Flash Flood Warning 12, NWS Fort Worth, TX

Area Forecast Discussion
National Weather Service Fort Worth TX
613 AM CDT Sat May 18 2019

.AVIATION...
/12z TAFs/

A swath of MVFR stratus currently overspreads all TAF sites which
will prevail through the morning hours. Some warm advection
showers have also developed within the I-35 corridor which will
intermittently produce some light rain over the next few hours.
Meanwhile, widespread convection ongoing across West Texas and the
Concho Valley will continue to congeal and organize and make a
run towards the I-35 corridor by late morning. Additional isolated
storms may develop ahead of this line as well. Overall,
confidence in timing of convection at the TAF sites is medium to
low at this point, but the most likely time for TAF sites to be
affected seems to be generally late this morning into the early
afternoon hours when a Tempo TSRA mention has been maintained. At
least a few hours of TS impacts are likely, and some storms at
the TAF sites could become severe with hail and/or damaging wind
gusts. Showers and possibly a lingering storm could continue
through as late as 00z at any of the TAF sites, although most
activity should be shifting east by early evening at the latest.

Overnight, a weak cold front will approach the DFW Metroplex TAF
sites. Some MVFR cigs may develop in advance of this feature, or
possibly even some patchy fog if surface winds become light enough
in the vicinity of the stalling front. For now will show some MVFR
cigs, but a vsby reduction may need to be introduced in subsequent
TAFs. The front will eventually cause a weak northerly wind shift
Sunday morning, and have shown this in the extended DFW TAF.

-Stalley

&&

.SHORT TERM... /Issued 250 AM CDT Sat May 18 2019/
/Through Tonight/

An episode of severe weather is expected today, although exactly
how convection and attendant threats will unfold remain in
question. There are three main points of concern discussed below,
all of which are being handled by varying degrees of quality by
latest guidance. Currently not a fan of any lone CAM solution, as
all are struggling with timing/placement of initiation within the
open warm sector. Despite this, larger scale factors can still
deliver some clues as to how things may proceed today.

Our current setup consists of a moisture-rich boundary layer (70F
dewpoints) which overspreads much of the Southern Plains, capped
by a rather stout inversion in the 700-750 mb layer. Above this
inversion, steep lapse rates of 8 C/km are contributing to rather
large MUCAPE values which will continue climbing today. A deep
upper low exists across the Intermountain west, with broad
troughing and southwest flow aloft over Texas. A lead disturbance
pivoting through this parent trough is aiding in ascent across the
Concho Valley along with increased lift associated with warm
advection from a strengthening nocturnal low-level jet. The
result of this lift can be seen on morning satellite and radar,
with an increase in convective initiation and cooling cloud tops
on all activity west of the forecast area. This region of
convection will be the first of three concerns for today.

An eventual vague organization of convection to our west is
expected to occur over the next 3-6 hours as it continues moving
eastward. This may take some time to occur, as mid-level wind
fields are still relatively weak with the fastest flow still
positioned farther to our northwest closer to the upper trough.
Shear in the lowest 3 km from San Angelo to Abilene is only about
25 kts at best at the current time based on latest objective
analysis, although these values should increase with time as wind
fields strengthen with the approaching upper trough. The increased
shear should allow a complex/cluster to materialize and move
northeastward into our western zones sometime around daybreak.
This cluster should roughly follow a trajectory along with the
lead upper disturbance responsible for its initiation, which means
it should have a tendency to track into our North Texas counties
while activity on the southern end struggles and weakens,
especially as the low-level jet begins weakening later this
morning. The severity of this convection remains in question, and
its possible it may remain weakly elevated during an unfavorable
time of day for surface-based convection. This may limit what
would otherwise be a decent threat for damaging winds. More robust
updrafts within the convective cluster would still be capable of
some marginally severe hail simply due to large elevated
instability, but the rather modest shear should limit individual
updraft intensity and longevity. If activity is able to persist
into mid/late morning and encounter better surface-based
instability, a tornado spin-up could be possible along any more
organized segments within this cluster/line.

The morning round of storms will have major implications on the
severe threat through midday and into the afternoon, which is the
second point of concern. The northeast track of the convective
cluster described above would largely leave our Central Texas
areas ripe with large diurnally-driven surface based instability
through midday. When coupled with increasing deep-layer shear,
a supercellular storm mode would be likely, and capable of all
severe hazards. Very large low-level instability and low LCL
heights could contribute to a more favorable tornado potential,
especially the farther east one's location. In addition, any
isolated supercell storms will be able to feed on rather
remarkable instability values, and should be capable of large,
potentially significant hail as well. The location for these
threats really will not be known until mid-morning when we see the
evolution of the aforementioned convective cluster and how much
contamination of the environment it may cause in its vicinity.
Any areas unaffected by early morning storms will need to pay
close attention to the weather around midday through the
afternoon. However, this threat will only exist if the morning
cluster behaves as advertised above. A slower motion with the
morning storms could cause the afternoon supercellular threat to
exist back west into the I-35 corridor as well. But a faster
motion would limit this threat to areas farther southeast. More on
this alternate scenario is discussed below.

The third point of concern is the potential for any redevelopment
along the dryline boundary late this afternoon which would mostly
occur across our northwest. Depending on how fast morning
convection exists our western CWA, this area should have enough
time to recover and diurnally destabilize ahead of a sharp surface
dryline. Low-level convergence and continued synoptic scale
ascent ahead of the upper trough would allow new storms to
possibly develop by late afternoon. These storms would also
likely be supercellular in nature, and capable of a renewed
severe threat west of I-35 and north of I-20 into the early
evening. This potential is rather low, and again, will hinge on
events earlier in the day.

A seemingly less likely (but not impossible) alternate scenario
is for a line of storms to develop from the early morning
convection to our west and spread through the entire forecast area
by late morning or early afternoon. Because of the reasons
previously discussed (trough lifts north, nocturnal jet weakens,
modest 0-3 km shear), a well-organized line does not seem like
the most likely solution at this time. However, if it were to
occur, it would feature a threat for strong/damaging winds across
much of the forecast area along with an embedded tornado threat,
particularly east of I-35. This scenario would virtually end the
chance for any new afternoon development anywhere in the forecast
area.

By late evening, all activity should be exiting the area to the
southeast. A weak surface cold front will nudge into our
northwestern zones with cooler and drier air behind it. This
should lead to a rather quiet overnight period heading into Sunday
morning with cooler lows than previous nights.

-Stalley

&&

.LONG TERM... /Issued 250 AM CDT Sat May 18 2019/
/Sunday through Next Weekend/

An upper low will drop southeast across California on Sunday and
approach North and Central Texas from the west on Monday.
Light east to southeast winds on Sunday will veer and pick up
speed on Monday as the approaching low induces and helps to
strengthen a lee surface trough, with speeds of 15 to 25 MPH
expected by Monday afternoon. A weak ripple ahead of the low
pressure system could generate isolated convection across the
northwest third of the region on Monday, but the presence of a
capping inversion should keep Monday POPs on the low side.

A stronger shortwave pivoting around the base of the upper low
will generate more widespread thunderstorms across Northwest Texas
Monday afternoon. Forcing for ascent produced by the disturbance
combined with a strong low level jet should maintain convection
Monday night as activity moves into northwestern portions of the
CWA. Shear and instability will be sufficient for some of these
thunderstorms to become severe with both a damaging wind and a
hail threat present. A potential for a few tornadoes will also
exist, though the tornado threat will be likely be higher
northwest of the region where earlier activity will have likely
been composed of supercells.

A QLCS type of complex should be ongoing Tuesday morning as
activity approaches the I-35 corridor. The upper low (along with
its pivoting shortwave) will race off to the northeast during he
day, taking the better forcing with it. This and the weakening low
level jet will cause convection to weaken some during the morning
hours, but activity should re-intensify by midday as outflow
pushes into a moist and unstable environment. A few strong to
severe thunderstorms will again be possible Tuesday afternoon
along an east of I-35. Training convection will become more
likely during this time along with the potential for heavy rain
and localized flash flooding. Activity will push off to our east
Tuesday evening, which may be followed by patchy dense fog Tuesday
night due to clearing skies and saturated soils.

The rest of the week still looks to be dominated by a ridge in the
southeast part of the CONUS and a trough in the southwest, with
isolated dryline convection possible in our west and northwest
counties each evening. Temperatures will be slightly above normal
due to the influence of the ridge, with highs approaching 90 each
afternoon. Slightly better rain chances will become possible next
weekend as a piece of the southwest trough breaks off and moves
east around the northwest periphery of the southeast ridge. Even
better rain chances may return early the following week as the
next upper low approaches.

30

&&

.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth    82  66  86  70  86 /  80  40   0   0  20
Waco                82  71  88  72  86 /  70  30   0  10   5
Paris               81  68  85  65  86 /  70  70   0   5  20
Denton              82  60  87  68  86 /  80  40   0   5  30
McKinney            81  64  85  68  86 /  80  50   0   5  20
Dallas              83  67  87  72  88 /  80  40   0   0  20
Terrell             82  69  86  70  87 /  80  60   0   5  10
Corsicana           82  72  89  72  87 /  80  60   0   5   5
Temple              82  71  90  72  87 /  60  30   5  10   0
Mineral Wells       85  57  87  67  86 /  70  30   0   0  30

&&

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

26/30

Hazardous Weather Outlook
National Weather Service Fort Worth TX
354 AM CDT Sat May 18 2019

TXZ091>095-100>107-115>123-129>135-141>148-156>162-174-175-190900-
Montague-Cooke-Grayson-Fannin-Lamar-Young-Jack-Wise-Denton-Collin-
Hunt-Delta-Hopkins-Stephens-Palo Pinto-Parker-Tarrant-Dallas-
Rockwall-Kaufman-Van Zandt-Rains-Eastland-Erath-Hood-Somervell-
Johnson-Ellis-Henderson-Comanche-Mills-Hamilton-Bosque-Hill-Navarro-
Freestone-Anderson-Lampasas-Coryell-Bell-McLennan-Falls-Limestone-
Leon-Milam-Robertson-
354 AM CDT Sat May 18 2019

This Hazardous Weather Outlook is for North and Central Texas.

.DAY ONE...Today and Tonight.
A cluster or line of storms will organize to the west and spread
into North Texas through the morning hours. The main hazard with
this complex will be strong winds, but hail and a brief tornado are
also possible.

By late morning and early afternoon, additional storms
may develop to the east and south of this complex. If they do
develop, these more isolated storms would be capable of all severe
threats including damaging winds, large hail, and tornadoes. The
development of these storms will depend on both the speed and
expanse of the morning complex.

Thunderstorms should be tapering off from west to east during the
early evening hours.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.
After a break in thunderstorm chances on Sunday, additional storms
are possible possible on Monday into Tuesday across the region.
Severe weather will be possible during this time as well.

Low thunderstorm chances will also exist on Wednesday afternoon.

.SPOTTER INFORMATION STATEMENT...
Spotter activation is likely across parts of North and Central Texas
today.

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