air. We
are sending up a special balloon to examine the strength of this
cap, which is expected to weaken a bit. This will be discussed
later. At this hour, the moist plume continues to stream
northward ahead of a sharpening dryline that was located across
the OK and TX panhandles and extended southward to near Lubbock
TX. This feature will progress eastward through the early
afternoon hours and should be the impetus for some strong to
severe storms across parts of North and Central TX later today.

The dryline should make noticeable progress towards the east,
into western Oklahoma as a conveyor of strong mid level flow
arrives from the west. Farther to the south across North and
Central TX, the overall motion of the dryline will mix a bit
slower towards the east. Model forecast soundings do indicate that
the cap will be quickly eroding through the early to mid
afternoon as heating at the surface and cooling aloft (evidenced
on experimental GOES-16 data) continue. The lack of strong large
scale forcing across the area,however, should allow initial
activity to be isolated to widely scattered. Right now, the most
likely timing for initial thunderstorm development will be between
2-3 PM out across western zones (west of highway 281 corridor).
Deep layer shear values (which are on the order of 40 knots)
oriented orthogonal to the dryline should ensure that storms that
initially develop will most likely be superceullar in nature. CAPE
values will be approaching the extreme end of the spectrum with
values forecast to near the 4,500-5,000 J/kg range.

Given the good potential for extreme updraft velocities, large to
giant hail (up to the size of baseballs) appears very realistic.
Forecast soundings also indicate that very dry air will be in
place which could further enhance the hail production. More
importantly, the presence of dry air at the mid-levels may also
support cold pool generation. As a result, we could see the
development of a storm complex or two which transition the hazards
from one of large to giant hail to that of damaging thunderstorm
outflows late this afternoon and into the early evening hours.
Overall, the tornado threat from a synoptic and even meso
standpoint doesn`t appear great, but it is non-zero, especially
initially when thunderstorms may be a bit more isolated and
discrete. This will be discussed a bit more in the next paragraph.

Strong to severe thunderstorms will likely continue into the
afternoon and evening hours closer towards the I-35 corridor
perhaps around the 6-7 PM time frame. By this time, the overall
mode SHOULD support mainly a large hail and damaging wind threat.
Around and just beyond this time frame, the low level wind
fields will intensify resulting in large, looping hodographs
favorable for a few tornadoes, should discrete storms remain.
There will also be a threat for brief spin ups along the leading
edge of any storm complex lines, with the assumption that
convective outflows don`t accelerate from the main batch of
convection. The best storm and thus severe potential beyond 8 PM,
should be near and just north of the I-20 corridor. The one
possibility, which warrants low PoPs farther south during the
evening hours, however, is if a large complex develops and the
cold pool continues to accelerate to the east.



/ISSUED 652 AM CDT Thu May 18 2017/
A strong push of Gulf moisture overnight has resulted in MVFR
stratus, which now blankets much of the region. Early morning
satellite imagery reveals cloud cover is gradually becoming more
splotchy, and our now-ascending morning balloon shows that the
saturated layer is actually quite thin just under the capping
inversion. As a result, it appears cigs should fluctuate roughly
around FL020 for a few more hours this morning before diurnal
mixing begins to quickly lift and scatter the cloud deck by around
17Z, potentially even earlier. As mixing ensues, a few streamer
showers will be possible, but these mainly look to impact Waco
later this morning. At Waco, it appears that mechanical mixing has
been too great to allow cigs to build down to IFR. As a result,
have removed any explicit mention from the TAF, although we can`t
rule out a brief dip towards 010 this morning.

The next aviation focus will then turn to thunderstorms, which are
expected to blossom along a dryline this afternoon. There are a
couple conflicting signals in recent high-resolution guidance
regarding storm placement, however. The main focused area of
ascent will remain displaced well to the north and west of the
region--across the Texas and Oklahoma Panhandles. As a result,
convergence along mesoscale features (the dryline) will be a
primary driver of convective initiation today, and where this will
occur is difficult to pinpoint. Earlier guidance seemed keen on
moving a complex of storms across the Metroplex late this
afternoon, but recent runs now seem to be favoring areas to the
south. Given these uncertainties, opted to continue a mention of
VCTS at all sites this afternoon, but a TEMPO for TSRA may be
warranted later as the presently nebulous convective trends
become more clear.

After this activity clears, VFR should prevail for a period this
evening before MVFR stratus once again surges across the TAF sites
late tonight.



.PREV DISCUSSION... /ISSUED 357 AM CDT Thu May 18 2017/
An upper level low currently over the Intermountain West will be
the main upper level feature of interest through the upcoming
weekend. This low will move south southeastward across Utah today,
then move east and then northeast across Colorado Friday and then
out into the Northern and Central Plains Saturday. As a trailing
trough moves east, a cold front will move southeast into the
northwestern parts of North Texas overnight Friday night and
through the southeastern part of the forecast area before sunset
Saturday. As a surface ridge that will build in behind the cold
front moves east Monday, the front will start to lift back
northward. We will be in northwesterly flow aloft for much of
next week which will keep on and off chances of showers and
thunderstorms in the forecast.

Today...Southerly surface surface winds of 15 to 25 mph will
continue to feed rich Gulf moisture northwards. A dryline is
expected to develop across West Texas which will be the main
focus for thunderstorm development. Expect scattered thunderstorms
to move east into North Texas mid to late afternoon and continue
to develop in areal extent through the evening. Given the strong
instability, steep lapse rates and moderate to strong effective
bulk shear, supercell thunderstorms are likely. These storms will
have the potential to produce golf ball size hail or larger and
damaging downburst winds. In addition, a few tornadoes will also
be possible.

There will be a low chance of showers and thunderstorms Friday
morning but the next significant round of thunderstorms is
expected Friday afternoon and Friday night. We will continue to
have deep moisture, high instabilities and moderate to strong
effective bulk shear in place. Expect showers and thunderstorms
develop ahead of the dryline that should approach the western
parts of the forecast area mid to late afternoon. These
thunderstorms will move into the forecast area. Additional
development is expected overnight Friday night ahead of the cold
front. Once again, large hail and damaging winds are expected with
some of these storms and a few tornadoes cannot be ruled out.

As the cold front moves southeast overnight Friday night into
Saturday, there will also be the potential for some locally heavy
rain that may result in some flash flooding. Rain chances will
decrease across areas north of I-20 Sunday and Sunday night.
As the front lifts back to the north on Monday, chances of showers
and thunderstorms will return area wide. Another cold front will
move through the region overnight Monday night into Tuesday.



Dallas-Ft. Worth    90  72  85  71  80 /  30  50  40  60  70
Waco                90  72  87  72  83 /  30  30  40  40  70
Paris               86  69  84  69  78 /  20  40  40  50  70
Denton              90  71  84  67  78 /  40  50  40  60  60
McKinney            88  70  84  69  79 /  30  50  40  60  70
Dallas              90  72  86  72  81 /  30  50  40  50  70
Terrell             88  72  86  71  81 /  20  40  40  40  70
Corsicana           89  72  88  72  82 /  20  30  40  30  70
Temple              90  72  87  72  84 /  30  30  40  30  60
Mineral Wells       88  70  84  67  77 /  50  50  50  60  60




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